Calcareous sponges are largely confined to the littoral and the shallow seas, although a few have been dredged at depths down to 600 ft. or beyond, to a maximum depth of 2,700 ft. Hexactinellida is typically deep-sea, the majority of species living at depths greater than 600 ft., but around the Indo-Pacific islands they seem to be fairly common in shallower waters, up to 120 ft. The Tetraxonida range from mid-tide level to the greatest depths so far dredged, and probably occur in the deepest trenches in the oceans.
The density of populations of littoral sponges varies according to the range of the tide as well as the substratum and other environmental factors. Where the tidal rise and fall is small there are few sponges, or where there is a racing current giving a scouring action. They are almost non-existent also on sandy beaches, although Hymeniacidon perlevis can grow half-buried in sand, or on mud-flats, where Halichondria coalita will also grow half buried.
Pebbly beaches also are barren. Much the same relation between substratum and sponge is seen for shallow and deep seas, the sponges usually requiring a solid substratum on which to grow, but there are many species adapted to life on fine mud or ooze.
These have either a long stalk embedded in the mud, which holds the body well up from the substratum, as in Stylocordyla borealis, or there is a tuft or raft of long spicules serving the same purpose, as in Pheronema carpenteri or Euplectella aspergillum.
One of the most remarkable adaptations is seen in the Single-rod Sponge (Monoraphis chuni), a deep-sea hexactinellid in the Indian Ocean, which has one spicule a yard long with the sponge itself situated near the upper end, the lower end being embedded in the mud of the sea floor.
In the shallow seas or at middle depths in the deep seas there is a profusion of sponges in tropical and subtropical waters, and again in polar waters, with less profusion in temperate waters.
At depths of around 600 ft. and in the deeper seas sponges tend to be spaced out, and it is probably in these situations, where individuals are widely separated, that asexual reproduction offers the greater advantages.
The commercial sponges (i.e. the bath sponge and related species) reach their maximum development in special areas. The group Keratosa, to which they belong, is almost entirely confined to tropical and subtropical seas, with very few species ranging into temperate or polar waters. Commercial sponges are most numerous in the Mediterranean, especially in the eastern half, and in the Gulf of Mexico, especially around the Bahamas and Mexico.
Outside these broad generalizations, it is not easy to plot the geographical distribution. The sponge fauna of the Arctic spreads south around the continental shelf of north-west Europe, north-east and north-west America, and north-east Asia. Further south it merges into subtropical faunas, typified by those found in the Mediterranean and the Gulf of Mexico, and the subtropical faunas, in turn, merge into tropical faunas.
There is a similar succession starting from the Antarctic and passing northwards to the Equator. The genera tend to be cosmopolitan and the species differ, although in outward form there is little to differentiate them, the identification being largely based on differences in their skeletons.
Siliceous spicules of a tetraxonid sponge (Geodia sp.), showing the large four-rayed spicules and the needle-shaped spicules, known as megascleres, and two small spherical microscleres.
The sponge Hymeniacidon perlevis has a wide distribution in the northern hemisphere and is also found on the coasts of South Africa, Australia, and New Zealand. In the littoral, it forms low cushions, tan or bright red, but offshore tends to assume a larger size with finger-shaped surface processes.
The Sea-orange (Tethya aurantium) lives in sheltered spots on rocks in the littoral and down to depths of 100 fathoms. It commonly reproduces asexually by means of stalked buds, which become detached, float away, and settle on the sea bed subsequently.
Desmacidon fruticosus is a siliceous sponge, with its main skeleton a network of needle-shaped spicules (oxea), living in the shallow seas of the eastern North Atlantic. It is remarkable for the amount of mucus it exudes and for having a prawn living in its cloacal cavities, and feeding on its tissues.