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(Bowerbank, 1866)

Species Overview

Axinella dissimils (Bowerbank, 1866) A. polypoides > is a branching erect yellow-orange sponge, with firm consistency and smooth surface. Branches are frequently in one plane, are typically undulating and somewhat flattened in cross section. The sponge normally occurs in the deeper sublittoral on rocks.

Taxonomic Description

Colour: Yellow, orange, orange-red; turns grey in alcohol.
Shape, size, surface and consistency: Branching-erect, up to 40 cm high (axinella_dissimilis_large.jpg). The diameter of the branches varies unevenly along their length (resembling annulations). The branches are up to 2 cm in thickness, usually oval in cross-section, and may coalesce. In stunted specimens, coalescence may be so pronounced that it results in lamellate forms (axinella_dissimilis_div2.jpg). Thick stalk. Surface even, velvety because of the quantity of projecting spicules which are of uniform length. The oscules are small and scattered over the branches and stalk. They have a number of short, shallow, surface grooves converging on them. These stellate grooves (see photograph axinella_dissimilis_div3.jpg ) can be seen readily in preserved material but not underwater (the grooves are covered with a thin membrane which collapses when the specimen is removed from the water). Smell none. Contraction not noticeable. Consistency moderately firm, moderately elastic; surface (but not central core) cracks when axis is bent through 90° (cf. Raspailia, Stelligera and Haliclona oculata in which the surface does not crack with this degree of bending).
Spicules: (axinella_dissimilis_spics.jpg) Megascleres: Styles: about 375-450 (MCS "Sponge V" gives 220-360) x 12 µm, and oxeas: about 220-400 (MCS "Sponge V" gives 180-210) x 12 µm—usually both are present. Variations in size between styles and oxeas can occur—both can be the same length, or the styles can be longer than the oxeas, but the reverse is rarely the case.
Microscleres: Trichodragmata: 22-25 µm long (Cabioch, 1968), scattered throughout the skeleton. These are difficult to see for the first time and need a microscope of good resolution. They look like small ill-defined smudges even under high magnification (see also Donadey et al., 1990).
Skeleton: (Axinella Vosmaer) The ectosomal brushes may consist of styles only, and these are often longer than the styles of the main skeleton. The overall skeletal architecture is plumose and the axial skeleton of longitudinally orientated megascleres forms a stiff core (axinella_dissimilis_cross.jpg ) along the centre of each branch. There is a softer extra-axial skeleton of thin fibres, which radiate at right angles from the core towards the surface (axinella_dissimilis_long.jpg). These fibres sometimes splay out in plumose fashion to end as brushes at the surface (the majority piercing the surface). The amount of spongin encasing the spicules is variable.
Ecology: Mainly on upward facing clean or silty rock, usually in deep (30-108 m) water but also shallower; typically found in the circalittoral in SW Britain, but in Jersey also occurs in the lower infralittoral, competing directly with foliaceous algae. Prefers clean oceanic water, but tolerates some silt. Reported on shingle and muddy sand; confirmation would be valuable. Other sponges, commonly Sycon spp., can be found growing on the branches.
Distribution: Typically a southern species. West coast of France and SW Britain, as far north as Anglesey. Furthermore, the Atlantic coast of Ireland as far north as Rathlin Is., Co. Antrim. Not yet known from North Sea coasts.
Etymology: dissimilis = not resembling others.
Type specimen information: BMNH 1867:7:26:81, 1877:5:21:335 (+ slides: BMNH 1867:3:11:78). MCS voucher: Mc1377, Guernsey, Channel Islands.

Remarks

The presence of stellate oscular grooves (not just clear tracts caused by channels beneath the surface) is diagnostic of a closely related group of branched Axinellid species including A. polypoides.
Some authorities (e.g. van Soest et al. 2000) considered this species to be a junior synonym of the Mediterranean Axinella polypoides Schmidt, 1862. NE Atlantic studies (Ackers et al., 1992; Boury-Esnault in Donadey et al., 1990) favour a separate species A. dissimilis (Bowerbank, 1866) (Isodictya dissimilis), confined to the Eastern Atlantic, differing from A. polypoides in having the oscules in rows along the branches. Possibly, such individuals could be no more than stressed and malformed examples of the Mediterranean A. polypoides (A. polypoides Schmidt). A further difference given by Boury-Esnault is the fact that Mediterranean specimens turn brown in alcohol, whereas East Atlantic specimens turn grey. The recent discovery of trichodragmata of 12-21 µm long in Mediterranean specimens (Donadey et al., 1990) as well as similarity in lectin content (Bretting et al., 1981) of Atlantic and Mediterranean specimens, are still throwing doubt over the distitinctness of both. A further technical complication is that the name distorta Bowerbank, 1866 (Halichondria distorta) has page priority over dissimilis Bowerbank.
Source: Ackers et al., 1985, 1992 (S.M. Stone, B.E. Picton, D. Moss, J.D. Guiterman).

Axinella dissimilis