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(Montagu, 1818)

Species Overview

Dysidea fragilis (Montagu, 1818) is a strongly conulose, grey sponge forming lobate or digitate cushions. Elastic when compressed. It lacks a spicule skeleton, in stead it has a network of fibres loaded with sand grains, broken spicules and other foreign material (microscopic examination). It is a common sponge along most coasts of Western Europe.

Taxonomic Description

Colour: Usually whitish or grey but can be brown. In cases where sand grains predominate over organic matter the sponge can assume the colour of sand.
Shape, size, surface and consistency: Form variable (Dysidea fragilis MCS2). Cushions to massive-lobose forms, usually less than 15 cm across, but can be over 15 cm in tidal rocky estuaries. Surface smooth, conulose, because of spongin fibres lifting the surface layer. The projections are sometimes lighter in colour than the main surface The surface may be slippery. Oscules scattered, variable in size. Contraction not noticeable. Consistency variable. Usually moderately soft and fully elastic. The elasticity depends on the ratio of spongin (elastic) to inorganic matter. Usually tough. No smell.
Spicules: None of its own. Foreign spicules may be incorporated into the skeleton.
Skeleton: (Dysidea fragilis skel) A reticulation of spongin fibres partially or wholly obscured by foreign inclusions (enclosed in the spongin fibres and sometimes loose in the matrix), e.g. sand grains, spicules of other sponges, diatom valves, etc. The reticulation consists of stout ascending fibres up to 250 µm diameter or more, with a secondary network connecting these main fibres; thinner fibres about 30 µm in diameter. The spongin is not always visible, unless the section has been stained.
Associations: The barnacle Acasta spongites forms a specific association.
Ecology: On rock, maybe in crevices, widespread but seldom dominant. On shell and gravel. Tolerates some silt.
Distribution: Atlantic coasts of Europe; Arctic; Mediterranean. Elsewhere reported from many parts of the world, but these records need critical evaluation.
Etymology: The name refers to the fact that this species is easily damaged (in dry condition).
Type specimen information: Burton (1934) proposed to name Johnston's (1842) specimen in the British Museum, reg. no. 30:7:3:447, which bears the label "Spongia fragilis type" as neotype (Dysidea fragilis neotype); MCS voucher BELUM Mc70. Strangford Lough, N Ireland.

Remarks

Dysidea pallescens is a Mediterranean species reported from Lough Hyne, SW Ireland. It is apparently pink-purple in colour with relatively large oscules, but its record needs further substantiation. D. avara is more intensely coloured, often darker; the conules are much higher and further apart. The skeleton is more open-meshed and primary fibres are thicker.
The conulose surface, great elasticity, coupled with the colour (whitish) are useful diagnostic features. If any of these features is absent then demonstration of the three dimensional spongin net, with inclusions, under the microscope is essential for confident identification. Pleraplysilla spinifera could be confused with thinly encrusting Dysidea, it differs in the absence of anastomosed spongin fibres (microscopic examination). Ulosa stuposa is a very polymorphic species, and in certain forms can superficially resemble Dysidea, even to the point of becoming loaded internally with sand grains. However Ulosa can be distinguished by always having styles coring the spongin fibres.
Sources: Arndt (1935); Hiscock et al., 1983[1984]; Ackers et al., 1992 (S.M. Stone, B.E. Picton and D. Moss)

Dysidea fragilis