Marine Biology (2+1)

Sessile Epifauna

Many organisms are attached to the substrate throughout their maturity and have no mobility at all. Included are both solitary organisms and colonial ones, with as many as hundreds of individuals merged into large condominium-style skeletal complexes. Some can torn up and moved, then reattached and still carry on, where as others expire when uprooted. Virtually, all sessile epifauna are filter feeders, relying on currents to carry their food to them.

There is a variety of sessile solitary invertebrates that attaches to hard substrates, typically bedrock. These include barnacles, oysters, some brachiopods, and mussels; sponges, sea anemones, and sea-lilies. Although all are macroscopic, there is some range in size, from sponges only a few millimeters across to large sea lilies with arms that may be meters long.

Brachiopods and mussels (Pelecypoda) are both bivalves and filter feeders. Brachiopods (lamp shell) attach with a stem like foot that extends from near the hinge line that holds the shells together. Mussels are about the same size and they attach themselves to a hard surface with strong thread like structures, called byssus threads which develop at the hinge line. The mussels are especially well attached and can withstand vigorous wave and current action.

Anemones (Coelenterata) and lilies (Echinodermata, crinoid) belong to different phyla and have markedly contrasting anatomies, but there are some similarities in their feeding activities. Both have multiple appendages that serve as the primary food gathering mechanisms. Anemones are carnivores; they grasp their prey, then envelop and digest it. Some have sticky substances or toxins in their appendages to aid in capture and submission of their pray. By contrast, the sea lilies have sophisticated system of circulating plankton and organic debris to their centrally located mouth.

Barnacles are crustaceans (Arthropoda) that exist in two different forms, both sessile. Encrusting barnacles have their calcareous shells attached directly to the hand substrate, their soft part extended during feeding and retracted for protection. These are the barnacles that encrust boats, bridges, and other marine structures. The goose-necked barnacles have fleshy, stalk like structures that attach to the substrate and emanate from the shell that contains the soft parts of the organism. Both feed by removing small particles of organic debris from the water.

There are also colonial varieties of sessile benthic animals. Primary members of this group are the corals, sea whips, sea fans and bryozoans. Bryozoans are small and may be encrusting or delicately branching. They have calcareous external skeletons forming lacy structures that have given this phylum the nick name “moss animals”. The corals, sea whips and sea fans are all coelenterates, the same phylum as the sea anemones. Sea whips and fans do not have hand, articulated skeleton and so they disintegrate upon expiration, where as the corals have massive calcareous skeletons that house numerous individuals. All feed by filtering plankton and organic debris from the water.