Echinoderes wilberti, Anguas-Escalante & Herranz & Martínez-Arce & De Jesús-Navarrete & Sørensen, 2023

Anguas-Escalante, Abril, Herranz, María, Martínez-Arce, Arely, De Jesús-Navarrete, Alberto & Sørensen, Martin V., 2023, New Echinoderes (Kinorhyncha: Cyclorhagida) from Mexico: Molecular barcoding demonstrate species delimitation between highly similar morphospecies, Zoologischer Anzeiger (Zool. Anz.) 302, pp. 146-165 : 163-164

4.1. Morphology of Echinoderes wilberti sp. nov. and putatively closely related species

Results of samplings in Xcalak, Quintana Roo, have shown that the locality hosts two closely related Echinoderes species. Both E. horni and E. wilberti sp. nov. share arrangement of spines and tubes, morphometry, general appearance, segment appearance, glandular cell outlets type 1 pattern, and sensory spots pattern. After examination through LM, SEM, and confocal microscopy, only two morphological differences were found: subdorsal tubes on segment 2 (present in E. wilberti sp. nov.; absent in E. horni ) and midlateral sensory spots on segment 9 (present in E. wilberti sp. nov.; likely absent in E. horni , but it should be noted that we perhaps were unable to visualise all the sensory spots in this species). Another morphologically similar and most likely closely related species is Echinoderes parahorni Cepeda et al., 2019c , described from the Dominican Republic. The main differences between them are also found on segment 2, where E. parahorni has glandular cell outlets type 2 in subdorsal positions – the same positions as we find the tubes in E. wilberti sp. nov. E. parahorni was not observed in samples from Xcalak, but in the Dominican Republic it co-occurs with E. horni ( Cepeda et al., 2019c) .

The three species share two other interesting and uncommon characteristics: the presence of paraventral bristles on segments 3 to 6, and the lack of spines in the dorsal series. Paraventral bristles were first noticed in the Mediterranean species Echinoderes shahmaranae Sørensen et al., 2021 . After observing these conspicuous bristles in this new species, all other available species of Echinoderidae were re-examined, and paraventral bristles were found in thirteen additional species of Echinoderes ( Sørensen et al., 2021) . However, out of these, only four species had the bristles restricted to exactly segments 3 to 6: E. belenae , Echinoderes hispanicus Pardos et al., 1998 , E. horni , and E. parahorni . E. belenae and E. hispanicus differ in many ways (i.e., middorsal spines, tubes on segment 2, tubes on segment 5, tubes on segment 8, etc) from the two latter, and the bristles do not indicate any close relationship between the four. But the presence of these bristles in E. horni and E. parahorni are well in line with the close similarity between the species, and it was therefore not surprising to find the exact same bristles on segments 3 to 6 in E. wilberti sp. nov.

Complete absence of spines in the dorsal series can be found in fourteen additional congeners ( Yamasaki et al., 2020; Kennedy et al., 2022). However, twelve of these species belong (or potentially belong) to the Echinoderes coulli group, and nothing indicates a close relationship with the three species addressed here. Out of fourteen species without middorsal spines, only Echinoderes malakhovi Adrianov, 1999 in Adrianov & Malakhov (1999) and Echinoderes multisetosus Adrianov, 1989 are not part of the E. coulli group, and the species could potentially be close to E. horni , E. parahorni , and E. wilberti sp. nov. However, E. malakhovi was illustrated with a series of subdorsal and paradorsal structures on segments 4 to 9 ( Adrianov & Malakhov, 1999) that could be interpreted as glandular cell outlets type 2, and this would speak against a close relationship. As for E. multisetosus potential presence or absence of glandular cell outlets type 2 is not reported ( Adrianov, 1989). However the species differ considerably from E. wilberti sp. nov. First of all, its trunk shape is much more cylindrical, closer to the trunk shape in the typical E. coulli group species. In addition, it has lateral accessory tubes on segment 7, and ventromedial tubes or spines on segment 9. Such tubes/spines are not present in E. wilberti sp. nov. The species are interesting though, and if re-examined it should be considered as a potential relative to the three.

It is clear that E. horni , E. parahorni , and E. wilberti sp. nov. share numerous similarities, and only differ regarding subdorsal cuticular structures on segment 2. Thus, we would like to propose a new species group, the E. horni group, to accommodate the three species. Species of the new group should be characterized by the lack of acicular spines in the dorsal series, tubes in lateroventral (or ventrolateral) positions on segment 2, in lateroventral positions on segment 5, in lateral accessory positions on segment 8, and in laterodorsal positions (but showing sexual dimorphism in size) on segment 10; spines are present in lateroventral positions on segments 6 to 9, and bristle-like cuticular hairs are present in paraventral positions on segments 3 to 6.