Molluscs are one of the largest invertebrate groups and are usually quite conspicuous wherever they are. They are a very varied group with some widely different members such as octopus, oysters and snails. They usually require damp conditions to be active, though many are able to seal up their shell for short or extended dry periods emerging when it rains or when the tide comes back in.
Mollusca - Mollusks Molluska or Molluscs - from Molluscus - thin-shelled
Tridacna maxima - the giant clam, a Pelycopod
A typical bivalve mollusc seen here end-on embedded in a coral reef, the blue region is soft tissues which are extruded and spill over the top of the two shells when they open. Tridacna like most bivalves is a filter feeder.
picture - Ahmed Abdul Rahman used under CC4 Attribution-Share Alike International license.
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 Jurgen Schoner - GNU Free Documentation License, Version 1.2
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 as with land and sea slugs.
Pelycypoda 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.
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 bright corals on which they often live, for others the colouring is warning.
Clione, a small (2-3cm long) shell-less Gastropod
This group is 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 can appear 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 it is not so easy to see why some very different members are all Molluscs, there are sub-groups which are quite characteristic making identification usually not so difficult.
- 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 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.
Kingdom - Animalia
Phylum - Mollusca
The group includes:
- Snails - land and aquatic
- Pteropods - Thecosomata - sea butterflies
What do molluscs eat?
Virtually every feeding method 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.
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 number from 16 to thousands.
The radula rasps away at plant material moving back and forth 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 ambush.
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. Here they are used as second hand weapons against any small fish or other animal that might fancy a nibble of the gill as a tasty morsel.
Cephalopoda - highly adapted as predators. They have large eyes which are arguably the most sophisticated in the animal kingdom (more so than mammals or birds) 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. In squid and cuttlefish the suckers have a horny ring to them and may have a hook in the middle as well, they are also 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 beak-like 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, 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 depend largely on where they live, it includes almost all animals that can be killed and subdued, including fish, crabs, 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.
Pelycypoda (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 1ÃƒÆ’Ã†â€™Ãƒâ€¦Ã‚Â½ÃƒÆ’Ã¢â‚¬Å¡Ãƒâ€šÃ‚Â¼m 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 highly complex gills with their coating of beating cilia generating currents in a co-ordinated fashion. Filtered materials and tiny organisms are immobilized by 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.
Birds such as 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 it at the stone until the shell breaks and the snail can be eaten.
Many molluscs are eaten by other molluscs as already mentioned
A Molluscan radula seen with
an electron microscope
The radula shown immediately above and below 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.
Top banner - shell variability in the gastropod snail Theba geminata, picture courtesy H. Zell used under CC3 Attribution Share-Alike Unported license.