Ophiosphalma glabrum

Ophiosphalma glabrum is a deep-sea brittle star of the family Ophiosphalmidae, recorded in the eastern Pacific at depths of 878 to 5203 m, from British Columbia to Peru ^[1],[2]. It lives on soft sediment, apparently as a generalist deposit feeder, is probably gonochoric and, judging by oocyte size, probably has larvae that feed on egg yolk reserves ^[3].

The disc is pentagonal and covered with scales of varying size, the smallest at the centre and the largest toward the edge. The radial shields are large, longer than wide, and separated by a row of scales bigger than those of the rest of the disc. There are 6 to 7 oral papillae on each side of the jaw, three visible pairs of tentacle pores and three rounded tentacle scales per pore; preserved animals are cream-coloured ^[1]. The original authors noted that some specimens may show 4 or 5 pairs of tentacle pores ^[1].

In specimens measured in the Clarion-Clipperton zone the disc diameter was 5.7 to 25.2 mm; older works cited in that study give maxima of 35 to 40 mm ^[3]. The family is recognised by many disc scales, small primary plates, genital slits longer than in Ophiomusa and three (rarely 2 to 5) visible pairs of tentacle pores ^[4]. The species was described as Ophiomusium glabrum and moved to Ophiosphalma in 2016, the year in which Ophiosphalma fimbriatum was also treated as a junior synonym of it ^[5],[1].

Similar species

• Ophiosphalma jolliense: Ophiosphalma jolliense has a flattened disc, arms with a granular look and a single rounded tentacle scale per pore, against three in Ophiosphalma glabrum ^[1].
• Ophiomusa lymani: Species of Ophiomusa have two visible pairs of tentacle pores and very short genital slits, whereas Ophiosphalma glabrum has three pairs and long slits ^[4],[1].

It has been reported from British Columbia (Canada) to Mexico, Isla del Coco, Panama, Colombia, Malpelo Island, Ecuador, the Galapagos Islands and Peru, at depths of 878 to 5203 m ^[1]. In the Pacific waters of Costa Rica it was recorded between 1157 and 3400 m, and together with one sea cucumber it had the widest depth range in that review and was one of only two species found below 3200 m ^[2].

In the TALUD project off Mexico it was found between 1848 and 2125 m, off the Pacific coast of Baja California and off Colima and Guerrero; a single station yielded 312 specimens ^[1]. On the north-eastern Pacific abyssal plain, inside the Clarion-Clipperton zone, it is one of the more frequent brittle stars on seabeds with polymetallic nodules ^[3].

A histological study of 76 individuals, collected with a remotely operated vehicle in 2015 and 2019, showed that the sex ratio did not differ from 1 to 1 and that no hermaphrodites were present, so the species is probably gonochoric ^[3]. The smallest mature individual had a disc diameter of 12.07 mm if male and 14.14 mm if female ^[3].

The largest oocytes measured 453 μm and held abundant yolk, which points to larvae that feed on their reserves and could disperse even where food is scarce ^[3]. Among females, 56 % had nearly ripe ovaries and none was ripe; among males, 49 % were developing and 8 % ripe ^[3].

It is epibenthic on soft sediment (burial of 0 to 2 mm, according to works cited in the study) and is regarded as probably a generalist deposit feeder, like the related genus Ophiomusa ^[3]. Specimens were found singly, with no grouping by size, unlike the brittle star Ophiacantha cosmica, which forms groups on stalked sponges ^[3]. At one Mexican station (1858 to 1879 m) it occurred with Ophiomusa lymani, Amphiophiura oligopora and Histampica duplicata ^[1].

The nodule-bearing seabeds where it lives in the tropical north-eastern Pacific are under threat from deep-sea mining. The authors warn that, like other deposit feeders, it could be affected by changes in the sediment, such as compaction or the release of heavy metals, and that abyssal communities adapted to a stable setting may cope poorly with such disturbance ^[3].