Axinella flustra (Topsent, 1892a) is a small (max. 7 cm) lamellate or irregularly flatly branching yellowish sponge, with a tuberculate and hispid surface. It is stalked and has a distinct axis. It differs from other branching axinellids in having thin-blades/branches, somewhat resembling the foliose bryozoan Flustra (hence its name), and by possession of trichodragmas (microscopic examination). It is a rare southern species occurring at greater depths in the rocky sublittoral.
Colour: Beige, greyish yellow.
Shape, size, surface and consistency: Branching-erect, tbe branches being very flattened (about 2-4 mm thick) and lamellate, all in the same plane, with well-rounded ends. There is usually a pronounced basal stalk ca. 1 cm long to about 7 cm in height. Surface even, slightly tuberculate and slightly hispid, with moderate friction. Oscules inconspicuous, regularly distributed over one surface of lamella. Faint star-shaped canal patterns are visible in situ. Consistency compact with flexible branches.
Spicules: (Axinella flustra spics) Megascleres consisting of styles and oxeas. The styles are very variable in size: 300-685-900 x 7-20 µm, and are usually bent abruptly in the lower one third to one quarter of their length. The oxeas are stout, and often strongly curved by an abrupt bend in their centres: 150-180-220 x 4-15 µm. Microscleres are trichodragmata: up to 40 x 8 µm. They can be difficult to see.
Skeleton: The axial skeleton is compressed, conforming to the outline of the branches. The core consists of numerous, irregularly disposed, slender oxeas, cemented by variable quantities of spongin. At the periphery of the axial skeleton numerous stouter oxeas orientated parallel to the surface of the axis, are found. Variations on the structure of the axial skeleton occur with styles sometimes present and the peripheral oxeas are not always clearly differentiated. Large styles echinate the axial skeleton, their heads in the axis, with their points piercing the sponge surface to give the hispid appearance.
Ecology: Found in rocky areas, often togetber with Axinella dissimilis, from ca. 25-320 m. More habitat information is required.
Distribution: Rare in the British Isles, only having been recorded from Ireland (at ca.40 m on the west side of the Aran Islands); known from Bretagne, northern Spain; Azores, Cape Verde islands.
Etymology: Named after its resemblance with the Bryozoan Flustra.
Type specimen information: The type is in the Muséum National d'Histoire Naturelle, Paris; MCS voucher: BELUM Mc1315. Aran Islands, Galway.
Flabellate forms of Axinella dissimilis, Axinella infundibuliformis and Phakellia ventilabrum have thicker branches. Endectyon delaubenfelsi has a similar low growing, erect habit, but with a more complicated branching arrangement.
Source: Ackers et al., 1992 (B.E. Picton, D. Moss)