We all know a crustacean when we see one, don’t we? They are those seafoods that have to be peeled to get at the delicious bits; like prawns, crabs and rock lobsters. But even these common crustaceans have stages in their life cycles that look so different that perhaps their mothers wouldn’t recognize them. Can you match any of the babies pictured to their adults?
Crabs, prawns and lobsters make up only a tiny fraction of a huge and very varied group of animals, most of which are marine. Other members of the group include barnacles, land-dwelling woodlice, and tiny marine copepods, the mainstay of most food webs in the ocean.
So how do we recognize a crustacean? All have a tough outer covering like insects but with the addition of calcium salts to give added hardness. The covering is great for protection but has to be moulted regularly to allow the animal to grow.
Unlike insects, crustaceans have two pairs of antennae in front of the mouth and most have appendages with two branches, except when they are claws or used for walking. The appendages can be modified for many purposes such as swimming, grasping, filter feeding, breathing, sperm transfer and egg brooding.
Filter feeders use limbs with numerous bristles to beat the water and trap food particles. Many species have gills attached to their appendages. In our seafood types there are usually seven rows of branched gills under the carapace which covers the back of the animal’s head and thorax.
Most crustaceans (but not barnacles) have separate sexes. The males transfer their sperm to the females who then usually brood the fertilized eggs for a while.
The first stage of development of the young crustacean is a nauplius larva. It has no segments, one central eye in the front part of its head, and three pairs of appendages, all with bristles for swimming. However, most nauplii remain in the egg and later stages of development are the ones that hatch and spend time as part of the plankton. With each succeeding moult more segments are added to the larval bodies and eventually they take on a miniature adult form.
How did you go matching the larvae to their parents? The sketches (not to scale) are of stages in the lives of a rock lobster (A), a prawn (B) and a crab (C).
