Hairy Seaweed Crab

This crustacean has a hard rounded-triangular shell, or carapace, that is covered in hair-like projections that vary in size. The shell's length is 1.3 times the size of its width, the shell's surface has many tubercles with spines running to the front of the shell going to the sides. The edges of the carapace have nine outward directed sharp spines: 3 supraorbital, 3 hepatic and 3 brachial. Two spines at the front of the crab's head form a V-shape and are covered with small bristles. Some of the bristles along the carapace are short and hooked while others are long and straight. The abdomen has seven segments in both sexes with the male abdomen being more narrow with the segments being broader than longer. The legs are longest in the front and get progressively shorter towards the back.

The seaweed crab is antipodean as it is native to New Zealand and south-east Australia.

Crabs are typically scavengers, meaning that they eat whatever they can find, and as omnivores this includes both plants and other animals. In a study conducted on seaweed crabs living on the coast of the South Island of New Zealand, the foreguts were opened among a sample of crabs to determine what foods had been consumed. Algae, including calcareous red algae and branched brown algae, was identified as the most commonly eaten substance, as it was found in nearly all of the crabs sampled. Other common foods included isopods, amphipods, and decapods (e.g. small fish, crayfish, other crabs). Bivalves, sponges, bryozoans, and more were consumed as well but not commonly. Meat consumed is usually from dead animals because seaweed crabs are slow moving and not very strong.

The female seaweed crab lays fertilized eggs from which larvae develop and break free. The eggs are bright orange when freshly laid and turn brown as they approach time to hatch, and are approximately 0.75mm across. There are multiple stages of the larva before they reach maturity including pre-zoea, zoea 1, zoea 2, and megalopa. When the larva hatches it is enclosed in a thin and transparent covering that is shed shortly after. Then this comes the first zoea stage during which the larva is about 3.0mm. Zoea 1 lasts 8–16 days before the exoskeleton of the larva is shed and it enters zoea 2. At zoea 2 the larva is about 3.5mm long. The larva transforms into megalopa after 12–15 days at a temperature of 16 degrees Celsius. After 20 days, the megalopa larva moults into a juvenile crab.