Crabs are one of those creatures that fire the imagination. They are alien and monstrous in some settings, yet fun and cute in others–who can resist a hermit crab?
If we look at the stars, there is a whole constellation named in honor of these animals.
Certain crabs can also make a very tasty meal!
This page takes a look at some the most interesting types of crabs and how they live. It is not so much an exploration of the different families of crabs as an examination of what kinds of places crabs live with examples of individual species.
There is also an overview of the species that are most important to human beings–whether as food, as a pest or simply as objects of curiosity.
The typical crab:
- has ten jointed legs (crabs are decapods)
- the front two legs are usually claws (often called ‘pincers’ or ‘pinchers’). Sometimes, one of these will be massive and fearsome
- has two eyes on stalks
- breathes through gills which will work in water or air (as long as the gills stay moist)
- has a hard, jointed shell (sometimes called an exoskeleton)
- grows by molting its old shell and then ‘inflating’ a new, bigger exoskeleton
- produces eggs that often the female will carry until they hatch into tiny larvae, These will be free swimming, feeding in the sea before becoming adult crabs.
The sea shore is one of the best places to find crabs. They are good at coping with the constant changes that the tides bring to life on the shoreline. A crab can breathe underwater but can also breathe out of water for long periods. Whether the tide is in or out, these animals can survive.
Crabs need to cope with threats from the air, land and sea. Birds will eat them, fish will eat them, even people will eat them.
The crab’s tough shell and spines provide some defense against predators. They are also good at hiding during daylight hours and at low tide. Some will bury themselves in sand or mud. Some hide in rock pools. Others retreat into the water as the tide ebbs. These behaviors stop them from drying out and from being eaten.
Pictured above is the Rock Crab (or Maine Crab) that you find along the eastern coast of the US. It makes good eating.
In the UK, the Edible Crab (Cancer pagurus) is sometimes found on shore but is more common just below the waterline, to depths of 100 meters.
Below is a video of a Striped Shore Crab (common on the west coast of America) scavenging for food on a rock. Like most crabs, this crab will eat any food it finds (they are omnivores). This includes algae, worms, mollusks, clams and even fungi.
Striped Shore Crab, Eating
The video shows the crab eating algae and any other food encrusted on the rock surface.
Many swimming crabs make good food. The Blue Swimmer Crab (Portunus pelagicus) is one example that lives mainly in river estuaries in Asia and Australia.
It buries itself in mud or sand as it waits for the high tide, then emerges and swims strongly in search of food. A pair of flattened legs at the back of the crab make great paddles.
It will eat shellfish, algae and small fish. The video below shows a Blue Swimmer Crab eating a clam.
I can also tell you from personal experience that it makes a very fine meal. Crab curry is a delicacy in many Asian countries.
A factsheet on Portunus pelagicus from Singapore: wildsingapore./portunidae/pelagicus
A Blue Swimmer Feeding on a Clam
The power of a crabs’ claws are well illustrated below.
Christopher Columbus was the first European to come across crabs living in the middle of the Atlantic Ocean. The Columbus Crab (Planes minutus) clings onto weeds or other floating material like goose barnacles and, sometimes, even loggerhead turtles. It eats algae and invertebrates.
In the Pacific, there are two similar species–the Brown Pacific Weed Crab and the Blue Pacific Weed Crab. Not much is known about these creatures but they have the ability to change color to suit their backgrounds and escape being noticed. They are quite small at around 3 inches across, fully grown.
Some species are well suited to live in deep water.
Red King Crabs (Paralithodes camtschaticus) are an important food resource in Alaskan waters and the North Pacific.
Often they are found on the sides of underwater mountains. They usually stay within a certain depth range and spread to other underwater mountains by producing swimming larvae.
The Japanese Spider Crab is another deep sea dweller and is able to live at depths of 600 meters.
Many deep sea crabs are very large. Stretch out the spindly legs of a Spider Crab and the creature will be wider than a person is tall.
There are a few crabs that are entirely at home on land. Their gills are enclosed in water and perform in a similar way to lungs, allowing the crab to breathe efficiently. Most species need to return to the sea to breed.
One example is the Halloween Crab (Gecarcinus quadratus), pictured, which can be kept as a pet.
There are many mangrove dwelling crabs of the genus Sesarma that can live very happily out of water. They do not even need to go back to the sea to spawn.
In Caribbean countries, a common occurrence is an invasion of towns and villages by land crabs during their annual migration. Tens of thousands of land crabs head back to the sea from the forests to breed, choking roads and gardens. Many species of the most common land crab (scientific name Gecarcoidea) are toxic, so people do not eat them.
A Lot of Crabs!
Christmas Island is a remote island in the Indian Ocean. It too, has masses of land crabs as you can see in the video below.
These are the biggest land dwelling crabs. They can weigh up to 4 kg (9.0 lb) and are about the size of a cat. They will eat almost any kind of food they come across, from fruit to dead animals. They are even capable of opening and eating a coconut, hence their name.
There are many species that live in fresh water–especially in the streams and billabongs of Australia–but also on every other continent.
The Southern European crab, Potamon fluviatile, pictured, has been eaten by people since Roman times.
Unfortunately, freshwater crabs are threatened by human activities more than most groups of animals and many species are in danger of becoming extinct.
Hermit crabs can be found in many habitats, on land, on shorelines and in deeper waters.
The back half of a hermit crab is soft and very vulnerable to predators. This does not matter because hermit crabs are clever enough to use a tough ready-made shell to protect the softer parts.
Usually, they find an unoccupied sea snail shell of the right size and simply reverse into it. When they grow too big they find another shell.
Why do they do this?
- it saves them the energy of growing a complete exoskeleton for themselves
- a sea snail shell is very tough and the hermit crab can withdraw completely into its protection if they are threatened.
I will be adding pictures below of the most colorful, unusual or memorable carbs that I come across.
Charles Darwin probably came across the Sally Lightfoot Crab on his voyage in the Beagle. It is common on the Galapagos Islands, off the coast of South America.