An introduction to crabs

Typical crab features

Crabs, along with spiders, scorpions, millipedes and centipedes are included in the wide-ranging primary division (phylum) of the animal kingdom named Arthropoda. Arthropods are animals that have a jointed hard external skeleton. Crabs are further grouped into a sub-phylum called Crustacea which simply refers to the hard external skeleton. Having two pairs of antennae distinguishes crustaceans from other arthropods. There are around 30,000 species which include crabs, lobsters, shrimps, prawns, barnacles and slaters. True crabs make up the infraorder Brachyura and hermit crabs and the robber crab make up the infraorder Anomura. 

Growing and moulting

Having a crusty skeleton on the outside of their body requires crabs to shed their outer shell periodically and replace it with a new and larger one for their bigger body size as they grow.  This process is called moulting, and occupies a significant part of a crab's life. Splitting of the old shell and emergence is achieved by swelling the interior body and new soft shell by absorbing water. This is an arduous and time consuming process for those species that are now adapted to live on land. Emergence is usually backwards through a split in the rear of the old carapace. The crab emerges with a new, but soft shell. They usually hide until the new shell hardens. The old shell is usually eaten to retain calcium salts and to replenish valuable organic materials. 

Getting around

The legs of arthropods are adapted for all types of locomotion ; walking, pushing, running, swimming, and burrowing.  Crabs have jointed limbs, like a segmented series of hinged cylinders. Each hinged section only moves in one plane but the combination of a series of hinges that can move in different planes results in limbs that can move in almost every direction. Brachyuran crabs walk using eight legs. The two taxa of anomuran crabs have different styles of walking. Robber crabs use six legs and hermit crabs use four legs. The claws are usually held off the ground when crabs walk, except when they are walking slowly, and then the claws may touch the ground, giving the appearance of being used as another pair of legs.

Sensory organs

Crabs have compound eyes on stalks consisting of several thousand optical units. The eyes can be lowered into sockets at the front of the carapace for protection. Crabs appear to have good vision; some species are able to detect movement up to 30 metres away. Bristles and hairs all over their body and legs  act as touch receptors. Bristles generally signal contact with hard surfaces , whilst other shorter hairs are sensitive to water or air currents. Crabs find food by using chemical stimuli. Their antennae have "smell detectors" that sense chemicals that stimulate the search for food. Crab mouthparts have receptors which are sensitive to particular chemicals. Crabs can hear using the bristles on their legs or through vibrations, and some species produce a variety of sounds. Crabs use their sense organs in combination to find food and mates and to flee predators.

Breathing

Crabs that live in water use gills to breathe, and crabs that lead a terrestrial existence, while retaining gills,  have also evolved lungs with which they breathe. Gills, and lungs, work because the oxygen molecule is very small. During respiration oxygen molecules first dissolve into a layer of moisture surrounding a thin membrane. Then the oxygen molecules, because they are so small, to cross through the membrane into the blood of the animal. As long as a crab can keep its gills or lungs moist, they are able to diffuse oxygen from the air or water. Compared to their marine counterparts, terrestrial crabs have reduced gills, larger lungs and enlarged branchial chambers.

Circulatory system

A crab's circulatory system is fairly simple and is classified as an open circulatory system, i.e., they have no blood vessels. The circulatory system is the primary means of transporting oxygen and food to the cells, and carbon dioxide and waste products from them. Crabs' organs are surrounded by open spaces (sinuses)  and when the animal moves, these spaces contract and expand there by aiding in circulating the blood throughout the organs and bodies. ​Their blood bathes the internal organs directly  and is oxygenated when it passes through the branchial chamber housing the gills and lungs. 

Stepping on to land

Crustaceans evolved in the sea and most present day crustaceans remain in the marine environment. Some species have however ventured forth on to the land. These are crabs we describe as terrestrial or land crabs. There is a varying degree of adaptation to a terrestrial existence exhibited among crab species and it is difficult to decide the characteristics and habits that typify a crab as a land crab.  Compared to their insect relatives, even the most terrestrial of land crabs have poor resistance to water loss, and it is only by modifying their activity patterns significantly to conserve water, that they are able to survive out of an aquatic environment.



The terrestrial crab populations of Christmas Island form the most conspicuous and unusual element of the island ecosystem. The island has the highest diversity of terrestrial and semi-terrestrial crabs in the world. These crab populations drive important ecological processes and significantly influence the floristic species and the structure of the island rainforest.

More detailed information on crab physiology and biology with accompanying images is presented in the book

"Crabs of Christmas Island" by Max Orchard

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Images by Max Orchard unless indicated otherwise