Every now and then I like to highlight groups of organisms of which little interest has been placed and molluscs fall into this category.

They have recently gained negative attention with the arrival of the Giant African Land snail in this country.

Also most people just do not like them because they are "slimy" and "ugly". However, these organisms play an important part in the functioning of our terrestrial and aquatic ecosystems. In this article I will go through the different types of molluscs and the characteristics that distinguish them, some of the ecosystem services they provide and the reasons why mollusc species are declining.

There is estimated over 100,000 species of molluscs that currently exist on the planet. They have also been around for a long time which is evident by their presence in the fossil record.

Out of the several subgroups of molluscs there are three major ones; gastropods, which include snails and slugs; 2) bivalves, which contains animals like mussels, oysters, clams and scallops and 3) cephalopods, which include squid and octopus.

So how can you tell if an animal is a mollusc? All molluscs share three characteristic structures, a visceral mass, mantle and a foot.

The visceral mass can be thought of as its "guts" and contains the digestive, excretory and reproductive organs. The mantle is the protective covering of the organism and can secrete a shell as in oysters, clams and snails. The foot is a muscular appendage filled with nerves and is used to get food and attach the animal to the substrate, which is necessary for species that occupy rocky shores that experience heavy wave action.

In bivalves the foot is tongue-shaped and is used for pushing the animal along and for digging, as these animals bury themselves in the sand to avoid predators. In cephalopods, the foot is divided into tentacles. Also, about 99% of known mollusc species have shells. The shells can form a coiled "horn" as in snails, two halves as in bivalves or as an internal structural rod found in squids referred to as a quill.

At this point, this group of animals get interesting as where a mollusc lives and its diet influences its anatomy.

Some molluscs like snails possess a radula. This is their version of a tongue, but unlike ours which is relatively smooth, it is covered in backward facing barbs, which are used to scrape algae which it feeds on from rocks, tear vegetation or grasp prey like earthworms.

Molluscs also do not have haemoglobin, but haemocyanin so that their blood is blue in colour when it is carrying oxygen and colourless when it is not. Also most species possess what is referred to as an open circulatory system, that is, their blood is not contained in vessels and it just flows freely around the organs in the visceral mass.

But in molluscs like squids and octopuses, it is contained in vessels and therefore they are referred to as having a closed circulatory system.

The outer "skin" of some molluscs, like cephalopods contains special pigment cells which cause the animal to change colour to startle predators and allow them to escape.

Squids also change colour when they move in a group, which is thought to be a form of communication with each other.

But besides their interesting anatomy, molluscs perform many valuable ecosystem services, for instance, species low down in the food chain provide food for animals like birds and small mammals.

Those found in aquatic ecosystems act as food for fish, some of which are of commercial importance like trout and salmon. As a source of food, they are very nutritious as they are low in fat and high in protein. Molluscs also filter water thus keeping it clean and healthy for other animals and for human consumption.

Molluscs are also good indicator species as they are soft bodied or have permeable skin; therefore they are susceptible to changes in the environment, both in the air and water. Therefore they provide an early warning of habitat deterioration.

The bites and stings of some mollusc species are quite painful and may result in death or necrosis, which is the death of the skin surrounding the area of the bite. However, the biotoxins contained in some species are used in the pharmaceutical sector; for example, those produced by cone snails are used to develop drugs for severe pain in cancer and AIDS patients.

However, these organisms are declining, with mollusc species comprising 41% of the 736 documented animal extinctions in the year 2007, with most of the endangered species being found in freshwater or on land.

There are two main reasons for the decline of mollusc species. The first reason is through habitat destruction, by activities such as the clearing of forests, pollution and dam construction. Deforestation in particular does not only affects terrestrial species but those occupying freshwater ecosystems, because without the trees, loose soil is washed into rivers and streams.

This silt-filled water is difficult for the molluscs to filter.

The second reason for their decline is the introduction of non-native or exotic species which compete with the native species for habitat, food and other resources.

These exotic species also sometimes bring with them diseases which can wipe out the native species population in an area.

I hope that this article has raised your awareness when it comes to the biodiversity of our planet. Molluscs are also a good example that all organisms no matter how small or how "ugly" or "slimy", contribute to the functioning of the ecosystems on our planet.

Finally, I would like to emphasise that our best weapon in the fight to preserve our biodiversity is by increasing public awareness of the flora and fauna that exist and their role in our ecosystems.