Ophiomitrella ” thuyi, Stöhr, 2025

Stöhr, Sabine, 2025, Two new species of brittle stars (Echinodermata: Ophiuroidea), living epizoic on a stalked crinoid, European Journal of Taxonomy 1022, pp. 176-201 : 187-188

“ Ophiomitrella ” thuyi sp. nov.

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Fig. 2

Diagnosis

The dorsal disc is densely covered by low round granules, except on the radial shields, which are ¼ as long as the disc diameter. There are three spiniform lateral oral papillae. The adoral shields are trapezoid, larger than the oral shield, positioned only proximal to the oral shield. There are up to eight arm spines that do not meet dorsally.

Etymology

The specific epithet honours palaeontologist Ben Thuy whose work has greatly advanced the understanding of ophiuroid evolution.

Type material

Holotype

TONGA – N Ha’apai group • 19°03.7′ S, 174°18.98′ E; depth 523–806 m; 14 Jun. 2000; Anders Warén leg.; Bordau stn DW1595; Warén dredge; on Neogymnocrinus richeri (SMNH-225978); MNHN, MNHN-IE-2023-165 .

Paratype

TONGA – N Ha’apai group • 1 spec.; same collection data as for holotype; SMNH, SMNH-Type-10004 .

Description

Holotype DORSAL DISC. Rounded pentagonal, 4.4 mm dd, dorsal scales obscured by spherical granules of variable size. Radial shields bell-shaped, about ¼ as long as dd, not bearing granules, pairs of radial shields separated by single column of narrow scales bearing few granules.

VENTRAL DISC. Covered by overlapping round scales, few scattered granules similar to dorsal ones. Three lateral oral papillae, two proximal ones spiniform pointed, distalmost one shorter, compressed, blunt, all roughly thorny. In mouth angle, an unidentified compartment plate (see Hendler 2018). Lyman’s ossicle likely absent. Ventralmost tooth larger than papillae, flattened, pointed, roughly thorny. Upper teeth larger still, with increasingly rounded tip. Adoral shields limited to proximal edges of oral shield, trapezoid, with straight edges, proximal edge longer than distal edge, sides slanting. Oral shield rounded rhombic, about twice as wide as long, with small median process in distal edge, smaller than adoral shield. Madreporic shield longer, with central depression.

ARMS. Dorsal arm plates fan-shaped, with convex distal edge, separated. Lateral arm plates arch around arm, meeting dorsally and ventrally. Up to eight conical, pointed, all over thorny, hollow arm spines, second dorsal spine longest, up to two arm segments long, ventral spines shorter than one arm segment. Spines do not meet across dorsal arm. Spine articular structures bordered by wavy ridge with unperforated stereom. First ventral arm plate narrow, extending into mouth slit, with median furrow. Other ventral arm plates rounded pentagonal, with strongly convex distal edge, acute proximal angle, slightly overlapping, distally a band of unperforated stereom. Single oval tentacle scale, closing tentacle pore completely.

COLOURATION. Dark green, similar to host N. richeri , faded in alcohol.

Paratype

Disc rounded pentagonal, 4.4 mm dd, fewer disc granules than holotype, interradially three columns of round scales, central column overlaps outer two. Other characters as in holotype.

Remarks

“ Ophiomitrella ” thuyi sp. nov. falls within the group of species with low round disc granules. It is closest to O. tenuis in its granule covered disc and same number of arm spines, but the granules in O. tenuis are smaller, slightly taller, less spherical, and somewhat thorny, and the arm spines form a continuous row across the dorsal arm. Its radial shields are smaller (1/6 of dd), the adoral shields are crescent-shaped, narrower, smaller than its oral shield, the madreporite is larger and has a domed centre. “ Ophiomitrella ” thuyi resembles O. diaphora (H.L. Clark, 1911) in the size of the radial shields, the large adoral shields and disc granules of variable size, but that species is considerably larger, the radial shields are widely separated, the dorsal arm plates are rhombic, it has up to seven arm spines and four lateral oral papillae, the distalmost one enlarged. Among other species with similar radial shields and large adoral shields, O. mensa O’Hara & Stöhr, 2006 has stump-like disc granules with widened head, in O. globulifera (Koehler, 1896) the radial shields are widely separated by large scales and it reaches a larger size. All other species with round disc granules have smaller radial shields and/or crescent-shaped adoral shields ( Table 2 ). There is some similarity to the specimens identified as O. conferta by O’Hara & Stöhr (2006), but they differ from the true O. conferta (Koehler, 1922) ( Fig. 1G–H) and from O. thuy in having large globular disc granules with short peduncle. Last but not least, none of the other species in Ophiomitrella and Ophiosemnotes are known to live on N. richeri , and the two new species described here may be the first records of epizoic ophiuroids on N. richeri .