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.

Websites and Resources: National Oceanic and Atmospheric Administration noaa.gov/ocean ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Ocean World oceanworld.tamu.edu ; Woods Hole Oceanographic Institute whoi.edu ; Cousteau Society cousteau.org ; Montery Bay Aquarium montereybayaquarium.org

Websites and Resources on Fish and Marine Life: MarineBio marinebio.org/oceans/creatures ; Census of Marine Life coml.org/image-gallery ; Marine Life Images marinelifeimages.com/photostore/index ; Marine Species Gallery scuba-equipment-usa.com/marine

Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) coris.noaa.gov ; International Coral Reef Initiative icriforum.org ; Wikipedia article Wikipedia ; Coral Reef Alliance coral.org ; Global Coral reef Alliance globalcoral.org ; Coral Reef Pictures squidoo.com/coral-reef-pictures ; The Global Coral Reef Monitoring Network; the International Coral Reef Action Network.

Book: “The Secret Life of Lobsters” by Trevor Corson (HarperCollins 2004)

Crustacean Characteristics

Anatomy of a shrimp
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.

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.

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.


20120519-spiny lobster TucacasBajoMeroPanulirusArgus.JPG
spiny lobster
There are two main kinds of lobsters: 1) American lobsters, with large claws; and 2) spiny lobsters without big claws. Spiny lobsters are sometimes called rock lobsters or crayfish. They are found in tropical oceans all over the world.

Lobsters go through larval stage. Superlobster is a term sometimes used to describe thumb-nail-size larvae just before they advance to the adult stage. The look like lobsters but can swim like fish. It is the only time in their lives that lobsters can swim forward. Their primary goal is to find a crevice to hide in. After that goal is achieved the lobster spends the next several years hiding there.

The largest lobster on record is an American lobster trapped off Nova Scotia in 1977. It weighed 44 pounds and 6 ounces and was 3½ feet long. These days it is unusual to catch one that weighs more than three pounds. Occasionally you get bright blue American lobsters due to a genetic defect.

Lobster Features

Lobsters can walk slowly forward but move more quickly backwards with jet propulsion and quick movements of its tail. At night lobster search for food while fish that are active during the day sleep in some of the caves occupied by lobsters.

Lobsters have compound eyes that move about on stalks and long antennae that sense water movements. The antennae, claws and body are covered by thousands of sensory hairs that are used to find food and detect predators. Using high speed video, dyes and lasers, scientists discovered that hairs trap “odor plumes,” and by moving their antennae lobsters collect a wide variety of data based on the “odor plumes.” The Navy is trying the technique as a possible way to detect mines and explosives in the sea.

Lobsters periodically shed their shells to make way for their growing bodies. It can take several weeks to grow a new shell and this is when the lobsters are most vulnerable. If a lobster loses a limb. A new one grows back. Molting of lobsters can be an arduous task. The lobster exoskeleton includes teeth in the stomach to grind food, This is ripped out before the shell come free. Sometimes lobsters can not successfully shed their shells and die.

A Norwegian study found that lobsters most likely do not feel pian when they are dropped in hot water. Other studies have turned up no evidence of anything resembling pain receptors. The lobster nervous system is about as complex as that of a fruit fly.

Lobster Conga Lines and Other Lobster Behavior

Durer drawing of a lobster
Spiny lobster spend most of their time in caves or crevasses in the reef, with only their antennae protruding. Often characterized as being solitary loners, they are actually quite social, preferring to hang out in caves with others. They fight a lot though and often vent their anger by peeing at their rivals using a bladder that is on their head.

Spiny lobsters produce a rasping noise when threatened by conger eel by rubbing their hard antennae along a toothed spike that projects from their head between their eyes. All lobsters who hear the noise take cover in their caves. In colder, deeper water the body temperatures of lobsters drops. This helps them conserve energy and lower their food needs at a time of the year when food supplies are low.

Spiny lobsters in coral reefs off Florida and the Bahamas form massive conga lines with up to 50 individuals and migrate to relatively warm water when the first autumn storms stir up the water. The lobsters march across the sandy sea floor in a single file, head to tail, head to tail, and so on, heading towards deep water where they are safe from churning water of the storm. They maintain contact through touch with their antennae or sight of the lobster in front of them. Forming lines helps them from getting knocked around, reduces drag and provide protection from predators. Those that get left behind are munched on by parrot fish or triggerfish.

Lobster Reproduction

ancient lobster claw rhyton
Females produce several thousand eggs. After spawning, the female secretes a glue and fastens the eggs to her swimmerets (fringed paddles that hang from abdomen) and carry them for up to 12 months. When the eggs hatch larvae look nothing like adults. The larvae molt three times in 15 an 18 days and begin to look like adults. When they are one year old they have molted 14 to 17 times and are two to three inches long. They reach adulthood around the age of five.

American lobster females are thought to mate just once if that in their lifetimes. It is not known how long it takes them to reach sexual maturity, estimates range from five to nine years.

Describing lobster sex Trevor Corson, author of a book about lobsters, told U.S. News and World Report, “The female sprays urine into the male’s apartment, basically drugging him into submission. Then she move in with him and get PMS — premolting syndrome. She gets irritable, shoves a lot of gravel around the place. He is understanding and tender; he waits until she molts, until her legs can stand, then he turns her on her back and mounts her. The female has a seminal receptacle, a kind of fanny pac. the male guides his swimmerets own into the pouch. Then he rolls some sperm packets into a plug for her seminal receptacle so no other males can get there.”

There is a lot about lobsters that remains a mystery. For example, there have been record harvests of American lobster in the Gulf of Maine but empty traps in further southern around Cape Cod. No one knows why. Some think it is related to overfishing of fish such as cod, haddock and hake that feed on lobsters.


Antarctic krill
Krill are shrimp-like crustaceans that form huge masses in the opens sea. They feed while swimming, trapping food particles in hairlike structures, and are the primary food sources for a number of marine animals including baleen whales, dolphins, penguins and other seabirds. Krill have the unusual distinction of being one of the few creature that can reverse molt into smaller shells when food is scarce.

According to some studies krill numbers in the southern seas and Antarctica in 2004 were a fifth of what they were in 1975. This may be a direct effect of global warming. Declines of seabirds such as murres and auklets has been linked to declines in krill populations. Tiny Cassin’s auklets, a relative of puffins, have traditionally fed their young krill after they hatched, but in recent years the krill have shown up late, causing the young birds to starve. The late appearance of the krill is blamed on climate change changes that have caused weak winds that in turn prevent upwelling of nutrient-rich waters from deep in the ocean, depriving the krill of food. The absence of krill has also led to a collapse of rockfish population which in turn have led to a decline in populations of murres that feed on them.

Krill is rich in omega 3 fatty acids. Norwegian and Canadian companies are already marketing krill health pills. The crustaceans are also harvested for special enzymes that can be use by surgeons to clean wounds, and also to clean contact lenses.

There is some discussion of major human harvesting of krill. As it stand now only a about 100,000 tons is taken each year but some predict that figure could rise to a several million tons in the not to distant future, especially as demand for protein sources increases. Already there are concerns about the overfishing of krill. According to some studies krill numbers in Antarctica in 2004 were on fifth what they were in 1975.

China has sent ships to the Antarctic the harvest swarms of krill as protein source for the fish farming industry. Environmentalist are worried because they are targeting shoreline areas where seals and penguins rely on krill as one their primary food sources.

Chinese Going for the Krill in the Southern Ocean

northern krill
Conservationists are warning of a potential disaster in Antarctica as China seeks to exploit the world’s last untapped ocean. Chinese trawlers are preparing to sail for the Southern Ocean to harvest krill, the shrimp-like creatures on which the continent’s colonies of penguins and seals survive. [Source: Frank Pope, Times of London July 18 2010]

Two ships were sent this year to fish the swarms of krill that converge off the coast of Antarctica, which also sustains the blue whales that cruise in deeper waters. China is planning a new expedition as part of a 5-year exploration program to investigate the potential of krill to provide protein and omega-3 oils to fuel its booming fish-farming industry.

Depletion of the food could have disastrous consequences. Marine scientists complain that overfishing had consumed some 95 percent of large fish in many of the world’s seas and pushed some species to the brink of collapse. Although the Chinese krill catches have so far been small — at about 115,000 tonnes a year from a stock that scientists estimate could support a harvest of 3.5 million tonnes — they disguise the wide dispersal of the krill.

Gerry Leape of the Pew Environment Trust says: “The problem is one of prey depletion for land based krill predators. As they are forced to move farther offshore to feed, this could impact on their reproductive success.” The inefficiency in farming and food and the risks to the environment are exhibited once more.


Shrimp are small crustaceans. They have long feelers, a double fishlike tail, and ten paddle-likelegs for walking on the bottom. Its other appendages are used for swimming. A true shrimp has two appendages on the side of its head. The prawn has a single sharp beak. Prawns are usually larger than shrimp.

Shrimps are found in a variety of habitats: reefs, mangroves, coastal areas. Those served at restaurants usually live on the ocean floor and are caught in coastal areas with dredging nets or raised in shrimp farms.

Shrimp are caught in sandy and slightly muddy water where the salinity favors the growth of plankton. Shrimp fishing stops when shrimps lay their eggs and the larvae hatch. The larvae migrate to an area in the sea where the salinity is low. The main shrimp season begins when the shrimp swim quickly from low to high saline water.

Kinds of Shrimp

Many of the shrimp consumed by humans and fish are opossum shrimp. Found mostly in esturine or marine waters, they are free swimmers with long, soft elongated bodies and distinctive movement sensors at the base of an inner pair of flaplike appendages on either side of their tail fan. Many are pale or translucent. Some are red.

Cleaner shrimp pick off parasites, fungus and pests from fish. Some fish pull up to sections of reef inhabited with cleaner shrimp like cars pulling up to a car wash and wait on line for their turn to be cleaned. The shrimp even climb into the mouth's of moral eels to clean their teeth. They also provide free medical service by cleaning parasites which congregate around the fish's open wounds and drive away small predators that feed on the fishes eggs.

There are all kinds of shrimp out there. Pistol shrimp produce a loud cracking noise by dislocating their claws. King shrimp are often found in places where dead sea plants, mud and fine sand accumulate. Caribbean peppermint shrimp are born male and become female as they mature. "Females" with sperm-producing organs and ducts that can be used to fertilize other "females." Spot prawns have a similar life cycle. The larger the prawn the more eggs she can carry.

A number of shrimp have symbiotic relations with other marine life. Commensal shrimps live among the tube-feet on starfish's arm, grazing on dead skin cells, mucus and other detritus. Sea urchins that look like they trapped inside a small localized blizzard are in fact surrounded by hundreds of tiny shrimp in the process of laying their eggs. Some of these shrimp have kangaroo-like pouches. Shrimpfish are small fish that search for minuscule shrimp and inhale them in their tubelike mouths.

Mantis Shrimp and Snapping Shrimp

Mantis shrimp are among the most colorful shrimp. They come in a variety of specular psychedelic colors. One is aptly named the peacock shrimp. They also have very acute sight when it come to color.

Mantis shrimp possess the most complex eyes in nature. They rotate separately 180 degrees like a chameleon and provide superb spatial perception. They also provide extraordinary “trinocular vision" using visual receptors in three distinct bands and 16 different kinds of light-sensing retinal cells, including four for ultraviolet lights and polarized light. In contrast humans have only four kinds of retinal cell types and can not see ultraviolet or polarized light. In addition, mantis shrimp are believed to have eight cone types in their eyes for detecting color will most fish have four.

Mantis shrimp eyes process much of the information they receive before it even reaches the brain, reducing the work load on the brain. Each tiny dome on the surface of the shrimp’s eye is a separate cornea that admits light. The cornea connects to a crystalline cone and photoreceptors called a rhabdon. The most specialized light processing occurs in the middle photoreceptors. These give detailed readings from available light.

Mantis shrimp have specialized claws that fold up like knife blades. Lethal to marine life and dangerous to humans, they are strong enough to break glass and flip out in fraction of a second to spear or smash fish, crabs or rival mantis shrimp. One biologist told National Geographic, "I had a letter from a South African surgeon who picked one up while diving. His finger was so badly mangled that they had to amputated."

Snapping shrimp produce a noise that is so loud that submarines use the noise to hide from sonar. The shrimp make the noise through "cativation," which is normally produced by the turbulence caused by objects moving extremely quickly through water. Snapping shrimp have a relatively giant claw that snaps, producing a stream of water that moves at 70mph. The pressure of the water causes tiny bubbles to expand. Within a microsecond the pressure is equalized and the bubbles compress, producing a loud sound and a shock wave, powerful enough to stun prey.

Snapping shrimp are heard much than they are observed. They often live inside sponges and are the only known marine species that live in colonies that resemble the colonies of bees and wasps. The colony often consists of a two parents and a whole bunch of grown male children.

Shrimp Fishing and the Destruction it Causes

20120519-Trawl_catch_of_myctophids_and_glass_shrimp the_bottom_200_meters_depth.jpg
Trawl catch of myctophids and glass
shrimp at 200 meters bottom depth
Shrimp is a multi-billion global business and increasing becoming one in which the developing world is feeding the developed world. In 2001, shrimp overtook tuna as the No. 1 seafood in the United States. Much of the world’s shrimp comes from places like Thailand, China, India, Latin America and Southeast Asia.

Wild shrimp and prawns are often caught with bottom trawls, weighted nets that are dragged along the ocean floor. Environmentalist condemn the practice because it damages the sea floor, tearing it up like a bulldozer. Studies show that sea life declines in places where shrimp and prawn trawlers operate. Environmentalist are pushing for the use of shrimp traps which sit on the ocean floor and do harm other marine life.

Prawn and shrimp fishing operations are responsible for one third of the world’s discarded catch. In some cases 10 pounds of bycatch is thrown out for every one pound of shrimp caught. Some studies have shown that shrimp make up only five percent of the material pulled up by drag nets. Dead fish caught by shrimp fishermen is thrown overboard.

Shrimp fishing is particularly dangerous to sea horses and turtles. Trawling kills an estimated 150,000 sea turtles a year. See turtles

Shrimp Farms

20120515-shrimp farm Construction_Pekalongan.jpg
shrimp farm construction
One quarter of all the shrimp produced is raised on shrimp farms. Most are raised in giant, man-made rectangular shrimp farming ponds and pens that are filled with coastal sea water directed and controlled by dikes. The shrimp are fed shrimp feed that is produced on industrial levels. The shrimp are harvested about twice a year

Until recently, exporting shrimp and prawns was lucrative businesses. Black tiger shrimp are often the preferred species. But may are raising white shrimp, which cost less to produce and are immune to certain diseases.

Thailand, Ecuador and the Philippines were shrimp farming pioneers. Now shrimp farming is a major industry in Brazil, China, India, Central America and throughout Southeast Asia.

Indoor shrimp ponds have been developed with temperature and water-quality controls. These are seen as the future because the shrimp can been harvested five times a year and are shielded better from viruses and many environmental problems are reduced. Production is almost more consistent and predictable.

Shrimp farming is being threatened by oversupply. Farmers are sometimes unable to sell all their catch. So many shrimp are raised the price has collapsed. Farmers are working much hard to raise fewer shrimp. On top of that viruses devastated farmed stocks in Ecuador and China in the mid 1990s and Thailand in the early 2000s while farmers have been hit with tighter restrictions on the use of antibiotics.

Environmental Costs of Shrimp Farming

20120515-ShrimpFarming_Honduras_L7_1987-99 1.jpg
Coastal area of Honduras in 1987
There are a number of environmental problems associated with shrimp farming. Shrimps ponds often have no lining so salt water percolates through sandy soil, contaminating fresh water ground water supplies and aquifers. Waste water from the farms are fed into canals that empty into rivers used for drinking water and into the sea. Diseases caused by overcrowding at the farms are often treated with chloramphenicol — a powerful antibiotic with no known safe level of human consumption. In some places, particularly in Thailand, shrimp farms generate so much pollution that the farms are abandoned and the land is unable to produce anything else.

Many mangrove swamps have been destroyed to make away for shrimp farming ponds, degrading places where many young fish live. This, consequently, has harmed fisherman by reducing the number of fish they catch. Huge, export-oriented prawn and fish hatcheries have destroyed some local fishing operations.

Coastal area of Honduras in 1999
after shrimp farming
Forests have been cleared and wetlands and agriculture land have been appropriated for shrimp farms. One environmentalist told the New York Times, “This is basically a cut-and-kill system. They buy up the land, create dikes, use chemicals, and kill everything off. Then when they done, they leave and move up and down the coast, looking for more land.”

World Heritage sites in Bangladesh and the Philippines have been cleared to make way for shrimp farming ponds. A report by the London-based Environmental Justice Foundation (EJF) called the pollution and deforestion problems related to shrimp farming to be a “shocking environmental crisis.” Acknowledging the economic importance of the shrimp industry, many environmentalists are pushing for environmentally-friendly versions of shrimp farming rather than categorically condemning the practice.

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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