Marine Education Society of Australasia Home Page

banner image for Life on Australian Seashores Website

Decapod Crabs

Along the Eastern Warm Temperate Zone in south-eastern Australia on rocky ocean shores, there are many types of crab from different families. Typical shore crabs belong to two major groups; the Grapsid Shore Crabs and the tall-eyed Ocypode Crabs.

Graphic of Decapod crab parts

Grapsid Shore Crabs - Grapsidae

The Grapsids have invaded the largest number of shore habitats. They may be distinguished by their broad fronts, short eyes and almost square-shaped carapace.

Some grapsids can leave the water for long periods of time. As the deoxygenated and carbon dioxide loaded water is expelled from the gill chambers, it is spread over spongy areas on either side of the mouth where it picks up fresh oxygen.

Appendages at the entrance of the gill cavity near the base of the claws (chelipeds) drives the oxygen recharged water back to the gills.

Most grapsids have equal-sized chelae (claws), unless one has been broken off and is regrowing. Male grapsids usually have much larger chelae than females of the same species.

Some grapsid shore crabs which are found along our shores are:

Small photo of Variegated Shore CrabVariegated Shore Crab, Leptograpsus variegatus


Small photo of Red Bait CrabRed Bait Crab, Plagusia chabrus


Small photo of SowrieSowrie, Plagusia glabra


Small photo of Smooth Shore CrabSmooth Shore Crab, Cyclograpsus audouinii


Ocypodid Shore Crabs - Ocypodidae

The other major group of shore crabs are the Ocypodids, These are the Ghost Crabs and Fiddler Crabs and relatives. They are mostly all tall-eyed, with the bases of their eyes close together at the front of the carapace. Ocypodid crabs live on sandy beaches, mudflats and in mangrove swamps. They are active burrowers and many live in colonies. The male Ghost Crabs and especially the Fiddler Crabs have unequal sized chelae.

Ocypodids are not normally found on rocky ocean shores.

Spider Crabs - Majidae

These Majidae crabs are commonly known as the Spider Crabs. The seaweed crabs of this family are covered with long hooked hairs, which allow the crab to snip off living pieces of algae and attach them to the hairs. A decorated seaweed crab with algae, sponges and other marine growths is excellently camouflaged. It only gives itself away when it moves.

One seaweed crab found in the intertidal zone in south-eastern Australia is the:

Small photo of Seaweed decorator CraabSeaweed Decorator Crab, Naxia tumida


Black Fingered Crabs - Xanthidae

The black-fingered crabs are the largest group of crabs in Australian seas. Xanthids are very common in tropical waters, especially on the Great Barrier Reef.

Most members of the group are almost oval in shape, being slightly wider than long. The distinguishing feature of the group, is the dark coloured fingers of the chelae.

The most common black-fingered crab on rocky ocean shores in south-eastern Australia is the:

Small photo of a Reef CrabReef Crab, Ozius truncatus


Swimming Crabs - Portunidae

The portunid swimming crabs have a broad carapace and flattened hind legs which form swimming paddles and sometimes digging spades. None of the portunids come out onto the shore, but they do enter shallow water to hunt for live food. The most common estuarine swimming crab is the Blue Swimmer Crab, Portunus pelagicus.

On the rocky ocean shore one of the most attractive crabs is the:

Small photo of a Tubercled Shore CrabTubercled Shore Crab, Nectocarcinus tuberculosus


Regrowth of Limbs

Decapod crabs can regrow their limbs if they are broken off. In fact there is a special "weakness plane" across the leg, so that if a predator grasps the limb, the leg breaks off, and special muscles contract and pinch off the nerves and stem the loss of fluid. It takes about six or so moults to regrow a severed limb.

For more information on Crustaceans visit MESA Crustaceans

Home Page

Environmental Factors
Water Currents
Wind Effects
Wave Strength
Tide Effects

Home Page
Rocky Shores
Tidal Levels
Intertidal Zonation
Environmental Factors
Biological Factors
Feeding Relationships




photo of Keith DaveyLife on Australian Seashores
by Keith Davey (C) 2000

Learning Consultant - Media
The University of Newcastle

email at

Scientific Consultant: Phil Colman
site created 01.01.98 : updated 01.04.2000