Hymeniacidon kitchingi (Burton, 1935) is a beige-yellowish irregularly encrusting sponge with characteristic surface projections which may grow out irregularly, resembling e.g. Halichondria (H.) bowerbanki. Unlike that species it is very soft, weak, and has a less distinctly detachable skin, and the spicules are styles, not oxeas. So far the species has been recognized only along northern and western coasts of the British Isles. It occurs in the sublittoral, on rocks and sessile organisms.
Colour: Greyish beige, with a tinge of pink or purple in encrusting specimens, more yellowish or orangy in more elaborate specimens. Yellow in alcohol.
Shape, size, surface and consistency: Very variable, irregular. Thin sheets may have thin tassel-like outgrowths reminiscent of Halichondria (H.) bowerbanki, but are thinner. Encrusting forms resemble Haliclona (R.) viscosa or Haliclona (R.) cinerea, with lobate growths each with oscules. Tassel-like forms similar to Iophon nigricans have been observed in sheltered conditions. Size may be up to 2 x 3 x 2 cm. Surface smooth or rugose, or smoothly lobate, with a porous appearance similar to that of Haliclona (R.) viscosa. The apertures may be the most constant feature of the external appearance of this species. The oscules have a raised transparent rim with a few converging transparent excurrent canals. The oscular rims may develop short tassels reminiscent of Amphilectus fucorum. However, in thinly encrusting specimens the oscules may not be apparent. Oscules become inconspicuous on collection. Consistency very soft, compressible, may turn liquid when sirred with a needle.
Spicules: Slender styles, often flexuous, typically ca. 220 x 3-6 µm. According to the original description the distal ends narrow to a point in semi-discrete steps. This feature is not visible in all specimens, and occurs in only about 10 % of spicules in most specimens.
Skeleton: (Hymeniacidon kitchingi skel) Slender styles often forming wisp-like bundles running towards the surface; otherwise somewhat irregularly arranged. The ectosomal skeleton is barely developed, consisting of the partly tangentially running whispy endings of the choanosomal bundles.
Ecology: On rocks, algae (Laminaria holdfasts), bryozoans such as Cellaria and Pentapora, etc, in sites sheltered from wave action, but with moderate tidal streams. Shallow subtidal, infralittoral.
Distribution: Shetland, W. coast of Scotland, Ireland, probably not uncommon.
Etymology: Named after the collector of the type specimen, Dr Kitching.
Type specimen information: BMNH 1934:9:26:79 (wet), Kyle Scotnish, Loch Swen, W coast of Scotland; MCS voucher: BELUM Mc617, Ard Bay, Galway, W Ireland.
The identity of this species rests on its thin styles (with distinctive narrowing to the point), together with soft texture, and halichondroid skeleton. Burton (1935) erected Rhaphidostyla with the present species as the type species. He also assigned Phakellia incisa Schmidt, 1868 (=Dictyonella incisa according to Topsent (1938) and Pulitzer-Finali (1978)), Phakellia plicata Schmidt, 1862 (= Dictyonella plicata), Clathria pelligera Schmidt, 1862 (= Scopalina pelligera), and Stylotella marsilii Topsent, 1925a (= Dictyonella marsilii). From this, it would seem that Rhaphidostyla is a junior synonym of Dictyonella were it not for the fact that the type species R. kitchingi is not at all like Dictyonella spp.
In his description of R. kitchingi Burton emphasized the telescoped endings of its spicules. This feature, however, is of wide occurrence in many different demosponges. In other aspects there is great similarity between Rhaphidostyla kitchingi and members of the genus Hymeniacidon.
It is very similar to H. perlevis (Montagu, 1818), which occurs sympatrically, but may be distinguished on life-colour (orange in perlevis) and smaller and thinner spicules (up to 400 µm in perlevis). Until Western European Hymeniacidon have been revised, it seems wise to recognize this doubtful species as Hymeniacidon kitchingi.
Sources: Burton, 1935; Van Soest et al., 1981; Van Soest et al., 1990; Ackers et al., 1992.