Ophioderma Müller & Troschel, 1840

Stöhr, Sabine, Weber, Alexandra Anh-Thu, Boissin, Emilie & Chenuil, Anne, 2020, Resolving the Ophioderma longicauda (Echinodermata: Ophiuroidea) cryptic species complex: five sisters, three of them new, European Journal of Taxonomy 600, pp. 1-37 : 6-8

Genus Ophioderma Müller & Troschel, 1840

Type species

Ophioderma longicauda ( Bruzelius, 1805) .

Type locality

Mediterranean Sea.

Diagnosis

Disc with dense granule cover dorsally and ventrally, including jaws; oral shields (and in some species adoral shields) naked. Arm spines numerous, up to half arm joint length. Multiple dorsal arm plates present in several species. Two short genital slits to each bursa. Single rows of numerous oral papillae. Lyman’s ossicle present. Oral plate ridge spines present. Dental plate in several pieces, with nonperforating sockets of two types, round and small for tooth papillae, wide slit-like for teeth. Dissociated radial shield triangular, with distal abradial process, large internal pore and several smaller external pores. Lateral arm plates with two short spurs on the outer proximal edge and corresponding spurs on the inner distal edge. Adradial genital plate elongated, curved, proximal end with depression and round knob. Abradial genital plate flat, trapezoidal, about half as long as adradial plate.

Remarks

Multiple dorsal arm plates are known from several species of Ophioderma and previous authors have assumed that they developed by division or fragmentation ( Ziesenhenne 1955; Hendler et al. 1995). Hendler (2018) questioned this assumption because the morphogenesis of these plates is not yet known and recommended microstructural analysis of the stereom. Although this question lies outside the scope of this study, we have made some observations that may help to understand the process. In young specimens of all studied species, all joints are covered by a single wide dorsal arm plate. In larger specimens, additional plates appear on the proximal (= oldest) joints and they align as if they were a single plate. Thin hair-like cracks were observed on some joints in smaller specimens with otherwise entire plates. The number of plates increases on the basal joints and distalwards with growth. There does not seem to be a pre-determined division plane and the split can occur along the mid-longitudinal of the first-formed plate or offset to left or right, seemingly at random. The separate plates appear to continue to grow, obtaining various shapes, and all their edges become rounded. When the number of plates increases, a mosaic pattern appears where plates are no longer aligned across the arm, and it may be difficult to decide to which arm joint a particular plate belongs. Fragmentation seems a plausible explanation for the formation of multiple plates that replace the first plate. The additional plates that are not in line with the original plate may form either independently or from additional divisions. Ziesenhenne (1955) suggested that broken arm plates may be the result of mechanical injury from falling stones, and this may be an alternative explanation for the existence of multiple arm plates or there may be two mechanisms at work. Possibly, only multiple plates on proximal arm joints should be considered as species-specific characters, whereas the occurrence of a few cracked plates further out on the arms is more likely the result of injury. Due to these open questions, we have refrained from using the terms fragmentation or division when describing the multiple dorsal arm plates. With regard to dental plates, Hendler (2018) found that fragmentation occurs in one species of Ophioderma , and it is likely that this applies to the whole genus.