Form: Polymorphic, varying from a cushion to almost a branching-repent form. A cushion can give rise to oscular chimneys with oscules at the top or, more typically, to a profusion of simple, solid, tassel-like branches with the oscules mainly along their length (cf. Halichondria panicea). In some sheltered localities the branches grow over other organisms and loop like bramble stolons, attaching to any suitable object they encounter.
Colour: Usually buff or cream, never green.
Smell: Not obvious.
Consistency: Soft and moderately elastic. Branches do not break even if bent through 180 deg. (cf. Halichondria panicea).
Surface: Smooth or uneven, often with a translucence which enables the outline of some deeper structures to be seen. In a collected specimen the translucence and consistency are reminiscent of parchment. Surface spicules are sometimes more or less parallel and united into widely spaced fibres, or the spicules are arranged in no obvious order.
Apertures: Oscules are "usually small", (but see photo 59) and either at the tops of chimneys, whose shape is uneven, or along the sides of branches. Larger oscular chimneys (up to 1 cm tall) tend to have an ill defined translucent band running up one side. The apical termination of this band (canal?) contributes to the unevenness of the oscular rims (cf. Halichondria panicea).
Contraction: Not noticeable.
Skeleton: Halichondroid. The main skeleton is a confused reticulation of oxea of variable size with a slight tendency to form fibres 4 - 10 spicules thick. The ectosomal skeleton is typically a regular reticulation of similar spicules arranged tangentially, these either lying singly and parallel to one another, or formed into fibres: this regular arrangement can become confused and lost. The surface spicules help to reinforce a well-defined ectosomal membrane. Sub-surface spaces are not as well developed as in Halichondria panicea. Very small quantities of spongin are present.
Spicules: Simple. Megascleres are slightly curved slender oxea only: 400-(500)-600 x 12Ám, and 200-(253)-320 x 2.5Ám. The spicules are rather more slender and show a wider size range than in Haliclona spp., with which certain forms of Halichondria may be confused. No microscleres.
Habitat: On rock or other animals, even ascidian tests. It reaches its maximal development in harbours and estuaries, being very tolerant of muddy and brackish conditions where it tends to replace Halichondria panicea. (But there are reports of both Halichondria species occurring together in silty conditions, e.g. from Sussex.) Halichondria bowerbanki can be partly embedded in mud. Halichondria bowerbanki occurs typically from the upper infralittoral downwards, and is never (?) found on the shore. Halichondria panicea occurs from the shore to the lower infralittoral, rarely deeper.
Distribution: "Arctic; Norway; Belgium; British Isles; France; Mediterranean." A common species in the British Isles.
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 variable nature of Halichondria bowerbanki can make it very difficult to identify positively. The most likely confusion is with Halichondria panicea. The table with Halichondria panicea will help to separate the two species. Some growth forms are peculiar to one or other of the species and these are indicated. The growth of tassels in Halichondria bowerbanki seems to be a good character, but these are not present all year round. Frequently however, it is impossible to be certain to which of the two species a specimen belongs. However larval differences prove that two species are involved here. The cushion forms, with modest oscular chimneys, can be mistaken for Myxilla incrustans (q.v.). The confusion arises when a specimen of Halichondria bowerbanki has an 'open' sub-surface appearance, which is reminiscent of the labyrinthine channels of M. incrustans.
Voucher: BELUM : Mc150. Strangford Lough, Down.
Editors: D. Moss, B.E. Picton.
|Picton, B.E., Morrow, C.C. & van Soest, R.W.B., 2011. [In] Sponges of Britain and Ireland |
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