(Bowerbank, 1866)
Species Overview
Axinella subdola (Bowerbank, 1866) is a yellow or orange, thinly and irregularly branching sponges, up to 25 cm high. Typically the branches arise at right angles to the main stem. The branches are rouned. The stalk is wiry. It resemblesto some extent other branching sponges such as Axinella species, and Haliclona oculata , but the combination of a wiry stalk and thin branches is characteristic. From Adreus fascicularis which is also thin-branched, it differs in having the branches at right angles and untidilly arranged (and Adreus has asters as microscleres). This is a southern species known mostly from the south coasts of the British Isles.
Taxonomic Description
Colour: Pale yellow, deep orange, orange, brownish-yellow or golden-yellow when alive; becomes creamy white in alcohol.
Shape, size, surface and consistency: Irregularly branching-erect (Axinella subdola MCS3). The branches sometimes coalesce but never produce the lamellate forms found in other Axinellid sponges. The side branches tend to arise at right angles to the main stem which gives it an untidy, 'bushy' appearence (Axinella subdola MCS2). The branches are circular in cross-section (Axinella subdola div.1), borne on a thinner, wiry stalk. Up to 25 cm high. Surface even, slippery, smooth; in older specimens minutely hispid. The oscules are small, not numerous, and disposed irregularly on the branches. They become inconspicuous on collection. Considerable lateral contraction of the branches occurs on collection. This is in contrast to the other axinellids, which do not contract markedly in this manner. Consistency: the branches are firm, tough and elastic but collapse if compressed when alive. The stalk is wiry.
Spicules: (Axinella subdola spics) The megascleres are smooth, usually straight, styles, which vary in size: 270-(440)-560 x 5 µm. There are no microscleres.
Skeleton: A plumose axial core of loose fibres of long styles, which are surrounded by a dense sub-surface layer of irregularly arranged spicules. Some of these lie at right angles to the surface (occasionally piercing it); others are tangential to it. The intervening space between the two main skeletal areas is occasionally bridged by thin, almost imperceptible, fibres of spicules. Small amounts of spongin are present.
Ecology: Found in sheltered and semi-exposed conditions. On horizontal and slightly inclined rock, covered with a layer of silt. Out in the open on sheltered bottom plains, or at the bottom of gullies, which create relatively sheltered conditions in otherwise exposed situations. Circalittoral (?): at Lundy at 10 m or deeper. Raspailia hispida , Raspailia ramosa and Axinella polypoides are other branching species often seen in the vicinity.
Distribution: British lsles. Also reported from Sea of Japan; Sea of Okhotsk. In the UK this is a southern species, recently recorded from the Scilly Isles; Lundy; Devon; and Skomer. The type locality is Guernsey.
Etymology: Subdolus (Greek) = deceitful, referring to the problem the original author had in assessing its affiliation.
Type specimen information: Holotype BMNH 1929:11:27:1, Guernsey. MCS voucher BELUM: Mc481, Skomer Island.
Remarks
Alive the appearance lies between that of Axinella polypoides and Haliclona oculata . However, the oscules of Axinella subdola are less distinctive and not arranged in rows. In the field the best identifying character is probably the wiry stalk, which can be felt if not seen among the underlying 'turf'. The untidy appearence of the sub-branching, caused by the secondary growth of short stubby branchlets, may also prove to be a useful character, but this needs testing. The side branches come off more or less at right angles—a very characteristic feature. There is a slight possibility of confusion with Adreus fascicularis which is much more wiry and on average a smaller sponge which does not branch 'untidily' as does Axinella subdola . Also, Adreus fascicularis has distinctive lines/grooves running along the branches.
Sources: Burton, 1930b; Ackers et al., 1992(S.M. Stonc, G. Ackers, B.E. Picton, D. Moss).