What are Seaweed Crabs

Crab Profile: Seaweed Crab

The seaweed crab is an interesting and unique looking little creature which lives in the coastal waters of south eastern Australia. They can be found in shallow water and sheltered areas on rocky shorelines from central New South Wales, Victoria, Tasmania, and the Houtman Abrolhos along the coast to Western Australia. Their habitat consists of intertidal reefs and rock pools, among algae, sponge and sea grass. Although common in these waters and shorelines, and with different sizes from small or minor seaweed crabs to the larger hairy seaweed crab, they are rarely seen due to their unique camouflage techniques.


The carapace of the seaweed crab is its hard shell-like covering which protects the crabs head and body cavity known as its thorax. The thorax is the crab’s body area between its head and abdomen. The carapace of the seaweed crab is chitinous, meaning the organic material that makes up the carapace. The seaweed crabs carapace is covered with fine hooked hairs (called hook setae), spines and bumps. The seaweed crab decorates itself with pieces of seaweed, sponge, algae, bryozoans and hydroids which it pinches off into little pieces and sticks to the hairs and spines on its carapace and legs for very effective camouflage. This camouflage is why the seaweed crab is often called a decorator crab or decopod from its scientific order Decapoda.  Some of the plant materials the crab sticks to itself will grow on its carapace, making the seaweed crab look much bigger than it actually is. Another interesting fact in this crab profile is that as the crab’s body grows, it must shed its too small carapace for a new, larger addition which has been growing under the original. This shedding is called molting. Molting usually happens every few months and the warmer the water and younger the seaweed crab is will cause faster growing and more frequent molting.  As the seaweed crab molts, it often removes the camouflage from the outgrown shell and saves it to decorate the new one. This decoration help protect the crab from predators as it is slow moving and has weak claws. Many times, small crustaceans, worms and sponge live in the seaweed crabs decorations. The sponge helps to protect the crab from predators as well, due to the neurotoxins that it releases.

Seaweed crab dimensions and colors

The seaweed crab is approximately 30 to 40 mm wide and can have up to 5 spines which project from each side. Being part of the spider crab family, the seaweed crab has 8 legs which are a bit flat, long and spindly yet the smaller types have shorter legs which they tuck in under its body. Due to its camouflage techniques, the seaweed crab original coloring is often hard to interpret. With all the bits and pieces of its camouflage removed, the seaweed crab’s carapace is usually yellowish brown, brownish green or just brown. Their claw color is very close to their carapace color but can have a yellow or orange tint.


Due to being a much slower crab, they do not actively hunt. Seaweed crabs feast on dying or dead mussels and dead paua. They also eat decaying seaweed and other sea grass. These crabs are scavengers and omnivorous, which means they eat meat or flesh, as well as plant material.

Other seaweed crab profile facts

Seaweed crabs have a lifespan of around two years. There are approximately 43 different types of spider crabs found in the waters off Australia along with 170 or so different species of crabs. The seaweed crab is not in danger of extinction.  Seaweed crabs are typically nocturnal but you can find them during the day. This nighttime activity adds to their safety.

Scientific crab profile information

The taxonomy of the seaweed crab is as follows:





Habitat-Native to Australia

Status-Not a Threatened Species

 Where to see

The good news about these crabs is that although they are native to Australia, you do not have to travel far to see them. You can view seaweed crabs in some of the larger aquariums and aquatic zoos around the United States.  Each seaweed crab, being distinct because of its decorating techniques and choices, is always an exciting attraction.