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Johnston, 1842

Species Overview

Halisarca dujardini Johnston (1842) forms greyish-or ochre-yellow slippery-slimy sheets or cushions, under rocks, in empty shells and on brown algae. There are no spicules or other skeletal structures (microscopic examination necessary). It differs from the equally spicule-less Oscarella rubra in lacking surface lobules: the surface is smooth and even. Unlike Oscarella, it has a firm and elastic consistency. It is common along most coasts of Western Europe.

Taxonomic Description

Colour: Ochraceous yellow, greyish, yellowish or creamy fawn.
Shape, size, surface and consistency: Slimy sheets up to 4 x 5 cm, or slimy cushions generally surmounted by a short, narrow oscular chimney. Surface slippery, smooth, composed of a distinct cuticle. Mottled with pale spots or pores where the sponge is expanded. Oscules are sparsely distributed, frequently at the ends of short, cylindrical chimneys. Normally 1-2 mm wide. Consistency fairly firm and elastic. The oscular chimneys can close up, and the pores become indistinct.
Spicules: None.
Skeleton: No spicules of any kind are present. Sections of the sponge sometimes reveal the presence of a network of fibrils in the soft matrix, helping to strengthen it. Sections cleared in clove oil reveal that the choanocyte chambers are elongated (up to 60-125 µm long and about 25 µm wide) and sometimes are branching; typically they radiate from exhalant canals which form the centre of aquiferous stars, but during the reproductive period the aquiferous system tends to become disorganized to the extent that choanocyte chambers remain only in the peripheral regions of the sponge.
Reproduction: May to September.
Ecology: On small stones, in the empty shells of lamellibranchs, on the carapace of crabs (e.g Inachus, Macropodia) at the base of gorgonians, from 0-50 m depth or more (80 m: Lévi; 300 m: Arndt). At low water spring tide levels on the shore it occurs under overhangs, in fissures aud on holdfasts of Laminaria. At mean tide levels it is found under stones and boulders. In rock pools it may occur on Fucus serratus and Ascophyllum nodosum, but possibly a different species is involved here (see below).
Distribution: Atlantic coasts of Europe; Greenland, Mediterranean.
Etymology: Named after the French biologist Dujardin, who was the first to describe this sponge (as Halisarca).
Type specimen information: No data.

Remarks

Superficially these thin slimy sheets, greyish brown in colour, can be confused with Hymedesmia coriacea but the absence of spicules distinguishes Halisarca from it. It can also be easily distinguished from the other spicule-less slime sponge, Oscarella rubra, by the presence of a superficial cuticle, the absence of small lobules on the surface and the presence of elongated, not spherical, choanocyte chambers.
Two other species of Halisarca were described from Bretagne by Topsent (1893), H. sputum and Lévi (1956: 87), Halisarca metschnikowi. Since their discovery, they have not been positively identified or redescribed other than by Lévi, and although Lévi is an authority on sponges, the distinctness of metschnikowi from dujardini needs further corroboration. The two can apparently only be distinguisbed by minute details of gametogenesis and larval development, the value for species discrimination of which is untested.
Halisarca rubra Hanitsch (1895) is an Oscarella.
Sources: Arndt, 1935; Lévi, 1956; Ackers et al., 1985, 1992 (W. Clifford Jones, S.M. Stone, D. Moss, B.E. Picton).

Halisarca dujardini