Ophiacantha Müller & Troschel, 1842

Thuy, Ben, 2013, Temporary expansion to shelf depths rather than an onshore-offshore trend: the shallow-water rise and demise of the modern deep-sea brittle star family Ophiacanthidae (Echinodermata: Ophiuroidea), European Journal of Taxonomy 48, pp. 1-242 : 98-99

Genus Ophiacantha Müller & Troschel, 1842

Type species

Ophiacantha spinulosa Müller & Troschel, 1842 [junior synonym of Ophiacantha bidentata (Bruzelius, 1805) ], by original designation.

Diagnosis

LAPs with dorsal and ventral lobes of spine articulations fused into continuous volute; ventral part of LAPs not protruding ventro-proximalwards; generally no more than one spur on the outer proximal and inner distal edges; ridge on inner side composed of long, oblique, straight to slightly bent main part with a pointed dorsal tip and with ventrally pointing, vertical to slightly oblique or bent extension generally at least as long as half of the main part; in many cases, main part and ventrally pointing extension fused into large, irregularly triangular knob; tentacle notch small.

Remarks

Ophiacantha is one of the most speciose ( Stöhr et al. 2012), but also one of the most heterogeneous genera among the extant ophiuroids ( O’Hara & Stöhr 2006). This morphological heterogeneity is also reflected in LAP morphology, as already pointed out by Thuy & Stöhr (2011), with some species sharing greater similarities in LAP morphology to species of other ophiacanthid genera than with their congeners. Thus, working out a comprehensive and at the same time meaningful LAP morphological diagnosis of Ophiacantha is a major challenge. To make matters worse, most characters observed in the LAPs of Ophiacantha also occur in other genera. The most stringent approach is to focus on the LAP morphology of the type species, Ophiacantha bidentata , on the basis of which the LAP morphologies of the other species are assessed.

The LAPs of O. bidentata were described in detail and illustrated by Thuy & Stöhr (2011). They are characterised by a ridge on their inner side, which consists of a main oblique, straight to slightly proximally bent part with a pointed dorsal tip and fused dorsally to a ventrally pointing, near-vertical to slightly oblique extension close to the proximal edge of the LAP longer than half of the main part of the ridge. In some cases, the main part of the ridge and its ventrally pointing dorsal extension are merged into a single, large, irregularly triangular knob. In combination with the large ear-shaped spine articulations on a strongly elevated distal portion of the LAP, with dorsal and ventral lobes merged into a continuous volute, and proximally sharply bordered by the distalmost lamella of a well-developed vertical striation on part of the outer surface, the shape of this ridge is distinctive. This is important to note since slightly modified versions of ridge shape are also found in other, not necessarily closely related ophiacanthids (e.g. Ophiomitrella Verrill, 1899 ; Ophioplinthaca Verrill, 1899 , “ Ophiophthalmus ” Matsumoto, 1917). Many species of Ophiacantha share the above-mentioned combination of features, which can be thus considered as the Ophiacantha LAP morphology in its proper sense. Whether the species in question also share similar general skeletal morphologies, and thus potentially represent a true clade with the Ophiacanthidae , remains to be investigated.

A number of extant species of Ophiacantha conflict with the type species in terms of LAP morphology. Particularly striking is the occurrence in a number of species of Ophiacantha of a ridge shape otherwise exclusively found in Ophiotreta and Ophiopristis . The species in question include Ophiacantha anomala G.O. Sars, 1872 , O. rosea Lyman, 1878 , O. spectabilis G.O. Sars, 1872 and O. vivipara Ljungman, 1872 . Remarkably, these four species are known to have closely similar general skeletal morphologies in common ( Paterson 1985; O’Hara & Stöhr 2006), thus most probably form a phylogenetically consistent group. A detailed reassessment of the species in question is likely to result in separation at the generic level, which would gain support from evidence of LAP morphology.

Other species currently assigned to Ophiacantha differ from the type species in displaying generally lower LAPs (smaller height/width ratios), proximally separated dorsal and ventral lobes of the spine articulations which thus become slightly horizontally elongate rather than round, a single protruding spur in the middle of the outer proximal and inner distal edges, and a densely meshed or even granulated stereom covering the outer surface rather than a vertical striation. At the same time, they share the highly distinctive shape of the ridge on the inner side with Ophiacantha bidentata . Such taxa include Ophiacantha smitti Ljungman, 1872 , O. pentagona Koehler, 1897 , O. prionota H.L. Clark, 1911 and O. levispina Lyman, 1878 . Whether this grouping is artificial or, indeed, reflects general skeletal morphological similarities is in need of further study. Remarkably, the species in question are closer to species of Ophiolebes Lyman, 1878 in terms of LAP morphology than to their congeners.

According to the revised cladistic phylogeny presented here, Ophiacantha is sister taxon to the Ophiolebes - Ophialcaea clade. Unfortunately, LAPs of extant species of Ophialcaea Verrill, 1899 could not be examined using the standards set by Thuy & Stöhr (2011). Ophiolebes , however, is shown here to share largely similar LAP morphologies with Ophiacantha , in particular with the species mentioned above.Similar to those of the latter, the LAPs of Ophiolebes display a ridge that is superficially comparable to the one seen in Ophiacantha bidentata . The main distinction between the LAPs of Ophiolebes and those of O. bidentata and similar congeners are the proximally separated dorsal and ventral lobes of the spine articulations.

To conclude, as currently understood, the genus Ophiacantha obviously unites a number of fundamentally different LAP morphologies, some of which definitely reflect differences at the generic level to be worked out in detail by future studies. It is very difficult to characterise the LAP morphology of the genus convincingly, even if restricted to the type species and species with morphologically similar LAPs. The ridge shape described above for Ophiacantha bidentata , along with spine articulations freestanding on the elevated distal edge of the LAP, with dorsal and ventral lobes fused into a continuous volute, and bordered proximally by a not exceedingly thickened distalmost lamella is a combination which probably comes closest to a diagnosis of the LAP morphology of Ophiacantha in its proper sense. For the purpose of the present, only fossil LAPs displaying this combination of characters are assigned to Ophiacantha .