Hymedesmia (Hymedesmia) prostrata Thiele 1903

Hymedesmia (Hymedesmia) prostrata Thiele, 1903

(Figs 9 A–E)

Hymedesmia prostrata Thiele, 1903: 955, fig. 20.

Material examined. MNHN DCL 4042–B, Jean-Louis Etienne Expédition Clipperton 2005, station 18, 55 m, on dead corals, 20–01–2005, 1 specimen.

MNHN DCL 4049–A, Jean-Louis Etienne Expédition Clipperton 2005, station 42, 8 m, on dead corals, 31–01– 2005, 1 specimen.

Description. Thinly encrusting (Fig. 9 A), microhispid, covering areas of several cm2. No apparent oscules or porefields. Consistency soft, easily damaged.

Skeleton. Megascleres are arrenged erect on the substrate (hymedesmioid), loose bundles or single tornotes at the surface.

Spicules. Anisotornotes, acanthostyles and arcuate isochelae.

Tornotes (Fig. 9 B) with one end mucronate and slightly swollen, the other end tapering evenly rounded, 114– 137.3 –156 x 1.5–2.5 µm.

Large acanthostyles (Fig. 9 C) densely spined at the head, more sparingly spined towards the pointed end, 149– 186.2 –233 x 7 – 8.4 –10 µm.

Small acanthostyles (Fig. 9 D) heavily spined all over, 57– 65.3 – 78 x 4 – 5.6 –6 µm.

Arcuate isochelae (Fig. 9 E) in a large size range but not divisible in categories, alae frequently somewhat irregular or partially split, 12– 17.5 –21 µm.

Ecology and distribution. Encrusting undersides of corals over a wide depth range; reported from Île Clipperton, Ternate and Aldabra.

Remarks. The identification with Thiele’s Ternate species is made on the basis of generally similar spicular characters, although the tornotes of Thiele’s specimen were slightly longer (200 µm). Sizes of acanthostyles (240 and 90 µm) are close and particularly the aniso-condition of the tornotes is similar to our specimen. Hymedesmia records from shallow Indo-Pacific waters are few.

Sponges of the genus Hymedesmia are typical deep-water / cold-water species, with relatively few known from shallow-water and even fewer from shallow coral reef environments. Most species have been described from the North Atlantic, and only half a dozen were recorded from the Pacific and none from the tropical East Pacific. The nearest recorded shallow-water species is Hymedesmia sp. from Hawaii (Bergquist 1977), apparently blue colored, but no further data were provided. As far as is known H. (H.) prostrata is never blue-colored. A record from Indonesia, apart from Thiele’s from Ternate, is Hymedesmia (Hymedesmia) mertoni Hentschel, 1912 from the Arafura Sea. This has similar megasclere sizes and shapes, but differs sharply from our species in possessing two categories of both isochelae and sigmas. Eastward, Sim & Bakus (1986) report the occurrence of the North Atlantic deep sea species Hymedesmia (Hymedesmia) levis Lundbeck (1910) from the coast of California; their specimen has tornotes in the form of tylotes, unlike our tornotes, and their acanthostyles were considerably shorter than ours. It is unlikely that their material from 9 m depth is conspecific with Lundbeck’s deep sea species.

Further away to the south (Patagonia) two species described by Thiele (1905), Hymedesmia irritans and Hymedesmia laevis were assigned to other genera (Forcepia Carter, 1874 and Hymenancora Lundbeck, 1910, respectively) by Van Soest et al. 2008. In the South West Pacific, northern New Zealand, three species of Hymedesmia have been recorded, viz. Hymedesmia (Hymedesmia) lundbecki Dendy, 1924, Hymedesmia (Hymedesmia) microstrongyla Bergquist & Fromont, 1988 and Hymedesmia (Hymedesmia) anisostrongyloxea Bergquist & Fromont, 1988. All three differ from H. (H.) prostrata in having larger, more robust chelae, but H.(H.) microstrongyla is closely similar to our material in other aspects.

To the north, in Japanese waters, occurs Hymedesmia (Hymedesmia) uchinourensis Hoshino, 1981, which differs clearly in having the shaft of the chelae ‘serrated’ (bluntly spined) quite unlike the commonplace smooth chelae of our species. Two deep-water species from New Caledonia, Hymedesmia (Hymedesmia) spiniarcuata Lévi & Lévi, 1983 and Hymedesmia (Hymedesmia) brachyrhabda Lévi & Lévi, 1983 also have spined chelae. All three should perhaps be transferred to the genus Acanthancora Topsent, 1927 because of the spined condition of the chelae.

Shallow-water species from the Indian Ocean, e.g. Hymedesmia (Hymedesmia) tenuissima (Dendy, 1905 as Myxilla) (subsequently recorded from the Great Barrier Reef by Burton 1934), and Hymedesmia (Hymedesmia) dichela (Hentschel, 1911 as Leptosia) are not only far removed from the present location but they also differ in spicule sizes (shorter acanthostyles and larger chelae in tenuissima) and categories (two sizes of chelae in dichela).

Hymedesmia (Hymedesmia) prostrata was also recorded by Lévi (1961) from the Western Indian Ocean. Spicule categories and sizes are closely similar to ours. Lévi (1961) suggests that Hymedesmia (Hymedesmia) lancifera Topsent, 1906 from the Western Indian Ocean is a junior synonym.