Previous articles have often focused on mobile animals, particularly large ones such as fish, but often it is the fixed life – or sessile life, to use the term the biologists use – that gives the underwater world its colour and character, and makes up much of the marine environment. It is interesting also that there is a far greater range of creatures represented among the sessile life than you find on land, including many that have no land-based relatives. There are hard and soft corals, sponges, ascidians, tube worms, anemones, hydroids, attached molluscs, barnacles and bryozoans. They provide a varied environment of homes, hiding places and food for many other creatures.

A sessile lifestyle has advantages when living in a liquid medium that contains vast quantities of potential food. All a sessile animal has to do is to find a secure spot to anchor itself and then filter out the food from the passing sea water. But there are problems. The organism has to find a surface to anchor which is secure enough and which is exposed to passing nutrient-rich water, it has to protect itself from being overgrown by other sessile creatures and, importantly, it has to avoid being eaten. Competition among these groups of animals for space and food on the rocks is intense.

Two common groups of sessile creatures are sponges and ascidians. The sponge is a primitive life form that has no close relatives. Many have no fixed shape, adopting whatever shape is best suited to wave intensity, with a tendency to low encrusting types on rocky shores, with large masses, structures or tubes in deeper water. They are often brilliantly coloured, particularly those in deeper waters. You can see them around rock pools and when snorkelling, as well as when diving.

Sponges are an assemblage of tiny cells, some of which have the function of moving water into the sponge by beating minute whip-like hairs. The water passes through canals within the sponge, which extracts microscopic food matter. Their power to pump water is extraordinary. Their shape is supported by millions of tiny splinter-like structures known as spicules, embedded in the sponge mass. Sponge ‘skeletons’ are often washed up on beaches after storms.

Sponges are of great interest to medical science as they have a capacity to repel invaders and predators and hence scientists seek to use their chemical compounds to make drugs that can inhibit disease.

Sponges and ascidians live a similar life-style but they are completely different animals. An ascidian known as the humble sea-squirt, or cunjevoi, is familiar to rock-pool fossickers and fishers and the squirt of water it emits when prodded always amuses small children. Snorkelers can get a different view of these animals by floating over massed sea squirts on rocks just below the surface at high tide. As they feed, their siphons open up, showing a beautiful pink colour in their internal tissues.

The cunjevoi we see close to shore has many colourful relatives in slightly deeper water, which adopt interesting strategies to make a living. Some, the sea tulips, extend themselves on long stalks, seeking to get away from the more crowded environment close to the bottom. The giant sea tulip (Pyura spinifera) common in our area, is always found living in association with an encrusting sponge; and usually with another ascidian (Cnemidocarpa pedata) also sponge-encrusted, living at its base!

Others work together in colonies, drawing water in individually, but expelling it through a common siphon. One thing not generally known about ascidians is that they are more closely related to vertebrates (including humans) than other invertebrates such as crabs or molluscs. This is because the larvae of the ascidian has a rudimentary backbone or notochord that is used for support until it has found a suitable surface on which to attach.

In addition to these colourful groups, marine life observers on the South Coast also commonly see beautiful tube worms, brilliant jewel anemones, lace-like bryozoans, fern-like hydroids and crusty barnacles, as well as soft and hard corals. All of these provide a varied and attractive landscape for people to explore.