Spicule Morphology

Commonly the spongin skeletons decay and unfused spicular skeletons disintegrate shortly after death leaving only a selection of hard parts, such as spicules (Fig. 11.5). Spicule morphology is thus a fundamental means of identification of those spiculate forms. Spicules may be large (megascleres), acting as part of the skeleton, or small (microscleres), scattered throughout the sponge and rarely preserved. Five basic types of spicule have been recognized:

1 Monaxons: single axial forms that may grow in one (monactinal) or two (diactinal) directions.

2 Tetraxons (hexactines): four-rayed forms that may have axes of equal length (calthrop).

3 Triaxons: six-rayed forms that form regular networks within the Hexactinellida or glass sponges.

4 Desmas: irregular-shaped forms with ends modified to articulate with one another.

5 Polyaxons: multirayed forms including spherical or star-shaped spicules.

Despite the flexibility of a typical bath sponge, sponges are skeletal organisms. Skeletons are composed of a colloidal jelly or spongin, a horny organic material; calcareous or siliceous spicules may occur with or without spongin. These structures support the body shape and provide a framework for the rather disparate cells of the sponge. In simple terms, the sponge animal functions as a colonial, loosely-integrated protist, but with a higher degree of physiological integration.

Three basic levels of chamber organization have been recognized among the sponges (Fig. 11.2), and these provide a useful guide to their shape. The simple ascon sponges are sacs with a single chamber lined by flagellate cells, whereas the sycon grade has a number of simple chambers with a single central para-gaster. The leucon grade is the most common where a series of sycon chambers access a large central paragaster.

Autecology: life as a sponge_

Sponges are part of the sedentary benthos, with large exhalant openings, communicating upwards with the water column. When not resting, the sponge sucks in water through its upward-facing ostia, forming inhalant currents; material is then pumped out of the animal though the exhalant opening. The group is entirely aquatic, living attached in a range of environments from the abyssal depths of oceans to the moist barks of trees in the humid tropics. Most Paleozoic and early Mesozoic forms have been collected from shallow-water environments, although like many other groups they expanded into deep-water environments during the Ordovician where they remained an important part of the benthos.

Today, sponges occupy a wider range of environments than in the past. Modern hexac-tinellids prefer depths of 200-600 m, probably extending down onto the abyssal plains and into submarine trenches, whereas the calcareous sponges are most common in depths of less than 100 m. The modern calcified sponges are either deep-reef or, more often, cave dwellers, lurking in the shadows of submarine crevices at depths of 5-200 m, mainly in the Caribbean although the group occurs elsewhere, including the Mediterranean. The meadows of Antarctic cold-water sponges can comprise up to 75% of the living benthos in seas under the ice sheets.

Sponges use a variety of substrates. Clionid sponge borings, producing the trace fossil Entobia (see p. 523) in mollusk shells, have a long geological history and today Cliona is commonly associated with many oyster beds. Spicules themselves can form mat-like substrates that when colonized form local pockets of biodiversity. Although almost all sponges are fixed filter feeders, some deep-water forms are carnivorous: their long barbed spicules entangle fish and arthropods, and the sponge tissue rapidly grows over the prey to digest it. Moreover some encrusting sponges can crawl slowly over the surface in search of food. Few predators attack sponges, although some fishes, snails, starfish and turtles have been observed eating their soft tissues in the tropics; and some organisms have used sponges as a refuge, including hermit crabs, while dolphins sometimes use sponges to protect their snouts when investigating crevices.

Synecology: sponges and sponge reefs through time

Sponges and corals are the major components of modern and ancient reefs (Wood 1990). The first sponges probably appeared in the Late Proterozoic as clusters of flagellate cells. But the evolution of the main groups of fossil sponges is intimately related to their participation in reef ecosystems (Fig. 11.6). Particular grades of organization were suited to special environmental conditions and sponges can possess a rigid, reef-building skeleton by the fusion of strong spicules or by the development of an additional basal calcareous skeleton. The Cambrian sponge fauna, of thin-walled and weakly-fused spiculate demo-sponges and hexactinellids together with early calcisponges, is mainly cosmopolitan, having a wide geographic distribution. In contrast, Ordovician sponge faunas are characterized by the heavier, thick-walled demosponges that continued to dominate Silurian faunas in carbonate environments; siliciclastic facies were dominated by the hexactinellids. The demo-sponges, however, became less important as the stromatoporoids together with rugose and tabulate corals began to sneak into these sorts of niches. Hexactinellids were locally abundant during the Late Devonian, and in the l00 200

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