Achlyonice longicornis, Bohn, Jens Michael, 2006

Bohn, Jens Michael, 2006, Crinoidea and Holothuroidea (Echinodermata) of the abyssal Angola Basin — Results of the DIVA­ 1 expedition of FS " Meteor " (Cruise M 48 / 1), Zootaxa 1276, pp. 1-31 : 14-16

Achlyonice longicornis spec. nov.

( Figs 7 , 8 A–E)

Material. Holotype ( ZSM 20043073), FS "Meteor", DIVA­1, station M 48/1­351, 16°25.2' S, 5°27.1' E, 5387 m to 16°33.2' S, 5°27.3' E, 5385 m, Agassiz trawl, 30 Jul, 2000.

Description. The holotype is more or less ovoid ( Figs 8 A–B), 35 mm long, about 10 mm wide (across ventral sole), and of a dirty­white colour (preserved). The mouth is ventral and the anus is terminal, dorsally of the ventral sole. Ten tentacles encompass the mouth, eight (?) of which are connected by a membrane, while at least the two posteriormost tentacles are free. Terminal discs of tentacles with numerous minute processes. Nine pairs of tube feet border the posterior two­thirds of the ventral sole ( Figs 8 A–B). The anterior five pairs are conical and bear a terminal disc consisting of four lobes ( Fig. 8 D). They are well separated from each other and slightly decrease in size from anterior to posterior. The remaining four pairs, all of about equal size, are clustered brim­like at the posterior end of the ventral sole. A conspicuous dorsal velum, about as long as the body, is situated close to the anterior end of the specimen ( Figs 8 A, C). It is composed of four papillae, of which the long median pair are fully fused along their length, while the outer papillae are much shorter and separate in their distal part, forming small lateral lobes. A third pair of small and free papillae is present close to the base of the velum.

A calcareous ring seems to be missing. The single polian vesicle is large. The unpaired left gonad consists of richly branched short tubules opening into a well developed common duct. The anterior part of the intestine is straight, while the posterior forms a loop.

Calcareous deposits are straight or slightly curved spinous rods ( Fig. 8 E), up to 470 long, which are present in the tentacles the dorsal papillae and the tube feet; but none were found in the body wall (some of the deposits are in the process of dissolving, due to acidic fixation fluid).

Remarks. Achlyonice longicornis differs from all other known elpidiid species by the combined occurrence of the following two characteristics: the peculiar triangular velum with two small lateral lobes and the large spinous rods present in the tentacles, the dorsal papillae and the tube feet.

Only one species is known to possess a somewhat similar velum, the single specimen of Peniagone stabilis Koehler and Vaney, 1905 collected in the Bay of Bengal. It is characterised by a triangular velum consisting of one large and two small papillae and a posterior lobated border of fused tube feet ( Koehler & Vaney 1905). It differs from Achlyonice longicornis by the much shorter velum, which is only 5 mm high (with both specimens being of similar size) and by its calcareous deposits, which are four­armed and have one central apophysis and one apophysis on each arm.

The generic assignment of the new species is somewhat ambiguous. Three out of five species of the genus Achlyonice Théel, 1879 and all species of the genus Ellipinion Hérouard, 1923 are known to possess rod­shaped, often spinous deposits and a velum. Ellipinion species differ from Achlyonice by the additional presence of small C­shaped deposits and constantly 10 tentacles. Hansen (1975) characterises Achlyonice as follows: tentacles 10–12; deposits tripartite, rod­shaped or absent; calcareous ring consisting of five isolated pieces, each having a varying number of arms. Absence of C­shaped deposits assigns the new species to Achlyonice rather than to Ellipinion .

Within this genus there are two species, Achlyonice monactinica Ohshima, 1915 and Achlyonice myriamae Gebruk, 1997 , which share some similarities with A. longicornis . All three have the anterior tentacles connected by a membrane and the calcareous deposits are rods ( Ohshima 1915; Gebruk 1997). Most obviously, A. longicornis differs from A. myriamae and A. monactinica by: different tentacle numbers, both species have 12 tentacles as opposed to A. longicornis , which has only ten; its peculiar dominant triangular velum; the restriction of rod deposits to the tentacles, the dorsal papillae and the tube feet, which are present throughout the body wall in the other species. Furthermore, A. myriamae has in addition to rod deposits also tripartite deposits, which are lacking in A. monactinica as well as in A. longicornis .

Distribution. ( Fig. 7 ) So far, this species is only known from the type locality, Atlantic Ocean, Angola Basin, 5385–5387 m.

Etymology. The name, longicornis , refers to the characteristic, horn­like shape of the dorsal velum.

Figures

FIGURE 7. Distribution of Peniagone purpurea (Théel, 1882) and Achlyonice longicornis spec. nov.

FIGURE 8. Achlyonice longicornis spec. nov. (A) Right lateral view (schematic). (B) Ventral view (schematic). (C) Velum. (D) Distal end of tubefoot. (E) Rods from dorsal papillae. Figs A – C to the same scale.