Form: Very polymorphic, varying from thin sheets, massive forms and cushions to branching-repent forms (eg. a 3-dimensional clump of anastomosing hollow branches). Prone to giving off stout branching processes which develop into oscular chimneys. At certain seasons of the year slender, almost filamentous, branching processes (the 'albescens' form of Bowerbank) are produced. The photographs indicate the diversity of form.
Colour: Green in well-lit sites owing to the presence of algae. Cream-yellow in shaded sites. "Ash-grey, brown or (rarely) brownish-red when alive."
Smell: Distinctive, like freshly deposited kelp on the strandline. This smell, once learned, is of very great help in determining difficult cases. (Halichondria bowerbanki does not smell like this.)
Consistency: Compressible, resilient but crumbly - i.e. when handled can readily break (both cushion and branching forms), until skeleton has become well consolidated. Branches break if bent through 20 degrees (this is helpful in separating it from Halichondria bowerbanki.
Surface: Usually smooth, often with a slightly glassy appearance especially when air is trapped below the surface in the sub-ectosomal system. Surface spicules are often united into fibres, which form a light regular net visible with the naked eye, although a x10 hand lens may be needed. This net is not always present. Even.
Apertures: Oscules are circular, raised on conules in the cushion form (eg. photo 4) and often regularly spaced; but are at the branch tips in the branched forms (photo 6), in addition to being sited along the branches. In some forms (? affected by currents) the oscules can be flush with the surface, or in lines on ridges. Oscular rims are of regular outline (?).
Contraction: Not noticeable.
Skeleton: Halichondroid. Main skeleton is a confused incipient reticulation of megascleres of variable size with little tendency to form ascending fibres and connecting branches. The ectosomal skeleton is typically a regular reticulation of tangentially arranged fibres of spicules which are similar to those of the main skeleton; but it can become confused. The spicules help to reinforce a well defined ectosomal membrane; the ectosomal fibres are 2 - 5 megascleres thick. A well marked system of sub-ectosomal spaces can be present, setting off the ectosomal skeleton from the confused interior. Very little spongin is present.
Spicules: Megascleres of slightly curved, slender, oxea only; typically 200-(265)-320Ám long but can vary between 100 and 480Ám. These are more slender and show a wider size range than those of typical Haliclona spp., with which certain forms of Halichondria might be confused. There are no microscleres.
Habitat: An opportunistic species, found on the shore down to the circalittoral, which adapts to a wide range of niches. Usually on rock or any other hard substrate, e.g. crab carapaces, shells, etc. It sometimes grows profusely on the stipes of Laminaria hyperborea (in clean water habitats affected by tide and wave induced water movement); often well developed in tidal rivers and harbours. (? the faster the currents the better developed is the Halichondria panicea?)
Distribution: "Arctic; Atlantic coasts of Europe; Mediterranean; etc."
Distribution Map: NBN map : National Biodiversity Network mapping facility, data for UK.
Identity: Shore sheet form. If the surface net is obvious, the colour is green, and it looks like the photograph then the identity is probably reliable. Other forms. Sections must be examined microscopically, and even then identification is not certain (smell is helpful). If the surface has a translucent quality (i.e. it is possible to see something of the underlying tissues through it), the surface spicule net is not obvious, the texture is pliable and the body skeleton is composed of spicules with a tendency to form bundles, then suspect Halichondria bowerbanki (q.v.). Other closely related species which should be considered when dealing with this Halichondria complex are Halichondria (Topsentia) difficilis (Lundbeck 1902:28), a deep water form with the distinctive characteristic of turning a deep purple on collection; Halichondria agglomerans Cabioch, 1968:226 and Raspaigella (Spongosorites) genitrix Schmidt, 1870:41. See Table on following page for differences from Halichondria bowerbanki. As there is so much overlap several workers have tried to pin point one character which they consider most helpful in separating the two species. Both the translucence of the surface of Halichondria bowerbanki and the surface net of Halichondria panicea have been used. However both these features are unreliable. For the worker with access to fresh material, the difference in smell is probably the most reliable character, although even this must be treated with caution. Several characters need to agree before the identity of a specimen can be reasonably certain.
Voucher: BELUM : Mc109. Portrush, Co Antrim.
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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