Ophiacanthidae (Paterson, 1985)

Martynov, Alexander, 2010, Reassessment of the classification of the Ophiuroidea (Echinodermata), based on morphological characters. I. General character evaluation and delineation of the families Ophiomyxidae and Ophiacanthidae 2697, Zootaxa 2697, pp. 1-154 : 126-127

The family Ophiacanthidae — traditional definitions and newly applied characters

The family Ophiacanthidae is one of the largest among Ophiuroidea and externally characterized by a disk covered with various spinelets in most species, often quite densely placed, long numerous arm spines, and oral papillae arranged without a gap in relation to the apical papilla; the latter typically is relatively narrow and conical ( Koehler 1914; Fell 1960). Few taxa have been studied internally and have been reported to possess a dental plate with non-perforating, elongated, narrow tooth sockets ( Murakami 1963; Bartsch 1982, 1987), relatively short jaws ( Murakami 1963) and well developed adradial and abradial genital plates, and simple articulation surfaces of the adradial genital plate and radial shield in form of a low elongated elevation ( Matsumoto 1917). Paterson (1985) has added to the diagnosis of the family a particular shape of the spine arm articulations with a distinct volute-shaped lobe dorsal to the muscle opening. However, his study was based on few ophiacanthid species and has not until now been supplemented by additional taxa. The genera Ophiocymbium and Ophiomyces , having a clearly different pattern of the articulations from the other ophiacanthid taxa, were nevertheless retained within the family Ophiacanthidae ( Paterson 1985) . Although the characteristic shape of the spine articulations is a promising feature for testing the monophyly of the Ophiacanthidae , further study ( Smith et al. 1995) suggested paraphyly of this family.

This study shows that most species from the well-known ophiacanthid genera, i.e. Ophiacantha , Ophiolebes , Ophiosemnotes , Ophiocamax , Ophioplinthaca and others, all have essentially similar articulations with a distinct distal volute-shaped lobe and the sigmoidal fold ( Figs 28A–L; 45A–C; 47E–F; 50F–G; 58A–B; 59C–E; 60F–G; 68A–B; 69E–H; 70E–F; 78F, H). Although each of the listed taxa has peculiarities in the shape of the articulation ridge, general constructions are basically the same in all studied species. For instance, within eight studied species of the genus Ophiacantha , O. spectabilis G.O. Sars, 1871 ( Fig. 28F) is characterized by a more circular shape of the articulations with closed proximal edge, whereas in O. abyssicola G.O. Sars, 1871 ( Fig. 28E) they are more elongated with partially open proximal edge, but all species have a volute-shaped distal lobe.

Most of the genera of the subfamily Ophiotominae , including the type genus of the subfamily — Ophiotoma , and a genus that has recently been placed in the subfamily in question – Ophiolimna , were investigated in the present study. Some of these genera have not only the typical ophiacanthid articulation artuclations sigmoidal fold, but also unfragmented dental plates with elongated slit-shaped sockets, holding relatively broad and massive teeth, for instance Ophiotoma assimilis Koehler, 1904 and Ophiotoma alberti (Koehler, 1896) ( Figs 13D–F). Nevertheless, at least one internal character of Ophiotoma assimilis — a massive rounded articulation surface of the adradial genital plate — suggests an ophiomyxid appearance (compare Figs 6 A and B). Thus according to some important characters, i.e. spine articulations and dental plate, some of the studied ophitomin taxa clearly belong to the family Ophiacanthidae , whereas the soft disk, large tentacle pores and shape of the genital plate articulation surface are possibly plesiomorphic characters indicating an ophiomyxid relationship.

In contrast, within the subfamily Ophiotominae there is another group of genera that is characterized by the disk covered with numerous small scales, without spinelets nor granules dorsally or ventrally. The dorsal arm plates are well-developed, apical papillae usually spiniform and forming a small cluster. Oral papillae differ between various genera of this group, for instance in Ophiologimus they are numerous and rather spiniform ( Figs 48B; 49B), whereas Ophiocymbium has distinctly block-shaped distal oral papillae ( Figs 40B, D). The present study has added to the characteristics of both genera, a poorly developed adradial genital plate and absence of the abradial genital plate ( Figs 4G, H).

The position of these genera within the family Ophiacanthidae has never been challenged. This study has examined this group of genera for both arm spine articulations and other internal characters. At least all species of the genera Ophiocymbium and Ophiologimus do not possess any characters that would allow them being included in the family Ophiacanthidae . For instance, spine articulations in both Ophiocymbium and Ophiologimus lack the volute-shaped appearance and the sigmoidal fold characteristic of the family Ophiacanthidae (including subfamily Ophiotominae ). Instead, these two genera possess rounded or slightly elongated articulations with no nerve opening or a nerve opening at a small distance near the muscle opening ( Figs 27H–N). Dental plates of both Ophiocymbium and Ophiologimus have few round sockets, bearing spiniform teeth ( Figs 12E–J; 15B, C), and thus clearly differ from the relatively massive ophiacanthid teeth placed on the narrowly elongated sockets (including the type genus of the subfamily Ophiotominae , Ophiotoma ) ( Fig. 13). All Ophiacanthidae (including Ophiotominae s.str.) have a well-developed adradial genital plate with distinct abradial genital plate ( Figs 6, 7; 47H; 67C; 78G), whereas Ophiocymbium and Ophiologimus have a poorly developed adradial genital plate (sometimes entirely reduced) without abradial genital plate ( Figs 4G–H). Thus, at least Ophiocymbium and Ophiologimus should be excluded from the family Ophiacanthidae and placed in the family Ophiomyxidae , because they share with the latter similar spine articulations, dental plate, radial shield and genital plate. It is important to note that the articulations of the type species of the genus Ophiocymbium , O. cavernosum , are essentially similar to the articulations of Ophiomyxa , the type genus of the family Ophiomyxidae (compare Figs 27H and 27G).

Some ophiomyxid genera (e.g. Ophioscolex ) also have poorly developed radial shield, adradial and abradial genital plates, as shown above for the type species, O. glacialis Müller & Troschel, 1842 ( Figs 4I, U). The genus Ophiocymbium clearly has some similarities with Ophioplexa gen. nov. in general disk appearance: very small genital slits under the adoral shields, slit-like grooves between the proximalmost parts of the arms and the ventral disk, and poorly developed genital plate and radial shield. Unlike most genera of the subfamily Ophiotominae , the newly discovered Ophioplexa gen. nov., despite presenting some superficial ophiacanthid features, e.g. numerous small disk scales without evident radial shields, combined with well defined dorsal arm plates, lacks any characters of true ophiacanthids (including subfamily Ophiotominae ), i.e. the ophiacanthid sigmoidal fold, dental plate with regularly placed elongated sockets and well-developed genital plates and radial shield. Instead, the new genus has narrowly elongated arm spine articulations ( Figs 56A–H), a distinct dental plate with several articulations and irregularly placed, small, rounded sockets, holding numerous spiniform teeth ( Figs 56M–N), and extremely poorly developed adradial genital plate and radial shield (including total absence of the abradial genital plate) ( Figs 4J, K, P).