Ophiolimna, Verrill, 1899

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 : 131-134

Ophiacanthid genus Ophiolimna and their relationship to the family Ophiodermatidae : the strongest controversy of the ophiuroid taxonomy

The following case of three externally similar genera currently considered within three different families highlights problems in ophiuroid taxonomy in general and elucidates the relationship of the families Ophiacanthidae and Ophiodermatidae . There are several genera with externally similar appearance, including fine dorsal granules, granules on ventral interradii and similar granules covering the adoral and/or oral shields, which have puzzled taxonomists through the history of ophiuroid systematics. These genera are Ophiolimna Verrill, 1899 (currently a member of the subfamily Ophiotominae ) and Ophioconis Lütken, 1869 , which, although currently considered within two different families ( Ophiacanthidae and Ophiodermatidae respectively), have a long history of taxonomic association with each other ( Figs 81A–D). Lyman (1878, 1882) described species belonging to both, in modern sense, genera Ophiolimna and Ophioconis , under the same generic name, Ophioconis . At the same time, the type species of the genus Ophiolimna , O. bairdi , was described by Lyman (1883) within the genus Ophiacantha .

Verrill (1899) created the genus Ophiolimna within the family Ophiacanthidae , but included only two species and did not discuss the relationship with Ophioconis and the Ophiodermatidae . H.L. Clark (1911) already suspected that some species of the genus Ophioconis really belong to the family Ophiacanthidae , and especially emphasized the similarity between Ophiacantha and his species Ophioconis papillata . Despite this, the genus Ophioconis was placed by him within the family Ophiodermatidae , at the beginning of the classification, thus separate from ophiacanthids. Simultaneously in the same work, Clark placed O. bairdi ( Lyman, 1883) and O. lambda (H.L. Clark, 1911) within the genus Ophiacantha (and referred the family Ophiacanthidae close to the end of the classification), but a close resemblance to Ophioconis was mentioned. Clark (1911: 230) even stated that “This species is very near Ophioconis , and it is virtually impossible to find any character which would warrant their being placed in separate families…”. Matsumoto (1917) for the first time clearly distinguished the genera Ophioconis , which he referred to the Ophiodermatidae , and Ophiolimna , which he considered within Ophiacanthidae . Matsumoto also further divided Ophioconis into three genera, Ophioconis s. str., Ophiuroconis , and Ophiurodon . The latter two genera he placed into the subfamily Ophiarachninae , whereas Ophioconis s. str. was placed into Ophiodermatinae s. str. In the phylogenetic part of his work, most interestingly, Matsumoto considered Ophiarachninae as a very close taxon to the family Ophiacanthidae .

Almost half a century later, Dyakonov (1954) created the genus and species Toporkovia fragilis based on a single small imperfect specimen and placed it within the family Ophiodermatidae . Further this genus was included by Fell (1960) into his “Synoptic keys…” in the family Ophiodermatidae , thus supporting Dyakonov’s decision. Soon afterwards, Fell (1961) considered the Antarctic species Ophioconis antarctica ( Lyman, 1878) as a member of the genus Toporkovia , as illustration of the idea of bipolarity. Stöhr & O’Hara (2003) mentioned the great similarity as well as the differences between Ophiolimna antarctica , previously considered as Ophiodermatidae , and O. bairdi , which was always considered as a member of the family Ophiacanthidae . O’Hara and Stöhr (2006) gave a detailed taxonomic history of the genus Ophiolimna (including O. antarctica and O. papillata ) considering it as member of the family Ophiacanthidae , and finally considered Toporkovia as synonym of this genus. However, until now there are no definite conclusions on the relationship between Ophioconis , currently placed in the Ophiodermatidae , and Ophiolimna , currently in Ophiacanthidae . Besides some minor differences in arm spines, tentacle scales etc., the presumably ophiodermatid Ophioconis is well distinguished from Ophiolimna by the presence of strong, very wide hyaline teeth. However, some species, e.g. Ophioconis miliaria Lyman, 1878 do not possess such teeth, at least in a well-defined condition. It may therefore be that Ophiuroconis is distinguished from Ophioconis by the triangular and pointed instead of widened teeth. In contrast, Ophiurodon Matsumoto, 1915 is currently recognized as a synonym (or genus with doubtful status) of Ophioconis (A.M. Clark 1965; A.M. Clark & Rowe 1971; Baker & Devaney 1981).

Besides the above mentioned taxa, there is another group, some genera of which are externally very similar to the genera Ophioconis and Ophiolimna , but have never been considered close to the latter or synonymized with it. It is the ophioleucid group of the family Ophiuridae . The external similarity between some ophioleucids, e.g. Ophiostriatus from one hand and Ophioconis and Ophiolimna from another ( Fig. 81), is remarkable and includes dorsal and ventral fine granulation, dorsal spinelets intermingled with granules, and striations on the lateral arm plates ( Stöhr & O’Hara 2003; O’Hara & Stöhr 2006; present study). The genus Ophiostriatus was placed within the group Ophioleucidae (e.g. Madsen 1983), and therefore at least formally was never considered as an ophiacanthid or ophiodermatid. Thus, these three genera, Ophiolimna , Ophioconis and Ophiostriatus , represent a very interesting case, because they have an exceptionally similar external appearance, but are currently considered within three different families. Obviously it is necessary to use additional characters to enlighten the relationship of these three genera. Conclusions of such an investigation may serve as a test for the validity of the spine articulations as a character as well. In the present study many of the microstructural characters of Ophiolimna bairdi , O. antarctica , Ophiolimna perfida , Ophioconis vivipara Mortensen, 1925 , Ophioconis permixta Koehler, 1905 , Ophiconis cf. cincta Brock, 1888 , Ophioconis cupida Koehler, 1905 , Ophioconis sp. , Ophiostriatus striatus (Mortensen, 1933) and an undescribed species of the genus Ophiostriatus were used. The shape of the jaws is very similar between Ophiolimna and Ophioconis in proportions and especially in the presence of a peculiar folded area on the proximal adradial surface ( Figs 82D, E). In contrast, the shape of the jaws of Ophiostriatus is greatly different from both Ophiolimna and Ophioconis : it is very elongated and without proximal folds ( Fig. 82F). The dental plate of Ophiolimna is entire, imperforate, with regular elongated grooves ( Fig. 82G). Ophioconis also has an imperforate dental plate, with very weakly developed regular transverse grooves, however the dental plate is naturally fragmented into three pieces ( Figs 82H; 15J). The dental plate of Ophiostriatus is entire, but perforated by several small round holes, arranged quite irregularly, in single, two or three rows ( Fig. 82I). The articulation between adradial genital plate and radial shield differs between all three genera. The articulation surface of the adradial genital plate in Ophiolimna bairdi has an elongated, ill-defined elevated ridge ( Fig. 82J), Ophioconis possesses a well developed asymmetrically placed ball-like condyle, which is clearly separated from the remaining articulation area ( Fig. 82K), whereas Ophiolimna antarctica ( Fig. 47G) and O. perfida ( Fig. 6E) have an intermediate condition between O. bairdi and Ophioconis . Finally, Ophiostriatus has a quite complicated structure appearing as a horseshoe-shaped deep groove around the small but distinct condyle ( Fig. 82L).

On the articulation surface of the radial shield in Ophiolimna there is an elongated elevated ridge ( Figs 6M; 47I), in Ophioconis several indistinct longitudinal folds, in Ophiostriatus also a quite complicated structure with two separate pits and with a condyle in between. The spine articulations of Ophiolimna and Ophioconis are very similar, both volute-shaped with large muscle opening and small nerve opening ( Figs 82A, B). The differences are mainly in the larger and more defined lobe of the articulation in Ophioconis , with a more round nerve opening than a rather slit-shaped one in Ophiolimna . In other species of Ophioconis , Ophioconis sp. , this lobe is even more developed, whereas the opposite walls of the articulations are more reduced, a condition that is very similar to some “advanced” ophiodermatids ( Fig. 30A). Unlike Ophiolimna and Ophioconis , the articulations of Ophiostriatus are very different in having a peculiar irregularly rhomboid shape comprised of a very large muscle and small nerve openings ( Fig. 82C). Other special features of the articulations of Ophiostriatus include the characteristically curved distal wall fused with the edge of the lateral plate. Considering all above listed characters, Ophiolimna is a member of the family Ophiacanthidae . Ophioconis is similar to Ophiolimna not only in the external characters, but also in the shape of the jaws, the presence of folds on the adradial surface, and an imperforate dental plate with elongated sockets. However, Ophioconis has a peculiar character of the family Ophiodermatidae , the fragmented dental plate. Also the hylenated teeth of the genus Ophioconis are notable. The articulation surface of the adradial genital plate of Ophioconis has a well-developed asymmetrical ball-like condyle and is also more similar to the Ophiodermatidae than to Ophiacanthidae . At the same time, the arm spine articulations of Ophioconis are very similar and not distinguishable from Ophiolimna . All internal characters of the genus Ophiostriatus are very different from Ophiolimna and Ophioconis . The shape of the jaws and dental plate of Ophiostriatus are typical for the family Ophiuridae and the ophioleucid group. The articulations of Ophiostriatus are essentially concurrent with that group too. Thus, considering these three genera, it is possible to conclude that the significant similarity of Ophiostriatus on the one hand and Ophiolimna and Ophioconis on the other hand is clearly a remarkable case of convergence. The external similarity between Ophiolimna and Ophioconis is well supported by the shape of the articulation tubercles and partially supported by internal characters. The similarity includes small details, essentially alike between all studied species of Ophiolimna , and several species of Ophioconis (having a fragmented ophiodermatid dental plate and very wide hyaline teeth), including the general appearance of the articulations, which have a peculiar bridge of narrow stereom strips connecting the proximal border of the articulation with the striated stereom structure at a considerable angle ( Figs 45A–F; 82A–B) (connecting ridge). A presupposition that such structures, similar in several complicated details, have evolved independently is hardly plausible. Whereas all external and internal characters of Ophiolimna do not contradict those known for the family Ophiacanthidae , the fragmented dental plate and genital plate articulation surface of the genus Ophioconis are shared with the family Ophiodermatidae . A fragmented dental plate, being a plesiomorphic feature, has also reappeared within some other advanced ophiuroid families, e.g. Amphiuridae ( Amphilimna multispina , present study). Thus, the genus Ophioconis is an intermediate taxon between two large, well-defined traditional families Ophiacanthidae and Ophiodermatidae .

The presence of a similar articulation surface of the adradial genital plate in other Ophiolimna species , O. antarctica ( Fig. 47G), O. perfida ( Fig. 6E) and some other ophiotomin ophiacanthids (e.g. Ophiotoma ) ( Fig. 6C) make the limits between Ophiacanthidae and Ophiodermatidae even more uncertain. It is also one of the reasons for the long taxonomic association and confusion between the species now considered within Ophiolimna and Ophioconis . Therefore, phylogenetically, the large and well-defined, mostly tropical family Ophiodermatidae is rooted within the deep and cold water genus Ophiolimna of the family Ophiacanthidae . At least one studied ophiodermatid, e.g. Ophiopeza spinosa , has an entire dental plate ( Fig. 14O) and thus, even with regard to this character, these families are poorly distinguished. Another consequence of this conclusion is that one of the peculiar characters of almost all ophiodermatids, the fine granulations of both the dorsal and ventral sides (including oral and adoral shields) are similar to the finely granulated disk surface and several papilla-like tubercles on the oral shields of the genus Ophiolimna . The similarity of the spine articulation between the genera Ophiolimna and Ophioconis agrees with the similarity of other internal characters and therefore can be used as an indication of the relationship. Thus, three large ophiuroid families, one predominantly deep-water— Ophiacanthidae —and two tropical shallow-water— Ophiodermatidae and Ophiocomidae —are closely related and have some intermediate genera. Separation of these families into different orders, infraorders or even superfamilies is highly artificial.

Other families assigned to the different superfamilies of the infraorder Gnathophiurida , i.e. superfamily Gnathophiuroidea (families Amphiuridae , Ophiactidae and Ophiotrichidae ) on the one hand and the superfamily Ophiocomoidea (families Ophiocomidae and Ophionereididae ) on the other ( Smith et al. 1995) have a clearly intermediate group, the family Ophionereididae , externally still very similar to the family Ophiocomidae , but with respect to the jaw morphology and the arm spine articulation ridge pattern similar to the family Amphiuridae (compare Figs 33A–C and 31). The family Ophiolepididae considered by Matsumoto (1917) (in broad sense together with the taxa of the family Ophiuridae ) within the order Chilophiurida and separated by Smith et al. (1995) into an independent infraorder, has an almost typical amphiurid-ophiactid dental plate (especially in the genus Ophiozonella ) and subparallel reduced articulations ( Figs 34B–D), and appears part of the Amphiuridae-Ophionereididae line.