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Echinoderms from Southern New Zealand

Ophiomisidium Koehler, 1914 — Ophiomisidium irene sp. nov. (Figures 21 and. 22)

Ophiomisidium Koehler, 1914

Ophiomisidium irene sp. nov. (Figures 21 and. 22)

Dimensions: R, 4·0 mm.; r, 1·5 mm.; ratio R/r, 2·7. The holotype and syntypes are all similar.

Disc: pentagonal, flattened above and below. The aboral surface of the disc is made up of a symmetrical mosaic of 26 plates, all polygonal and interlocking. These comprise the following; At the centre lies the pentagonal dorso-central: it is surrounded by a ring of live hexagonal primary radials; outside these is a ring of fifteen plates, five of them pentagonal primary interradials. the remainder the five pairs of radial shields. The latter are pentagonal and contiguous. A transversely elongate secondary interradial intervenes between adjoining pairs of radial shields. The adoral surface is largely made up of the first lateral arm-plates. The oral shield is vestigial, rhomboid. Adoral plates are longer than broad, contiguous over most of their length, meeting the oral shield only on their distal borders, their lateral borders emarginated by the first large tentacle-pore (whose scale they carry). The oral plates are broader than long, broadly contiguous with each other as Well as with the adoral plates. Oral papillae represented only by a narrow marginal bar along each oral plate.

Anms: short, expanded at the base, tapering distally to a slender extremity, nine arm-segments. There are five upper arm-plates; the first fan-shaped, contiguous with the radial shields, and overlapping upon the second; the third, fourth, and fifth also fan-shaped, but not contiguous, and diminishing in size rapidly in that order. Distal arm-segments without upper arm-plates. Lateral arm-plates well developed on all segments, the first far the largest, the second much smaller, the third and succeeding ones tapering more gradually. The first lateral arm-plates of each side are broadly contiguous not only with each other, but also with their corresponding member of the adjoining arm. Hence they form a closed ring completely investing the oral skeleton. They comprise most of the interradial area below, all but obliterate the oral shields, and completely obliterate the genital clefts. The lateral plates meet above and below on all segments except the first, where they meet below only. The first and second lateral plates bear on their free edges two flattened scales, which evidently represent modified spines; the scales form a fringe round the whole disc. The third and fourth lateral plates carry each only one short, stumpy spine, and thereafter there are no more spines. There are five lower arm-plates, the innermost largest, the second and third moderately large, the outer two minute. The first three lower arm-plates are more or less pentagonal, having their lateral borders emarginated by the three large tentaclepores.

page 29
Ophiomisidium irene sp. nov.

Ophiomisidium irene sp. nov.

Fig. 21.—Aboral view. Fig. 22.—Adoral view.

Abbreviations: A, adoral plate. D, upper arm-plates. DC, dorso-central primary plate of disc. I, primary interradial plate of disc. II, secondary interradial plate of disc. L, lateral arm-plates. LA, spines of basal lateral arm-plate, modified as flattened scales. O, oral shield. PL, oral plate. R, primary radial plate of disc. S, radial shield. V, lower arm-plate.

page 30 There is only one large tentacle-scale to each of the three tentacle-pores, flattened, rounded, scale-like. The first tentacle-scale is borne by the adjoining adoral plate, the second and third scales by the first and second lateral arm-plates respectively.

Colour, in spirit, white; the outer parts of the aboral primary plates are porcellanous, their inner parts translucent.

Type locality: Discovery station 2733, Chatham Rise, 30 metres, November 4, 1950; sixteen individuals.

Holotype: Zoology Museum, Victoria University College.

Koehler (1914) founded Ophiomisidium to accommodate three species specially characterized by the entry of the arm-plates into the formation of the oral surface of the disc. The first lateral arm-plate makes up most of the interradius, the first lower arm-plate is large, and is succeeded by two others not quite so large.

Of the three species included, O. pulchellum (Lyman, 1878) and O. speciosum Koehler (1914) have the first lateral arm-plates less enlarged, so that the genital clefts are still present. O. flabellum (Lyman, 1878), known from the continental shelf off New South Wales, has the first lateral arm-plate so large as to obliterate the genital clefts, and the oral shields are rudimentary. Ophiomisidium irene therefore belongs to the O. flabellum type in this regard. It is distinguished from the latter species in many respects, notably the more attenuated distal arm region, the larger upper arm-plates, the more compact form of the primary interradial plates, and by the fact that the spines of the first two lateral plates are modified into large rounded scales.

The delicate porcellanous and translucent lustre of the plates and the symmetry of the whole animal combine to make this tiny sea-star a microscopic object of great beauty. Although no species of Ophiomisidium has hitherto been taken in New Zealand waters, representatives are probably very common, since no fewer than sixteen specimens were taken in the single Discovery haul at station 2733.