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dungess crab
Crustaceans are a diverse group that include crabs, lobsters, shrimps, krill, prawns, water fleas, copepods, barnacles, and few land creatures such as woodlice. They are generally water creatures that have tough shells and no backbone and breath through gills. They are generally scavengers that feed on detritus. Their shells are made of chitin, the same material that makes up insect shells.

Crustaceans belong to the phylum of arthropods along with insects, centipedes, millipedes and arachnids (including spiders and scorpions). Arthropods account for three fourths of all known animals. All have exoskeletons made of chitin; a body divided into segments and protected by cuticle; jointed legs arranged in pairs; an open circulatory system with organs bathed in a liquid called hemolymph that is pumped around the body by the heart; and a nervous system comprised of paired nerve chords.

The first crustaceans appeared about 500 million years ago when trilobites dominated the seas. Early varieties were similar to trilobites except they had two pair of antennae rather one. Today there are about 35,000 different species of crustacean — four times as many as the total number of bird species. Most are found among rocks and reefs. Some of those found in coral reefs are quite colorful.

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Hermit crab
According to the Census of Marine Life, completed in October 2011, crabs, lobsters and other crustaceans are now believed to be the most common species in the seas of Australia and Japan, whose waters are thought of as the most varied .

Websites and Resources: National Oceanic and Atmospheric Administration ; Smithsonian Oceans Portal ; Ocean World ; Woods Hole Oceanographic Institute ; Cousteau Society ; Montery Bay Aquarium

Websites and Resources on Fish and Marine Life: MarineBio ; Census of Marine Life ; Marine Life Images ; Marine Species Gallery

Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) ; International Coral Reef Initiative ; Wikipedia article Wikipedia ; Coral Reef Alliance ; Global Coral reef Alliance ; Coral Reef Pictures ; The Global Coral Reef Monitoring Network; the International Coral Reef Action Network.

Crustacean Characteristics

Crustaceans have two pairs of antennae and compound eyes on stalks. Their head and thorax is often covered by a shield or carapace and the front of this extends to form a projection called the rostrum. Crustaceans employ a number of strategies when feeding. Large species capture prey and kill it by crushing, stunning or tearing it apart. Other are filter feeders that use their thorical appendages to set up currents in front of their mouth that draw in water which can be filtered for small particles of food. Yet others use their appendages to scavenge or root through sand, mud, algae and other materials.

Crustaceans have series of paired appendages we call legs which are powered by internal muscles within the exoskeleton. These have been adapted for their particular needs. Many have evolved their front legs into claws and pincers (known to scientists as chelipeds). The middle legs are generally used for paddling or walking.

Fiddler crab anatomy

Crustacean appendages have two branches and have a number of functions, including movements, sensing, respirations and egg-brooding. The first pair, often claws or pincers, are used for defense, handling food and even sexual communication. Thoracic appendages called perepods typically have gills. The basal part of some appendages help in walking while abdominal segment often have paired swimming appendages called pleopods or swimmerets.

Crustacean leg muscles are attached to prongs near the points inside the exoskeleton. The joints can only move in one plane. To get around this limitation, joints are often grouped in twos or threes on each limp. They are often close together, each operating in different planes, which allows the limb to move in a variety of directions.

Crustacean Exoskeletons

The chitin exoskeleton of crustaceans is strengthened with calcium carbonate. Because they operate almost as well on land as in water, many kinds of crustaceans emerge from the water at beaches and shores or easilt survive when exposed by low tides.

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ghost crab
Because the shells can't expand or grow, crustaceans must periodically shed their shells and grow new ones. Before a crustacean molts its absorbs much of the calcium carbonate from its old shell into its blood. This weakens the old shell and allows it to be shed more easily.

The new shell is secreted in the form of wrinkled skin underneath the old shell which splits open and remains mostly intact, resembling a translucent ghost of its former occupant, as the animal crawls out. The animal grows and swells its body by absorbing water. The skin swells and stretches out the wrinkles and hardens gradually into shell. While the shell is hardening the crustacean is vulnerable to attacks and must hide.

The limbs of most crustaceans grow back if they are lost. A quarter of male crabs in one survey lost their claws in combat. Some species of crustaceans can grow back the first set of lost limbs but not a second set. Shellfish like lobsters, crabs and shrimp turn red when cooked because they accumulate red pigment from eating certain plankton and algae. The pigments bond with proteins in the shell, making them invisible until cooking breaks the bond and reveals the red.

Barnacles, Copepods and Sea Spiders

Barnacles clamp on to rocks with a cement that is so strong scientists are studying it to make strong-water-resilient glues and other commercial uses. They are filter feeders who prefer strong currents to bring lots of nutrients passing their way. They are often found on wharfs, docks and ships.

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Fiddler crab
Copepods are flealike seas creatures and one of the most abundant and diverse animal groups on Earth. One sampling of an area the size of a bathroom (5.4 square meters) deep in the South Atlantic Ocean in the mid 2000s turned up 700 species of copepod, 99 percent of them unknown to science.

Sea spiders are not crustaceans but I didn’t know where else to put them so I put them here. Also Pantopoda or pycnogonids, they are marine arthropods of class Pycnogonida, found especially in the Mediterranean and Caribbean Seas, as well as the Arctic and Antarctic Oceans. There are over 1300 known species, ranging in size from 1 to 10 millimetres (0.039 to 0.39 in) to over 90 cm (35 in) in some deep water species. Those found in relatively shallow depths tend to be small while those in Antarctic waters can grow quite large. Some small ones are so small some to their organs are on the their legs. [Source: Wikipedia]

Although "sea spiders" are not true spiders, or even arachnids, their traditional classification as chelicerates would place them closer to true spiders than to other well known arthropod groups, such as insects or crustaceans. However this is in dispute, as genetic evidence suggests they may even be an ancient sister group to all other living arthropods. Most sea spiders survive by sucking juices of coral, sponges and sea anemones. [Ibid]


Most crabs are scavengers that feed on detritus, but some feed on plant material and others feed on small sea creatures. There are marine crabs, freshwater crabs and terrestrial crabs. Their basic design has not changed much in 200 million years and they have many similarities with insects.

Crabs have exoskeleton and compound eyes like insects, one set of claws, four pairs of legs, eyes mounted on stalks and a hard carapace enclosing gills and a soft body. Crabs scuttle along sideways on land but there movement in the water is more complex and varies according the conditions in the water.

Crabs are basically marine or at least water creatures They breath by passing oxygen-laden water through gill chambers within their shells. Crabs that spend a lot of time out of the water take in water when they are submerged and retain water within their gill chamber inside their carapace when they leave water. This a relatively small volume of water. Many species beat the water into a froth so it can hold more oxygen — which is drawn in from the air — which is absorbed in the gill chamber. Their exoskeletons prevent them drying out.

Crab carapaces can be round, triangular, rectangular or oval. The abdomen is short, flat and tightly curled so it fits under the carapace. Crabs molt by squeezing their bodies through a crack across the rear of the shell.

Spider crabs fighting
Female crabs can only mate during the short period after she sheds her shell and is waiting for her new shell to harden. Males are alerted by biochemical signals given off by female when this occurs. The first male on the scene climbs quickly on her back and fights of rivals until mating takes place.

Crab larvae (called “zoea” ) hatch from eggs. At first they are a form of plankton. They are very small and swim freely and look like nothing like crabs and go through several larval stages, each with a bizarre-looking creature. After several moltings they begin to look like a crab. Most crab larvae are consumed by predators. Very make it to adulthood. It takes about ten moltings for larvae to become a full fledged crab.

Some crabs have become so adept at living outside of water they don't like the water. During high tide some crabs immerse themselves in their holes and plug them and place a bubble of air underneath the plug to sustain them until tide goes out.

Kinds of Crabs

There are around 6,500 species of crab. They come in an astounding variety. Some look like terrifying monsters. Others resemble daddy long legs spiders, helmeted hockey players or wolves. The greatest collection of specimens is at the National Museum of Nation History in Paris.

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juvenile Galapagos crabs
The smallest species, the pea crab, is only a half an inch across. The large species, the Japanese spider crab, can measure up to three meters from claw to claw and have legs that are 1½ meters long and a carapace about a half meter long. Crabs have been observed at depths of 20,000 feet in the sea and 6,000 feet above sea level on land.

Spider crabs have a triangular carapace; long, slender legs; pincer-bearing length that look similar to their other legs. They often used their claws to attach pieces of sponges, seaweed, hydroid or other organic material on their shells as a form of camouflage. Some species cover their entire bodies and are quite difficult for divers and predators to find. When the move to new surroundings they will change their camouflage to match their new environment.

Some species have Velcro-like hairs and attach sea weed and even sea anemones to themselves as camouflage. Swimming crabs hold the fish they catch in their claw like a cigarette and then eat them head first. Arrowhead crabs a kind of spider crab, have eyes so far part they can almost see in a complete circle. Spotted crabs found in the Great Barrier Reef fiercely defend their territory during the day but at night you can pick them with your hands.

Ghost crabs, sand crabs and other crabs that hang out at beaches tend to hang out in the hole during high ride when their holes are submerged and emerge during low tide. Their holes are often surrounded by excavated sand and little round balls that are remains of their feeding. They feed by picking up sand with their claws and sifting through it for detritus. As they do this they roll the sand into little balls. While they do this they need to keep an eye out for sea birds and other predators that feed on them.

Ghost crabs live holes and feed on organic matter found in the mud. They keep the lookout for predators with eyes stalks covered by multiple eyes that allow the crabs to see 360̊. They sometimes climb trees to get at insects.

Fiddler Crabs, Mangrove Crab, See Mangrove.

Horseshoe Crabs

Horseshoe crabs are not really crabs, There are more closely related to spiders and scorpions. Resembling a helmet with a spike tail, they grow slowly and molt around 17 or 18 times — reaching the size of a silver dollar when they are two and fit into the palms of a person’s hand when they are five.

Horseshoe crabs are regarded as living fossils, They have been around more the 300 million years — before the dinosaurs. Their blood has the extraordinary ability to clot when harmful bacteria is present. A substance called LAL, which is derived from horseshoe crab blood, is used in vaccines, allergy shots, nasal sprays, eye drips and medical equipment such as needles and tubes to make sure they are bacteria-free.

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Horseshoe crabs mating
Horseshoe crabs have been called a “Swiss army knife of jointed limbs.” They have specialized pairs of legs for swimming, walking, grasping, shredding and defending themselves. They have been called “biological bulldozers” for the way they dig up the ocean floor or root out small shellfish and worms to eat. The structure of their eyes is similar to that of humans. They can see well in the dark.

Horseshoe crabs need to keep their primitive gills moist to survive. If they get tuned over their back they fold themselves in half, with their tails up in the air to conserve water. Their tail looks dangerous but it isn’t. It is used to steer and turn over.

Females reach sexual maturity at around age 10. During high tides in the full-moon cycle of May and June, female horseshoe crabs come ashore and lay eggs. One female can lay around 34,000 eggs at one time and as many as 100,0000 in one season. Only a handful of larvae survive until adulthood. Many of those that don’t provide food to a number of animals.

Horseshoe crabs play a vital role providing food for migratory shorebirds. The red knot is a sandpiper-like bird that flies 15,000 kilometers between Tierra del Fuego and the Arctic. Critical to their trip is a stopover in Delaware Bay to feats on horseshoe crab eggs, The birds know when and where to find the eggs, in some cases doubling their weight after the feed on them. But these days horseshoe crabs and horseshoe crab eggs, by conch and eel fishermen as bait, are harder to find and a critical way station the red knots is threatened.

Hermit Crabs

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Hermit crab in crown conch

Hermit crabs are not true crabs in that they lack a hard protective shell. They have a long abdomen, which is soft and spiral shaped. They place their abdomen in an empty gastropod shell and anchor themselves to it with hooks on a pair of leglike organs.

Hermit crabs tend to live along shores of sandy or rocky coasts. They can be found both on the land in the water. Land hermit crabs are often bigger than ones that spend most of their time in the water. Hermit crabs in mangroves spend a lot of their time on land near their burrows. Once a year they migrate en mass to the sea to breed. Females lay thousands of eggs in the sea that hatch into larvae that spend six weeks in the water before coming on land.

Hermit crabs make their homes in whelks, winkles, snails and other single-shell mollusks (gastropods). An individual crab snags the shell by curling it soft rear end into the spiral tunnel of the shell. It can withdraw into the shell, closing of the entrance if necessary with its claws.

Hermit crabs walk around, dragging their shells behind them. When they grow too large for their shell they move to a new shell. Sponges sometimes make their homes on the shells of hermit crabs. feeding on the remains of the crab's meals and adding to the crab's disguise. Sometime worms take up permanent residence in the shells and snatch food from the hermit crab's claw when the crab is eating.

It is not unusual for sea anemones to hitch rides on the shell. These keep predators of the crab away. When the crab changes shells it sometimes lifts the anemone with its claws to the new shell. In the Indian Ocean there are hermit crabs that carry sea anemones on each claw.

Fresh Water Crabs, Coconut Crabs and Land Crabs

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Caribbean hermit crab
Fresh water crabs are tied to the ocean for reproduction. They return to the sea to mate and produce eggs. Young crabs go through larval stages to become crabs and then move to freshwater rivers, stream and lakes. A few species avoid returning to the sea by raising their young inside their shells while they pass through the larval stages and then releases them as small crabs. Freshwater crabs tend to near streams or marshes but can stay on land for long periods of time especially after it rains.

Coconut crabs, or robber crabs, are land-living crustaceans that are so large they can embrace the trunk of palm tree between its outstretched legs. At the back of the main carapace there is an opening to an air chamber lined with moist skin through which the crab absorbs oxygen. It returns to the sea to lay its eggs but otherwise lives out its life on land.

Coconut crabs are the world’s largest terrestrial arthropods. Males can weigh up to five kilograms. Their large clawed legs can span 90 centimeters. They reportedly climb up palm trees and use their gigantic pincers to cut down the young coconuts. There is little evidence to support this claim. Even if they could removing the coconut husk and breaking open the nut is beyond the capability.

Coconut crabs are a kind of land hermit crabs that doesn’t wear shell. They are primarily found on land, living holes in the ground and feeding on food they can forage or scavenge. They are good climbers but are slow on land. Coconut crabs and other land crabs such as mangrove crabs do well on islands, at least party because there are no vertebrae predators to threaten them.

Coconut crabs spend their youth in the sea and move onto land when they mature. For a long time there was some debate as to whether the creatures laid their eggs on land or at sea. The dispute was settled in 2009 when a Japanese photographer took a picture of a female guarding her red-colored eggs in a cave in Okinawa.

Coconut crabs once lived throughout the western Pacific and eastern Indian Ocean. There numbers have sharply declined and now they are only found in a few places. The fact they are easy to catch and good to eat is the primary reason their numbers have shrunk. On some islands they are regarded as delicacies and are becoming increasingly popular with tourists. In some places they are kept as pets.

Image Source: National Oceanic and Atmospheric Administration (NOAA); Wikimedia Commons

Text Sources: Mostly National Geographic articles. Also the New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Natural History magazine, Discover magazine, Times of London, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated March 2011

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