Axinella dissimilis (Bowerbank, 1866)


Family : Axinellidae

Form: Branching-erect, the diameter of the branches varying evenly or unevenly along their length (resembling annulations). The branches are usually oval in cross-section, and may coalesce (see photo. no. 35). In stunted specimens, coalescence may be so pronounced that it results in lamellate forms. Thick stalk.

Colour: Yellow, pale orange, or buff. Turns brown in alcohol.

Smell: None.

Consistency: Moderately firm, moderately elastic; surface (but not central core) cracks when axis is bent through 90 deg. (Cf. Raspailia, Stelligera and Haliclona oculata in which the surface does not crack with this degree of bending.)

Surface: Even. Velvety because of the quantity of projecting spicules which are of uniform length.

Apertures: The oscules are small and regularly spaced along the sides of the branches in flat fan-like specimens or at the edges of lamellate forms. They are placed in depressions and are often opposite, producing the annulate appearance described above. They have a number of short, shallow, surface grooves converging on them. These stellate grooves 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). See photograph. A major distinction in Axinella polypoides is the fact that the osculae are arranged all round the branches.

Contraction: Not noticeable.

Internal characters

Skeleton: Plumose. An axial skeleton of longitudinally orientated megascleres forms a stiff core 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. 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.

Spicules: The megascleres are styles (a) 220-(305)-360Ám and oxea (b) 180-(200)-210Ám - usually both are present. Variations in size between styles and oxea can occur - both can be the same length, or the styles can be longer than the oxea, but the reverse is rarely the case - see note on the variability of the spicule complement under Axinella infundibuliformis. The ectosomal brushes may consist of styles only, and these are often longer than the styles of the main skeleton. Microscleres are trichodragmata (c) 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.

Habitat: Mainly on upward facing clean or silty rock, usually in deep (30m) water but also shallower - typically found in the circalittoral in SW Britain, but in Jersey and Brittany also occurs in the lower infralittoral, competing directly with foliaceous algae. Prefers clean oceanic water, but tolerates silt. Also reported "on shingle and muddy sand" (??) - confirmation would be valuable.

Ecology: Other sponges, commonly Scypha spp., can be found growing on the branches.

Distribution: Typically a southern species, "Madeira, Spain, France, British Isles, (Mediterranean - probably only Axinella polypoides present)". Recently known from SW. Britain, as far north as Anglesey, and the Atlantic coast of Ireland as far north as Rathlin Is., Co. Antrim; and with one recent confirmed Scottish record from Loch Scridain, Mull. Not known from North Sea coasts of U.K.

Distribution Map from NBN: Grid map (fast) : Interactive map (slower, requires login to view records) : National Biodiversity Network mapping facility, data for UK.

Identity: The presence of stellate oscular grooves is diagnostic of a closely related group of branched Axinellid species including Axinella dissimilis. A number of other branched axinellids have been reported from the continental channel coast but not yet from Britain. See also Axinella damicornis and Axinella flustra. Specimens on the south side of Skokholm Is, Dyfed, were recently observed to fall into two morphotypes, with smaller, hand-shaped specimens and larger irregular and sparsely branched specimens growing alongside each other. Critical observations and photographs of this phenomenon from other sites would be interesting.

Voucher: BELUM : Mc1377. Guernsey, Channel Is.

Editors: J.D. Guiterman, D. Moss, B.E. Picton.

 Picton, B.E., Morrow, C.C. & van Soest, R.W.B., 2011. [In] Sponges of Britain and Ireland