Establishment of a new subfamily of Idyanthidae Lang, 1944 with the description of a new species of Pseudometeorina George & Wiest, 2015 (Copepoda, Harpacticoida) from the Eratosthenes Seamount (eastern Mediterranean Sea)

The description of Pseudometeorina eratosthenica sp. nov. from the eastern Mediterranean Eratosthenes Seamount was the release for a detailed phylogenetic comparison of a group of derived representatives of the family Idyanthidae Lang, 1944, namely Aspinothorax Moura & Martínez Arbizu, 2003, Meteorina George, 2004, Pseudometeorina George & Wiest, 2015, and Styracothorax Huys, 1993. The named genera share the derived presence of a 2-segmented P1 exopod as well as the transverse elongation of the bases of the swimming legs P1–P4, being these five deviations unique for Idyanthidae and therefore hypothesized here as synapomorphic for the above-named taxa. Consequently, they are pooled in the newly established Aspinothoracinae Moura & Martínez Arbizu, 2003 subfam. nov. The new species described from Eratosthenes Seamount doubtlessly belongs to Pseudometeorina, sharing with P. mystica George & Wiest, 2015, a set of 24 synapomorphies. Otherwise, P. eratosthenica sp. nov. provides nine autapomorphies that justify its establishment as distinct species. A detailed phylogenetic analysis, based on 131 morphological characters, is presented. Moreover, a key to the species of Aspinothoracinae subfam. nov. is provided.

Page 3 of 26 35 2012). Sampling and sample treatment are described in detail by  and George (2022). The single male specimen of Pseudometeorina eratosthenica sp. nov. was collected on 29.12.2009 with a multiple corer at the eastern transect area E1, station #1112-7 (Fig. 1, grey star).
For the identification of the species, the individual was embedded in glycerol. The dissection was made using a Leica MZ 12-5 stereo microscope; the species identification and drawings were made by means of a camera lucida on a Leica DMR compound microscope equipped with differential interference contrast.
The type specimen is deposited in the collection of the Senckenberg Forschungsinstitut und Naturmuseum Frankfurt/Main, Germany.
For the phylogenetic evaluation, the morphological comparison was based on the respective original species descriptions and on the type material of P. mystica and Meteorina magnifica.
The phylogenetic analysis strictly adopts the concept of "consequent phylogenetics" (Hennig 1982;Ax 1984Ax , 1988Ax , 1995Sudhaus and Rehfeld 1992;Wägele 2001). It consists of a detailed and comparative character discussion that enables the formulation of an intersubjective hypothesis (Ax 1984). The characterization of monophyla respectively sister groups results from the recognition of exclusive (autapomorphies) or shared deviations (synapomorphies), whilst ancestral (shared) characters ([sym]plesiomorphies) are irrelevant. Computer programs were used neither for the phylogenetic analysis nor for the generation of the presented cladogram (Fig. 8). The latter simply presents the graphic and condensed result of the phylogenetic analysis.
Concerning the number of segments and setae/spines, the generally accepted principle of oligomerization (Huys and Boxshall 1991;Seifried 2003), which postulates that the reduction of segments/elements constitutes the relatively more derived state, was adopted here.

Generic diagnosis
Idyanthidae, Aspinothoracinae Moura & Martínez Arbizu, 2003. Habitus long, tapering posteriorly, with conspicuous podoplean boundary between pro-and urosoma. Sexual dimorphism in A1, A2, and P5; uncertain for P4; female with genital double somite resulting from the fusion of last thoracic (P6-bearing) and first abdominal somite. Cphth massive, reaching more than 1/3 of total body length, dorsally with a row of spinules along the posterior margin and 3 pairs of lateral processes, the first and second ones smaller than the third pair, located in anterior half; the third pair on lateral half of cphth, long and distally curved, with some sensilla arising from small socles, and with fine setules or long spinules along anterior and posterior margin. Rostrum fused to cphth, elongate, with a rounded tip that bears a tube pore; frontally and/or laterally with fine setules and a pair of sensilla. Free thoracic somites carrying P2-P4 each with a pair of pronounced, backwardly curved lateral processes that bear long setules/spinules on their posterior and/or anterior margins; in addition, P2-and P3-bearing somites with a row of spinules on their posterior margins, P4-bearing somite posteriorly also with the spinulose row or with dentate frill. P5-bearing somite until penultimate abdominal somite laterally with setulose tufts; posterior margins of first four urosomal somites dorsally bare or with dentate frills; preanal somite dorsally with peculiar frill that consists of finger-shaped or smooth triangular appendages that are variable in appearance. Telson is small, broader than long, with an anal operculum that lacks accompanying sensilla. FR long and slender, of peculiar shape, about three times longer than broadest width, with 7 setae: I and II located closely together, arising from the laterally produced pedestal, being I smaller than II; III as long as II, also arising from laterally extended outgrowth, resulting in a concave shape of the outer furcal margin between Setae I/II and III. IV as long as III, on the outer apical margin; V longest seta, on the apical edge; VI at the most as long as I, on the inner apical margin. Setae I-VI bipinnate. VII inserting subapically from the small pedestal on the dorsal surface, tri-articulate and bare. Antennule 6-segmented in the female, 8-segmented and subchirocer in the male, with weak geniculation between the sixth and seventh segment. Antenna with basis, 2-segmented endopod and 1-segmented female exopod (unknown for P. eratosthenica) but 3-segmented male exopod. Basis and enp-1 lacking abexopodal seta. Enp-1 as long as the basis, enp-2 is longer than enp-1, with 3 bipinnate spines on the inner margin; apically with 4 long setae, 3 of which geniculate; additionally, with 1 small seta on each the inner and outer apical edge. Mouthparts except mxp atrophied; gnathobase of md unknown, md palp with slender but remarkably elongated basis, endoand exopod. Mxl minute, with 2 setae at most; mx small, with 2 or 3 endites. Mxp slender, prehensile, consisting of syncoxa, basis, and 1-2-segmented endopod that carries 1 long curved claw and 2 setae. Swimming legs 1-4 with transversely elongated bases. P1 not prehensile, with 2-segmented exopod and 1-2-segmented endopod. P2-P4 with 3-segmented exo-and endopods, no sexual dimorphism in the male P2. P5 with small baseoendopod; endopodal lobe completely reduced, represented by 2 setae, which are very small in the male. Exopod distinct or fused to baseoendopod, long and slender, with 2 outer, 1 inner and 3 apical setae in the female, and with 1 inner and 3 apical setae in the male. Female genital field small, partly covered by sclerotized plate; the female P6 forming a single plate situated in front of the genital field. Male P6 of P. mystica unknown, in P. eratosthenica sp. nov. small, both limbs fused medially and bearing 1 long biplumose seta each.

Male description
Habitus (Fig. 2) long, tapering posteriorly, body length from the rostrum to the end of FR about 446 μm. Cphth massive, reaching more than 1/3 of total body length, dorsally with a row of spinules along the posterior margin and 3 pairs of lateral processes, the first and second ones small, located in the anterior half; third pair on lateral half of cphth, long and distally curved, with some sensilla arising from small socles, and with fine setules along anterior and posterior margin. Rostrum (Figs. 2, 3b) fused to cphth, elongate, with a rounded tip that bears a tube pore; laterally with fine setules and a pair of sensilla. Free thoracic somites carrying P2-P4 each with a pair of pronounced, backwardly curved lateral processes that bear long setules on their proximal margins; in addition, each somite with a row of spinules on its posterior margin. All free thoracic somites except penultimate somite and telson with a single pore on the posterior margin. P5-bearing somite until penultimate abdominal somite laterally with setulose tufts; P5-and P6-bearing somites additionally with tube pore but lacking spinules on posterior margins, which are present on the first two abdominal somites. The frill on the penultimate abdominal somite is formed by smooth triangular appendages that are variable in appearance (Figs. 2,3a). Telson is small, broader than long, with an anal operculum that lacks accompanying sensilla (Fig. 3a). FR (Figs. 2,3a) long and slender, of peculiar shape, about three times longer than broadest width, with 7 setae: I and II located closely together, arising from the laterally produced pedestal, being I smaller than II; III as long as II, also extended laterally, resulting in a concave shape of the FR between Setae I/II and III. IV apically on the outer margin, as long as III; V longest seta, on the apical margin; VI shortest seta, on the inner apical margin. Setae I-VI bipinnate. VII inserting subapically from the small pedestal on the dorsal side, tri-articulate and bare. A1 ( Fig. 3b) 8-segmented, subchirocer, with weak geniculation between the sixth and seventh segment; segments of different lengths. First segment with few spinules on the anterior margin and with 1 biplumose seta on the apical anterior margin. Second segment is small, with 1 bare seta; the third segment is longest, with fine, long spinules along the posterior margin, and with 5 bare setae (1 seta broken in Fig. 3b) on the anterior margin. Fourth segment trapezoid in lateral view, with 1 apical bare seta on anterior margin; fifth segment minute, discoid, also with 1 apical bare seta. Sixth segment slightly swollen, with 5 bare setae on the anterior margin, one of which inserted apically on a small socle; additionally, with 2 long bare setae arising together with an aes from a strong pedestal, and with 6 short bipinnate spines. Seventh segment small, with 2 short bipinnate spines and 1 tiny bare seta; eighth segment slightly longer than seventh one, with cuticular tooth apically, and with 2 small setae on anterior margin, setae, 6 bi-articulate setae, and 2 long setae forming an apical acrothek with a small aes. Setal formula: 1-1; 2-1; 3-5; 4-1; 5-1; 6-11(+ 2 + aes); 7-3; 8-8(+ 2 + aes). A2 ( Fig. 4a) comprising coxa, basis, 2-segmented endopod, and 3-segmented exopod. Coxa squarish, with few slender spinules along the posterior margin; basis elongate, with few long spinules along the anterior margin; enp-1 as long as the basis, with few spinules on the anterior margin. Enp-2 is longer than enp-1, with a row of spinules and 3 bipinnate spines on the inner margin; apically with 4 long setae, 3 of which geniculate; additionally, with 1 small seta on each the inner and outer apical edge. Subapically with few spinules on the outer margin.
The apparently atrophied area of the mouthparts is shown in the semi-schematic Fig. 4b. It is notable that the former three pairs of mouthparts are very small, and the mxl and part of the mx are overlapped by the supposed labrum. Moreover, the md gnathobase(?) seems to be strongly atrophied, reduced to 1 weak tooth, and seemingly fused to the body. Due to that condition, it was foreseen from further dissection in order to avoid the destruction of the limbs. Thus, neither the md gnathobase nor the mxl were recognized unambiguously and thus not drawn in detail. The md palp ( Fig. 4c; both palps broken) is comparatively massive; it consists of an unarmed, elongate basis that carries each a 1-segmented elongated exo-and endopod. The endopod bears 1 lateral and 4 apical setae, all of which bipinnate; the exopod is equipped with 1 lateral bipinnate seta; apically it bears 2 setae, the innermost being biplumose, the outermost bare.
Mx (Fig. 4d) is very small, about 20 µm long; syncoxa with 3 endites, the proximal one knob-like, without setae; the middle endite as long as the proximal one, with 2 apical bipinnate setae; the distal endite largest, apically with 2 bare setae. Basis slightly longer than distal endite, apically with 1 strong bipinnate spine and a bare seta. Endopod lost.
Mxp ( Fig. 4e) strong, prehensile. Syncoxa (Fig. 4b) and allobasis elongate, of nearly the same length. Syncoxa with several slender spinules, lacking apical seta; allobasis with a row of long spinules along the anterior margin. Endopod slightly elongate, armed with a long rounded apical claw, 1 small bare outer seta arising from projection, and 1 long and geniculate bare inner seta. P1 ( Fig. 5a) with small coxa, transversely elongated basis, 2-segmented exopod, and 1-segmented endopod. Intercoxal sclerite is narrow and broader than long. Basis with 1 outer biplumose and 1 inner bare seta; endopod nearly as long as exp-1, with 1 bare biplumose seta apically. Exp-1 with few outer spinules and 1 biplumose outer seta; exp-2 is almost twice as long as exp-1, with 1 biplumose outer seta at the half length of the segment, and with 4 bare geniculate setae apically, additionally with 1 biplumose inner seta adverse of the outer one.
P2-P4 ( Fig. 6a-c) with coxae carrying fine spinules on the outer margin; bases transversely elongated, with 1 outer bipinnate seta arising from the small pedestal (exemplary shown in Fig. 6a). Exo-and endopods 3-segmented; as the female remains unknown, sexual dimorphism cannot yet be detected. Nonetheless, the male P2 endopod does not show the "typical" idyanthid transformation; it deviates, however, somewhat from the P3 and P4 endopods in the shape of its third segment (Fig. 6a). Exps-1 and exps-2 with few inner and/or outer spinules as depicted, and with 1 inner and 1 outer seta; P2 exp-3 with 3 outer, 2 apical, and 2 inner setae; P3 exp-3 with 3 outer, 2 apical, and 3 inner setae; P4 exp-3 with 2 outer, 2 apical, and 3 inner setae. As exemplary shown in Fig. 6a for the P2, all outer exopodal setae are biplumose in their proximal, but bipinnate in their distal half. The outermost apical seta shows an outer bipinnate ornamentation but is biplumose on its inner margin; all remaining setae are biplumose. The endopods lack any spinules; all setae are biplumose. The enps-1 bear 1 subapical inner seta; the enps-2 of P3 and P4 also carry 1 inner seta, whilst the P2 enp-2 is equipped with 2 inner setae. The enps-3 bear 1 outer, 2 apical, and 2 inner setae. The setal formula of P2-P4 is given in Table 1. P5 (Fig. 5b) is small and elongate, baseoendopod and exopod fused. Baseoendopod without outer seta; endopodal lobe strongly reduced, with 2 small bare setae. Exopodal lobe with 1 inner bare seta on its proximal half, and with 3 apical setae, the outer and inner ones bipinnate. P6 (Fig. 5c) is small, both limbs fused medially and bearing 1 long biplumose seta each.
Female unknown

Discussion
Background Moura and Martínez Arbizu (2003) discussed and substantiated a close phylogenetic relationship of Aspinothorax insolentis and Styracothorax gladiator, concluding that both species belong to the Idyanthinae Lang, 1944, which at that time was placed in the Tisbidae Stebbing, 1910 Seifried (2003) adopted Moura and Martínez Arbizu's (2003) hypothesis and added Styracothorax to the Idyanthidae (ignoring, for whatever reason, Aspinothorax). Subsequently, George (2004) described Meteorina magnifica from the Great Meteor Seamount plateau (302 m water depth). He assigned the species to the Idyanthidae, noting remarkable similarities between S. gladiator and M. marina and thus indicating a potentially close relationship of these species.
Subsequently, Bröhldick (2005) described Nematovorax gebkelinae Bröhldick, 2005 from the south-eastern Angola deep-sea basin (5389-5427 m water depth). She allocated N. gebkelinae to the Idyanthidae because of the abovelisted family autapomorphies 3-5. Furthermore, Bröhldick (2005) assumed that N. gebkelinae might take a basal position within that family, because compared with other idyanthid taxa, it presents several features that correspond to the supposed idyanthid ground pattern (cf. Seifried 2003), which on its part had to be partially upgraded to accommodate the species (Bröhldick 2005). In contrast, the assignation of S. gladiator, A. insolentis, and M. magnifica to the Idyanthidae caused certain difficulties, because these species show further deviations especially in characters 1, 2, 3, and/or 4 (Moura and Martínez Arbizu 2003;Seifried 2003;George 2004). Thus, Seifried's (2003: 98) statement "…it is not possible to confirm the derived morphology of Idyanthidae.", which was referenced to Styracothorax, has to be extended to all three species. Nevertheless, despite the impossibility to confirm all idyanthid autapomorphies for S. gladiator, A. insolentis, and M. marina due to their further deviation, all three species do share character 5 (a sexually dimorphic male P2), i.e., the main and quite characteristic autapomorphy of the Idyanthidimorpha Seifried, 2003(= Idyanthidae + Zosimeidae Seifried, 2003 combined with the additional idyanthid autapomorphy of the loss of the inner setae in the male P2 enp-3. Thus, their affiliation to the Idyanthidae remains unquestioned (Moura and Martínez Arbizu 2003;Seifried 2003;George 2004;Wells 2007).
With respect to Pseudometeorina, the circumstances become more complicated, because neither P. mystica nor the here-described P. eratosthenica sp. nov. share any of the above-listed five idyanthid autapomorphies. Both species present a strongly derived md, mxl, and mx, which are remarkably atrophied. That condition inhibits the confirmation of characters 1 and 2. Besides, like in A. insolentis and S. gladiator, the P1 is considerably derived, bearing 2-segmented limbs that differ remarkably from the characteristic idyanthidimorph/idyanthid shape, leading to a disutility also of character 3. Moreover, and like in M. marina, the proximal outer spine in the female P5 exopod is not displaced towards the basis; additionally, also the distal outer spine did not shift towards the subapical edge of the exopod, so character 4 can neither be used for the allocation of Pseudometeorina to Idyanthidae. The main discrepancy is, however, the shape of the male P2 enp-3 (character 5). It lacks the characteristic idyanthidimorph apomorphic transformation of the apical elements but still retains the two inner setae that are lost in idyanthid males. So, the male P2 is identical with that of the female-Pseudometeorina shows no sexual dimorphism in the P2, and it was mainly this discrepancy that forced George and Wiest (2015) to assign P. mystica as species incertae sedis to the Idyanthidae. Though, in the following, it shall be proven that despite the missing idyanthid autapomorphies, Pseudometeorina in fact belongs to Idyanthidae. Within that taxon, it is most closely related with Meteorina, with which it shares a series of deviations, i.e., synapomorphies.
The most peculiar feature of M. magnifica is, without doubt, its general body shape (Fig. 7a). The large cphth, the remarkable long rostrum, and the shape and ornamentation of very characteristic lateral cuticular processes on the cphth and the prosomal body somites are noteworthy. Thus, the detection of a few specimens presenting an almost identical body shape (Fig. 7b) in a station of the Guinea deep-sea basin (George and Wiest 2015) led to the initial assumption of a new record of M. meteorina in that region. Subsequent detailed examination of the material revealed, however, several remarkable differences between the deep-sea and the seamount specimens, despite the impressive similarity of the general appearance. These differences prompted George and Wiest (2015) to assign the species name Pseudometeorina mystica for the Guinean deep-sea specimens.
The record of the here presented Pseudometeorina eratosthenica sp. nov. from Eratosthenes Seamount provides a third species of Idyanthidae showing a body shape (Figs. 2, 7c) that is-apart from marginal differences in size and ornamentation-nearly equal to that of M. magnifica and P. mystica, in particular with respect to the cphth and the body processes. These morphological features are not only unique within the Idyanthidae but in all Harpacticoida and are thus seen here as derived characters. Their similarity is such that a convergent development is regarded as quite improbable. Consequently, it is hypothesized that the development of these deviations reflects a close phylogenetic relationship of M. magnifica, P. mystica, and P. eratosthenica sp. nov.

Phylogenetic analysis
The given circumstances-(a) the assignment of A. insolentis, S. gladiator, and M. marina to the Idyanthidae, (b) their presumed close relationship, (c) the striking morphological similarity of M. marina, P. mystica, and P. eratosthenica sp. nov., and (d) the ambiguous assignation of Pseudometeorina to the Idyanthidae-induced the here presented phylogenetic attempt. Although a detailed phylogenetic analysis of the Idyanthidae would go beyond the scope of the present contribution, in the following, a detailed comparative evaluation of the phylogenetic relationship between the above-named species is provided, based on 131 morphological characters ( Table 2). As a premise, the phylogenetic characterization of the Idyanthidae as a monophylum provided by Seifried (2003) as well as the affiliation of the further derived A. insolentis, S. gladiator, and M. magnifica to that taxon and their supposed close relationship as discussed by Moura and Martínez Arbizu (2003), Seifried (2003) and George (2004) formed the base for the here presented argumentation. Consequently, the following hypothesis is formulated: Aspinothorax, Meteorina, Pseudometeorina, and Styracothorax form a monophyletic group of quite derived Idyanthidae; the deviations are such, that they often go beyond the recognized family autapomorphies. Moreover, the deviations may include secondary transformations into states that resemble ancestral ones.
In the following, to avoid an exceeding character discussion, only those presumed apomorphies are discussed in detail that seems to be ambiguous or shall be justified as synapomorphies. Instead, the presumed autapomorphies of single taxa, particularly of Aspinothorax, Styracothorax, Meteorina, and Pseudometeorina, are so extraordinary and exclusively linked to the respective taxa that a brief argumentation is considered sufficient to confirm their autapomorphic state.
The result of the phylogenetic analysis is presented in Fig. 8. It shows that-based on the above-mentioned premise of a monophylum Idyanthidae (Table 2, Fig. 8: character 1)-Tachidiella kimi can be characterized by four derived characters (Table 2, Fig. 8: characters 2-5) that are absent in all remaining studied species and described in detail by Lee and Huys (1999). To clear the potential state of characters 2-5 as aut-or synapomorphies of the remaining idyanthid species is not the aim of the contribution on hand; here, they simply serve to unambiguously characterize T. kimi and to distinguish that species from the other species under study.

Aspinothorax Moura & Martínez Arbizu, 2003
Aspinothorax presents a set of 12 autapomorphies (Table 2, Fig. 8: characters 17-28). These refer to the ovoid shape of the cephalothoracic shield (character 17), the remarkably diminished size of the tergites on the pedigerous somites P2-P4 (characters 18-20), the fusion of the maxillular endopod with the basis (character 21), the only 1-segmented P1 (character 22), only 2-segmented P2-P4 endopods (characters 23-25), the only 1-segmented P4 exopod (character 26), the transformation of the male P6 into an asetose single plate (character 27), and the shape and length of the FR (character 28). All 12 characters are clearly derived in comparison with the remaining Idyanthidae and exclusively present in Aspinothorax; only characters 22 and 27 are also present in P. eratosthenica sp. nov. and M. magnifica, respectively. As these species are, however, characterized by their own autapomorphies that are missing in Aspinothorax, the simultaneous development of characters 22 and 27 in the different species is regarded here as convergence.

The Styracothorax-Pseudometeorina clade
The Styracothorax-Pseudometeorina clade is characterized by eight unambiguous autapomorphies (Table 2, Fig. 8, characters 29-36). Characters 29-33 refer to the development of lateral cuticular processes on the cphth and the pedigerous body somites. The evolution of cuticular processes on the cphth and body somites took place in several harpacticoid major taxa (e.g., in Aegisthidae Giesbrecht, 1893 [part.], Ancorabolidae [part.], Argestidae [part.], Cletodidae [part.], Laophontidae T. Scott, 1904 [part.]). Their distribution along many different Harpacticoida, in combination with the restriction of their development to certain groups within the respective major taxa, doubtlessly points towards a multiple and independent evolution of cuticular processes in Harpacticoida. In Idyanthidae, only Meteorina, Pseudometeorina, and Styracothorax have evolved lateral cuticular processes, which is therefore interpreted here as a synapomorphic state.
Further supposed autapomorphies of the Styracothorax-Pseudometeorina clade are: -the remarkably elongated and narrowed rostrum (character 34); the rostrum of the remaining Idyanthidae remains rather short and triangular in shape, as observable in Aspinothorax, Dactylopia, Idyanthe, Idyella, Idyellopsis, and Nematovorax (cf. Sars 1909;Becker 1974;Moura and Martínez Arbizu 2003;Bröhldick 2005;Kornev and Chertoprud 2008); -the secondary diminution of the former elongated exopod of the mxl (character 35); according to Seifried (2003; see above character 2), one autapomorphy of the Idyanthidae is the elongation of the maxillular exopod. In the derived Styracothorax-Pseudometeorina clade the mxl exopod is, however, small and does not surpass the length of the endopod. That reduction of the exopodal size is hypothesized here as a further deviation that resembles an ancestral, pre-idyanthid state. Such reduction is also notable in Nematovorax but considered here as convergent development, as Nematovorax lacks at least characters 12-16 (cf. Bröhldick 2005) and thus cannot be assigned even to the higher-leveled Aspinothoracinae. -the presence of at most two setae on the mxl exopod (character 36); the ground pattern of Idyanthidae encloses a mxl exopod bearing three setae; this plesiomorphic state is retained in Aspinothorax, Idyanthe, Idyellopsis, Nematovorax, and Tachidiella (e.g., Sars 1909;Brady 1910;Lee and Huys 1999;Moura and Martínez Arbizu 2003;Bröhldick 2005;Kornev and Chertoprud 2008). Instead, the Styracothorax-Pseudometeorina clade lost one seta, which is considered here as a shared deviation. Admittedly, however, also certain Idyella species (not confirmed for all species) show a mxl endopod that bears two setae (cf. Sars 1905Sars , 1920, whilst I. nilmaensis retains only one seta (Kornev and Chertoprud 2008). That circumstance might suggest a close relationship of Idyella and the Styracothorax-Pseudometeorina clade. Nevertheless, this was the one and only apomorphy shared by these taxa, which would simultaneously exclude Aspinothorax (three setae on mxl exopod) that on its part shares a series of deviations (

Styracothorax Huys, 1993
Styracothorax can unambiguously be characterized by 21 exclusive characters. Most of them (Table 2, Fig. 8: characters 37-56) focus on the presence and development of peculiar denticles, dorsal cuticular processes, tri-cuspidate processes, and/or strong spinules on the cphth and the body somites. These striking structures are exclusively found in Styracothorax and therefore interpreted as autapomorphic characters of that taxon. Additionally, Styracothorax is the only idyanthid taxon presenting a 2-segmented maxillar exopod (character 57) (Huys 1993), which is 3-segmented in the remaining Idyanthidae (e.g., Lee and Huys 1999;Moura and Martínez Arbizu 2003;George 2004;Bröhldick 2005), with exception of Pseudometeorina, whose mouthparts are strongly atrophied (see below). Thus, that deviation is also assumed to be autapomorphic for Styracothorax.

The Meteorina-Pseudometeorina clade
The Meteorina-Pseudometeorina clade presents a set of 13 autapomorphies ( Table 2, Fig. 8: characters 58-70), most of which (characters 58, 59, 61-65) are related to the further development of the lateral cuticular processes on the cphth and the body somites that have been discussed briefly in the first part of the discussion. As shown in Fig. 7, Meteorina and both Pseudometeorina species show an almost identical pattern of cuticular processes; moreover, these processes strongly resemble each other with respect to the size, shape, and even the ornamentation. In contrast, they differ remarkably from the processes exhibited in the supposed sister group Styracothorax. As such characteristic features have solely been found in Meteorina and Pseudometeorina (not even in any other Harpacticoida), it is concluded here that they originate from the last common ancestor of the Meteorina-Pseudometeorina clade, thus constituting an unequivocal complex of synapomorphies for Meteorina and Pseudometeorina. That assumption is supported by further derived characters: -FR elongate; at least as long as the last three abdominal somites together (character 60): most Idyanthidae present short furcal rami, being only slightly longer than broad (e.g., Tachidiella, Idyella, Nematovorax, Styracothorax) (cf. Huys 1993;Lee and Huys 1999;Bröhldick 2005;Kornev and Chertoprud 2008), which is considered as part of the idyanthid ground pattern. Instead, the Meteorina-Pseudometeorina clade is characterized by considerably elongated FR. This is hypothesized here as synapomorphic for Meteorina and Pseudometeorina. The concurrent elongation of the FR in Aspinothorax is regarded as convergent development; -Mandibular basis remarkably elongated, more than twice of its maximal width (character 66): Idyanthidae bear a rather compact md basis (e.g., Aspinothorax, Nematovorax, and Tachidiella) (cf. Lee and Huys 1999;Moura and Martínez Arbizu 2003;Bröhldick 2005). A certain elongation is already noticeable in some Idyella species and in Styracothorax (Sars 1920;Huys 1993;Kornev and Chertoprud 2008); however, only Meteorina and the Pseudometeorina species present a md basis that is more than two times longer than its broadest width. This is considered as autapomorphic for the Meteorina-Pseudometeorina clade; -Mandibular exopod 1-segmented (character 67): Idyanthidae are characterized by a 2-segmented md exopod. Instead, the Meteorina-Pseudometeorina clade presents a 1-segmented md exopod, which is regarded here as autapomorphy. Likewise, Aspinothorax shows a 1-segmented exopod on the md, but as shown above, that taxon branches off more basally, presenting several autapomorphies but lacking the apomorphies of the further derived taxa. Therefore, it is assumed that the loss of 1 exopodal segment occurred independently in Aspinothorax and the Meteorina-Pseudometeorina clade; -Mandibular exopod(-1) with at most three lateral setae (character 68): the idyanthid ground pattern encloses a mandibular exopod(-1) equipped with four lateral setae (Seifried 2003). That condition is retained by most Idyanthidae, except for Aspinothorax, Nematovorax, and the Meteorina-Pseudometeorina clade; in the latter, only P. mystica bears three lateral setae, whilst P. eratosthenica sp. nov. carries only one lateral seta and M. magnifica lost all lateral setae. In combination with the remaining 14 apomorphies, the loss of (at least) one lateral seta is regarded as an additional autapomorphy for the Meteorina-Pseudometeorina clade. However, reduction of setal elements occurs often in Harpacticoida, so it is assumed here that like for other characters, the simultaneous reduction of one exopodal lateral seta in both Aspinothorax (and Nematovorax) is due to convergence; -Maxillar middle endite reduced in size, with at most two apical setae (character 69): the ground pattern of Idyanthidae encloses a middle maxillar endite that bears three setae (Seifried 2003). That condition is retained in all here studied taxa (cf. Lee and Huys 1999;Moura and Martínez Arbizu 2003;Bröhldick 2005;Kornev and Chertoprud 2008) except Meteorina and Pseudometeorina, which lost one seta (cf. George 2004;George and Wiest 2015;present contribution). That reduction is hypothesized as synapomorphic for both genera; -Maxillar distal endite with at most two setae (character 70): like for character 69, also the loss of one seta on the distal maxillar endite (= two setae present) is seen here as deviation shared by Meteorina and Pseudometeorina, whilst all other studied species still retain the ancestral condition of three setae.

Meteorina George, 2004
The Meteorina-Pseudometeorina clade splits into the remaining two genera. Meteorina presents a set of 18 unambiguous autapomorphies (Table 2, Fig. 8, characters 71-88), most of which (characters 71-81) referring to the exclusive ornamentation of the cphth and body somites. The remaining characters relate to the FR seta VII showing a bifid tip (character 82), the strong diminution of the mandibular exopod (character 83), combined with the loss of lateral setae (character 84), and the retention of only one apical seta (character 85), the loss of three setae on the maxillular endopod (= three setae present) (character 86), the development of an elongated prehensile P1 endopod that carries a well-developed claw on the enp-3 (character 87), combined with a remarkable elongation of the P1 enp-1 that surpasses the length of the whole exopod (character 88). Admittedly, a 3-segmented P1 endopod (likewise the exopod) forms part of the idyanthid ground pattern. Nevertheless, the development of a 3-segmented endopod in Meteorina is interpreted here as a secondary evolution into a derived state that only resembles the ancestral one. This assumption is substantiated by the special shape of the endopod: Meteorina lacks the characteristic idyanthid P1 with a 3-segmented exopod and the first endopodal segment being clearly broadened on the level of the inner seta in the middle of the segment; in contrast, in Meteorina, the exopod is 2-segmented (see Table 2, character 13), and the enp-1 is remarkably elongated, becoming a long and slender segment that surpasses the length of the exopod and carries its inner seta in its distal quarter; the enp-3 bears just two elements, one of which tiny and the other produced into a strong serrated claw. Thus, as Meteorina is clearly embedded in the group of derived Idyanthidae due to the arguments presented in the contribution on hand, its P1 is seen here as the exclusive deviation of Meteorina. Characters 71-88 evolved exclusively in Meteorina, whereas the remainder of the Idyanthidae presents comparatively ancestral conditions, with the exception of characters related to the mouthparts that are even further derived in Pseudometeorina (see below).

Pseudometeorina George & Wiest, 2015
As stated above, seven out of the ten genera currently assigned to the Idyanthidae are monotypic. With respect to the here studied taxa, this applies to Aspinothorax, Meteorina, and Styracothorax. The finding of P. eratosthenica sp. nov., however, now allows the phylogenetic characterization of the formerly also monotypic taxon Pseudometeorina. That genus presents a large set of 24 autapomorphies (Table 2, Fig. 8, characters 89-112), of which only three (characters 102-104) show a perhaps convergent development in single other species. Characters 89-101 relate to the shape and ornamentation of the cphth, the body somites and their respective cuticular processes, and to the shape of the FR. These features deviate remarkably from those of representatives that are considered as rather original ones like e.g., Idyanthe, Nematovorax, and Tachidiella, because their morphology resembles that of the supposed idyanthid ground pattern as proposed by Seifried (2003). Furthermore, compared with Aspinothorax, Meteorina, and Styracothorax, characters 89-101 are observable only in the Pseudometeorina species. Their morphology eminently suggests a further deviation instead of the retention of ancestral conditions, so they are hypothesized here as unambiguous autapomorphies of Pseudometeorina.
Further deviations regard the loss of an abexopodal seta on the A2 basis (character 102) and on the enp-1 (character 103), the loss of setae on the mandibular basis (character 104), the loss of one apical seta on the mandibular endopod (character 105), the remarkable elongation of the mandibular endo-and exopod (characters 106, 107), the strong atrophy of the mxl (character 108), resulting in a minute lobe that apparently carries just two short setae (cf. Figure 4b), the complete loss of an endopod in the atrophied mx (character 109), the loss of one syncoxal seta in the mxp (character 110), the retention of only two setae and a claw in the maxillipedal endopod (character 111), and the loss of the sexual dimorphism in the male P2 (character 112).
Characters 102, 103: Idyanthidae retain an abexopodal seta on both the A2 basis and the enp-1 (Seifried 2003). This is still observable in the here studied species, with exception of Pseudometeorina. Nonetheless, also Aspinothorax, and Styracothorax lost the basal abexopodal seta (character 102) (Huys 1993;Moura and Martínez Arbizu 2003), whilst Tachidiella reduced the respective seta in the enp-1 (character 103) (Lee and Huys 1999). Nevertheless, as stated above, single setae are often and independently reduced in Harpacticoida, and as demonstrated in the discussion above, the named species are clearly separated from Pseudometeorina and characterized by own apomorphic characters. Therefore, the simultaneous loss of the abexopodal setae is interpreted here as synapomorphic for both Pseudometeorina species but as convergent development in Tachidiella, Aspinothorax, and Styracothorax.
Characters 104-111 refer to the mouthparts, which show deviations in Pseudometeorina in comparison with most remaining Idyanthidae. The genus presents a mandibular basis lacking any setation (character 104), whilst the remaining Idyanthidae bear three basal setae. This derived setal lack is also noticeable in Styracothorax. Though, as that genus lacks the 13 synapomorphies of Pseudometeorina and Meteorina (Table 2, characters 58-70), the loss of setae in the mandibular basis is considered here as convergent development in Pseudometeorina and Styracothorax. Moreover, the former presents a mandibular endopod that bears four apical setae (character 105) instead of five; likewise, its mandibular endo-and exopod are long and slender (characters 106, 107), giving to the mandibular palp an elongated and remarkable stretched aspect, whilst in the remaining Idyanthidae it retains a rather compact and short shape, and also characters 108-111 clearly reflect the atrophy of the mxl, the mx, and the mxp in Pseudometeorina, which in the remaining here studied species are developed according to the idyanthid ground pattern as proposed by Seifried (2003).
Character 112, i.e., the here assumed complete loss of a sexually dimorphic male P2, may constitute the most striking characteristic of Pseudometeorina. One might assume that the absence of a sexually modified male P2 endopod represented a rather ancestral condition. But as the expression of the typical idyanthid sexual dimorphism is explicitly interpreted as one main autapomorphy of the family (Seifried 2003), its absence in Pseudometeorina demanded the exclusion of that genus from the Idyanthidae, despite the here discussed deviations shared with Aspinothorax, Styracothorax, and especially with Meteorina. The alternative, here adopted assumption is that Pseudometeorina in a secondary step lost the male sexual dimorphism in the P2. Such a loss definitely occurs in Harpacticoida and has been documented in some representatives, e.g., of Laophontidae T. Scott, 1904(e.g. Lang 1965Mielke 1981;Wells and Rao 1987;Fuentes-Reinés and Suárez-Morales 2017). In the present context, it is therefore regarded as an unambiguous autapomorphy of Pseudometeorina.

Establishment of Pseudometeorina eratosthenica sp. nov.
The specimen collected from Eratosthenes Seamount shares the derived characters 89-112 (except characters 102-104) exclusively with Pseudometeorina mystica. Its allocation into Pseudometeorina is hence well-justified. However, both species differ from each other by means of several distinct deviations, here considered as respective autapomorphies. These relate to the shape and ornamentation of the posterior margins of the cphth and the body somites (characters 113-120: apomorphies corresponding to P. mystica; characters 123-127: apomorphies corresponding to P. eratosthenica sp. nov.) (cf. Table 2), the setation of the mandibular endo-and exopod (character 121: md endopod with three apical setae in P. mystica but with four setae in P. eratosthenica sp. nov.; character 128: md exopod with one lateral seta in P. eratosthenica sp. nov. but with three lateral setae in P. mystica), the composition and/or ornamentation of the maxillar endites (character 122: middle maxillar endite lost in P. mystica, 1-segmented in P. eratosthenica sp. nov.; character 129: proximal maxillar endite minute, knob-like, without setae in P. eratosthenica sp. nov. but of regular size and bilobed, with two setae in P. mystica), and the maxillipedal syncoxa lacking any setae in P. eratosthenica sp. nov. These morphological differences between the deep-sea and the seamount specimens, which furthermore constitute autapomorphies of each of them, clearly support the hypothesis of the Eratosthenes specimen belonging to a distinct species: Pseudometeorina eratosthenica sp. nov.

Conclusion
The results of the exhaustive phylogenetic analysis as summarized and depicted in Fig. 8 support the above-formulated hypothesis: Aspinothorax, Meteorina, Pseudometeorina, and Styracothorax in fact form a monophylum within the Idyanthidae that can be verified by five unambiguous autapomorphies. The establishment of the Aspinothoracinae Moura & Martínez Arbizu, 2003 subfam. nov. for the unification of the above-named four genera is therefore well-justified. Moreover, each genus is characterized by exclusive autapomorphies. In the monotypic taxa, Aspinothorax, Meteorina, and Styracothorax, these exclusive deviations are identical with those of the assigned species. In contrast, the detection of Pseudometeorina eratosthenica sp. nov. enables the characterization of Pseudometeorina as a monophylum, which is verified by means of 24 derived characters considered here as synapomorphies of the two enclosed species P. mystica and P. eratosthenica sp. nov.
Funding Open Access funding enabled and organized by Projekt DEAL.

Conflict of interest
The author declares no competing interests.

Ethics approval No animal testing was performed during this study.
Sampling and field studies No special permits were needed at the time of sampling.
Data Availability All data generated or analysed during this study are included in this published article.
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