New Caledonia is an archipelago in the south-west Pacific. It is located 1500 km to the north of New Zealand and 1500 km to the east of Australia (Fig. 1). It comprises the main Island “Grande Terre”, the Loyalty Islands (Maré, Lifou, Tiga, and Ouvéa) and other smaller Islands, such as Ile des Pins and Ile Belep. Grande Terre, from which the samples for the current study were obtained, represents the emergent part of the Pacific Norfolk Ridge. New Caledonia lies just north of the Tropic of Capricorn within latitudes 18° and 23° south and longitudes 158° and 172° east (Rawling 2009).

Figure 1. 

Map showing position of the type locality of Caledromus robinsmithi gen. et sp. nov. on Grande Terre, the main island of the New Caledonian archipelago. Source: Google Maps.

The present material was collected by JH and KM during the 2018 expedition to New Caledonia.

Ostracods were collected by moving a rectangular hand net (28 × 14 cm, mesh size ~ 160 μm) either over sediment (to whirl up the top layers with living biota) or amidst aquatic vegetation. We measured pH (VWR/pH/1100H) and electrical conductivity (EC) / water temperature (VWR/CO/3100H) in situ. The position of the type locality on the Grand Terre is illustrated in Fig. 1.

Soft parts were separated from the valves using dissection needles and were then put in a drop of glycerine for the dissection of the appendages. The dissection was covered with a coverslip and sealed with transparent nail polish. Valves were stored dry in micropaleontological slides. Drawings of soft parts were made using a camera lucida (Olympus U-DA) attached to the microscope (Olympus CX-41). Carapace and valves were illustrated and measured using Scanning Electron Microscopy (SEM, Fei Qanta 200 ESEM, in the Royal Belgian Institute of Natural Sciences, Brussels, Belgium) in different views and details.

Type material is lodged in the Muséum national d’Histoire naturelle (MNHN, Paris, France) and in the Royal Belgian Institute of Natural Sciences (RBINS, Brussels, Belgium).

Chaetotaxy of the limbs follows the model proposed by Broodbakker and Danielopol (1982), revised for the A2 by Martens (1987) and Scharf et al. (2020). Names for the limbs were used according to Meisch (2000) except for caudal ramus, which follows Meisch (2007). Higher taxonomy of the Ostracoda follows Horne et al. (2002) and Meisch et al. (2019).

Valves and carapaces:

Cp carapace;

CpD carapace in dorsal view;

CpRL carapace in right lateral view;

CpV carapace in ventral view;

H height of valves;

L length of valves;

LVi left valve in internal view;

RVi right valve in internal view;

W width of valves.

Limbs:

a, b, d setae on T1;

A1 antennula;

A2 antenna;

alfa, beta, and gamma setae on Md-palp;

Att attachment of the CR;

CR caudal ramus;

d₁, d₂, d_p, e, f, g, h₁, h₂, h₃ setae on T2 and T3;

db, vb distal branches of Att;

ds1, ds2 lobes of distal shield of Hp;

g, t1-4, z1-3 setae on female A2;

G1-3, Gm, GM claws on female A2;

Ga, Gp, Sa, Sp claws and setae on the CR;

Hp hemipenis;

Lpp Left Prehensile palp;

Md mandibula;

Md-palp mandibular palp;

M/F Male/Female;

ms medial shield on Hp;

Mx1 maxillula;

T1 first thoracopod;

T2 second thoracopod;

T3 third thoracopod;

R Rome Organ;

Rpp Right Prehensile Palp;

Y, Ya, y3 aesthetascs.

Every taxonomic hierarchy is a working hypothesis, to be tested by using new characters or new methods, or by the discovery of new taxa. Especially in cases where different characters show different distribution patterns in taxa, it can be difficult to determine which characters show the correct phylogeny and which are homeomorphic (see for example, discussion in Almeida et al. 2023). Phylogenetic analyses, based on large sets of either morphological or molecular characters, or on both, are the best way to minimise subjective decisions, but such analyses are not always possible. A good example of a potential convergent evolution in the Herpetocypridinae is in the tribe Isocypridini. One of the morphological characters defining the genus Amphibolocypris is the claw-like subapical seta on the distal segment of the walking leg (T2), so that species in this genus have two distal claws on the T2. The exclusively southern African Amphibolocypris shares this character with the genus Platycypris, which is endemic to Australia (De Deckker 1981a). This shared character could be the result of common descent. However, it is also possible that Amphibolocypris and Platycypris are less closely related than it would seem, because a double end claw on T2 also occurs in other, unrelated genera, for example in Limanocypris Schornikov, 1961 (Limanocypridinae Hartmann & Puri, 1974) from Siberia, the Spanish endemic Candelacypris Baltanás, 2001 (Eucypridinae Bronshtein, 1947) and in Scottia Brady & Norman, 1889 (Scottiinae Bronshtein,1947). Here, we provide a brief overview of the major characters and their character states used in the taxonomic hierarchy of the Herpetocypridinae.

The most important synapomorphy to unite all Herpetocypridinae is the presence of a supporting triangular chitinous structure at the base of the CR Att (Triebel 1953; Martens 2001). Of course, this is only a workable character for Recent taxa, so we here refer to Danielopol et al. (1986) for features that allow the identification of fossil Herpetocypridinae.

The marginal valve anatomy, including the width of the fused zone, the morphology of the marginal pore canals (single or branched), the presence of false pore canals (as in Herpetocypris), and generally the presence or absence and the morphology of selvages and inner lists are important characters which define genera and tribes. The fact that valves in this group are bearing such clear identifiable characteristics also allows fossils to be readily identified (Danielopol et al. 1986). The largely inwardly displaced selvage and the wide fused zone in the LV of Herpetocypris is a case in point, and the distinctively raised, inwardly displaced selvage at the postero-ventral part of the RV in Thaicypris even more so. Several genera in the Stenocypridini, and the genus Stenocypris most of all, have clear marginal septa along at least the anterior valve margin, which is again a useful character for both neontologists and palaeontologists. It was this feature which allowed Horne and Martens (1998) to recognise some Mesozoic fossils, e.g., Mantelliana mantelli (Anderson, 1971), as being most likely a species of the extant genus Stenocypris. However, also other genera have marginal septa, e.g., species of the distantly related Cypretta Vara, 1895 (Cyprettinae, Hartmann, 1963) and in Pseudocypretta Klie, 1932 (Cypridopsinae, Kaufmann, 1900). Shearn et al. (2014) discovered a new character within the LV of Ilyodromus species, namely the presence of one or two ventral inner pegs, as remnants or fortifications of an ancient inner list. Here, we find similar, albeit less pronounced, structures in Caledromus, even though these genera are presently placed in different tribes (Herpetocypridini and Psychrodromini, respectively).

Within the Herpetocypridinae, the Rome Organ on the A1 has a plastic morphology. For example, within the Herpetocypridini, the genera Candonocypris and Herpetocypris have invariably small Rome organs. Amongst species of Ilyodromus, however, this organ can be longer than the actual segment on which it is situated and can be two- and even three-segmented. Most other genera in the Herpetocypridinae have small Rome Organs, but in Ampullacypris (Stenocypridini) even this small organ can be two-segmented (De Deckker 1981b).

The length of the natatory setae on the A2 can vary greatly, even between species of the same genus (see for example Humphcypris). However, except for Caledromus robinsmithi where the five natatory setae have completely disappeared, in the other genera and species of the subfamily some small remnants always remain. Even in the presently described species without any trace of the natatory setae, the accompanying seta is still present and not reduced, which clearly shows that this seta has a developmental program which is unrelated to those of the actual natatory setae. The inverse situation occurs in Brasilocypria Almeida et al., 2023 (Cyclocypridinae) where the species have very long natatory setae, but where the accompanying seta has disappeared (Almeida et al. 2023).

In males of several species and genera of Herpetocypridinae (e.g., Herpetocypris, Humphcypris, Somalicypris, Caledromus, and others, but not in Thaicypris), the male A2 has a the larger claw Gm set with a row of long spines, which are absent in females. Some Cypridopsinae, e.g., the African Zonocypris G.W. Müller, 1898 and the Brazilian Cabelodopsis Higuti & Martens, 2012, have a similar morphology, but the hyperdeveloped claw occurs in females only. Smith et al. (2023) hypothesised that in the latter case these claws are used by the females to clean vegetal substrates for egg deposition. This is of course possible, but the present equivalent in male Herpetocypridinae most likely has a different function, maybe as part of mate recognition during copulation (see also discussion in Higuti and Martens 2012).

The shape of the second segment of the Mx1 palp is important in the taxonomy of the Herpetocypridinae, as species in the tribes Herpetocypridini and Isocypridini have spatulate segments, while species in the tribes Stenocypridini and Psychrodromini have cylindrical palp segment, which show up as rectangular structure in the dissections (see discussion on the position of the genus Paranacypris below).

The useful characters of the T1 are mostly those related to the morphology of the male prehensile palps, but also the presence/absence, shape and length of the setae d, b, and a can be useful in the taxonomy of several groups. In the present species, both males and females have asymmetrical setae b and d, with lengths varying between left and right sides. It is the first time that this is being reported in this group, but it is a feature which is rarely observed and described for both limbs of the same individual. The new genus Caledromus is characterised by the presence of a wide and broad female T1-palp with very small apical setae, an autapomorphic feature, and therefore useful to identify this taxon, but not helpful regarding taxonomic placement of the genus. Also, several other characters are autoapomorphic for one genus and not informative for phylogenetic studies. One example is the monospecific genus Stenocypria, where the sinuous posterior inner margin of the female valves of S. fischeri (Lilljeborg, 1883) results from a lobe-shaped expansion of the calcified inner lamella. This calcified lobe is absent in the males (Scharf et al. 2020). Another example is the presence of two f-setae on the T3 in the species of the genus Candonocypris. A similar, but unrelated, duplication of setae can be found in species of Chlamydotheca Saussure, 1858, where the T2 invariably has two e-setae (e.g., Chlamydotheca colombiensis Roessler, 1985).

The morphology of the female T1-palp in C. robinsmithi is interesting from an evolutionary point of view, as the morphology of this palp resembles that of the shape of prehensile palps in A-1 males. This could thus be a case of heterochronic development, of which several examples exist in non-marine ostracods, for example in the African and Australian terrestrial representatives of the subfamily Scottiinae Bronshtein, 1947 (see Martens et al. 2004).

The potential relevance of the presence/absence and length ratios of the d₁ and d₂ setae on the T2 for the identification at the genus level has already been stressed several times (see Martens 2007 for Cypridini, Martens et al. 1992 for Eucypridinae and others). In the present subfamily also, the application of this character proves to be useful, but again at the level of genus rather than at that of tribes.

The differences in the morphology of the seta Sa on the CR (with character states: simple seta, moveable claw-like seta, immobile spine, fully absent) were amongst the first reasons that drove Danielopol and McKenzie (1977) to distinguish between Ilyodromus and their new genus Psychrodromus. This distinction stands till the present and is now valid at the level of the tribes of the Herpetocypridinae, as for example, the above two genera belong to two different tribes.

The Hp, in species and genera in which males are known, comprises useful taxonomic information. Mostly, external Hp morphology can be used to identify species within genera. Here, however, the morphology of the inner spermiduct, with between 0 (in Caledromus) and 6 (in Somalicypris) additional postlabyrinthal coils (see definition above) allows divisions even at the level of tribes (see diagnoses in Martens, 2001). The illustrated Hp of Thaicypris (in Savatenalinton 2022c) has a weakly developed inner anatomy, which indicates that the male specimen might have been sub-adult. Nevertheless, the presence of a single additional coil of the postlabyrinthal spermiduct is visible. Finally, also the presence/absence of sclerotised hook-like structures on the ms of the Hp are taxonomically informative (in Psychrodromini present in all genera but Caledromus).

Based on the set of characters described above and on the diagnoses of the four tribes in the Herpetocypridinae (see Martens 2001; Shearn et al. 2017b), Caledromus is herewith lodged in the Psychrodromini, primarily because of the compact and broad Cp in dorsal view, the absence of marginal septa on the valves, the cylindrical (not spatulate) second palp segment of the Mx1-palp with L ~ 1.5× W, the long seta d₁ (d₁ = ~2 × d₂), and the symmetrical CR with an immobile spine Sp. The genus differs from all three other genera in the tribe by the complete absence of additional postlabyrinthal coils in the Hp, the complete absence of the natatory setae on the A2, and the broad, asymmetrical T1-palp with minute apical setae in the female.

Higuti et al. (2009) lodged Paranacypris in the tribe Psychrodromini of the Herpetocypridinae, based on several characters which showed a close relationship with the genus Psychrodromus. For this, they had to deal with two major characters in their genus which deviated from the diagnosis of this tribe. Firstly, the complete disappearance of seta d₂ on T2. Higuti et al. (2009) argued that in the Psychrodromini this seta was already reduced when compared to seta d₁, so that the complete disappearance was just one evolutionary step further. Secondly, the spatulate second segment of the Mx1-palp, which is typical of the Herpetocypridini, was considered a result of mosaic evolution. The diagnosis of the tribe Psychrodromini was adapted to allow inclusion of Paranacypris. Here, based on the discovery and relevance of Caledromus, we reconsider this decision and now lodge Paranacypris in the Herpetocypridini. This is based on its closer relationship with Ilyodromus, namely because of the spatulate second segment of the Mx1-palp, and the long Rome Organ on the A1 (Table 1). The stout gamma-seta on the Md-palp stands as a character which is not decisive for either of the two tribes.

Although several ostracod species from the New Caledonian expeditions still await description, the ones that were reported from this archipelago (see table in Kisseih et al. 2020) already provide some ideas on the zoogeographical affinities of its ostracod fauna. The species of Stenocypris and Strandesia, Cyprinotus drubea Martens et al., 2019 and Cypris granulata Daday, 1910 can already qualify as belonging to circumtropical lineages. Kennethia major (Méhes, 1939) clearly has Australian affinities. The same is true for Candonocypris novaezelandiae (Baird, 1843), but this species has meanwhile “conquered the world” as a successful alien invasive species (see Scharf et al. 2014). With Caledromus robinsmithi, however, we have a species and genus which shows, because of its affinity with Psychrodromus, more pronounced Palaearctic connections. Therefore, it seems that the geographical position of the New Caledonian archipelago, isolated as this region may seem to be, has been prone to invasions by lineages with different zoogeographical origins. Further research on thus far undescribed radiations will further illuminate the geographical origin of the New Caledonian non-marine ostracods.

No conflict of interest was declared.

No ethical statement was reported.

No funding was reported.

KM and JH conceptualised the project and collected the material. VGF and NMA provided the illustrations and the descriptions. All authors contributed to writing of the manuscript.

Koen Martens https://orcid.org/0000-0001-8680-973X

Vitor Góis Ferreira https://orcid.org/0000-0002-5244-7707

Nadiny Martins de Almeida https://orcid.org/0000-0002-4380-125X

Janet Higuti https://orcid.org/0000-0002-3721-9562

All of the data that support the findings of this study are available in the main text or Supplementary Information.