Gastropods are second only to the insects in terms of varieties. Snails and slugs are gastropods and they are classified under the class of Mollusca. You can find many species of sea snails and sea slugs, freshwater snails, freshwater limpets, land snails and land slugs. Each group of gastropods have different structure of body parts, behaviour and eating habits. Therefore it is hard to generalise all the gastropods. The world’s biggest gastropod is a snail – the Giant Ghana Snail.
All of the gastropods live in different places. Example, in gardens, woodlands, deserts, mountains, small drains, big rivers, lakes, estuaries, mudflats, rocky seashore, sandy subtidal and in the oceans. You may be amazed to know that they are found at very cold places like Arctic and Antarctic to warmer places like tropics. They can be brought from one habitat to another habitat by a bird like Great Tit. There are about 30,000 species of marine gastropods and about 5000 species of freshwater and salty water gastropods. At the moment about 4000 species of freshwater snails have been discovered. There are 444 currently discovered extinct species of gastropods.
Snails have shell which will allow the soft parts of their bodies to be tucked in. Those gastropods without shell and those with a very small or internal shell are known as slugs. Gastropods live in places where calcium carbonate are available for them to use to make their pretty shell. In places where calcium carbonate is not found there will be slugs and some snails with thin clear shell.
Gastropods has a head with two to four tentacles with eyes and a foot which help to push away debris as the snail crawls. Most shelled gastropods have one pieces of shell. They are either coiled or spiralled. The land slugs have small shells or no shell at all. Some sea slugs are very brightly coloured. They are very pretty. Their bright colour is there to warn their predators that they are poisonous. So beware of them! These bright colours also help them to camouflage themselves very well among the sea weeds and sponges.
Gastropods can smell, see and balance themselves but the sad thing is they can’t hear like us. For the land snails and slugs, their smell sensor are located on the tip of their 4 tentacles and they are the most important sensory organ for them. Their eyes are either at the tip or the base of the tentacles. Vision is not the most important sense for the land snails and slugs. Do you know why? It is because they are active at night and rest during the day.
Gastropods have small teeth which are called radula. The meat eating and plant eating snails and slugs use the radula to graze algae off rock surfaces. Most marine gastropods are burrowers. They have siphon to taste the water and to locate prey from a distance. Those with siphons are normally predators or scavengers. Majority of all marine gastropods breathe with a gill, but many freshwater species and most land species have a lung. There are plant eating, predatory meat eating, scavengers, parasites marine gastropods.
Land gastropods species chew up leaves, bark, fruit and decomposing animals. As for marine species they scrape algae off the rocks on the sea floor. A few sea slugs are plant eating and some are meat eating. Cone shells and ghost slug are predatory meat eating gastropods.