Big Bank Shoals of the Timor Sea
An environmental resource atlas

FILTER-FEEDING ECOSYSTEMS

Biology and ecology of sponges

Structure

The outer surface of the sponge is covered in tough, flattened cells called pinacocytes. The inner surface is covered in a lining of flagellated collar cells or choanocytes. Between these two layers is a gelatinous mesochyme which contains several types of free-moving cells, or amoebocytes, and various spicules which provide reinforcement and stiffening. The minute spicules which form the skeletal framework can be crystalline or organic fibres. Crystalline spicules are either calcium carbonate (CaCO₃) or siliceous material (chiefly H₂Si₃O₇). Spicules are of many types, shapes and sizes and are used as a diagnostic tool in the classification of sponges. For example, some deep-sea glass sponges (Hexactinellida) are characterised by spicules which become fused together in a framework. Bath sponges, and some other horny sponges, are examples of sponges which display fine, interconnected and irregular fibres of spongin, which are composed of collagen.

Figure 34. The highly varied sponge morphologies. a: encrusting, b: vase, c: columnar, d: bowl (C. Wolff and L. De Vantier).

The body of a sponge is perforated with numerous pores, each of which form a canal through a tubular cell called a porocyte. Water is circulated through these openings by the beating of the choanocytes’ flagellae (Figure 35). Water flows into an internal cavity, the spongocoel, where waterborne food is trapped by the choanocytes and is then extracted by feeding cells called archaeocytes. The water then exits through a large, excurrent pore, the osculum.

Sponge spicules (AIMS).

The simplest type of sponges are those which are vase-shaped. These simple structures are called asconoid sponges and are small, no more than a few centimetres high. The asconoid architecture imposes limitations on the size that the sponge can attain. If the sponge becomes too big it would contain more water than its collar cells could efficiently move.

This problem has been solved by the evolution of sponges that have repeated infoldings of the flagellated layer to increase surface area. These are called syconoid sponges (Figure 35).

Figure 35. The differing structures of sponges showing the direction of water flow.

The majority of sponges have further increased the surface area of the flagellated layer through the formation of many small chambers within which the collar cells are located. These are called leuconoid sponges and are characterised by a system of canals which carry water to the flagellated chambers. The water leaves the sponge through converging excurrent canals opening to the exterior osculum. These sponges do not possess a spongocoel, like the asconoid and syconoid sponges. These modifications, have allowed sponges to attain much greater sizes and numerous, different morphologies (Figure 34).

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Last updated - 1 September 98

Copyright ©1996-1998 Australian Institute of Marine Science

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