Porifera (sponges)

On this page, types of sponge cell, canal systems, skeleton.

There are four Classes of sponge, Class Hexactinellida, Class Calcarea, Class Demospongiae, Class Sclerospongiae

Overview of the sponges

The sponges are an ancient group with a fossil record back as far as the Precambrian. There are about 10,000 known species of marine sponges, and about 150 freshwater sponges, with 5 British freshwater species. They are relatively abundant in all marine waters at all depths. They vary in size from a few millimetres to over two metres across.

Their cells are structured around a system of pores, chambers and canals through which water is moved by the action of the flagellae of the choanocyte cells. The large pores called oscula (singular - osculum) are water outlets, and the small pores called ostia are water inlets.

Sponges have the ability to grow continuously, but lack an immune system. However when they are exposed to x-rays, even in amounts 100 times greater than the human lethal dose, the sponges do not develop tumours. Also their cells continue to divide throughout their entire life. (bioRxiv,doi.org/f4dp)

Types of sponge cell

Pinaocytes. These are thin, flat epithelial cells covering the exterior surface and some interior surfaces. When they are located around pores they help to regulate water flow.

Choanocytes (see above) are responsible for maintaining the current of water through the sponge, and for processing food particles in the water current. The water current brings in oxygen and food, and removes waste. Eggs and sperm are also carried out of the sponge on this water current. The rate of water flow can be regulated by changing the size of the osculum. Choanocytes line the central cavity and small cavities in the canal systems (more below). In the drawings below the choanocyte layer is shown as a thickened black line. Each choanocyte has one flagellum ringed by a contractile collar (see above).

Archaeocytes digest food particles passed from the Choanocytes, secrete spicules, spongin and collagen.

Canal systems

There are three types of canal systems, in the following drawings the arrows show the direction of water flow:

• Asconoids have the simplest system, see the drawing above. These sponges are small and tube-shaped. The water enters through tiny ostia into one large internal cavity called a spongocoel, and is expelled through one large osculum. This type of canal system is found in the Calcarea Class (see below)

• Synconoids, like asconiods, have a single, large osculum, but their body is thicker. The drawing above shows a simple type on the left and a more complex type on the right. The water enters through numerous small ostia, and passes through incurrent canals before reaching the large central cavity. This system is found in Calcarea (see below) and Hexactinellida.

• Leuconoids have the most complex structure (see above). They have many small ostia. The ostia lead to numerous incurrent canals, but there is no large central cavity.

Sponge skeleton

The skeleton of spicules and spongin provides the support to keep the pores open. The spicules can be either siliceous or calcareous of a variety of shapes (see above), and can be used in identifying species. The spongin is a form of collagen.

Regeneration. Sponges have the amazing ability to regenerate an entire individual from just a few cells.

Class Calcarea

As the name suggests sponges in this class have spicules of calcium carbonate. The spicules are either free or fused. These sponges tend to be relatively small, mostly less than 10 cm, and tubular or vase-shaped. All three types of canal system occur in this class. There are about 100 species, mainly marine in water no deeper than 1000m.

Class Sclerospongiae

This is a small group of sponges that resemble corals. They are usually found in dark tunnels in coral reefs. The skeleton consists of siliceous spicules and spongin on a thick basal layer of calcium carbonate. All in this class have the leuconoid canal system.

Class Demospongiae Class Hexactinellida