Molluscs
There are three major types or Classes of molluscs that occur on coral reefs: bivalves, gastropods and cephalopods.
Bivalves
The body of a bivalve shell is flattened and is attached to both its hinged shells. Bivalves do not have a head. The largest of all bivalves is the Giant clam. Other bivalves include scallops, oysters and mussels.
Gastropods
Gastropods are the most common type of molluscs on coral reefs. Most of the body of a typical gastropod is hidden within its shell, which offers protection from predators. Cone shells are hunters. Their rasp-like tongue (common to most molluscs) has been modified into a hollow harpoon filled with venom that they use to kill prey. Cone shell venom can affect humans. Anyone stung by one should seek medical attention.
The colourful nudibranches have discarded the characteristic shell of other snails and rely on toxins within their skin for protection. For many their bright colours serve as a warning to predators while others use camouflage to evade predation. The name nudibranch means 'naked gill' and refers to the gill structure on their backs. There are two main groups of nudibranches - the dorids and aeolids. Dorids feed mainly on sponges. Aeolids are easily identified by the tubular projections on their back. This type of nudibranch feeds on anemones, corals and hydroids.
Cephalopods
The nautilus is the only cephalopod with a true external shell. The inside of the shell is divided into many gas-filled chambers. Buoyancy is controlled by taking in or pumping out water. The nautilus spends daylight in deep cool waters, rising at night to feed on crustaceans on the reef. They have more than ninety tentacles.
Octopi, cuttlefishes, squids and nautiluses are all cephalopods. They are the most highly advanced form of mollusc and the most intelligent vertebrate group. All cephalopods except nautiluses have an ink sac. Ink is expelled to confuse predators.
Squids are torpedo shaped and adapted for fast swimming. They propel themselves by jetting water through a siphon in the body and hunt for fish with other squids. They are noted for their rapid colour change.