Authors: H. Visser & H.H. Veldhuijzen van Zanten
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Scientific name:

Porifera

Vernacular name:

Zoetwatersponzen, Freshwater Sponges, Spongiaires, Süsswasserschwämme, Poriferi, Ferskvannssvamp


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family Planariidae - family Planariidae
family Adinetidae - family Adinetidae

(Sponges)
Fam. Spongillidae (Freshwater sponges)

Description
Animals which are usually hardly recognizable as such, forming an encrusting growth on any firm submerged object, like branches, timber, aquatic vegetation and pebbles. Size and shape of freshwater sponges is enormously variable, and depends, apart from differences between species, on age and various ecological conditions. In still waters sponges are characterized by a large number of finger-shaped projections, while in rivers it forms encrustations on the underside of stones, for instance. The surface is pierced by many small holes of various sizes, while internally it is a maze of interconnected and ramifying spaces, channels, and chambers. Sponge tissues are relatively unspecialized and consists of an inner and an outer layer of epithelium. The space between these two layers is filled with a gelatinous non-cellular matrix (mesoglea), traversed by a variety of amoebocytes, and containing needle-like siliceous spicules which support the body. Water movement through the sponge results from the work of flagellated cells which are grouped in special chambers within the sponge. Adult animals are hermaphroditic or alternately male or female, larvae and sperm-cells are planktonic.
Larvae, which are produced during the summer, are ciliated all over, and have a brief period of some twenty-four hours of free movement before settling. After settling the outer layer of ciliated cells migrates inward, to form the lining of the internal cavities, responsible for the movement of water through the sponge.
The colour of these animals ranges from yellowish-white to brownish-red. Greenish colours result from inter- and intracellular algae which become more abundant under conditions of strong light.
Measurements: Colonies may reach dimensions of some 10-50 cm across.

Ecology
Sponges are likely to be encountered in clean rivers, lakes and large ponds, attached to any firm, permanent substratum, but rarely on water plants. Sponges are often the first organisms to die when waters become polluted. Most sponges also die off in winter, the next years growth ensured by the asexually produced gemmules. Sexual reproduction occurs in late spring and during summer, resulting in tiny, free-swimming, ciliated larvae which soon settle to grow into new sponges.

Taxonomy
Most freshwater species of this phylum are placed in a single family, the Spongillidae. The 3 genera of Lake Baikal freshwater sponges have been placed in a separate family Lubomirskiidae. Four other genera from deep lakes (e.g. Lake Ohrid) may also belong to the latter family.
Species of freshwater sponge are identified through study of the gemmules, spherical resting stages which are usually present throughout the growing season, at which time they are deposited in a basal layer or found scattered throughout the sponge body. The outer (pneumatic) layer of these gemmules contains characteristic siliceous spicules, in the subfamily Spongillinae these are needle-like and beset with thorns, in the subfamily Meyeninae these needles have disc-like structures at opposing ends.
Simon (in Illies, 1978a) listed 18 species from Western Europe belonging to 5 genera.
Genera: Ephydatia Lamouroux, 1816; Eunapius Gray, 1867; Heteromeyenia Potts, 1881; Spongilla Lamarck, 1816; Trochospongilla Vejdowsky, 1883.

Distribution
Spongillids occur worldwide in streams, rivers, ponds and lakes. A number of species have been reported throughout Western Europe though not from Iceland.

(after Pennak, 1953; Mellanby, 1963; Simon, 1978; Hartman, 1982; De Pauw and Vannevel, 1991)

phylum Porifera (Freshwater Sponges)
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