Molluscs

Article | Updated 2 years ago

Animals such as squid, mussels, scallops, octopus and sea slugs belong to the phylum Mollusca. These animals all have soft bodies that do not contain bones. Most molluscs have a hard, chalky shell. However, some, like squid and cuttlefish, may have a fragile shell located inside the animal, or the shell may be absent, as in some sea slugs.

Many species of molluscs are collected for a variety of uses, particularly for food and for use as ornaments - pearls form inside the bodies of many molluscs, with the most valuable coming from tropical pearl shells, and some molluscs such as abalone and oysters are considered delicacies around the world.

There are approximately 100,000 molluscan species worldwide – on land, in freshwater and in the seas – and over 1,200 marine species have been recorded from the varied marine habitats of the Dampier Archipelago.

Nudibranchs

The vividly coloured nudibranchs, or sea slugs, belong to a group of gastropod molluscs (Class: Gastropoda) known as the Opisthobranchs. The opisthobranchs are quite varied in that some possess a shell (internal or external), some have the merest remnant of a shell and others have no shell at all.

Nudibranchs do not possess a shell and rely on toxic chemical secretions for protection. Many, especially the tropical species, are brightly coloured allowing them to advertise their toxicity and inedibility. Others with more subdued colouring can avoid predators by blending into their environment.

All nudibranchs are carnivorous and feed on other molluscs, sponges, corals and sea anemones.

Image of Dermatobranchus ornatus.

Dermatobranchus ornatus.
Image copyright Clay Bryce, WA Museum. 

Image of Chromodoris striatella.

Chromodoris striatella.
Image copyright Clay Bryce, WA Museum. 

Image of Chromodoris coi.

Chromodoris coi.
Image copyright Clay Bryce, WA Museum. 

Image of Chromodoris cf. africana.

Chromodoris cf. africana.
Image copyright Clay Bryce, WA Museum. 

Image of Ceratosoma tenue.

Ceratosoma tenue.
Image copyright Clay Bryce, WA Museum. 

Periwinkles

This picture of the periwinkle Littoraria pallescens on a branch of a mangrove tree illustrates the ability of this group of gastropods to temporarily live out of water. Its ability to survive out of water allows it to escape predators, such as fishes and crabs.

During the Dampier Archipelago surveys, L. pallescens was found in or near mangrove habitats, which is typical of this species. Other species of periwinkles are generally found along shorelines feeding on the algae growing on rocks or plants between the low and high tide marks.

Image of Littoraria pallescens.

Littoraria pallescens.
Image copyright Clay Bryce, WA Museum. 

Chitons

Chitons (Class: Polyplacophora) are flat and generally oval in shape, with eight overlapping plates or valves embedded in the tough skin of their backs. Some species also possess protective spines embedded around the edge of this tough skin.

Underneath is a large muscular foot which enables the animal to cling tightly to rocks and, therefore, avoid being swept away by waves.

Many species of chitons are found on rocks along ocean shorelines and are able to survive when exposed to air during low tides. Most species of chitons feed on algae from the rocks on which they live.

Image of Acanthopleura spinosa.

Acanthopleura spinosa.
Image copyright Clay Bryce, WA Museum. 

Cuttlefish

Cuttlefish are closely related to both squid and octopus (Class: Cephalopoda). Their shells, known as cuttlebones, are located just under the skin of their backs. These shells aid in buoyancy control as it contains fluid and air. By changing the balance of fluid and air in the small spaces of the shell, cuttlefish can move up or down in the water column. The shells of deceased cuttlefish are commonly found washed up on beaches.

Cuttlefish have eight arms and two tentacles that can be withdrawn into skin pockets when not being used to capture prey. When avoiding predators and when hunting, cuttlefish can rapidly change colour to become camouflaged against their surroundings. They are also able to produce ink that, when released into the water, distracts predators and allows the cuttlefish to escape.

Cuttlefish usually swim by the undulation of the fins that run along the sides of their bodies. However, if fast movement is required, they draw water into a cavity in their bodies and force it out quickly, achieving jet propulsion.

Image of Cuttlefish (Sepia sp.).

Cuttlefish (Sepia sp.).
Image copyright Sue Morrison, WA Museum. 

Octopus

Octopuses belong to the same group of molluscs as squid and cuttlefish (Class: Cephalopoda). Unlike most other molluscs, octopus do not posses either an internal or external shell. They have a soft body that allows them to squeeze into relatively small spaces such as crevices in reefs or among rocks.

Octopuses have eight arms (“octopus” meaning eight feet) that are lined with suckers. These arms are used to smell out, to seize and to ‘smother’ prey. Inside the mouth of an octopus are two jaws that resemble and operate like the hard beak of a parrot. Octopuses generally remain hidden during the day and emerge at the night to feed.

Some species, such as the blue-ringed octopuses, with their distinctive pattern of bright blue rings and/or lines, produce venomous saliva that can subdue and kill prey. Although relatively small in size, they have the potential to kill an adult human.

Image of Blue-ringed Octopus (Hapalochlaena sp.).

Blue-ringed Octopus (Hapalochlaena sp.).
Image copyright Clay Bryce, WA Museum. 

Image of Blue-ringed Octopus (Hapalochlaena sp.).

Blue-ringed Octopus (Hapalochlaena sp.).
Image copyright Clay Bryce, WA Museum.