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What are crustaceans?

Crustaceans belong to a subphylum of the Arthropoda, and are among the most successful animal groups with almost 52,000 described species. They are as abundant in the oceans as insects are on land. It is estimated that the tiny marine copepod crustaceans make up over half of all animals in the world by sheer numbers, and krill (the preferred food of some types of whales) has one of the greatest biomasses on the planet. As such, crustaceans are a crucial component of most marine food webs.

Crustaceans vary enormously in their sizes, shapes, and lifestyles - from transparent microscopic copepods to colourful shrimps and lobsters. The largest of all Australian arthropods, the Giant Crab (Pseudocarcinus gigas), weighs up to 14 kg, reaches nearly 40 cm across the shell, and has a claw as big as a human forearm. While predominantly marine, some crustaceans have also moved into freshwater, and a few groups have adapted to life on land, such as terrestrial crabs, terrestrial hermit crabs, woodlice (Isopoda) and leaf-hoppers (Amphipoda).

Despite their diversity of form, crustaceans are united by the special first larval form known as the nauplius, though in some forms some or all larval stages can be very short-lived, and even take place within the egg before release. Crustaceans also differ from other arthropods (such as insectsspiders and other arachnids) by having two pairs of antennae (although one pair can sometimes be tiny). Most large crustaceans have well-developed gills, but some smaller species respire directly through the body wall. Like other arthropods, crustaceans have a stiff exoskeleton that must be molted to allow the animal to grow. This is a time when many crustaceans are particularly vulnerable to attack from predators because their new larger shell still needs to harden into protective armour.

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