Typical Molluscs

Tridacna - the giant clam, a Pelycopod
A typical bivalve mollusc seen here end-on embedded in a coral reef, the blue region is the soft tissues which are outlined on each side by the two shells that make it a bi-valve. Tridacna like most bivalves is a filter feeder.

Helix sp. a common garden Gastropod
Scourge of gardeners and beloved of illustrators of children's books. The land snail approach to life is remarkably successful, eat many kinds of plants, only come out when it's safe, hide the rest of the time, when times get hard retreat into shell and seal yourself in for a while.
picture used permission of Jürgen Schoner
published under GNU Free Documentation License, Version 1.2

Octopus - a Cephalopod  Mollusc
Members of the Cephalopods have either no shell (as here) or a very modified internalized shell as in squid and cuttlefish

Octopus suckers
These help to give the octopus great manual dexterity as visitors to marine aquaria where a resident octopus that has been trained to open screw top jars for a fishy treat will attest.

A marine Gastropod Mollusc
Gastropods usually glide along on their single large foot, they often have a number of soft appendages, eyes, tentacles etc. that may emerge when they are moving. They are able to withdraw all soft parts into the shell for protection, sometimes being able to seal the shell opening with a hardened disc or clamping down onto a rock for protection using the foot as a holdfast. Some members of the group have lost or internalized their shell.

Pelycopoda or bivalves, these fresh water mussels are amongst the most efficient of filter feeders removing microscopic organisms from the water with sophisticated cilia covered gills. Water is drawn in through one opening and pushed out through the other so as not to be filtered more than once.

Cool Molluscs

This Nautilus belongs to a very ancient group of Cephalopod Molluscs. The animal is similar to a squid or octopus except it lives in a large shell that grows with the animal. The shell is compartmentalized and the animal lives in the last and largest chamber, the rest of the shell functions as an adjustable buoyancy device.

Nudibranchs (literally naked gills) are a colourful and shell-less group of Gastropods.
They are amongst the most colourful of any animals on earth often with contrasting colours and patterns, sometimes this helps to camouflage them against the coral on which they often live, for others the colouring is warning. See here for more dazzling examples.

Clione, a small (2-3cm long) shell-less Gastropod
This group is now called Thecosomata, formerly Pteropoda, though commonly known as sea butterflies or sea angels.
They are planktonic though can move about by a slow flapping motion of their "wings" which are actually parapodia developed from the foot. I first saw these in a huge swarm of thousands while diving beneath Antarctic Ice, an ethereal experience.

The group has members that superficially can be very different to each other, though they do have fundamental features in common. These characteristic features are not so easy to identify by the non-expert.

While the features themselves are not so easy to see for what they are, the members of the Mollusca belong to sub-groups which themselves are quite characteristic and so identification is usually not difficult.

They are one of the largest invertebrate groups and are usually quite conspicuous.

• There is a muscular "foot" which may be very modified
• The visceral mass is covered by a fold of the body called the mantle which may secrete a calcium carbonate shell, which may be external or internal
• There may be a radula present, this is a unique tongue-like organ for scraping and is covered in hundreds, sometimes thousands of microscopic teeth
• Body divided into head, foot and visceral mass
• No metameric segmentation - not a characteristic in itself, but helps distinguish the molluscs from other animal groups
• sexual reproduction

Typically they have shells but not hard appendages such as legs, antennae or other body parts.

The group includes:

• Gastropoda, usually have an external shell into which they can retreat, may be reduced or internalized though as in slugs
  • Snails - land and aquatic
  • Slugs
  • Nudibranchs
  • Pteropods - Thecosomata - sea butterflies
• Cephalopoda
  • Octopus
  • Squid
  • Cuttlefish
  • Nautilus
• Pelycopoda (Bivalvia or Lamellibranchia), two hinged shells that may be of equal of differing size and shape
  • Mussels
  • Clams
  • Oysters
  • Scallops
• Many other types too, but these are amongst the commonest encountered

What do molluscs eat?

Virtually every feeding habit is carried out by the molluscs, they may be herbivores, carnivores, scavengers, deposit feeders, filter feeders or parasites, these strategies are carried out by some members of almost all the major groups, there are some generalizations however that can be usefully made.

As the phylum is so varied, this is split down into the major groups:

Gastropoda - gastropod molluscs include the common garden snails and slugs that feed on plant matter. Gastropods possess a radula, a conveyor-belt like structure covered in teeth hardened with chitin that can vary from 16 to the thousands.

The radula rasps away at plant material breaking open cell walls to help in digestion. Some predatory gastropods have a radula that is designed to grind away at the shells of other molluscs such as bivalves to gain access to the contents when they inject a poison which relaxes the muscle holding the shells together allowing the predator to feed.

The most specialized of such predatory radulae are those in some cone shells where the teeth have developed into poison darts and are used to catch fish (by a slow ambush method).

Some Nudibranchs eat jellyfish, and coral animals that have stinging cells "nematocysts". The smaller of these are digested as food, though the larger nematocysts are passed unharmed and most surprisingly undischarged through the Nudibranchs digestive tract and then passed onto the vulnerable gills where they are used as second hand weapons against any small fish or other animal that may decide to take a nibble of a tasty readily available morsel.

Cephalopoda - this group are highly adapted as predators. They have large eyes which are arguably the most sophisticated in the animal kingdom with which to find their prey. They also have tentacles, 8 in octopus, 10 in squid and cuttlefish and 38 in Nautilus with which to grasp their prey, all but the Nautilus have suckers on their tentacles that in the squid and cuttlefish have a horny ring to them and may have a hook in the middle as well as being able to exert a suction pressure due to associated musculature.

Cephalopods have a radula which functions like a tongue drawing back pieces of tissue bitten off by the powerful beaklike jaws which are their main feeding apparatus. The beak can be manipulated and used with great dexterity to bite the back of the neck of fish for example to immediately subdue them.

Octopuses have a bite that may be poisonous and may also inject protein digesting enzymes in the way that spiders do, the prey is held when immobilized until it is semi-digested before being eaten. The small blue-ringed octopus lives in shallow water in the Indo-Pacific where it feeds on crustaceans, it's venom is highly toxic and has resulted in human fatalities.

The diets of Cephalopods depends largely on where they live, it includes almost all vertebrates and invertebrates that can be killed and subdued. This includes fish, crustaceans such as crabs and shrimps and snails. Some octopuses can drill a hole in gastropod shells using their radula and then inject poison before eating the occupant with their beak.

Pelycopoda (bivalves)- the radula has disappeared in this group, their feeding apparatus consists of very highly specialized and adapted gills that allow them to function as filter feeders.

Water is brought to the gills where plankton as small as 1μm can be separated out, this is at the size level of bacteria, much smaller than can be removed from the water by other filter feeders that use various forms of finely fringed appendages.

The Pelycopods are able to do this as the water to be filtered passes over the highly complex gills which have a coating of cilia beating currents in a co-ordinated fashion. Filtered organisms are finally immobilized in mucous and taken to the digestive organs.

What eats molluscs?

Molluscs are eaten by many predators from other animal groups, there are some very specialized ways of getting into shells and overcoming defences.

Sperm Whales eat a diet of large and giant squid. The scars from suckers found on the skin of some sperm whales imply that they encounter, attack and presumably kill and eat deep sea squid that are larger than have ever been seen by man. The beaks of these squid are indigestible and accumulate in the stomachs of sperm whales where they may eventually be egested as "ambergris" - greatly prized by the perfume industry.

Thrushes have a great liking of garden snails of various kinds. they take the snail they have found to a favoured stone known as an "anvil" grasp the snail in their beak and repeatedly throw the snail at the stone until the shell breaks and the snail can be eaten.

Many molluscs are specialized to eat other molluscs. The radula in particular can be put to good use in many species to grind holes through the protective shells of tightly closed up bivalves or into the top of gastropods such as limpets that are clamped tightly to rocks. Once a small hole has been made poison is often injected, usually into the main muscle that is holding the mollusc clamped together or to a rock so relaxing its hold before the contents are eaten.

Starfish of many kinds are adept at feeding on those clamped together bivalves and clamped down gastropods. The starfish start out with brute force, using their myriad tube feet, they prise open the shells or shell and rock substrate until a small gap is opened, at this point the starfish everts it's stomach into the hapless prey which is then digested before being absorbed along with the stomach.

A Molluscan radula seen with an electron microscope

The radula shown above is a typical rasping radula that might be found in a herbivorous snail, the hundreds of tiny teeth are constantly replaced as they wear away

Amongst the most specialized of radulae, these have been developed into hollow poison delivering darts.