Coral Reef Mollusks

Coral reef mollusks are members of a diverse and abundant group of mainly aquatic invertebrate animals (Phylum Mollusca). The vast majority of these animals are benthic (bottom dwelling) invertebrates, but there are a few open water swimmers included as well.

Three main groups (taxa) of mollusks are typically widly distributed throughout coral reef ecosystems:

Gastropods (snails, chitons, nudibranchs)
Bivalves (clams, mussels, scallops)
Cephalopods (squid, cuttlefish, octopus)

Here, we provide a brief introduction to each of these important members of the reef community.

Gastropods

Marine snails are common coral reef mollusks found on reefs and in sand plain and seagrass meadow habitats as well.

The vast majority of these animals are herbivorous slow-moving benthic grazers with a one-piece shell, as exemplified by the highly-prized Caribbean conchs, now badly overfished throughout the region for both their meat and shells.

Most of the smaller species are well camouflaged or well-hidden, spending most of the time safely concealed beneath the sand.

Not all coral reef mollusks are herbivores. For example, a common octocoral predator of Caribbean reefs is the flamingo tongue (pictured left). This colorful carnivore is often found in plain sight grazing on sea fan polyps.

Some coral reef mollusks – particulalry gastropods – make excellent additions to marine aquariums, as they help keep unwanted algae in check.

By far, the most colorful (and popular with marine aquarium owners) of reef-dwelling gastropods are the nudibranchs (see photo, left).

These are a type of marine snail that lacks a protective shell and is often cloaked in brilliant colors that serve to warn predators of their foul taste or poisons.

They are not uncommon coral reef mollusks, but many species have cryptic coloration and are therefore nearly impossible to be spotted by divers.

On Indo-Pacific reefs, predatory snails called cone shells contains species that have the capacity to inject a neurotoxin that can be lethal to much larger animals, including humans.

Bivalves

Bivalve mollusks have a shell composed of more or less equal halves. These are active filter feeders, pumping water through strainers to remove food.

Clams and scallops are frequent in sand and seagrass habitats, depending upon concealment and the heavy shell to keep predators at bay. Nonetheless they are heavily hunted by reef fishes and sea stars.

The giant clams (Tridacna spp.) of the Indo-Pacific are the largest and undoubtedly the best-known of all bivalve coral reef reef mollusks. Large specimens may exceed 4 feet in diameter, with shells weighing hundreds of pounds. These simple creatures are filter feeders not predators, and pose no danger to humans.

Nonetheless, the giant clam gained unwarranted notoriety in early Hollywood films in which they were often depicted trapping and drowning unwary pearl divers. In reality, such a fate was about as likely as a diver being hit by a meteor.

Throughout much of their range, these animals have been overharvested in recent years, and are becoming increasingly rare.

Cephalopods

The cephalopods (squid, cuttlefish, octopus) are among the most highly advanced of all invertebrate animals.

These swift, intelligent predators have well-developed nervous systems complete with relatively large brains and eyes very much like our own.

Cephalopods are distinguished by their many tentacles, which number eight in octopi, and ten in squids and cuttlefishes. They are masters of color change and are capable of complex behaviors.

Squid and cuttlefish are the only coral reef mollusks that feed as free-swimming, open water hunters. They stalk and feed on a variety of prey, including smaller reef fishes.

Octopi are secretive, benthic creatures, generally remaining well-hidden during the day within caves and crevices of the reef, often so well camouflaged as to be virtually invisible even at close range.

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