Porifera

Sponges are frequently found on the lower shore in gullies and under rocky overhangs. Here they avoid desiccation and display a range of colours and growth forms giving them a superficial resemblance to plants. Indeed, it was not until the 18th century that they were classified as animals and as late as 1825 before the issue was put beyond doubt by the research of Dr R.E. Grant of Edinburgh.

Sponges are sessile, some forming encrusting, irregular growths covering large areas of rock, while others are vase-like and attached to the substratum at the base. They exhibit the cellular grade of organization in which the cells do not form tissues and organs so characteristic of higher animals, and as a result they are often referred to as primitive animals. Despite this simplicity, they exhibit a range of complexity in body structure and have colonized both freshwater and marine habitats, extending from the shore to the greatest depths.

The basic design of a Simple sponge is that in which the body wall is perforated by many tiny pores through which water enters the sponge, and a larger opening, the osculum, through which water leaves (Fig. 36c). The cavity of the sponge is lined by flagellated cells known as choanocytes; these maintain a flow of water through the body and filter out fine suspended food particles. The body wall is often greatly folded, and in some species is supported by a skeletal network of calcareous or silicious spicules which are of various shapes and sizes and important in taxonomy. Others have a skeleton of protein fibres known as spongin, while some have a combination of silicious spicules and spongin fibres.

Regeneration of sponges is well documented, particularly through the now famous experiment in which a sponge is forced through a very fme mesh and within a short time the dissociated cells aggregate to form many new sponges. Regeneration from fragments is an important form of asexual reproduction, others being the production of internal or external buds and the release of clusters of cells known as gemmules which develop into a new sponge.

Sponges also reproduce sexually. Most are hermaphroditic but cross-fertilization generally takes place. Sperm are discharged through the osculum and enter a neighbouring individual in the

76 J. D. Fish et al., A Student’s Guide to the Seashore© J.D. Fish & S. Fish 1989

PORIFERA

inhalent current. In some cases, fertilized eggs are discharged into the sea where they develop into ciliated larvae but in the majority, the larva develops within the sponge and is then released. Larval life varies from a few hours to a few weeks and the adult lives for one to several years, with some species showing regression and fragmentation during the winter months. Sponges often have a rich associated fauna of, for example, amphipod crustaceans and are preyed on by a variety of animals including sea-slugs and echi­noderms.

Accurate identification of sponges is difficult and sometimes impossible without dissection and microscopic examination of the skeleton and spicules. A small number of common species is included here and reliable identification of these can generally be made by reference to the external features described. Some species show considerable morphological variation from one habitat to another in response to such factors as water depth and speed of current flow.

Phylum PORIFERA

Class Calcarea (p. 78)

Class Hexactinellida

Class Demospongiae (p. 79)

Class Sclerospongiae

Sponges with calcium carbonate spicules. Restricted to firm sub­strata. Most species found in shal­low water. Sponges with silicious spicules mainly of six rays. Many colonize soft substrata. Found in deep water. Commonly known as the glass sponges. Sponges with silicious spicules, spongin fibres or both. Found on a wide range of substrata from rock to soft mud and some bore into calcareous material. Found in shallow and deep water; includes the majority of the common shore species. Sponges with skeleton of silicious spicules, spongin fibres and cal­cium carbonate. Few species. Found on coral reefs.

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Figure 36 (a) Scypha ciliata (b) Grantia compressa (c) diagrammatic section of sponge to show direction of flow of water (-» .

Class Calcarea

Grantia compressa (Fabricius) (Scypha compressa) Purse sponge (Fig. 36b)

Vase-shaped, sometimes rather elongate. Conspicuous terminal osculum; occasionally more than one. Up to 20 mm in height sometimes larger. Thin and flat. Colour varied, white, grey, yellow.

G. compressa is widely distributed in north-west Europe and around Britain in sheltered crevices and under boulders on the lower shore where it occurs singly or in groups, attached to a variety . of substrata. It is an annual species. Overwintering specimens die after releasing larvae in spring and summer, and sponges settling in spring reproduce in late summer and autumn and survive the winter to reproduce in the following spring. Specimens settling later in the year are believed to reproduce only once, in the spring.

Scypha ciliata (Fabricius) (Sycon ciliatum, Sycon coronatum) (Fig. 36a)

Body upright, vase-shaped or tubular; more cylindrical than G. compressa

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(p. 78). Height about 30 mm but much larger specimens have been recorded. Conspicuous terminal osculum surrounded by stiff spicules. Rough, 'hairy' outer surface, occasionally smooth. Yellow-grey colour.

S. ciliata is widely distributed in north-west Europe and around Britain where it is common on the lower shore. It is attached singly or in groups to stones and seaweeds, the largest specimens being found in sheltered situations. It is an annual. Overwintering speci­mens release larvae in the spring and some of these develop rapidly to reproduce in the autumn of the same year, before overwintering.

Leucosolenia sp. (Fig. 37a) Encrusting network from which upright, often branching tubes arise. Oscula conspicuous, terminal. Surface rough. Up to about 20 mm in height, but usually much smaller; grey-white in colour.

The genus Leucosolenia is widely distributed in north-west Europe and around Britain; the species are difficult to identify. The sponges are attached to seaweed and stones on the lower shore and in the sublittoral, sometimes with the purse sponge, Grantia compressa (p. 78). Leucosolenia is believed to be an annual, breeding twice, once in autumn and again the following summer.

Class Demosponglae

Suberites Beus (Linnaeus) (Suberites domuncula) Sulpher sponge, Sea-orange (Fig. 37b)

Large, thick, fleshy sponge with smooth surface. Usually rounded in shape and up to 200 mm across. Oscula conspicuous, few in number. Orange, yellow or brownish in colour.

S. ficus is widely distributed in north-west Europe attached to a variety of hard substrata on the lower shore and in the sublittoral. It is often found on shells inhabited by hermit crabs where it com­pletely encloses the shell. It has recently been suggested that S. ficus is a complex of three species separable by enzyme electrophoresis.

Halichondria panieea (Pallas) Breadcrumb sponge (PI. 2d) Encrusting; relatively smooth surface. Oscula well developed, sometimes raised well above surface of sponge. Large encrustations may be 20 mm thick. Very variable in form. Characteristically olive-green in colour but shades of yellow common. When broken has a distinctive smell of seaweed.

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A STUDENT'S GUIDE TO TIlE SEASHORE

Figure 37 (a) Leucosolenia sp. growing on red seaweed (b) Suberites ficus (c) MyxilIa incTustans.

H. panicea is widely distributed in north-west Europe and round Britain and is sometimes abundant. It is one of the most common sponges on the lower shore where it is found in gullies, under rocky overhangs and among the holdfasts of seaweeds. It covers extensive areas of rock and often overgrows other encrusting invertebrates. The form of the sponge varies, particularly in relation to current flow, and in turbulent areas it is much thicket than in sheltered waters, the faster current presumably enhancing the flow of water through the sponge. The typical olive-green colour of the species is due to the presence of green algal symbionts in the tissue; speci­mens from deeper water lack the symbionts and are yellow in colour. In thick specimens, only a thin outer layer is green, the deeper tissues being yellow. Large patches of Halichondria often have a diverse associated fauna and there is evidence to suggest that some amphipods, e.g. Caprella (p. 323), are attracted by chemicals given off by the sponge. The life-span of Halichondria is believed to be about three years and in some localities breeding has been recorded annually.

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PORIFERA

Myxilla incrustans ijohnston) (Fig. 37c) Thick, encrusting, up to 200 mm across and 50 mm thick. Oscula well developed, unevenly distributed and often on raised areas. Surface marked with channels, slimy. Yellow to orange in colour.

M. incrustans is widely distributed in north-west Europe, encrusting rocks and boulders on the lower shore and in the sublittoral. Specimens with embryos have been recorded during August and September.

Hymeniacidon perleve (Montagu) (Hymeniacidon sanguinea) (PI. 2e)

Encrusting; sutface rough and often grooved. Many small oscula, randomly arranged. Orange-red to deep red in colour, but both colour and form vary widely.

H. perleve is widely distributed in north-west Europe and around Britain, and like Halichondria (p. 79) is one of the commonest intertidal sponges. The largest encrustations are found on the lower shore and cushion-like growths have been recorded from the shal­low sublittoral. The species extends to the middle shore where it is found as small fragments. It is found on a variety of substrata including rocks, seaweed holdfasts and also on muddy-gravel where only the oscula can be seen on the surface of the sediment. Growth in the summer months is often followed by a reduction in size and by fragmentation of the sponge in winter. Off the south coast of England embryos have been recorded in the sponge from July to October, with larval release presumably taking place in late summer. Longevity is believed to be three or more years.

Ophlitaspongia seriata (Grant) (PI. 2f) Encrusting; flat and thin; growing in patches 50-100 mm across. Surface smooth. Oscula well-defined, mainly regular in distribution; with small collar. Orange-red, blood red in colour.

O. seriata is found on west and south-west coasts of Britain and may be locally common. It is found on the lower shore and sublittorally to depths of one or two metres. Embryos are found in the sponge during summer and larval life is about two days.

Limestone rocks and stones on the lower shore, and the empty shells of bivalve molluscs such as the oyster, are often riddled with small, round holes about 2 mm in diameter. These have probably

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been made by Qiona celata Grant, a boring sponge sometimes seen as yellowish-orange projections at the entrance to the holes. Boring is started by the larva and is brought about by secretions, possibly including carbonic anhydrase, from special cells which eat into the rock. A small chip of the substratum is surrounded by the cell and eventually undercut and removed in the exhalent current, the sponge gradually penetrating deeper into the rock.

Ackers, R.G., D. Moss, B.E. Picton & S.M.K. Stone 1985. Sponges of the British Isles (Sponge IV). A colour guide and working document. Ross-on-Wye: Marine Conser­vation Society.

Burton, M. 1963. A revision of the classification of the calcareous sponges. London: British Museum (Natural History).

Hiscock, K., S.M.K. Stone & J.D. George 1984. The marine fauna of Lundy Porifera (Sponges): a preliminary study. Report of the Lundy Field Society 34, 16-35.

Juniper, A.J. & R.D.Steele 1%9. Intertidal sponges of the Portsmouth area. Journal of Natural History 3, 153-63.

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