Review Article |
Corresponding author: Georgina D. Cepeda ( gcepeda@inidep.edu.ar ) Academic editor: Danielle Defaye
© 2016 Georgina D. Cepeda, Marina E. Sabatini, Cristina L. Scioscia, Fernando C. Ramírez, María Delia Viñas.
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:
Cepeda GD, Sabatini ME, Scioscia CL, Ramírez FC, Viñas MD (2016) On the uncertainty beneath the name Oithona similis Claus, 1866 (Copepoda, Cyclopoida). ZooKeys 552: 1-15. https://doi.org/10.3897/zookeys.552.6083
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The marine cyclopoid Oithona similissensu lato Claus, 1866, is considered to be one of the most abundant and ubiquitous copepods in the world. However, its minimal original diagnosis and the unclear connection with its (subjective) senior synonym Oithona helgolandica Claus, 1863, may have caused frequent misidentification of the species. Consequently, it seems possible that several closely related but distinct forms are being named O. similis or O. helgolandica without explicit and accurate discrimination. Here the current situation concerning the correct assignment of the two species is revised, the morphological characters commonly used to identify and distinguish each species are summarized, and the nomenclatural implications of indiscriminately using these names in current taxonomic and ecological practice is considered. It is not intended to upset a long-accepted name in its accustomed meaning but certainly the opposite. “In pursuit of the maximum stability compatible with taxonomic freedom” (International Commission of Zoological Nomenclature), we consider that reassessment of the diagnostic characters of O. similissensu stricto cannot be postponed much longer. While a consensus on taxonomy and nomenclatural matters can be attained, we strongly recommend specifically reporting the authority upon which the identification of either O. similis s.l. or O. helgolandica s.l. has been accomplished.
Nomenclature, Oithona helgolandica Claus, 1863, Oithona similis Claus, 1866, sequence databases, taxonomy
A global-scale baseline assessment of marine zooplankton biodiversity is critically needed to provide a contemporary benchmark against which future environmental changes can be evaluated (
Oithona similis was first described by Claus in 1866 from specimens collected in the Mediterranean Sea, near Nice, France. Three years earlier, the same author had described a very similar congener from waters off Helgoland (North Sea) that he named O. helgolandica (
In our opinion, a rather confusing subjective synonymy of the two names has developed in recent practice, and the junior name similis has been imposed over helgolandica by prevailing usage, which is in clear contravention of the Principle of Priority (International Commission on Zoological Nomenclature, hereafter
Among contemporary records, references to O. similis are plentiful from almost everywhere in the world’s oceans (
New approaches such as molecular tools are becoming increasingly attractive for identifying plankton. Advancements, however, depend largely on the provision of reference libraries with sequences coming from accurately identified individuals (
Original diagnoses of O. helgolandica and O. similis were in both cases brief and mainly based on the comparison with a third species, O. spinirostris Claus, 1863 (= O. plumifera Baird, 1843). Actually, the first description of the older species O. helgolandica makes real sense only when simultaneously looking at drawings by the same author of female O. spinirostris from Messina (Italy) (
When studying the copepod fauna from Naples,
Overlooking Giesbrecht’s hesitation and without any factual justification for his judgment,
In the same year,
In contrast,
Worldwide variation in the key characters commonly reported for the determination of O. similis/helgolandica s.l.
Species namea Location Reference |
Sex TL | Antennule | Swimming legs setationb | Urosome | ||
---|---|---|---|---|---|---|
P1 | P2 | P4 | ||||
O. helgolandica Helgoland (North Sea) ( |
F 0.75 | “Hardly reaching the end of thorax” | nd | nd | nd | Ur4 shorter than Ur3 and almost as long as Fu. Fu with short setae |
O. similis Nice (Mediterranean Sea) ( |
F ~1.0 | “Nearly reaching the base of the urosome” | nd | nd | nd | Fu with short colorless setae |
O. spinifrons Boeck, 1864 ? O. helgolandica Claus, 1863 ( |
F 0.85 | “About as long as the cephalothorax” | nd | nd | nd | Ur1 long, Ur2 and Ur4 about equal and of moderate length, Ur3 somewhat shorter. Fu shorter than any Ur segment |
O. similis Claus Naples (Mediterranean Sea) ( |
F 0.73–0.80 | “Barely to the genital openings” | (0,1,4;1,1,2)/ (1,1,5;0,0,1) |
(0,1,5;1,0,1)/ (1,2,5;0,0,1) |
(0,1,5;0,0,0)/ (1,2,4;0,0,1) |
Ur & Fu relative lengths: 5,12,5,4,5,3.5. CR 2.5 width |
M 0.51–0.61 | Geniculate | (0,1,4;1,1,2)/ (nd) |
(0,1,4;1,1,2)/ (nd) |
(0,1,4;1,1,2)/(nd) | nd | |
O. similis Claus, 1866 Christmas Islands (Indian Ocean) ( |
F 0.73–0.80 | nd | (nd;1,1,2)/ (nd) | (nd;1,0,1)/ (nd) | (nd) | nd |
O. helgolandica Claus, 1863 off Norway ( |
F 0.70–0.90 | “Extending scarcely beyond the anterior division of the body. Length 1.02 times prosome* | (0,1,4;1,1,2)/ (1,6;0,1) |
(0,1,5;1,0,1)/ (1,2,5;0,0,1) |
(0,1,5;0,0,1?)/ (1,2,4;0,0,1) |
Ur & Fu relative lengths: 5,13,5.5,5.5,4.5,4*. CR hardly shorter than Ur4 |
M 0.50–0.60 | Geniculate | (nd;1,1,2)/(nd) | (0,1,5;1,1,2)/ (1,2,5;0,0,1) |
(nd;1,1,2)/(nd) | nd | |
O. similis Claus Valdivia (SE Pacific) ( |
F 0.78 | “Hardly extending to the genital openings” | (0,1,5;1,1,2)/(nd) | (0,1,5;1,0,1)/ (nd) |
(0,1,5;0,0,1)/ (nd) |
nd |
M 0.67 | Geniculate | (0,1,5;1,1,2)/(nd) | (0,1,5;1,1,2)/(nd) | (0,1,5;1,1,2)/(nd) | nd | |
O. helgolandica Claus [= O. similis Claus] Adriatic Sea ( |
F 0.73–0.96 |
“Barely reaching the genital openings” | (nd;1,1,2)/(nd) | (nd;1,0,1)/(nd) | (nd;0,0,0)/(nd) | Fu shorter than Ur4 |
M 0.59–0.70 | nd | (nd;0,0,2)/(nd) | (nd;0,0,2)/(nd) | (nd;0,0,2)/(nd) | ||
O. similis Claus ?1863 O. helgolandica Various localities ( |
F 0.74–0.95 | “Reaching the genital openings, located a little before the middle of the genital segment” | (nd;1,1,2)/(nd) | (nd;1,0,1)/(nd) | (nd;0,0,1)/(nd) | nd |
M 0.60–0.70 | nd | (nd;1,1,2)/(nd) | (nd;1,1,2)/(nd) | (nd;1,1,2)/(nd) | nd | |
O. helgolandica Claus, 1863 (O. similis Claus,1863) Various localities ( |
F 0.73–0.96 | “Barely attains the genital openings” | (nd;1,1,2)/(nd) | (0,1,5;1,0,1)/(nd) | (nd;0,0,0)/(nd) | Fu shorter than Ur4. CR twice width |
M 0.59–0.70 | nd | nd | (0,1,5;1,1,2)/(nd) | nd | nd | |
O. similis Claus, 1866 Japan ( |
F 0.80 | “Reach to the genital pores” | (0,1,4;1,1,2)/(nd) | (0,1,5;1,0,1)/(nd) | (0,1,5;0,0,1)/(nd) | Fu more than twice width. CR equal width |
M 0.65 | Geniculate | (0,1,5;1,1,2)/(nd) | (0,1,5;1,1,2)/(nd) | (0,1,5;1,1,2)/(nd) | nd | |
O. helgolandica Claus, 1863 NE Pacific ( |
F 0.69–0.96 | “Barely reach to genital segment” | (0,1,5;1,1,2)/(nd) | (0,1,5;1,0,1)/(nd) | (0,1,5;0,0,1)/(nd) | Ur 0.75 prosome length. CR shorter than Ur4 |
M 0.50–0.70 | Geniculate | (0,1,5;1,1,2)/(nd) | (0,1,5;1,1,2)/(nd) | (0,1,5;1,1,2)/(nd) | nd | |
O. helgolandica Claus Messina Strait (Mediterranean Sea) ( |
F 0.78 | nd | (0,1,4;1,1,2)/(nd) | (0,1,5;1,0,1)/(nd) | (0,1,5;0,0,1)/(nd) | nd |
M 0.68 | Non geniculate§ | (0,1,4;1,1,2)/(nd) | (0,1,5;1,0,1)/(nd) | (0,1,5;0,0,1)/(nd) | nd | |
O. helgolandica Claus, 1863 Buenos Aires shelf (Argentine Sea) ( |
F 0.80 | “Reaches the genital openings” | (0,1,4;1,1,2)/(nd) | (0,1,5;1,0,1)/(nd) | (0,1,5;0,0,1)/(nd) | Ur1 2.5 width. Ur4 similar to Ur2 and Ur3. Fu slightly shorter than Ur4. CR twice width |
O. similis Claus, 1866 Río Deseado estuary (Argentine Sea) ( |
F 0.89–1.10 | nd | (nd;1,1,2)/(nd) | (nd;1,0,1)/(nd) | (nd;0,0,1)/(nd) | CR divergent |
M 0.50–0.67 | Geniculate | nd | nd | nd | nd | |
O. helgolandicasensu Sars, 1913 Gulf of Lion (Mediterranean Sea) ( |
F nd | nd | nd | (0,1,5;1,0,1)/ (1,2,5;0,0,1) |
(0,1,5;0,0,0)/ (1,2,4;0,0,1) |
nd |
M nd | nd | (0,1,3;1,1,2)/ (1,1,5;0,0,1) |
(0,1,5;1,1,2)/ (1,2,5;0,0,1) |
(0,1,5;1,1,2)/ (1,2,4;0,0,1) | nd | |
O. similis Claus, 1866 Suruga Bay (Japan) ( |
F 0.69–0.84 | “Extending to the end of thorax 5” | (0,1,4;1,1,2)/ (1,1,5;0,0,1) |
(1,0,5;1,0,1)/ (1,2,5;0,0,1) |
(0,1,5;0,0,1)/ (1,2,4;0,0,1) |
nd |
M 0.60–0.65 | nd | (0,1,4/5;1,1,2)/ (nd) |
(0,1,5;1,1,2)/(nd) | (0,1,5;1,1,2)/(nd) | nd | |
O. similis Claus, 1866 ? O. helgolandica Claus, 1863 Various localities ( |
F 0.78 | nd | (0,1,4;1,1,2)/ (1,1,5;0,0,1) |
(0,1,5;1,0,1)/(nd) | (nd;0,0,1)/(nd) | nd |
M 0.60–0.70 | nd | nd | nd | nd | nd | |
O. similis Claus, 1866 (SW Atlantic) ( |
F nd | Extending slightly beyond thorax 5* | nd | nd | nd | nd |
M 0.70 | nd | (nd;1,1,2)/(nd) | (nd;1,1,2)/(nd) | (nd;1,1,2)/(nd) | nd | |
O. similis Claus, 1866 Various localities ( |
F 0.68–0.96 | “Length 1.1–1.3 times prosome” | (0,1,4;1,1,2)/ (1,1,5;0,0,1) |
(0,1,5;1,0,1)/ (0/1,1/2,5;0,0,1) |
(0,1,5;0,0,1)/ (1,2,4;0,0,1) |
Ur & Fu relative lengths: 13,34,15,14,14,11. Ur4 1.1–1.3 width. CR 1.9–2.4 width |
O. similis Claus, 1866 Magallanes Strait (Argentina-Chile) ( |
F 0.80–0.92 | nd | (0,1,4;1,1,2)/ (1,1,5;0,0,1) |
(0,1,5;1,0,1)/ (1,2,5;0,0,1) |
(0,1,5;0,0,1)/ (1,2,4;0,0,1) |
Ur & Fu relative lengths: 15,36,14,12,12,11 |
O. aff. helgolandicasensu Sars, 1913 Buenos Aires and southern Patagonian shelves (Argentine Sea) (Our unpublished data) |
F | Extending to the genital openings. Length 1.1-times prosome. | (0,1,4;1,1,2)/ (1,6;0,1) |
(0,1,5;1,0,1)/ (1,2,5;0,0,1) |
(0,1,5;0,0,1)/ (1,2,4;0,0,1) |
Ur & Fu relative lengths: 13.5,34,16,14,13,10.5. Ur1 2.0–2.2 width. CR twice width |
M | Geniculate | (0,1,4;1,1,2)/nd | (0,1,5;1,1,2)/nd | (0,1,5;1,1,2)/nd | nd |
In a surprising twist,
More recently,
Given this state of the problem, many authors have subsequently either applied the Principle of Priority or followed
Most important morphological features usually used for the identification of O. similis / helgolandica s.l. have been: (i) body size, (ii) rostrum presence and direction, (iii) relative antennule length, (iv) exopod setation of swimming legs 1-4, and (v) relative lengths of the genital segment, anal segment, and furcae.
Morphological differences among specimens worldwide (Table
Former selected drawings of O. similis / helgolandica. A, B O. spinifrons Boeck, 1864 (=? O. helgolandica Claus), female body and “one of swimming feet” (= leg 4?) (after
Some subtle differences are apparent among published drawings labelled as O. similis s.l. and of O. helgolandica s.l. (Fig.
In our view, when specimens have been identified as O. similis s.l., insufficient attention has often been paid to: (i) presence/absence of the small distal outer spine on exopod segment 3 of leg 4, (ii) endopod segmentation of leg 1, and (iii) relative antennule length.
The distal outer spine on the last segment of the exopod of leg 4 is lacking in some early descriptions and drawings of O. similis / helgolandica (e.g.,
In the genus Oithona Baird, both rami of the first swimming leg are 3-segmented (sensu
Lastly, the antennule length relative to the prosome appears slightly variable across records worldwide (Table
From the genetic point of view, the still scarce molecular studies on Oithona also support the hypothesis that more than one form is reported under the same specific name, O. similis s.l.
The nomenclatural implications of the taxonomic uncertainty apparent from the discussions above are not minor. From a historical standpoint, it is clear that over the course of time a substantial number of copepodologists has come to consider that O. similis and O. helgolandica actually denote the same taxon. Prevailing use which, as shown, has depended upon individual judgment and opinion, has made that the junior synonym O. similis were very commonly imposed over the older O. helgolandica, contradicting the rules of priority (
On the other hand, morphological differences worldwide in the key characters commonly used for diagnosis (Table
In the absence of proper holotypes, the designation of neotypes probably will be required because of the points raised above (
(i) A neotype each for O. helgolandica s.s. Claus, 1863 and for O. similis s.s. Claus, 1866 will be needed if specimens from both localities are proved to be different.
(ii) The appointment of only one neotype will be necessary if specimens from Nice and Helgoland are substantially identical. Strictly speaking, in this situation the senior name O. helgolandica should be used because of the rules of priority. Nevertheless, in pursuit of stability and universality and to avoid causing further confusion, it would be still possible to maintain the use of the junior synonym, O. similis, as it has largely prevailed through time. To stabilise this, however, the matter must be referred to the ICZN for a ruling under the plenary powers (
There are not, in fact, conclusive fundamentals at present in support of an objective synonymy between the names O. similis and O. helgolandica. Hence, until the diagnostic characters are re-examined and the nomenclatural issues settled, we strongly recommend specifically reporting the authority upon which the identification of either O. similis s.l. or O. helgolandica s.l. has been undertaken. In this process, particular reference should be made for female specimens in respect to: (i) relative antennule length, (ii) presence/absence of the small distal outer spine on exopod segment 3 of leg 4, and (iii) endopod segmentation of leg 1.
After this review, we find astounding the extent of taxonomic and nomenclatural uncertainty surrounding the name O. similis. Poor original diagnosis and frequently the inability of authors to perceive minute morphological differences have very likely caused the assembly of several forms distinct at the species level into a single, nominal species. This circumstance on top of the persistent confusion with its likely sibling, O. helgolandica, may have led to a false impression of cosmopolitanism. It is possible that many cryptic species are veiled behind the apparent morphological homogeneity of their forms, and O. similis s.l. and O. helgolandica s.l. may be an example in an abundant and ecologically important group, the genus Oithona. Therefore, we encourage a profound revision of O. similis s.l. in order to bring the exact status of this species to light. In accomplishing this goal, species should not be renamed or newly assigned based on morphology alone without the support of molecular genetic sequence information.
The authors are grateful to Martin Ehrlich, Norberto Scarlato, and Katharina Faulhaber for kindly translating sections of
Paula Israilson helped prepare illustrations for publication. Comments and stimulus by Silvina Menú Marque, Ann Bucklin, and Charlie Miller on an early version of the manuscript are most appreciated. We thank two reviewers for their opinions and valuable suggestions which improved the manuscript. This is