One of the largest family of animals is the mollusc family.
The word ‘mollusc’ (sometimes spelled mollusk) means soft-bodied.
Most of them have a hard shell to protect their soft body.
Molluscs have been on Earth for millions of years.
Molluscs are among the earliest forms of life on Earth. In fact, the nautilus is considered to be a living fossil, unchanged for over 400 million years. Only a few species remain, and live at ocean depths of about 550 metres during the day, and shallower water at night.
Read about the nautilus:
Where do they live?
Some molluscs are land living, such as the snail and slug. Others live in water, such as water snails, sea slugs, octopus and squid, bivalves such as oysters, mussels and clams, chitons (say kigh-tun) and tusk shells. Molluscs live along rocky shores, in freshwater habitats and even in gardens.
Body and Behaviours
Molluscs have a soft body and are invertebrates, which means they have no backbone. Most have a shell, either outside or inside, and a radula (say rad-you-luh) that is like a tongue but with many tiny teeth. They have either a muscle called a foot, or tentacles.
Most ocean living molluscs can move by jet propulsion, squirting water from their body to move forward. Those with shells, such as clams and scallops, can move by clamping their shell closed and ejecting water. Some attach themselves to rocks or other surfaces and don't move.
Bivalve means ‘two valves’. The soft-bodied animal lives inside a double shell, or valve, with a hinge that opens and closes the valves. When the animal dies, the hinge usually breaks, and the empty shells wash up on the beach. Most bivalves are marine, or saltwater, creatures. They feed by passing water through their gills, filtering out the tiny plankton for food as the water passes through. Bivalves don’t move around much. Some attach themselves to hard places, some attach to rocks, some burrow in the sand.
The sea slug is sometimes known as the sea cucumber because of its shape and because it is usually very still, looking like a plant. Because of this habit of not moving very much, sea slugs are a common prey for many animals, such as crabs, lobsters, fish and humans.
There are thousands of different kinds of sea slug, ranging in size from a few millimetres to over 30 centimetres long. They are a variety of colours.
Sea slugs feed on algae, plankton and decomposing matter on the sea bed. This makes them a useful part of the marine ecosystem.
Chitons are hard to find as they are well camouflaged and generally hide in cracks in the rocks or under boulders, hiding from light. They are covered with eight overlapping plates that protect them from predators and from crashing waves. When disturbed, they clamp their muscular foot on the rocks and are very hard to move. They feed by using their radula to scrape algae off rocks. Their eyes are inside the shell. There are about 150 different kinds of chitons in Australia, and about 90% of them are endemic (found nowhere else).
Squid have a very soft body with a large head and brain. There are about 300 different kinds of squid, but scientists believe there are more to be found. The size depends on what species, or kind, of squid they are. They can move extremely fast through a sort of jet propulsion when water is rapidly forced out of its body cavity. Squid breathe through gills so they do not have to come to the surface to breathe. There are eight arms around the mouth, two of which are longer and can contract. Prey is caught and held by these two arms, and held by the other arms while the squid eats. The jaw of a squid is similar to a parrot’s beak, and it tears its prey apart. When escaping a predator, squid expel a cloud of black fluid known as ink, which makes it hard for the predator to see the squid.
There are five groups of squid:
Giant squid grow to about 9 metres in length. They prefer cooler waters and swim between about 300 and 1,000 metres below the surface near Australia, Newfoundland, New Zealand and Norway.
Colossal squid is the largest invertebrate of all, up to 14 metres long. It has sharp hooks at the end of the tentacles. It is found in just a few areas south of New Zealand, South America, Africa and in Antarctica.
Vampire squid is about 30cm long. The tentacles are connected by a web of skin, which makes it look like it is wearing a black cloak, like a vampire. Its eyes look red or blue depending on the light around it. It is found in warm, tropical areas and keeps to deep water where it is dark.
Humboldt squid grow to about 1.5 metres in length. It can change its body colour, and rapidly changes from red to white while hunting. It is found in areas of the Pacific Ocean, and lives in very deep water.
Japanese flying squid
Japanese flying squid are so called because it can cover about 30 metres in a single leap above the surface of the sea. It is found in cooler waters, north of the Pacific and in the Bering Strait.
Octopus are found around the world in warmer tropical and temperate waters. There are about 300 different species, or kinds, of octopus.
An octopus has a globe shaped head and eight limbs or tentacles. Recently, scientists have found that two of these are more like legs, allowing the octopus to walk along the sea floor. If an octopus loses a tentacle, it can grow another. The tentacles have suckers to help grab and hold things. In its centre, an octopus has a hard beak that tears apart its prey.
While the octopus is invertebrate and has no internal skeleton, it does have a skull which protects its brain. Like the squid, the octopus uses jet propulsion for fast, forward movement. It releases a cloud of black ink to help it escape predators. Octopuses can squeeze themselves into tiny spaces.
An octopus lives for only about 18 months. They are solitary (live alone) and shelter in hollows in rocks or corals, or dig burrows. They move closer to the shore to mate, and two months after that a female releases about 100,000 to 500,000 eggs. She cleans the eggs, provides oxygen by squirting water, and protects them from predators until they hatch. Soon after the eggs hatch, both the male and female parents die. The young are carried by ocean currents, and feed on plankton. Only one or two will survive to become adults.
Watch a video about the octopus:
Watch a video about sea slugs:
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