Isopods are a large crustacean group with free-living species in marine freshwater and terrestrial environments. A few highly modified parasitic forms occur, especially on fish. Mostly small and drably coloured, they are not often noticed. Most are bottom dwellers, many found in soft sediments and in crevices from the shore to depths greater than 10,000m. Terrestrial isopods include the familiar slaters or wood lice.

• To be a small crustacean with many similar legs and no carapace. The word
• A body that is flattened from top to bottom (dorso-ventrally, the opposite to amphipods), and often oval in outline.
• Eyes that are large, compound and not stalked, or no eyes at all.
• Thoracic legs that are simple, without nippers (not chelate).
• Body segments.
• Gills that are associated with abdominal legs (not thoracic legs).

Slaters, pill bugs, wood lice, sea slaters , fish lice

If you can picture a slater then you can understand the body plan of all isopods. They all have flattened body with numerous similar legs, 19 almost equal segments, antennae which are either long or tucked underneath, and eyes that are compound and sessile. They are drably coloured, and some isopods can even change colour to blend in with their surroundings.

Most marine isopods are bottom dwellers. Many are found in soft sediments or crevices from intertidal shores to the deep ocean. One common species, Ligia, the sea slater lives high up on the shore on rocks or pilings. They swarm in large numbers and can move very rapidly when disturbed. Isopods occur in nearly all environments, and some are partly or wholly parasitic.

Isopods feed mostly on organic debris, but some browse on plant material or are predators of small animals. Others are external parasites on fish and feed on the fluids in the tissues of their host. Others lodge under the gill covers or in the mouth of larger fish. The giant benthic Bathynomus eats detritus and has such sharp mouth parts that it can chew through the toughest plastic bags. One group are parasitic on crustaceans such as crabs, prawns and shrimp, where they feed on body fluids and are often found inside the gill chamber of the host.

Any small, mobile marine predator may eat isopods as part of their diet. Fish are an important predator, especially on isopods that live in the gill chamber of fish.

Mature female isopods have a thin plate on each of the first 5 pairs of legs which overlap to form a brood chamber. Eggs are laid in this chamber and then fertilised by the male using special spikes on his 2nd. pair of abdominal legs. About 80 young are sheltered in the pouch before being released into the sea looking like miniature adults. Each adult probably lives about 3 years.

Isopods can be parasites on fish, crabs, prawns and shrimp. They attach themselves to the gills of fish and suck blood from their hosts. Fish with isopod parasites visit "cleaning stations" where the host allows small fish to enter its gill chambers and pick off the isopods. The first young isopod to find a shrimp host develops into a female, and those arriving later develop into tiny males.

Damage by timber boring isopods to wooden structures in the sea is a problem of major economic importance in Australian harbours. Several very small species are involved. They bore into wharf pilings, oyster stakes, submerged timbers of all kinds even sandstone and the lead covering of cables. A large untreated wharf pile has been known to collapse in less than 2 years. Modern treatments control the problem but never eradicate it.