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Research Article
A weighted taxonomic matrix key for species of the rotifer genus Synchaeta (Rotifera, Monogononta, Synchaetidae)
expand article infoTanja Wilke‡, Wilko H. Ahlrichs‡, Olaf R. P. Bininda-Emonds‡
‡ Carl von Ossietzky Universität Oldenburg, Oldenburg, Germany

Corresponding author: Tanja Wilke ( tanja.wilke1@uni-oldenburg.de )

Academic editor: Pavel Stoev
© 2019 Tanja Wilke, Wilko H. Ahlrichs, Olaf R. P. Bininda-Emonds.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation: Wilke T, Ahlrichs WH, Bininda-Emonds ORP (2019) A weighted taxonomic matrix key for species of the rotifer genus Synchaeta (Rotifera, Monogononta, Synchaetidae). ZooKeys 871: 1-40. https://doi.org/10.3897/zookeys.871.36435
ZooBank: urn:lsid:zoobank.org:pub:0A71AEE7-A92C-4DE8-A2F1-1355AF26CCA3
Open Access

Abstract

A new, weighted matrix identification key for 34 largely undisputed species of Synchaeta was created with the aim of providing comparable, detailed and diagnostic character sets for each species that can be applied to live and/or preserved specimens. As part of this process, 14 species of Synchaeta were intensively re-investigated with respect to their habitus and trophi morphology using binocular, light, and scanning electron microscopy, which, together with behavioural observations, revealed several new discriminating characters. Whenever possible, missing information for any character was added for the remaining species from the literature, with the two recently described species Synchaeta arcifera and Synchaeta squamadigitata being considered for the first time in an identification key. Beyond its completeness, our key has two distinct advantages. First, the characters are supported by detailed illustrations of their respective character states whenever possible to both simplify identification and minimize any uncertainty in the descriptions themselves. Second, the new approach of weighting the characters according to their reliability, robustness and/or ease of determination was employed. This latter approach is especially advantageous for soft-bodied rotifers such as species of Synchaeta, where, for example, several external characters can be influenced by preservation and are therefore less diagnostic or reliable. Although the key is as comprehensive as possible, information for many species remains missing for many characters, thereby highlighting the need for additional comprehensive and detailed species (re-)investigations within Synchaeta.

Keywords

Habitus, morphology, robust characters, species identification, swimming behaviour, trophi

Introduction

The rotifer genus Synchaeta (Monogononta, Synchaetidae) comprises approximately 37 (see Segers 2007) to 39 (see Jersabek et al. 2018) valid and truly planktonic species, of which approximately half occur in brackish and/or marine habitats (Hollowday 2002). Although their importance in aquatic food webs is unquestioned because of their often dominant role in the rotifer (Stemberger and Gilbert 1985) and metazooplankton communities (Arndt et al. 1990), specimens of Synchaeta in ecological studies are seldom identified to species level (Obertegger et al. 2006). To a large extent, this situation derives from the identification of and delimitation between species of this genus being regarded as being especially challenging (Pourriot 1965; Ruttner-Kolisko 1972; Koste 1978).

The several comprehensive revisions and keys of Synchaeta that exist (e.g., Voigt 1956–1957; Ruttner-Kolisko 1972; Koste 1978; and most recently Hollowday 2002) tend to be restricted in that they limit themselves to describing the most concise set of characters that delimit each species. Although this represents a useful simplifying strategy, the inherently incomplete data set it entails presents two distinct disadvantages. First, because additional, alternative characters are not presented for many species, their identification is impossible when the respective, diagnostic ones are deformed or not clearly visible (e.g., foot morphology when it is retracted). Second, and more importantly, direct comparisons among species are usually not possible because the species are often described using different sets of characters.

To address both sets of issues, we have developed a new identification key for Synchaeta, with the dual aims of making it both easy to use and as comprehensive as possible by providing large, comparable data sets for each species. To accomplish this, we thoroughly re-examined live and prepared specimens of 14 species and intensively researched the literature for all members of Synchaeta, including the most recently described species Synchaeta arcifera Xu, 1998 and Synchaeta squamadigitata De Smet, 2006, which are presented for the first time in a comparative identification key. In addition to an in-depth analysis of the habitus, we focussed on the trophi in particular because they are considered to be both species-specific (De Smet 1998; Fontaneto and Melone 2006; Wulfken et al. 2010) and less susceptible to conservation (Kutikova 1970 as cited in Obertegger et al. 2006). Detailed information about the trophi are therefore of great advantage in ecological studies, for example, where the material is necessarily fixed for practical reasons (Obertegger et al. 2006; Labuce and Strake 2017), with the consequence that the species identity of distorted or contracted rotifer specimens might be determinable only via their trophi (De Smet 1998; Segers 2004). By providing comparable data sets for each species, our taxonomic key also functions as a matrix key (also known as free access or multi-access key; see Hagedorn et al. 2010), which is better able to incorporate missing information when identifying species than the more traditional dichotomous key.

In addition, we weighted all characters within our matrix key according to their perceived discriminatory power. This strategy of focussing on more robust and diagnostically conclusive characters potentially facilitates accurate species identification by giving characters that are more susceptible to variation or artefacts (e.g., body shape, which is strongly affected by the pressure of a cover slip (Koste 1978), fixation/preservation (Ruttner-Kolisko 1972; Segers 2004) or by developing eggs and stomach content in soft-bodied rotifers) less impact than more constant and robust ones (e.g., the number and position of the lateral antennae.) To further simplify the identification process, we supported the characters with detailed illustrations and photographs of the respective character states whenever possible and introduce a consistent and distinctive terminology for homologous structures. Although the latter point seems obvious, the use of synonyms for homologous structures is a widespread problem, even within the same key. For example, Hollowday (2002: 90) variously denotes the apical receptors as “sensory frontal antennae”, “sensory antenna”, “frontal prominence with tuft of setae” or “sensory setae” in his identification key for species of Synchaeta.

Our purpose here is to deliver a comprehensive and robust key for Synchaeta by which a reliable identification of live and preserved specimens is feasible through a comprehensive and comparable morphological data set. In so doing, the present study not only confirmed existing discriminatory characters, but also re-described some of them more explicitly (e.g., foot shape and morphology of the apical receptors) as well as established several novel ones for species demarcation (as e.g., behaviour, morphology of the pedal glands, detailed fulcrum and ramus morphology).

Materials and methods

Study site and sampling

Using a 55-µm mesh plankton net, sampling for species of Synchaeta took place intermittently between June 2013 and August 2017 in northwest Germany from freshwater habitats in Oldenburg, Lower Saxony and Tecklenburger Land, North Rhine-Westphalia as well as from brackish and marine habitats in Wilhelmshaven, Lower Saxony (Table 1). Species of Synchaeta found in the samples (Table 2) were identified using the existing information in Rousselet (1902), Voigt (1956–1957), Ruttner-Kolisko (1972), Koste (1978) and Hollowday (2002).

Binocular and light microscopical (LM) investigations

Undisturbed, living specimens were initially observed in a petri dish using a binocular microscope to examine their (swimming) behaviour. For the LM analyses, single individuals were isolated and carefully sedated with carbonated water before being further immobilized through the slight pressure of a cover glass. For the latter step, extreme care was taken to avoid any deformation of the body, which could lead to morphological artefacts. All observations used differential interference contrast using a LEICA DMLB microscope and digital photographs were taken using a Canon EOS 5D Mark II camera.

Table 1.
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Sampling locations with their corresponding coordinates and habitat characterizations. Abbreviations: OL = Oldenburg, TL = Tecklenburger Land, WHV = Wilhelmshaven.

Location Coordinates Type of Habitat Salinity in PSU
Schlossteich (ST) OL 53.1603N; 8.1195E permanent freshwater pond 0
Löschteich (LT) OL 53.151957N; 8.166833E permanent freshwater pond 0
Haarenniederung (HN) OL 53.147092N; 8.171273E temporary freshwater winter puddle 0
Haarenstau (HS) OL 53.155623N; 8.105789E temporary freshwater winter puddle 0
Heiliges Meer (HM) TL 52.351944N; 7.633611E permanent freshwater lake 0
Banter See (BS) WHV 53.50906N; 8.1143116E tide-independent, brackish basin ca. 8
Yachthafen (YH) WHV 53.5097712N; 8.1216346E tide-independent, brackish basin ca. 20
Nassauhafen (NH) WHV 53.5129901N; 8.1458015E North Sea coast, marine habitat ca. 30
Table 2.
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Sampled species of Synchaeta. Abbreviations: BS = Banter See; HM = Heiliges Meer; HN = Haarenniederung; HS = Haarenstau; LT = Löschteich; NH = Nassauhafen; ST = Schlossteich; YH = Yachthafen.

Species Location Date Number of specimens examined
From freshwater habitats
S. grandis ST June 2016 18
S. kitina HM April 2017 ca. 25
S. longipes ST June and July 2013 15
S. oblonga ST March 2016 ca. 70
S. pectinata ST April to June 2013 and 2015 ca. 120
S. stylata ST June to August 2016 and 2017 ca. 50
S. tremula ST, LT March to May 2016 ca. 90
S. tremuloida HS, HN November to January 2015/2016 ca. 70
From brackish and marine habitats
S. triophthalma NH April 2016 ca. 25
S. hutchingsi YH August 2017 18
S. grimpei YH, NH April 2016 13
S. gyrina BS, NH January and April 2016 ca. 60
S. baltica BS, NH January and April 2016 ca. 35
S. vorax NH April 2016 17

Scanning electron microscopical (SEM) investigations

For SEM examinations of the habitus, single specimens were initially sedated with carbonated water before being euthanized with 1% OsO4 buffered in 0.1 M NaCa cacodylate buffer and fixed with 240 mOsmol picric acid-formaldehyde (Melone and Ricci 1995). To examine the isolated trophi, the surrounding tissue of selected specimens was dissolved according to the protocol of Kleinow et al. (1990) by transferring them into a droplet of dissolving agent (0.1 g dithiothreitol added to a 5-ml stock solution of 5.2 g sodium dodecyl sulphate + 0.24 g NH4HCO3 in 100 ml aqua dest; AppliChem, Darmstadt, Germany) for ca. 15 min before being rinsed with distilled water subsequently. Thereafter, samples of either the habitus or the trophi were dehydrated using an ascending, graded ethanol series. Following critical-point drying, each sample was attached onto an SEM stub and coated with gold-palladium before being examined on a Hitachi S-3200N SEM.

Illustrations

All new illustrations of the habitus were made using Adobe Illustrator CS4 based on representative digital photographs. References from drawings that we have obtained and modified from literature are listed below each illustration.

Included species and information from the literature

Overall, 34 species of Synchaeta were considered in our key, with six species that are recognized by either Segers (2007) and/or Jersabek et al. (2018) being excluded (see lists below). Information about species that we did not find in our samples derive from their respective initial descriptions and from the literature, with an emphasis on Rousselet (1902), Lauterborn (1905), Lie-Pettersen (1905), Peters (1931), Voigt (1956–1957), Ruttner-Kolisko (1972), Koste (1978) and Hollowday (2002). Information or interpretations that we have made from illustrations or photographs that are derived from other sources than the above-mentioned literature are indicated below each table (Tables 3–8). Species that are in urgent need of re-investigation because of inconsistent, ambiguous or highly incomplete descriptions (see Tables 3–8; “?“) and/or species that are known exclusively from preserved material are indicated with an asterisk in the following lists.

List of recognized freshwater species (Fig. 1A–M)

Synchaeta pectinata Ehrenberg, 1832 (Fig. 1A)

Synchaeta grandis Zacharias, 1893 (Fig. 1B)

Synchaeta oblonga Ehrenberg, 1832 (Fig. 1C)

Synchaeta tremula (Müller, 1786) (Fig. 1D)

Synchaeta tremuloida Pourriot, 1965 (Fig. 1E)

Synchaeta prominula Kutikova & Vassiljeva, 1982 (Fig. 1F)*

Synchaeta kitina Rousselet, 1902 (Fig. 1G)

Synchaeta stylata Wierzejski, 1893 (Fig. 1H)

Synchaeta longipes Gosse, 1887 (Fig. 1I)

Synchaeta verrucosa Nipkow, 1961 (Fig. 1J)

Synchaeta lakowitziana Lucks, 1930 (Fig. 1K)*

Synchaeta pachypoida Kutikova & Vassiljeva, 1982 (Fig. 1L)*

Synchaeta pachypoda Jashnov, 1922 (Fig. 1M)*

List of recognized brackish, marine or inland saline species (Fig. 2A–U)

Synchaeta grimpei Remane, 1929 (Fig. 2A)

Synchaeta baltica Ehrenberg, 1834 (Fig. 2B)

Synchaeta johanseni Harring, 1921 (Fig. 2C)*

Synchaeta bicornis Smith, 1904 (Fig. 2D)*

Synchaeta gyrina Hood, 1887 (Fig. 2E)

Synchaeta triophthalma Lauterborn, 1894 (Fig. 2F)

Synchaeta cecilia Rousselet, 1902 (Fig. 2G)

Synchaeta vorax Rousselet, 1902 (Fig. 2H)

Synchaeta fennica Rousselet, 1909 (Fig. 2I)*

Synchaeta cylindrica Althaus, 1957 (Fig. 2J)*

Synchaeta tavina Hood, 1893 (Fig. 2K)*

Synchaeta neapolitana Rousselet, 1902 (Fig. 2L)*

Synchaeta hutchingsi Brownell, 1988 (Fig. 2M)

Synchaeta atlantica Zelinka, 1907 (Fig. 2N)*

Synchaeta rousseleti Zelinka, 1927 (Fig. 2O)*

Synchaeta glacialis Smirnov, 1932 (Fig. 2P)*

Synchaeta hyperborea Smirnov, 1932 (Fig. 2Q)*

Synchaeta arcifera Xu, 1998 (Fig. 2R)*

Synchaeta tamara Smirnov, 1932 (Fig. 2S)*

Synchaeta bacillifera Smirnov, 1933 (Fig. 2T)*

Synchaeta squamadigitata De Smet, 2006 (Fig. 2U)*

Excluded species

In general, we excluded species of Synchaeta that are ranked as a species inquirenda (= species of doubtful identity) by Jersabek et al. (2018) or for which we strongly suspect this to be the case based on our own observations.

1. Synchaeta curvata Lie-Pettersen, 1905: Insufficient description (Hollowday 2002) and currently ranked as a species inquirenda in Segers (2007).

2. Synchaeta elsteri Hauer, 1963: Insufficient description based on preserved specimens (Hollowday 2002). This species was also ranked as a species inquirenda in Segers (2007).

3. Synchaeta jollyae Shiel & Koste, 1993: Described based on preserved specimens and synonymy with S. stylata suspected (Wilke et al. 2018a).

4. Synchaeta littoralis Rousselet, 1902: Synonymy with S. oblonga suspected (Koste 1978; Hollowday 2002; Wilke et al. 2018b). This species was also ranked as a species inquirenda in Segers (2007).

5. Synchaeta monopus Plate, 1889: Existing descriptions are insufficient, inconsistent and made exclusively on the basis of preserved and presumably deformed specimens (Wilke et al. 2019).

6. Synchaeta rufina Kutikova & Vassiljeva, 1982: Synonymy with S. oblonga suspected (Wilke et al. 2018b).

Figure 1. 

Species of Synchaeta from freshwater habitats. A S. pectinata B S. grandis C S. oblonga D S. tremula E S. tremuloida F S. prominula G S. kitina H S. stylata I S. longipes J S. verrucosa K S. lakowitziana L S. pachypoida M S. pachypoda. Drawings modified from: F, L, M Kutikova and Vassiljeva (1982) J Jersabek et al. (2003b) K Lucks (1930). Scale bar: 100 µm.

Character clarification, character weighting, and species identification

To support the written descriptions, the morphology for each character state is also exemplified both through illustrations as well as the naming of at least one exemplar species that possesses the respective state.

The character states are represented in detailed tables (Tables 3–8) and in a numerical list for each species where the text is formatted according to the perceived reliability and/or discriminatory power of the states:

1. “?”: The character state is unknown or ambiguous for the respective species – further examinations are required.

2. brackets: The character state rarely occurs in the species.

3. italics: The character is variably expressed within the species or its interpretation is either subjective or can be easily misunderstood because of potential artefacts that can arise during preparation. These characters should be applied with caution.

4. normal text: The character state is more or less robust, but shared by several, additional species of Synchaeta. Many characters of this quality are usually required for species demarcation in the form of a unique character set for each species.

5. blue color: The character state is robust and important insofar as it is unique for the species and/or shared by only a few, additional species of Synchaeta. Individual characters in this category typically exclude many other congeneric species to greatly simplify species demarcation.

6. bold: The character state is robust and species-specific (autapomorphy).

To simplify the identification process, blank character checklists and tables for recording character states are appended (Suppl. material 1: Tables S1, S2).

Figure 2. 

Species of Synchaeta from brackish, marine and inland saline habitats. A S. grimpei B S. baltica C S. johanseni D S. bicornis E S. gyrina F S. triophthalma G S. cecilia H S. vorax I S. fennica J S. cylindrica K S. tavina L S. neapolitana M S. hutchingsi N S. atlantica O S. rousseleti P S. glacialis Q S. hyperborea R S. arcifera S S. tamara T S. bacillifera U S. squamadigitata. Drawings modified from: C Harring (1921) D Koste (1981) G, K, L Rousselet (1902) I Rousselet (1909) J Althaus (1957) N Zelinka (1907) O Zelinka (1927) P, Q, S Smirnov (1932) and Friedrich and De Smet (2000) R Rougier and Pourriot (2006) T Smirnov (1933) U De Smet (2006). Scale bar: 100 µm.

Results

The characters are categorized into those for habitat and behaviour (characters 1–6), size (character 7), head and neck region (characters 8–16), trunk (characters 17–23), foot, pedal glands and toes (characters 24–37), sensory system (characters 38–50) and trophi (characters 51–60). The respective character states for each species are presented in Tables 3–8.

Identification characters

Habitat and behaviour (Table 3)

1. Habitat

a. freshwater (exemplar S. grandis)

b. brackish (exemplar S. bicornis)

c. marine (exemplar S. atlantica)

d. inland saline (exemplar S. cylindrica)

2. Swimming duration

a. exclusively pelagic (exemplar S. pectinata) or only adheres to objects transiently when disturbed (exemplar S. oblonga)

b. interrupted by frequent, long-lasting adherences to diverse objects (e.g., plants; exemplar S. tremula)

3. Adherence to diverse objects

a. absent (exemplar S. pectinata) or only transiently and only when disturbed (exemplar S. oblonga)

b. long-lasting adherence without any twisting movement about the longitudinal axis (exemplar S. kitina)

c. long-lasting adherence combined with a twisting movement about the longitudinal axis (exemplar S. tremula)

4. Swimming motion (always combined with a rotation about the longitudinal axis)

a. in a straight line (Fig. 3A; exemplar S. tremula, S. kitina)

b. slightly coiled (Fig. 3B; exemplar S. stylata, S. tremuloida)

c. distinctly coiled (Fig. 3C; exemplar S. pectinata, S. grandis)

5. Foot position while swimming

a. partly or fully retracted (Fig. 3D; exemplar S. baltica)

b. not retracted (Fig. 3E; exemplar S. tremula)

6. Directional changes while swimming

a. many sudden directional changes (exemplar S. stylata, S. oblonga)

b. few or no sudden directional changes (exemplar S. pectinata)

Figure 3. 

Habitat and behaviour. A–C Examples of the swimming behaviour A straight (S. tremula) B slightly coiled (S. stylata) C distinctly coiled (S. pectinata) D–E Foot position (thick lines) while swimming D foot retracted (S. baltica) E foot not retracted (S. tremula). Scale bar: 50 µm.

Size (Table 3)

7. Overall body length of mature specimens (measured from the apical field to the distal tips of the toes, excluding the cilia)

a. less than 250 µm

b. more than 250 µm

Head and neck region (Table 4)

8. Apical field – Width in relation to the trunk width1

a. as wide as the trunk (Fig. 4A; exemplar S. tremuloida)

b. wider than the trunk (Fig. 4B; exemplar S. longipes, S. triophthalma)

9. Apical field – Elevation with respect to auricles

a. level (Fig. 4C; exemplar S. grimpei) to slightly elevated (Fig. 4D; exemplar S. tremula)

b. intermediate (Fig. 4E; exemplar S. triophthalma)

c. strongly elevated; distinctly convex (Fig. 4F; exemplar S. grandis)

10. Dorsolateral styles – Elevation

a. not raised to very slightly raised (Fig. 4G; exemplar S. tremula)

b. intermediate (Fig. 4H; exemplar S. gyrina)

c. strongly raised (Fig. 4I; exemplar S. baltica)

11. Auricles – Size

a. not clearly distinct from the rotatory organ (Fig. 4J; exemplar S. grimpei)

b. small (Fig. 4K; exemplar S. tremula)

c. medium (Fig. 4L; exemplar S. oblonga)

d. large (Fig. 4M; exemplar S. grandis)

12. Auricles – Orientation

a. directed laterally (Fig. 4J, K; exemplar S. tremula)

b. directed semi-caudally (Fig. 4L; exemplar S. oblonga)

c. directed caudally (Fig. 4M; exemplar S. grandis)

13. Neck region – Demarcation of the head and trunk regions

a. gradual transition from the head into the trunk region; the neck is neither constricted nor distinctly offset (Fig. 4N; exemplar S. tremula)

b. demarcated; the neck is narrower than the head and trunk (Fig. 4O; exemplar S. tremuloida)

c. demarcated by a sharp constriction (Fig. 4P; exemplar S. pachypoda) or by distinct transversal folds (Fig. 4Q; exemplar S. oblonga)

14. Saccular appendages at the neck region (that compensate for pressure changes in the body fluid through contraction of the body)

a. absent (Fig. 4A, B; exemplar S. pectinata)

b. present (Fig. 4R; exemplar S. arcifera, S. bacillifera, S. bicornis, S. fennica)

15. Saccular appendages – Location

a. ventral (exemplar S. bacillifera)

b. dorsal (exemplar S. arcifera, S. bicornis, S. fennica)

c. absent (exemplar S. pectinata)

16. Head region – Colour2

a. colourless / transparent (Fig. 5A; exemplar S. pectinata)

b. mastax or parts thereof moderately yellow or orange (Fig. 5B; exemplar S. longipes)

c. parts of rotatory organ or auricles slightly yellow to orange (Fig. 5C; exemplar S. grandis)

Figure 4. 

Head and neck region. A, B Relative width of the apical field (dashed lines) A as wide as the trunk (S. tremuloida) B wider than the trunk (S. longipes) C–F Elevation of the apical field (top line) relative to the auricles (bottom line) C level (S. grimpei) D slightly elevated (S. tremula) E intermediate (S. oblonga) F strongly elevated (S. grandis) G–I Elevation of the dorsolateral styles (thick lines) G not or very slightly raised (S. tremula) H intermediate (S. gyrina) I strongly raised (S. baltica) J–M Auricle size (thick lines) and orientation (arrows) J not clearly demarcated from rotatory organ, directed laterally (S. grimpei) K small, directed laterally (S. tremula) L medium, directed semi-caudally (S. oblonga) M large, directed caudally (S. grandis) N–Q Separation of the head and trunk region N gradual transition, the head is not distinctly offset from the trunk (S. tremula) O head and trunk are demarcated by the narrower neck (S. tremuloida) P demarcation by a sharp constriction in the neck region (S. pachypoda) Q by distinct transversal folds (S. oblonga) R Presence of saccate appendages (thick lines) caudal to auricles (S. bicornis). Drawings modified from: P Kutikova and Vassiljeva (1982) R Koste (1981). Scale bars: 50 µm.

Trunk (Table 5)

17. Trunk region – Shape3

a. conical: trunk decreases gradually in width caudally (Fig. 6A; exemplar S. tremula)

b. cylindrical: trunk elongate, decreases in width only in its caudal quarter (Fig. 6B; exemplar S. tavina)

c. bell- (Fig. 6C: exemplar S. tremuloida) to wineglass-shaped (Fig. 6D; exemplar S. longipes), trunk is slightly bulbous and narrows abruptly in its caudal third.

18. Anal pseudosegment

a. distinct anal pseudosegment present (Fig. 6E; exemplar S. oblonga)

b. anal pseudosegment barely visible or absent (Fig. 6A–D; exemplar S. longipes)

19. Posteriodorsal saccate appendage on the integument (that compensates for pressure changes in the body fluid through contraction of the body)

a. present (Fig. 6F; exemplar S. arcifera)

b. absent (Fig. 6A–E; exemplar S. longipes)

20. Longitudinal striae on the dorsal trunk

a. present (Fig. 6G; exemplar S. tavina)

b. absent (Fig. 6H; exemplar S. pectinata)

21. Internal organs – Location

a. occupy entire trunk region (Fig. 6I; exemplar S. oblonga)

b. occupy middle and caudal trunk regions; the oesophagus is the only structure present in the anterior trunk region (Figs 6J, 7C; exemplar S. pectinata)

c. occupy middle trunk region; cloaca ends in posterior quarter of trunk, anteriorly to the lateral antennae (Fig. 6K; exemplar S. grimpei)

d. stomach and ovary each occupy separate sides of the trunk (Fig. 6L; exemplar S. longipes)

22. Violet globules in the body cavity

a. present (Fig. 7A, arrow; only known for S. baltica, S. bicornis, and S. grimpei, where the globules can also be absent)

b. absent (Fig. 7B, C; exemplar S. pectinata)

23. Oesophagus – Morphology

a. short oesophagus that widens in its caudal half to form a proventriculus (Fig. 7B; exemplar S. tremula, S. tremuloida)

b. highly tensile oesophagus, narrow or broad and of equal width, with numerous longitudinal striae (Fig. 7C; exemplar S. pectinata, S. baltica)

Figure 5. 

Head and neck region. A–C LM images showing different colours in the head region A colourless / transparent (S. pectinata) B mastax moderately yellow or orange (S. longipes) C parts of rotatory organ or auricles slightly yellow to orange (S. grandis). Scale bar: 25 µm.

Figure 6. 

Trunk region. A–D Morphology of the trunk region (thick lines) A conical (S. tremula) B cylindrical (S. tavina) C bell-shaped (S. tremuloida) D wineglass-shaped (S. longipes) E Presence of a distinct anal pseudosegment (thick line; S. oblonga) F Presence of a posteriodorsal saccate appendage (thick line; S. arcifera) G, H Longitudinal striae on the dorsal trunk G present (S. tavina) H absent (S. pectinata) I–L Location of the internal organs I occupy entire trunk region (S. oblonga) J occupy middle and caudal trunk regions, oesophagus only structure in the anterior trunk (S. pectinata) K occupy middle trunk region, cloaca ends anteriorly to the lateral antennae (S. grimpei) L stomach and ovary each occupy separate sides of the trunk (S. longipes). Drawings modified from B, G Rousselet (1902) F Rougier and Pourriot (2006). Scale bar: 50 µm.

Foot, pedal glands and toes (Table 6)

24. Foot – Orientation

a. directed dorsally (Fig. 8A; exemplar S. tremula, S. baltica, S. cecilia)

b. coplanar with the longitudinal axis of the body or directed very slightly ventrally (Fig. 8B; exemplar S. grimpei)

c. directed ventrally (Fig. 8C; exemplar S. longipes)

25. Foot – Shape

a. minute, less than one-tenth of the overall body length; as long as or shorter than the toes (Fig. 8D; exemplar S. atlantica)

b. triangular, medium; between one-fourth and one-sixth of the overall body length (Fig. 8E; exemplar S. triophthalma)

c. conical, short to medium; less than one-fourth of the overall body length, but always longer than the toes (Fig. 8F; exemplar S. tremula)

d. slender, medium to long; ca. one-third to one-fifth of the overall body length (Fig. 8G; exemplar S. longipes)

e. broad, long; ca. one-third of the overall body length (Fig. 8H; exemplar S. johanseni, S. baltica)

f. cylindrical, massive; approximately one-half of the overall body length (Fig. 8I; exemplar S. pachypoda, S. pachypoida

26. Pedal glands – Symmetry

a. asymmetrical; either of different size and shape or only singly present (Fig. 9A–C; exemplar S. cecilia, S. triophthalma)

b. symmetrical (Fig. 9D–I; exemplar S. tremula, S. pectinata)

27. Pedal gland(s) – Number and arrangement4

a. one single pedal gland (Fig. 9A; exemplar S. neapolitana)

b. pedal glands are paired but of different size and shape; one is rudimental (Fig. 9B–C; exemplar S. cecilia, S. hutchingsi, S. tamara, S. triophthalma)

c. two symmetrical glands are present (Fig. 9D–I; exemplar S. tremula, S. pectinata)

28. Pedal gland(s) – Length5

a. shorter than the foot (Fig. 9E; exemplar S. oblonga)

b. as long as the foot (Fig. 9A–D; exemplar S. tremula)

c. longer than the foot, extending into the caudal trunk region (Fig. 9F; exemplar S. atlantica, S. prominula, S. rousseleti)

29. Pedal gland(s) – Shape

a. tubular; of even width along their entire length (Fig. 9G; exemplar S. longipes)

b. club-shaped; voluminous proximally, decreasing gradually caudally (Fig. 9A, D; exemplar S. tremula)

c. voluminous proximally, decreasing abruptly caudally before widening again to form a reservoir in the distal half (Fig. 9E; exemplar S. oblonga)

d. tubular, suspended from the trunk (Fig. 9H; exemplar S. pachypoda)

e. each gland possesses two large and voluminous sections that are demarcated by a narrowing from one another; suspended from the trunk proximally (Fig. 9I; exemplar S. grimpei, S. pachypoida)

30. Pedal gland(s) – Opening

a. into the tip(s) of the toe(s) (Fig. 9A, B, D–H; exemplar S. tremula, S. pectinata)

b into a toe with the second into a spur (Fig. 9C; exemplar S. hutchingsi)

c. at the base of the toes (Fig. 9I; exemplar S. pachypoida)

31. Toes – Symmetry

a. asymmetrical; only one toe is present (Fig. 9J; exemplar S. hutchingsi, S. neapolitana, S. triophthalma) or two toes are of different size and shape (Fig. 9K; exemplar S. cecilia)

b. symmetrical paired toes (Fig. 9L–Q; exemplar S. tremula, S. pectinata)

32. Toe(s) – Number and arrangement

a. only one toe is present (Fig. 9J; exemplar S. hutchingsi, S. neapolitana, S. triophthalma)

b. paired toes present that are of different size and shape; one can be rudimental (Fig. 9K; exemplar S. cecilia, S. tamara)6

c. paired toes of equal size and shape (Fig. 9L–Q; exemplar S. tremula, S. pectinata)

33. Toe(s) – Size in relation to foot length

a. minute to small; less than one-tenth of the overall foot length (Fig. 9M; exemplar S. grandis)

b. medium to large; between one-tenth to one-quarter of the foot length (Fig. 9J–L; exemplar S. tremula)

c. very large; at least one-third of the foot length (Fig. 9N; exemplar S. pachypoda)

34. Toe(s) – Proximal separation

a. bases of the toes are widely separated (Fig. 9N, O; exemplar S. baltica, S. pachypoda)

b. bases of the toes are close to or in contact with one another (Fig. 9K–M, P, Q; exemplar S. tremula, S. pectinata)

c. only one toe is present (Fig. 9J; exemplar S. hutchingsi, S. neapolitana, S. triophthalma)

35. Toe(s) – Distal separation

a. tips are close to one another or only very slightly divergent (Fig. 9L; exemplar S. tremula)

b. tips are widely separated, distinctly divergent (Fig. 9P; exemplar S. gyrina)

c. only one toe is present (Fig. 9J; exemplar S. hutchingsi, S. neapolitana, S. triophthalma)

d. toes without tips; squamate, with rounded distal margin (Fig. 9Q; exemplar S. squamadigitata)

36. Additional foot appendages

a. none (Fig. 9J–Q; exemplar S. tremula, S. pectinata)

b. dorsolateral spur present (Fig. 9R; exemplar S. neapolitana)

c. ventral spine and dorsolateral spur present (Fig. 9S; exemplar S. hutchingsi)

37. Foot – Number of “pseudosegments”7

a. at least two (Fig. 9A, I; exemplar S. neapolitana, S. pachypoida, S. squamadigitata, S. tamara)

b. only one (Fig. 9B–H; exemplar S. tremula)

Figure 7. 

LM images of species of Synchaeta. A Presence of violet globules in the body cavity (arrow; S. baltica) B, C LM images of the habitus showing different morphologies of the oesophagus (arrows) B oesophagus widens to form a proventriculus (S. tremuloida) C oesophagus highly tensile with numerous longitudinal striae (S. pectinata). Scale bar: 50 µm.

Figure 8. 

Foot shape. A–C Orientation of the foot (grey areas and arrows) A directed dorsally (S. tremula) B coplanar with the longitudinal axis (S. grimpei) C directed ventrally (S. longipes) D–I Shape and size of the foot (grey areas) D minute, shorter than the toes (S. atlantica) E triangular, medium (S. triophthalma) F conical, medium (S. tremula) G slender, long (S. longipes) H broad, long (S. johanseni) I massive, cylindrical (S. pachypoda). Drawings modified from: D Zelinka (1907) H Harring (1921) I Kutikova and Vassiljeva (1982). Scale bar: 50 µm.

Figure 9. 

Foot, pedal glands and toes. A–I Presence of pseudosegments (arrows) and morphology of pedal glands (grey areas) A foot pseudosegmented, pedal gland single and of foot-length (S. neapolitana) B glands asymmetrical with the larger one being of foot-length, glands terminating in the toes (S. triophthalma) C glands asymmetrical with one gland terminating in the toe and one in the dorsal spur (S. hutchingsi) D symmetrical glands of foot-length, voluminous proximally and decreasing gradually in width moving caudally (S. tremula) E glands shorter than the foot, each spherical proximally and abruptly decreasing caudally before widening again to form a reservoir (S. oblonga) F glands longer than the foot (S. prominula), G glands of foot-length and tubular (S. longipes) H foot with wrinkles, glands tubular, suspended from the trunk (S. pachypoda) I foot pseudosegmented, each gland with two voluminous sections, suspended from the trunk, glands terminate proximally to the toes (S. pachypoida) J–Q Symmetry, size and separation of the toes J single toe (S. triophthalma) K asymmetrical, toes of different shape (S. cecilia) L symmetrical toes of medium size, bases of the toes are in contact (arrow), tips are close to one another or very slightly divergent (dashed arrows; S. tremula) M toes symmetrical, minute to small (S. grandis) N toes symmetrical, very large (S. pachypoda) N, O Bases of the toes widely separated (arrow; S. pachypoda, S. baltica) P bases of the toes in contact, tips distinctly divergent (dashed arrows; S. gyrina) Q toes without tips, squamate (S. squamadigitata) R, S Additional appendages of the foot (grey areas) R dorsolateral spur (dorsal view; S. neapolitana) S ventral spine and dorso-lateral spur (lateral view; S. hutchingsi). Drawings modified from: A, R Lie-Pettersen (1905) K Rousselet (1902) F, H–I, N Kutikova and Vassiljeva (1982) Q De Smet (2006).

Sensory system (Table 7)

38. Cerebral eye – Morphology

a. single (Fig. 10A; exemplar S. pectinata)

b. paired but partially fused or connected by pigment granules (Fig. 10B; exemplar S. triophthalma)

c. paired and distinctly separated from one another (Fig. 10C; exemplar S. oblonga, S. lakowitziana)

39. Cerebral eye – Size

a. small to medium, evenly shaped (Fig. 11A, B; exemplar S. pectinata)

b. large, irregularly shaped (Fig. 11C; exemplar S. baltica, S. hutchingsi)

40. Frontal aggregations of pigment granules8

a. present (Figs 10B, 11A; exemplar S. triophthalma)

b. absent (Figs 10A, C, 11B; exemplar S. pectinata)

41. Streams of pigment granules to the anterior margin of the apical field9

a. present (Figs 10B, 11C; exemplar S. triophthalma, S. baltica)

b. absent (Figs 10A, C, 11B; exemplar S. pectinata)

42. Apical receptors – Separation

a. Two ciliary tufts, the bases of which are not completely separated from one another (Fig. 10E, H; exemplar S. grandis, S. vorax)

b. Two ciliary tufts, the bases of which are slightly separated from one another (Fig. 10D; exemplar S. oblonga)

c. Two ciliary tufts that are distinctly separated from one another (Fig. 10F, G; exemplar S. triophthalma, S. pectinata)

43. Apical receptors – Elevation

a. on a flat or slight central elevation of the apical field (Fig. 10D; exemplar S. oblonga)

b. on a strong central elevation of the apical field (Fig. 10E; exemplar S. grandis)

c. on two bulges or pimples (Fig. 10F; exemplar S. triophthalma)

d. on strong, paired elevations (tentacles) (Fig. 10G; exemplar S. pectinata)

e. on a single, tubular elevation (Fig. 10H; exemplar S. fennica, S. johanseni, S. vorax)

44. Lateral and dorsolateral styles – Length10

a. minute (Fig. 10I; exemplar S. squamadigitata)

b. short (Fig. 10J; exemplar S. grimpei)

c. medium (Fig. 10K; exemplar S. pectinata)

d. long (Fig. 10L; exemplar S. vorax)

45. Dorsal antenna – Elevation

a. none to a slight elevation (Fig. 10M; exemplar S. oblonga)

b. distinct prominence to a snout-like projection (Fig. 10N; exemplar S. tremuloida)

46. Dorsal antenna – Basal opening

a. slit-shaped, longer than wide (Figs 10A, 12A; exemplar S. grandis, S. pectinata)

b. round (Figs 10B–C, 12B; exemplar S. tremula)

47. Lateral antenna(e) – Number

a. one; left lateral antenna is enlarged, right one is absent (Fig. 13A; exemplar S. hutchingsi, S. triophthalma)

b. one; right lateral antenna of normal size, left one is absent (Fig. 13B; exemplar S. tamara)

c. paired symmetrical lateral antennae of normal size (Fig. 13C; exemplar S. tremula)

48. Lateral antenna(e) – Location relative to the median transversal axis of the body

a. directly lateral on the median transverse axis (Fig. 13D; exemplar S. tremula)

b. ventrolateral to the median transverse axis (Fig. 13E; exemplar S. pectinata)

c. mid-dorsal and slightly displaced to the right of the body axis (Fig. 13F; exemplar S. tamara)

49. Lateral antenna(e) – Location relative to the longitudinal plane

a. in the posterior third of the trunk region (Fig. 13G; exemplar S. oblonga)

b. in the caudal-most trunk region at or near the base of the foot (Fig. 13H; exemplar S. tremula)

c. on lateral lobes caudally to the cloaca (Fig. 13I; exemplar S. grimpei)

50. Lateral antenna(e) – Base

a. surrounded by a tubular (Fig. 13J; exemplar S. johanseni) or papillary (Fig. 13K; exemplar S. oblonga) epidermal fold

b. surrounded by a low epidermal fold (Fig. 13L; exemplar S. pectinata)

Figure 10. 

Sensory system. A–C Head region showing the cerebral eye, pigment granules and the opening of the dorsal antenna A cerebral eye single, dorsal antenna opening slit-shaped (arrow; S. pectinata) B two partially fused cerebral eyes, frontal aggregations (arrow) and streams (dashed arrow) of pigment granules present (S. triophthalma) C cerebral eyes distinctly separated, dorsal antenna opening round (arrow; S. oblonga) D–H Morphology of the apical receptors (thickened lines, arrows) D receptors slightly separated, situated on a slight elevation centrally on the apical field (S. oblonga) E receptors incompletely separated, situated on a strong elevation centrally on the apical field (S. grandis) F receptors distinctly separated, each situated on a bulge (S. triophthalma) G receptors distinctly separated, each situated on a strong tentacle-like elevation (S. pectinata) H receptors incompletely separated, situated on a single tubular elevation (S. vorax) I–L Lengths of the lateral and dorsolateral styles (thickened lines) I minute (S. squamadigitata) J short (S. grimpei) K medium (S. pectinata) L long (S. vorax) M, N Elevation underlying the dorsal antenna (thickened lines) M not elevated to slightly elevated (S. oblonga) N distinct prominence (S. tremuloida). Drawings modified from: I De Smet (2006).

Figure 11. 

Cerebral eye and pigment granules. A–C LM images with regard to the cerebral eye (dashed arrows), frontal aggregations and streams of pigment granules (normal arrows) A Distinct frontal aggregations of pigment granules present, cerebral eye of normal size (S. triophthalma) B Frontal aggregations and streams of pigment granules are absent, cerebral eye of normal size (S. pectinata) C distinct streams of pigment granules are present, large cerebral eye (S. baltica). Scale bar: 20 µm.

Figure 12. 

Dorsal antenna. A, B REM images of the basal opening of the dorsal antenna A slit-shaped (S. pectinata) B round (S. tremula). Scale bar: 10 µm.

Figure 13. 

Location and morphology of the lateral antennae. A–C Number and size of the lateral antenna(e) (arrow) A single, enlarged left lateral antenna (S. hutchingsi) B single, right lateral antenna (S. tamara) C lateral antennae paired, symmetrical, and of normal size (S. tremula) D–F Location of the lateral antenna(e) (arrows) relative to the median transversal axis (dashed line) (lateral habitus is presented as a stylized drawing that is species independent) D directly lateral (e.g., S. tremula) E ventrolateral (e.g., S. oblonga) F mid-dorsal, single antenna slightly displaced to the right of the body axis (e.g., S. tamara) G–I Location of the lateral antennae relative to the longitudinal axis (arrows) G in the posterior third of the trunk region (S. oblonga) H in the caudal-most trunk region at or near the base of the foot (S. tremula) I on lateral lobes (dashed arrow) caudally to the cloaca and in the proximal third of the foot (S. grimpei) J–L The base of the lateral antennae (detail in inset) J surrounded by a tubular epidermal fold (S. johanseni) K surrounded by a papillary epidermal fold (S. oblonga) L surrounded by a low epidermal fold (S. pectinata). Drawings modified from: B Smirnov (1933) and Friedrich and De Smet (2000) J Harring (1921).

Trophi (Table 8)

51. Ramus11 (“unci”) teeth

a. Edentulous (Figs 14A, 15A; exemplar S. pectinata); plate plain, slightly serrated, fringed or corrugated (Fig. 14B, C; exemplar S. stylata)

b. With several distinct teeth (Figs 14D–G, 15B–F; exemplar S. gyrina, S. triophthalma)

52. Ramus*12 (“unci”) teeth – Shape

a. teeth absent (Figs 14A, B, 15A; exemplar S. pectinata, S. stylata)

b. one more or less distinct tooth, remainder serrated (Figs 14C, 15B; exemplar S. vorax)

c. one distinctly pointed single tooth, remainder slightly incised and blunt (Fig. 14D; exemplar S. verrucosa)

d. all teeth are distinctly incised (Figs 14E, 15C, D; exemplar S. gyrina, S. oblonga)

e. dorsal teeth are distinctly incised, ventral teeth are comb-like (Figs 14F, 15E; exemplar S. triophthalma)

f. dorsal teeth are comb-like, ventral teeth are distinctly incised (Figs 14G, 15F; exemplar S. baltica)

53. Ramus13 (“unci”) teeth – Separation

a. teeth are separated into two groups, either by a cleft (Fig. 15D, F, arrow; exemplar S. oblonga) and/or by their morphological distinctiveness (Figs 14F, G, 15E, F; exemplar S. triophthalma, S. baltica)

b. teeth are not separated so that a continuous row of teeth is present (Figs 14E and 15C; exemplar S. gyrina)

c. no distinct teeth present (Figs 14A, B, 15A; exemplar S. pectinata)

54. Spine of frontal hook14

a. absent (e.g., Fig. 14A, B; exemplar S. pectinata)

b. present (Fig. 14C, G, arrow; exemplar S. vorax, S. baltica)

55. Fulcrum – Height

a. of low to medium height (Figs 14H, I, 15G, H; exemplar S. tremula, S. oblonga)

b. high to very high (Figs 14J, 15I, J, 16B; exemplar S. longipes, S. vorax, S. grimpei)

56. Fulcrum – Overall shape

a. slender, blade-like (Figs 14H, 15G; exemplar S. tremula, S. pectinata)

b. machete-like (Figs 14I, 15H; exemplar S. oblonga)

c. robust, axe-shaped to semi-circular (Figs 14J, 15I, J, 16B; exemplar S. longipes, S. vorax, S. grimpei)

57. Fulcrum – Shape of the distal ventral margin

a. not oblique (Figs 14H, 15G; exemplar S. tremula)

b. oblique (Figs 14I, J, 15H–J, 16B; exemplar S. S. oblonga, S. longipes, S. vorax, S. grimpei)

58. Fulcrum – Presence of a distinct dorsal thickening (and lamellar ventral side)

a. present (Figs 15I, 16B; exemplar S. vorax, S. grimpei)

b. absent (or very weak) (Figs 15G, H, J; exemplar S. tremula, S. longipes)

59. Hypopharynx – Width

a. small to medium, robust (Figs 14K, 15K; exemplar S. tremula, S. oblonga)

b. broad to very broad, pointed laterally / dagger-like (Figs 14L, 15L; exemplar S. stylata)

60. Manubrium – Shape of cauda

a. of even width (Figs 14M, 15G; exemplar S. tremula) or narrowing slightly distally (Figs 14N, 15H; exemplar S. pectinata)

b. small (Fig. 14O; exemplar S. hutchingsi) or large (Figs 14P, 16B; exemplar S. grimpei, S. tremuloida) knob-like thickening at the distal end

c. oar blade shaped in the distal half (Fig. 14Q; exemplar S. oblonga)

d. spatulate or kinked at the distal end (Fig. 14R; exemplar S. glacialis)

61. Manubrium – Thickness of the cauda

a. very thin, slender (Fig. 16A; exemplar S. kitina, S. triophthalma)

b. medium or robust (Figs 15G, H, J, 16B; exemplar S. tremula, S. longipes, S. grimpei)

Figure 14. 

Trophi. A–G Morphology of the ramus (“unci”) teeth A teeth absent (S. pectinata) B teeth absent, margin slightly corrugated (S. stylata) C no distinct teeth, only a serrated plate, frontal hook with a spine (arrow; S. vorax) D one to two teeth are sharply pointed and remainder are blunt (S. verrucosa) E teeth distinctly incised, frontal hook with a spine (S. gyrina) F dorsal teeth distinctly incised and ventral comb-like (S. triophthalma) G dorsal teeth comb-like and ventral distinctly incised, frontal hook with a spine (arrow; S. baltica) H–J Shape and breadth (double headed-arrow) of the lateral fulcrum H blade-like, narrow, distal end not oblique (S. tremula) I machete-like, narrow, distal end oblique (dashed arrows; S. oblonga) J axe-shaped to semi-circular, broad to very broad, distal end oblique (dashed arrows; S. longipes) K–L Shape of the hypopharynx K small to medium, robust (S. tremula) L broad to very broad, laterally pointed / dagger-like (S. stylata) M–R Shape of the cauda of the manubrium M of even width (S. baltica) N slightly decreasing distally (S. pectinata) O with a small knob-like thickening distally (S. hutchingsi) P with a large knob-like thickening distally (S. grimpei) Q oar blade shaped at the distal half (S. oblonga) R cauda spatulate or kinked distally (S. glacialis). Drawings modified from: D Stemberger (1979) (sub. S. asymmetrica Koch-Althaus) and Jersabek et al. (2003a) R Friedrich and De Smet (2000).

Figure 15. 

SEM images of the trophi. A–F Ramus (“unci”) teeth (ventral sides of the trophi directed upwards) A teeth absent (S. pectinata) B no distinct teeth, only a serrated plate (S. vorax) C teeth distinctly incised (S. gyrina) D teeth distinctly incised and separated into two groups by a deep sulcus (arrow; S. oblonga) E dorsal group of teeth distinctly incised and ventral group of teeth comb-like (S. triophthalma) F dorsal teeth comb-like and ventral teeth distinctly incised, groups of teeth separated by a deep sulcus (arrow; S. baltica) G–J Shape of the lateral fulcrum (normal and dashed arrows) and thickness of the cauda (arrow-heads) (trophi from lateral view, ventral sides directed upwards) G fulcrum blade-like, narrow, distal end not oblique, cauda of medium thickness (S. tremula) H fulcrum machete-like, distal end oblique, cauda of medium thickness (S. oblonga) I fulcrum broad, with distinct dorsal thickening (dashed arrow) and ventral lamella (arrow; S. vorax) J fulcrum axe-shaped, very broad, with oblique distal end, cauda robust (S. longipes) K, L Shape of the hypopharynx (arrow) K small to medium, robust (S. tremula) L broad to very broad, laterally pointed / dagger-like (S. stylata). Scale bar: 20 µm.

Figure 16. 

LM images of the trophi. A, B Cauda (arrows) and fulcrum (dashed arrow) A very thin and slender cauda (S. kitina) B cauda medium with a large distal knob, fulcrum with dorsal thickening and ventral lamella (S. grimpei). Scale bar: 25 µm.

Weighted matrix key for Synchaeta – detailed tables

Table 3.
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Weighted character states for habitat, (swimming) behaviour and size of species of Synchaeta.

Category Habitat Behaviour Size
Characters Limnic, brackish, marine Swimming duration Adherence to objects Swimming motion Foot position while swimming Directional changes Overall size
Character number 1 2 3 4 5 6 7
S. arcifera b/c a? ? ? ? ? a
S. atlantica c a ? ? ? ? a
S. bacillifera c a ? ? ? ? b
S. baltica b/c a a b a b b
S. bicornis b a ? a a? a a/b
S. cecilia b/c b b/c a b a a
S. cylindrica d ? ? ? ? ? a
S. fennica b/c ? ? ? ? ? a/b
S. glacialis c a ? ? ? ? a
S. grandis a a a c b b b
S. grimpei b/c a a a b b b
S. gyrina b/c a† a† b/c b a a/b
S. hutchingsi b/c a a a/b b a a
S. hyperborea c a ? ? ? ? a/b
S. johanseni c a ? ? ? ? b
S. kitina a b b a a a a
S. lakowitziana a ? ? ? a? ? b
S. longipes a a a a b a a/b
S. neapolitana c ? ? ? ? ? a
S. oblonga a/(b) a† a† a/b a a a
S. pachypoda a ? ? ? b ? b
S. pachypoida a ? ? ? a ? b
S. pectinata a a a c b b b
S. prominula a ? ? ? ? ? a/b
S. rousseleti c a a ? ? ? a
S. squamadigitata c ? ? ? ? ? a
S. stylata a a a b b a a/b
S. tamara c a ? ? ? ? a/b
S. tavina b/c a/b‡ ? c ? a a
S. tremula a b c a b b a/b
S. tremuloida a b c b b a a/b
S. triophthalma b/c a a a b a a/b
S. verrucosa a ? ? c a ? a/b
S. vorax b/c a a a b a a/b

† Adheres to objects only when disturbed and then only for a short time and without any twisting movement. ‡ Adherence to objects observed by Lauterborn (1905) and Remane (1929), but not by Hood (1893).
Table 4.
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Weighted character states for the morphology of the head and neck region in species of Synchaeta.

Category Apical field Styles Auricles Neck Appendages Head
Character Width Elevation Elevation Size Orientation Demarcation Presence Location Colour
Character number 8 9 10 11 12 13 14 15 16
S. arcifera b b b? ? b a b b ?
S. atlantica b a/b a a a a/b a c ?
S. bacillifera b c b c/d c a b a ?
S. baltica b c c d c c a c c
S. bicornis b c c d b/c a b b a
S. cecilia a b b c a/b a/(c?) a c ?
S. cylindrica a b a/b b a/b c a c ?
S. fennica b c b c/d b/c c b b ?
S. glacialis a/b c a b b c a c ?
S. grandis b c b d c a a c a/c
S. grimpei b a a a a a a c a-c
S. gyrina a/b a b b a/(b) b/c a c a
S. hutchingsi b b b c a/b c a c a/b
S. hyperborea a c a b a/b c a c ?
S. johanseni b c b c/d b/c c? a c a
S. kitina b a/b b c a a a c a/b
S. lakowitziana † a/b b?/c b/c c b a/c a c ?
S. longipes b c c d c a a c b
S. neapolitana b b b/c d a/b c a c ?
S. oblonga b b b/c c b c a c a/b
S. pachypoda a/b a/b b b/c b c a c ?
S. pachypoida a/b a a b a c a c ?
S. pectinata b c a/b c/d c a a c a
S. prominula a b b b a/b b a c ?
S. rousseleti a a ? a a a a c ?
S. squamadigitata a/b c a c a c a c ?
S. stylata b c b d b/c a a c a
S. tamara a b a b a b/c a c ?
S. tavina a/b b b b a a/b a c a
S. tremula b a a b a a a c a/b
S. tremuloida a a a/b c a b a c a/b
S. triophthalma b b b/c d b a/(c?) a c a
S. verrucosa b c a d a/b c a c a
S. vorax b c c d b/c c a c a/c

† As already noted by Hollowday (2002), this species requires further revision (preferably on living, non-preserved specimens) because of inconsistencies in the published morphological data for it, especially for the neck region and the apical field.
Table 5.
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Weighted character states for the morphology of the trunk region in species of Synchaeta.

Category External morphology Internal morphology
Character Shape Anal-pseudosegment Appendages Longitudinal striae Internal organs Violet globules Oesophagus
Character number 17 18 19 20 21 22 23
S. arcifera ? ? a ? a ? ?
S. atlantica c a b ? a ? ?
S. bacillifera c a b ? ? ? ?
S. baltica c a b a a/b a/b b
S. bicornis a/b a b ? a/b a/b b
S. cecilia c b b a a b a
S. cylindrica b a b a a b ?
S. fennica a/b ? b ? a/b b b
S. glacialis b ? b ? a b a
S. grandis b b b a a/b b b
S. grimpei a b b a c a/b ?
S. gyrina c a b a a b a
S. hutchingsi a b b a a/b b a
S. hyperborea b b b ? a b a?
S. johanseni c ? b ? b b b
S. kitina a/c b b a a b a
S. lakowitziana b ? b ? a b a?
S. longipes c b b a a/d b ?
S. neapolitana a/c a b ? a/b b ?
S. oblonga c a† b a a b a
S. pachypoda b a? b a a b a?
S. pachypoida b b b a a b a?
S. pectinata a/c b b b b b b
S. prominula c ? b a a b a
S. rousseleti c b b ? a ? ?
S. squamadigitata c b b a b b b?
S. stylata c b b a a/b b b
S. tamara c a? b ? a b ?
S. tavina b b b a a b a
S. tremula a b b a a b a
S. tremuloida c a b a a b a
S. triophthalma a b b a (a)/b b a
S. verrucosa b/c ? b a b b a
S. vorax c b b a a b b

† In contrast to Hollowday (2002), we found that S. oblonga exhibits a foot with only one instead of two pseudosegments. The impression of two pseudosegments being present might derive from the distinct preanal-fold that distinctly overlaps the foot, which itself is predominantly withdrawn (Wilke et al. 2018b).
Table 6.
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Weighted character states for the morphology of the foot, pedal glands, and toes in species of Synchaeta.

Category Foot Pedal glands Toes Other
Character Orientation Shape and size Symmetry Arrangement Length Shape Opening Symmetry Arrangement Size Separation (prox.) Separation (dist.) Appendages Pseudosegments
Character number 24 25 26 27 28 29 30 31 32 33 34 35 36 37
S. arcifera ? a? b c a a? a b c c b a a b
S. atlantica c a b c c c ? b c c b a/b a b
S. bacillifera ? c/e b c a c a b c b b b a b
S. baltica a c/e b c a c a b c a a b a b
S. bicornis ? c b c b c? a b c b b b a b
S. cecilia a b/c a b b b ? a b b b a a b
S. cylindrica b c b c b c ? b c b b b a b
S. fennica ? c b c b a ? b c b b b a b
S. glacialis b c b c b a/b? a b c b/c b a/b a b
S. grandis c d b c b a a b c a b a a b
S. grimpei b e b c a e a b c a b b a b
S. gyrina b/c c b c a c a b c b b b a b
S. hutchingsi b/c b a b b b/c b a† a† b c c c† b
S. hyperborea b c/e b c b c a b c b b b a b
S. johanseni ? e b c a b/c‡ a b c a/b b b a b
S. kitina b b/c b c b b a b c b b a a b
S. lakowitziana ? e b c a c a b c b b b a b
S. longipes c d b c b a a b c b b a a b
S. neapolitana ? c a a§ b b a§ a| a| b c c b§ a
S. oblonga b/c c b c a¶ c a b c b b a/b a b
S. pachypoda ? f b c a d a b c c a b a b
S. pachypoida ? f b c a e c b c c ? b a a
S. pectinata c c b c a a a b c a/b b a a b
S. prominula ? c b c c b? ? b c b b a/b a b
S. rousseleti b ? b c c b ? b c c b b a b
S. squamadigitata ? c b c b a/b? ? b c b b d a a
S. stylata b d b c b a a b c a b a a b
S. tamara ? c a b a/b? b a a b b b ? a a
S. tavina b/c c b c b c# a b c b b a a b
S. tremula a c b c b b a b c b b a a b
S. tremuloida c c b c b b a b c b b a a b
S. triophthalma b b a b b b a a a b c c a b
S. verrucosa b e b c a c a b c b/c b b a b
S. vorax b d b c b a/b a b c b b a a b

† The dorsolateral spur in S. hutchingsi might represent a second toe that is turned upwards because one pedal gland terminates in the spur and eggs are carried attached to it (as is the case for true toes). ‡ “b” according to the drawing by Harring (1921) and “c” according to the LM image of an individual by Jersabek et al. (2003b). § It remains to be determined whether the pedal gland of S. neapolitana is truly single or is highly asymmetric, with a normal, but vestigial gland that was overseen. | It remains to be determined if the dorsolateral spur in S. neapolitana represents a true toe (as assumed by Rousselet 1902), potentially with a pedal gland terminating in it (contra Lie-Pettersen 1905). ¶ The pedal glands are shorter than the foot (contra Hollowday 2002). The impression of the pedal glands being of foot-length might derive from the foot being partly retracted, something that is typical of S. oblonga (Wilke et al. 2018b). # According to the LM image of an individual by Jersabek et al. (2003b).
Table 7.
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Weighted character states for the morphology of the sensory system in species of Synchaeta.

Category Eyes Apical receptors Styles Dorsal antenna Lateral antenna(e)
Character Morphology Size Frontal pigment granules Streams of pigment granules Separation Elevation Length of the styles Elevation Basal opening Number and size location (transversal) location (longitudinal) Bases of the antenna(e)
Character number 38 39 40 41 42 43 44 45 46 47 48 49 50
S. arcifera b a ? ? c ? c ? ? c a? b a
S. atlantica ? ? ? ? ? ? a ? ? ? ? ? ?
S. bacillifera a a ? ? ? ? b ? ? c ? a a
S. baltica a/b b a a a b c b b c b a a
S. bicornis b ? a a b b c/d a b ? ? ? ?
S. cecilia b a b b c? c? b/c b ? c a b b
S. cylindrica b a ? ? ? ? b/c b ? ? ? ? ?
S. fennica a/b a ? ? a e d b b c b a b
S. glacialis a/b a b b c c a a ? c a? a/b b
S. grandis a a b b a b a/b a a c b a b
S. grimpei a/b a a/b a/b b a b a b c a c b
S. gyrina b a/(b†) b a/b b a c a b c b a a
S. hutchingsi a/(b) b a/b a/b c c c a ? a a b b
S. hyperborea a/b a ? ? c c a a ? c a a/b b
S. johanseni ? ? ? ? a e c ? b c b a a
S. kitina b/c a a/b a/b c c c b b c a b b
S. lakowitziana c a b b b a/b b/c b b c ? a ?
S. longipes a/b a b b a b d a b c b a b
S. neapolitana a/b a b b c? c? c b? b c b a ?
S. oblonga a-c a a/b a/b b a c a b c b a a
S. pachypoda c a ? ? ? ? b/c ? b c ? a b
S. pachypoida b a ? ? ? ? b/c ? b c a? a a
S. pectinata a a b b c d c a a c b a b
S. prominula b ? ? a b a c ? b c a b b
S. rousseleti ? ? ? ? ? ? ? ? ? ? ? ? ?
S. squamadigitata b a ? ? c ? a a ? c ? a ?
S. stylata a/b a b b a b c a b c b a b
S. tamara b a ? ? b? a b/c ? ? b c b ?
S. tavina b/c a ? ? b a c b b c ? a b
S. tremula b a b b b/c a b/c b b c a b b
S. tremuloida b a b b b/c a b b b c a b b
S. triophthalma b a a a c c c b b a a b b
S. verrucosa c a a/b b ? ? b b ? c b a ?
S. vorax b a a/b a/b a e d b b c b a b

† Pale red aggregations of pigment granules located around the darkly pigmented cerebral eyes can make the latter appear large.
Table 8.
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Weighted character states for the morphology of the trophi in species of Synchaeta.

Category Ramus (“unci“) Unci Fulcrum Hyp. Cauda
Character Teeth presence Shape of the teeth Separation Frontal hook with spur Height Overall shape Distal margin Thickening Hypopharynx Shape Thickness
Character number 51 52 53 54 55 56 57 58 59 60 61
S. arcifera b e a a a a a b ? a/b a
S. atlantica b c/d b a a b b a? a b b
S. bacillifera b d a b a a ? ? ? ? ?
S. baltica b f a b a a a b a/b a b
S. bicornis b d a ? a a a ? ? ? ?
S. cecilia † b e a ? a a a b a a a
S. cylindrica b d b ? b b/c b b ? a b
S. fennica ‡ a b b b b c b a ? ? ?
S. glacialis § b d b b a/b b/c b a a d b
S. grandis a a/(b) b/c a a a b b b a b
S. grimpei a/b c b a b c b a a/b b b
S. gyrina b d b b a b b b a a/c b
S. hutchingsi b e a a a a a b ? a/b a
S. hyperborea § b d b b a b b a? a a/b b
S. johanseni| b f ? ? ? ? ? ? ? ? ?
S. kitina b e a a a a a b ? a a
S. lakowitziana b c/d b b a b ? ? ? ? ?
S. longipes a a/b b/c a b c b a a a b
S. neapolitana# b e a a a a a b ? ? ?
S. oblonga b d a b a b b b a a/c b
S. pachypoda b c b a ? ? ? ? a a b
S. pachypoida b c b a ? ? ? ? ? a b
S. pectinata a a c a a a a b a/b a b
S. prominula b d b ? ? ? ? ? a a/c b
S. rousseleti b c/d b a a b? b a? a a b
S. squamadigitata b d b b b b/c b a a b/d b
S. stylata a a c a a a b b b a/d b
S. tamara § a/b c/d b b b b/c b a a a ?
S. tavina b d ? ? b c b ? ? ? ?
S. tremula b d a b a a a b a a b
S. tremuloida b d a b a a a b a b b
S. triophthalma b e a a a a a b a a a
S. verrucosa †† a/b c b a b? b b a a/b a ?
S. vorax a b b b b c b a a a/c b

† Trophi according to an illustration from Arndt et al. (1990) and a LM image from Rougier et al. (2000) ‡ Trophi according to a LM image from Labuce and Strake (2017) § Trophi according to an SEM image from Friedrich and De Smet (2000) | Ramus according to a LM image of the habitus of an individual by Jersabek et al. (2003b), where the trophi were visible ¶ Trophi according to SEM and LM images from Obertegger et al. (2006) # Trophi according to LM image from Rougier et al. (2000) †† Trophi according to Jersabek et al. (2003a)

Weighted matrix key for Synchaeta – numerical list

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S. arcifera
1b/c 2a? ? ? ? ? 7a 8b 9b 10b? ? 12b
13a 14b 15b ? ? ? 19a ? 21a ? ? ?
25a? 26b 27c 28a 29a? 30a 31b 32c 33c 34b 35a 36a
37b 38b 39a ? ? 42c ? 44c ? ? 47c 48a?
49b 50a 51b 52e 53a 54a 55a 56a 57a 58b ? 60a/b
61a
S. atlantica
1c 2a ? ? ? ? 7a 8b 9a/b 10a 11a 12a
13a/b 14a 15c ? 17c 18a 19b ? 21a ? ? 24c
25a 26b 27c 28c 29c ? 31b 32c 33c 34b 35a/b 36a
37b ? ? ? ? ? ? 44a ? ? ? ?
? ? 51b 52c/d 53b 54a 55a 56b 57b 58a? 59a 60b
61b
S. bacillifera
1c 2a ? ? ? ? 7b 8b 9c 10b 11c/d 12c
13a 14b 15a ? 17c 18a 19b ? ? ? ? ?
25c/e 26b 27c 28a 29c 30a 31b 32c 33b 34b 35b 36a
37b 38a 39a ? ? ? ? 44b ? ? 47c ?
49a 50a 51b 52d 53a 54b 55a 56a ? ? ? ?
?
S. baltica
1b/c 2a 3a 4b 5a 6b 7b 8b 9c 10c 11d 12c
13c 14a 15c 16c 17c 18a 19b 20a 21a/b 22a/b 23b 24a
25c/e 26b 27c 28a 29c 30a 31b 32c 33a 34a 35b 36a
37b 38a/b 39b 40a 41a 42a 43b 44c 45b 46b 47c 48b
49a 50a 51b 52f 53a 54b 55a 56a 57a 58b 59a/b 60a
61b
S. bicornis
1b 2a ? 4a 5a? 6a 7a/b 8b 9c 10c 11d 12b/c
13a 14b 15b 16a 17a/b 18a 19b ? 21a/b 22a/b 23b ?
25c 26b 27c 28b 29c? 30a 31b 32c 33b 34b 35b 36a
37b 38b ? 40a 41a 42b 43b 44c/d 45a 46b ? ?
? ? 51b 52d 53a ? 55a 56a 57a ? ? ?
?
S. cecilia
1b/c 2b 3b/c 4a 5b 6a 7a 8a 9b 10b 11c 12a/b
13a(/c) 14a 15c ? 17c 18b 19b 20a 21a 22b 23a 24a
25b/c 26a 27b 28b 29b ? 31a 32b 33b 34b 35a 36a
37b 38b 39a 40b 41b 42c? 43c? 44b/c 45b ? 47c 48a
49b 50b 51b 52e 53a ? 55a 56a 57a 58b 59a 60a
61a
S. cylindrica
1d ? ? ? ? ? 7a 8a 9b 10a/b 11b 12a/b
13c 14a 15c ? 17b 18a 19b 20a 21a 22b ? 24b
25c 26b 27c 28b 29c ? 31b 32c 33b 34b 35b 36a
37b 38b 39a ? ? ? ? 44b/c 45b ? ? ?
? ? 51b 52d 53b ? 55b 56b/c 57b 58b ? 60a
61b
S. fennica
1b/c 2? ? ? ? ? 7a/b 8b 9c 10b 11c/d 12b/c
13c 14b 15b ? 17a/b ? 19b ? 21a/b 22b 23b ?
25c 26b 27c 28b 29a ? 31b 32c 33b 34b 35b 36a
37b 38a/b 39a ? ? 42a 43e 44d 45b 46b 47c 48b
49a 50b 51a 52b 53b 54b 55b 56c 57b 58a ? ?
?
S. glacialis
1c 2a ? ? ? ? 7a 8a/b 9c 10a 11b 12b
13c 14a 15c ? 17b ? 19b ? 21a 22b 23a 24b
25c 26b 27c 28b 29a/b? 30a 31b 32c 33b/c 34b 35a/b 36a
37b 38a/b 39a 40b 41b 42c 43c 44a 45a ? 47c 48a?
49a/b 50b 51b 52d 53b 54b 55a/b 56b/c 57b 58a 59a 60d
61b
S. grandis
1a 2a 3a 4c 5b 6b 7b 8b 9c 10b 11d 12c
13a 14a 15c 16a/c 17b 18b 19b 20a 21a/b 22b 23b 24c
25d 26b 27c 28b 29a 30a 31b 32c 33a 34b 35a 36a
37b 38a 39a 40b 41b 42a 43b 44a/b 45a 46a 47c 48b
49a 50b 51a 52a(/b) 53b/c 54a 55a 56a 57b 58b 59b 60a
61b
S. grimpei
1b/c 2a 3a 4a 5b 6b 7b 8b 9a 10a 11a 12a
13a 14a 15c 16a-c 17a 18b 19b 20a 21c 22a/b ? 24b
25e 26b 27c 28a 29e 30a 31b 32c 33a 34b 35b 36a
37b 38a/b 39a 40a/b 41a/b 42b 43a 44b 45a 46b 47c 48a
49c 50b 51a/b 52c 53b 54a 55b 56c 57b 58a 59a/b 60b
61b
S. gyrina
1b/c 2a 3a 4b/c 5b 6a 7a/b 8a/b 9a 10b 11b 12a(/b)
13b/c 14a 15c 16a 17c 18a 19b 20a 21a 22b 23a 24b/c
25c 26b 27c 28a 29c 30a 31b 32c 33b 34b 35b 36a
37b 38b 39a/(b) 40b 41a/b 42b 43a 44c 45a 46b 47c 48b
49a 50a 51b 52d 53b 54b 55a 56b 57b 58b 59a 60a/c
61b
S. hutchingsi
1b/c 2a 3a 4a/b 5b 6a 7a 8b 9b 10b 11c 12a/b
13c 14a 15c 16a/b 17a 18b 19b 20a 21a/b 22b 23a 24b/c
25b 26a 27b 28b 29b/c 30b 31a 32a 33b 34c 35c 36c
37b 38a(b) 39b 40a/b 41a/b 42c 43c 44c 45a ? 47a 48a
49b 50b 51b 52e 53a 54a 55a 56a 57a 58b ? 60a/b
61a
S. hyperborea
1c 2a ? ? ? ? 7a/b 8a 9c 10a 11b 12a/b
13c 14a 15c ? 17b 18b 19b ? 21a 22b 23a? 24b
25c/e 26b 27c 28b 29c 30a 31b 32c 33b 34b 35b 36a
37b 38a/b 39a ? ? 42c 43c 44a 45a ? 47c 48a
49a/b 50b 51b 52d 53b 54b 55a 56b 57b 58a? 59a 60a/b
61b
S. johanseni
1c 2a ? ? ? ? 7b 8b 9c 10b 11c/d 12b/c
13c? 14a 15c 16a 17c ? 19b ? 21b 22b 23b ?
25e 26b 27c 28a 29b/c 30a 31b 32c 33a/b 34b 35b 36a
37b ? ? ? ? 42a 43e 44c ? 46b 47c 48b
49a 50a 51b 52f ? ? ? ? ? ? ? ?
?
S. kitina
1a 2b 3b 4a 5a 6a 7a 8b 9a/b 10b 11c 12a
13a 14a 15c 16a/b 17a/c 18b 19b 20a 21a 22b 23a 24b
25b/c 26b 27c 28b 29b 30a 31b 32c 33b 34b 35a 36a
37b 38b/c 39a 40a/b 41a/b 42c 43c 44c 45b 46b 47c 48a
49b 50b 51b 52e 53a 54a 55a 56a 57a 58b ? 60a
61a
S. lakowitziana
1a ? ? ? 5a? ? 7b 8a/b 9b?/c 10b/c 11c 12b
13a/c 14a 15c ? 17b ? 19b ? 21a 22b 23a? ?
25e 26b 27c 28a 29c 30a 31b 32c 33b 34b 35b 36a
37b 38c 39a 40b 41b 42b 43a/b 44b/c 45b 46b 47c ?
49a ? 51b 52c/d 53b 54b 55a 56b ? ? ? ?
?
S. longipes
1a 2a 3a 4a 5b 6a 7a/b 8b 9c 10c 11d 12c
13a 14a 15c 16b 17c 18b 19b 20a 21a/d 22b ? 24c
25d 26b 27c 28b 29a 30a 31b 32c 33b 34b 35a 36a
37b 38a/b 39a 40b 41b 42a 43b 44d 45a 46b 47c 48b
49a 50b 51a 52a/b 53b/c 54a 55b 56c 57b 58a 59a 60a
61b
S. neapolitana
1c ? ? ? ? ? 7a 8b 9b 10b/c 11d 12a/b
13c 14a 15c ? 17a/c 18a 19b ? 21a/b 22b ? ?
25c 26a 27a 28b 29b 30a 31a 32a 33b 34c 35c 36b
37a 38a/b 39a 40b 41b 42c? 43c? 44c 45b? 46b 47c 48b
49a ? 51b 52e 53a 54a 55a 56a 57a 58b ? ?
?
S. oblonga
1a(/b) 2a 3a 4a/b 5a 6a 7a 8b 9b 10b/c 11c 12b
13c 14a 15c 16a/b 17c 18a 19b 20a 21a 22b 23a 24b/c
25c 26b 27c 28a 29c 30a 31b 32c 33b 34b 35a/b 36a
37b 38a-c 39a 40a/b 41a/b 42b 43a 44c 45a 46b 47c 48b
49a 50a 51b 52d 53a 54b 55a 56b 57b 58b 59a 60a/c
61b
S. pachypoda
1a ? ? ? 5b ? 7b 8a/b 9a/b 10b 11b/c 12b
13c 14a 15c ? 17b 18a? 19b 20a 21a 22b 23a? ?
25f 26b 27c 28a 29d 30a 31b 32c 33c 34a 35b 36a
37b 38c 39a ? ? ? ? 44b/c ? 46b 47c ?
49a 50b 51b 52c 53b 54a ? ? ? ? 59a 60a
61b
S. pachypoida
1a ? ? ? 5a ? 7b 8a/b 9a 10a 11b 12a
13c 14a 15c ? 17b 18b 19b 20a 21a 22b 23a? ?
25f 26b 27c 28a 29e 30c 31b 32c 33c ? 35b 36a
37a 38b 39a ? ? ? ? 44b/c ? 46b 47c 48a?
49a 50a 51b 52c 53b 54a ? ? ? ? ? 60a
61b
S. pectinata
1a 2a 3a 4c 5b 6b 7b 8b 9c 10a/b 11c/d 12c
13a 14a 15c 16a 17a/c 18b 19b 20b 21b 22b 23b 24c
25c 26b 27c 28a 29a 30a 31b 32c 33a/b 34b 35a 36a
37b 38a 39a 40b 41b 42c 43d 44c 45a 46a 47c 48b
49a 50b 51a 52a 53c 54a 55a 56a 57a 58b 59a/b 60a
61b
S. prominula
1a ? ? ? ? ? 7a/b 8a 9b 10b 11b 12a/b
13b 14a 15c ? 17c ? 19b 20a 21a 22b 23a ?
25c 26b 27c 28c 29b? ? 31b 32c 33b 34b 35a/b 36a
37b 38b ? ? 41a 42b 43a 44c ? 46b 47c 48a
49b 50b 51b 52d 53b ? ? ? ? ? 59a 60a/c
61b
S. rousseleti
1c 2a 3a ? ? ? 7a 8a 9a ? 11a 12a
13a 14a 15c ? 17c 18b 19b ? 21a ? ? 24b
? 26b 27c 28c 29b ? 31b 32c 33c 34b 35b 36a
37b ? ? ? ? ? ? ? ? ? ? ?
? ? 51b 52c/d 53b 54a 55a 56b? 57b 58a? 59a 60a
61b
S. squamadigitata
1c ? ? ? ? ? 7a 8a/b 9c 10a 11c 12a
13c 14a 15c ? 17c 18b 19b 20a 21b 22b 23b? ?
25c 26b 27c 28b 29a/b? ? 31b 32c 33b 34b 35d 36a
37a 38b 39a ? ? 42c ? 44a 45a ? 47c ?
49a ? 51b 52d 53b 54b 55b 56b/c 57b 58a 59a 60b/d
61b
S. stylata
1a 2a 3a 4b 5b 6a 7a/b 8b 9c 10b 11d 12b/c
13a 14a 15c 16a 17c 18b 19b 20a 21a/b 22b 23b 24b
25d 26b 27c 28b 29a 30a 31b 32c 33a 34b 35a 36a
37b 38a/b 39a 40b 41b 42a 43b 44c 45a 46b 47c 48b
49a 50b 51a 52a 53c 54a 55a 56a 57b 58b 59b 60a/d
61b
S. tamara
1c 2a ? ? ? ? 7a/b 8a 9b 10a 11b 12a
13b/c 14a 15c ? 17c 18a? 19b ? 21a 22b ? ?
25c 26a 27b 28a/b? 29b 30a 31a 32b 33b 34b ? 36a
37a 38b 39a ? ? 42b? 43a 44b/c ? ? 47b 48c
49b ? 51a/b 52c/d 53b 54b 55b 56b/c 57b 58a 59a 60a
?
S. tavina
1b/c 2a/b ? 4c ? 6a 7a 8a/b 9b 10b 11b 12a
13a/b 14a 15c 16a 17b 18b 19b 20a 21a 22b 23a 24b/c
25c 26b 27c 28b 29c 30a 31b 32c 33b 34b 35a 36a
37b 38b/c 39a ? ? 42b 43a 44c 45b 46b 47c ?
49a 50b 51b 52d ? ? 55b 56c 57b ? ? ?
?
S. tremula
1a 2b 3c 4a 5b 6b 7a/b 8b 9a 10a 11b 12a
13a 14a 15c 16a/b 17a 18b 19b 20a 21a 22b 23a 24a
25c 26b 27c 28b 29b 30a 31b 32c 33b 34b 35a 36a
37b 38b 39a 40b 41b 42b/c 43a 44b/c 45b 46b 47c 48a
49b 50b 51b 52d 53a 54b 55a 56a 57a 58b 59a 60a
61b
S. tremuloida
1a 2b 3c 4b 5b 6a 7a/b 8a 9a 10a/b 11c 12a
13b 14a 15c 16a/b 17c 18a 19b 20a 21a 22b 23a 24c
25c 26b 27c 28b 29b 30a 31b 32c 33b 34b 35a 36a
37b 38b 39a 40b 41b 42b/c 43a 44b 45b 46b 47c 48a
49b 50b 51b 52d 53a 54b 55a 56a 57a 58b 59a 60b
61b
S. triophthalma
1b/c 2a 3a 4a 5b 6a 7a/b 8b 9b 10b/c 11d 12b
13a(/c) 14a 15c 16a 17a 18b 19b 20a 21(a/)b 22b 23a 24b
25b 26a 27b 28b 29b 30a 31a 32a 33b 34c 35c 36a
37b 38b 39a 40a 41a 42c 43c 44c 45b 46b 47a 48a
49b 50b 51b 52e 53a 54a 55a 56a 57a 58b 59a 60a
61a
S. verrucosa
1a ? ? 4c 5a ? 7a/b 8b 9c 10a 11d 12a/b
13c 14a 15c 16a 17b/c ? 19b 20a 21b 22b 23a 24b
25e 26b 27c 28a 29c 30a 31b 32c 33b/c 34b 35b 36a
37b 38c 39a 40a/b 41b ? ? 44b 45b ? 47c 48b
49a ? 51a/b 52c 53b 54a 55b? 56b 57b 58a 59a/b 60a
?
S. vorax
1b/c 2a 3a 4a 5b 6a 7a/b 8b 9c 10c 11d 12b/c
13c 14a 15c 16a/c 17c 18b 19b 20a 21a 22b 23b 24b
25d 26b 27c 28b 29a/b 30a 31b 32c 33b 34b 35a 36a
37b 38b 39a 40a/b 41a/b 42a 43e 44d 45b 46b 47c 48b
49a 50b 51a 52b 53b 54b 55b 56c 57b 58a 59a 60a/c
61b

Discussion

Our weighted taxonomic matrix key constitutes the most comprehensive and comparable compilation of morphological and behavioural characters to date for the 34 species of Synchaeta that we consider to be valid. Through it, we hope to facilitate the reliable identification of both live as well as of preserved specimens, in part by highlighting those features that are more susceptible to the effects of preservation and, more generally, by indicating the reliability of different characters or individual character states for species identification.

In attempting to make our key as comprehensive as possible, we undertook detailed re-examinations of 14 species of Synchaeta (Wilke et al. 2017; Wilke et al. 2018a, Wilke et al. 2018b), supplemented by information from the literature where necessary. Nevertheless, we were restricted exclusively to literature information for 20 species of this genus, resulting in numerous cases of both missing information and uncertainty (indicated with a “?” in the tables 3–8). A pervasive problem in the literature is that many species have not been re-discovered since their initial description (e.g., S. atlantica and S. rousseleti; see Hollowday 2002) so that little information exists for them at all and that many species descriptions are extremely brief and exclusively restricted to the most obvious, diagnostic characters that discriminate the species from other known species and usually highly similar ones (e.g., S. tremuloida; see Pourriot 1965). Thus, it is not uncommon that important, but basic information is missing for many species such as for example the location and number of sensory antennae for S. atlantica, S. rousseleti and S. cylindrica (see Hollowday 2002), information that could also distinguish the species from new ones discovered in future. In addition, information is often missing or conflicting for those characters for which data are hard to obtain. For example, although the trophi are important for species identification in rotifers (De Smet 1998; Segers 2004), special skills and equipment are needed for their investigation (Telesh and Heerkloss 2002) such that they are often disregarded and so incompletely known for several species of Synchaeta (e.g., S. johanseni; see Hollowday 2002).

A further problem is that many illoricate species like those in Synchaeta have been described on the basis of preserved material only and, despite repeated calls not to do so (e.g., Donner 1959), fixation is commonly used in rotifer research (Labuce and Strake 2017), with its practical applications making it a necessary evil. However, in soft bodied rotifers such as Synchaeta, preservation is far more evil than it is necessary insofar as it causes deformations and/or distortions (Ruttner-Kolisko 1972; Koste 1978; Shiel and Koste 1993), with the consequence that species potentially include preservation-influenced characters in their respective descriptions. This, in turn, might explain why several species have only ever been found once (e.g., S. atlantica and S. rousseleti, both of which were described using preserved material). However, even more commonly reported members of Synchaeta were described on the basis of preserved material as well (see Hollowday 2002), including S. lakowitziana, which is “notoriously disputed” (Hollowday 2002; p. 103) for some aspects of its characteristic morphology (e.g., the sharp neck constriction) that are suspected to be a preparation artefact.

Altogether, these problems highlight the need for standardized and comprehensive species descriptions in Synchaeta as well as in rotifers more generally comprising morphological (habitus and trophi), behavioural and molecular data from both live and preserved specimens (e.g., in Wilke et al. 2017; Wilke et al. 2018a, Wilke et al. 2018b). Such an integrative approach ensures the most comprehensive data set for the respective species and facilitates an assessment of which characters are potentially affected by preservation-based deformations and to which degree. Depending on the context (e.g., ecological assessments), it will often be difficult to avoid preservation. However, knowledge of its specific effects and providing sets of characters that are robust to them will facilitate better species identification. As such, we hope that our weighted taxonomic matrix key for Synchaeta, both through its comprehensiveness as well as through its use of weighting to indicate character reliability and utility, will not only make species identification in Synchaeta easier, but will also serve as a model for future keys within rotifers.

Acknowledgements

We acknowledge the financial support provided by the Government of Lower Saxony. We further thank Christian Jersabek who helped to improve our manuscript.

References

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1 This character strongly depends on the amount of ingested food or presence of developing eggs in some species (e.g., S. gyrina; character state “a/b”).
2 The intensity of the colour is often variable within the species and a less intense colour can appear as colourless
3 The body shape is strongly influenced by the pressure of the cover glass or fixation/preservation (Koste 1978) as well as from developing eggs and stomach content.
4 Although the present character overlaps with the previous one, it is more specific. It should only be consulted when the state can be determined with certainty because the state “b” can be difficult to determine accurately in the case of a second rudimental gland that can be easily overseen. Otherwise, we recommend character 26 for species identification.
5 The length of the pedal glands is related to the foot length. Caution should be exercised with individuals that carry their foot retracted to any degree because in such situations the glands can appear longer than they really are and/or that they extend into the trunk.
6 The present character overlaps with the previous one but is more specific. It should only be consulted when the state can be determined with certainty because the state “b” can be difficult to determine accurately in the case of a second rudimental toe that can be easily overseen. Otherwise, we recommend character 31 for species identification.
7 A general re-examination is recommended here to verify if any apparent pseudosegmentation is actually an artefact caused by wrinkles (e.g., S. hyperborea; described as “wrinkled” in Friedrich and De Smet (2000), but “pseudosegmented” in Hollowday (2002)), by the insertion point of the spur (e.g., S. neapolitana) or by the preanal-fold overlapping the foot (e.g., S. oblonga; Wilke et al. 2018b).
8 The granules are often regarded as being frontal eyespots, but they are more likely frontal aggregations of pigmented granules of the retrocerebral organ (Wilke et al. in prep.)
9 The streams of pigment granules appear to be present in the ducts of the retrocerebral organ (Wilke et al. in prep.).
10 These states are admittedly subjective, especially in the absence of any reference point. However, for those species for which literature data only was available, it was often not possible to be more precise. As such, we have listed all states except the obvious “minute” as subjective in the associated table and highly recommend comparison with our illustrations of the exemplar species for each condition.
11 The toothed or edentulous plate always considered to be the uncus is actually the ramus (Wilke et al. in prep.).
12 See statement from character 51. This character overlaps with character 51 but is more specific. Trophi are difficult to examine in their detail, which is why the present character should only be applied when it can be answered with certainty. Otherwise, we recommend character 51 for species identification.
13 See statement from character 51.
14 The structure always described as the “frontal hook” actually represents the uncus (Wilke et al. in prep.).

Supplementary material

Supplementary material 1 

Blank character checklists and tables

Tanja Wilke, Wilko H. Ahlrichs, Olaf R.P. Bininda-Emonds

Data type: Microsoft Word file

Explanation note: Table S1. Checklist of all characters and their states. Table S2. Blank table for recording character states.

This dataset is made available under the Open Database License (http://opendatacommons.org/licenses/odbl/1.0/). The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this Dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited.
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