Reef Ecology Guide Phuket Thailand - Molluscs

The phylum Mollusc comprises snails and slugs, shells and cephalopods like octopus and squid. With more than 150,000 species they are one of the largest phylums in the animal kingdom.

Molluscs are characterized by a muscular mantle, a tooth-covered rasping tongue and a muscular foot, which may be modified for diverse uses. Most types possess a shell of calcium carbonate that is secreted throughout the lifespan.

However, many molluscs and some cephalopods have lost their shells entirely or it is much reduced. A well-developed set of gills used for respiration is another characteristic of most marine molluscs.

Sea shells are usually burrowers or are attached to the substrate. Because they are unable to actively search for food, most rely on their elaborate gill structure, to filter out tiny plants and animals.

Many species possess microscopic alga cells in their exposed mantles. The alga actually produces food for its host and apparently very little extra nutrients are required. The mantle also contains light and pressure sensitive spots that cause the shell to quickly close its valves if disturbed.

The sex is separate in most molluscs. Sea shells release clouds of eggs and sperm into open water. Somehow, perhaps by sensing changes of temperature or salinity, the spawning of the entire population of a local area is synchronized. The fertilized eggs hatch out as larvae, swimming or drifting in the sea for a period ranging from about 2-40 days, before finally settling on the bottom.

Snails and nudibranchs are by far the most common type of molluscs encountered on coral reefs. Most of the body of a typical sea snail is hidden within its shell, which offers protection from predators.Aside from the shell the only other body part usually seen is the muscular foot. The foot secretes mucus to cut down friction.

The feeding habits are just as variable as their appearance. In contrast to sea snails the evolutionary trend in nudibranchs has been towards a reduction or even complete loss of the protective shell.

The flesh of these bright colored creatures is frequently toxic or distasteful due to various chemical secretions. The bright coloration serves to warn predators of their inedible qualities. The majority of species feed on algae, sponges and cnidarians.

Members of this family have the most advanced nervous system of all invertebrate animals. Many live in the open ocean or in abyssal depths. Relatively few species are encountered on coral reefs.

All family members have in common an ability to eject ink, produced by a gland within the fleshy mantle, to deter and escape from potential predators.

Although outwardly cephalopods seem very unique, these animals have the same basic molluscan body plan, but with special modifications. Beside the well-known octopus the other members of this group are cuttlefish, squid and the nautilus.

The eyes of cephalopods are human-like and accurately register shapes, textures and colors. The keen eyesight and well developed brain make them deft hunters, feeding on fish, crustaceans and shelled molluscs.