Slaters and sand hoppers constitute about 20% of all crustacean species. Slaters (isopods) exhibit a greater range of body form than all the other crustaceans combined. Marine slaters range in appearance from wafer-like discs, to worm-like creatures and pill bugs that can roll into a ball. Most are small (3–10 millimetres), although the giant scavenging Bathynomus, a deep-water slater found off the Gulf of Mexico and Australia’s coast, reaches nearly half a metre in length.
Many marine slaters are scavengers – a scourge for commercial fishermen, as they attack dying fish in nets. Others are parasites and live permanently on or in the fish on which they feed. They show most of the typical traits of parasites such as loss of pigment, reduced eyes, strong attaching limbs and high reproductive capacity.
With a body flattened from side to side, sand hoppers (amphipods) not only look like fleas, but jump like them too. They are not restricted to beaches but may be found in many marine habitats as well as some inland sites.
Tongue worms (pentastomids) are scarcely recognisable as crustaceans, except when larvae. The adult form is like a worm or leech – a result of their highly evolved parasitic life in the respiratory tracts of reptiles and birds (and some fish).
Seed shrimps (ostracods) bear little resemblance to shrimps. They are tiny crustaceans with a distinct bivalved outer shell (usually referred to as the carapace), and they resemble miniscule cockles or clams. Because of their microscopic size, the group is not often featured in popular accounts and field guides. This is a predominantly marine group occurring in most benthic habitats in all the world’s oceans. In New Zealand, approximately 300 species are known.