Sessile versus mobile

Growth form and strategy

The facts that circalittoral rock provides a firm attachment, and that CFT biotopes are frequently in areas of strong wave action, mean that the sessile habit will confer advantages. It will prevent the organisms being swept away to what might be unfavourable conditions, and it will prevent them from being damaged by impact on rocks in the course of being moved by water action. Consequently, as is seen clearly in the section below, the great majority of prominent CFT species are sessile. Being sessile can be achieved in two ways. Most CFT species are attached permanently to the rock in one place - they are effectively glued to it. Sponges, tube worms, barnacles, byozoans and tunicates are all of this type: some of the adhesives which they use are highly efficient and are being commercially investigated. Others are sessile, but retain some powers of relocation. Anemones attach by a basal disc, but can creep slowly over the rock surface. Some bivalves attach by horny byssus threads, which they can discard and produce new ones to attach in another place. This sessile habit is possible only if the food supply comes to the animal (see below).

The prominent mobile CFT species consist mainly of decapod crustaceans, gastropod molluscs and echinoderms, and as grazers or predators these must be able to move to locate further food supplies. Even so, many of them are very well attached to the rocks, such as starfish and sea urchins with their many sucker-like tube feet. Others, such as the decapod crustaceans, are adept at finding refuges from the more severe water movement. There are other large mobile species, particularly fish, which are not considered part of the CFT community, but may nevertheless play an important structuring role.

In addition the CFT community includes a diverse cryptofauna of small organisms such as nemerteans, polychaete worms and amphipod crustaceans. These live permanently hidden amongst the larger sessile fauna, existing as grazers, micropredators or detritivores. They are not assessed by the standard visual survey techniques, and so do not feature in the biotope descriptions. They can only be collected by destructive sampling.

In terms of being prostrate or erect there is a conflict of interest for CFT species. They are mostly filter feeders, and they frequently live in vigorous water movement. A prostrate habit protects them from the worst of the water movement, as well as giving them a very robust morphotype. However, it tends to place them in the boundary layer with limited water movement, and it is the water which carries their food. Furthermore, if taller erect species are present, they will have precedence in access to the food supply. Conversely erect species will be more prone to damage by turbulence or currents, and more fully exposed to them: but they will be in a position to maximise food intake.

The outcome is predictable. Under conditions of extreme exposure robust low-growing forms predominate - barnacles, massive sponges, short hydroids, bryozoans and tube-building polychaetes. As exposure moderates the taller erect forms come into prominence - the sea fans, soft corals and the like. These still tolerate considerable exposure though - they have a tough yet flexible structure which enables them to withstand turbulence and strong currents without damage. In shelter erect forms still predominate, but different ones which can tolerate the sediment loading and the reduction in food-bringing currents: tunicates and erect sponges occur, delicate forms which would not withstand exposure.

Next Section                          References