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Crustacea are a large group of arthropods, comprising approximately 52,000 described species. They include various familiar animals, such as lobsters, crabs, shrimp, crayfish and barnacles. The majority are aquatic, thought a few groups have adapted to terrestrial life.





True shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Together with prawns, they are widely caught and farmed for human consumption.


Several types of shrimp are kept in home aquaria. Some are purely ornamental, while others are useful in controlling algae and removing debris.6 Freshwater shrimp commonly available for aquaria include the Japanese marsh shrimp (Caridina japonica, also called "Amano shrimp," as their use in aquaria was pioneered by Takashi Amano), and ghost or glass shrimp (Palaemonetes sp.). Popular saltwater shrimp include the cleaner shrimp Lysmata amboinensis, the fire shrimp (Lysmata debelius) and the Harlequin shrimp (Hymenocera picta).



Prawns are edible, shrimp-like crustaceans, belonging to the sub-order Dendrobranchiata . They are distinguished from the superficially similar shrimp by the gill structure which is branching in prawns (hence the name, dendro="tree"; branchia="gill"), but is lamellar in shrimp. The sister taxon to Dendrobranchiata is Pleocyemata, which contains all the true shrimp, crabs, lobsters, etc.



Crayfish are freshwater crustaceans resembling small lobsters, to which they are closely related. They are found in bodies of fresh water that do not freeze to the bottom, and which have shelter against predators. Most crayfish cannot tolerate polluted water.


Crayfish are sometimes kept as pets in freshwater aquaria. They prefer foods like shrimp pellets or various vegetables but will also eat tropical fish food, algae wafers, and even small fish that come too close, such as goldfish or minnows. Their disposition towards eating almost anything will also cause them to consume most aquarium plants in a fishtank; however, crayfish are fairly shy and may attempt to hide under leaves or rocks. When keeping a crayfish as a pet, remember to give it a hiding space. At night, some fish become less energetic and settle to the bottom. The crayfish might see it as a chance for an easy meal, or a threat, and injure or kill it with its claws. If you have a crayfish living with other fish species, you will not need to scoop out as many dead fish, as crayfish are effective scavengers. Crayfish are great escape artists and may try to climb out of the tank so any holes in the hood should be covered. In nations where imported alien crayfish are a danger to rivers, such as England, crayfish spread because specimens captured for aquariums in one river are often flung back into a different one.



Clawed lobsters comprise a family (Nephropidae, sometimes also Homaridae) of large marine crustaceans. Several different groups of crustaceans are known as lobsters, although the clawed lobsters (such as the genus Homarus) are most often associated with the name. Clawed lobsters are not closely related with spiny lobsters or slipper lobsters, which have no claws (chelae), or squat lobsters. The closest relatives of clawed lobsters are the reef lobster Enoplometopus and the three families of freshwater crayfish.


Lobsters live on rocky, sandy, or muddy bottoms from the shoreline to beyond the edge of the continental shelf. They generally live singly in crevices or in burrows under rocks.


Lobsters typically eat live food, consisting of fish, mollusks, other crustaceans, worms, and some plant life. Occasionally, they will scavenge if necessary, and may resort to cannibalism in captivity; however, this has not been observed in the wild. Lobster skin in the stomachs of lobsters has been found before, although this is because lobsters will eat their shed skin after molting. Lobsters grow throughout their lives and it is not unusual for a lobster to live for more than 100 years. They can thus reach impressive sizes. According to the Guinness World Records, the largest lobster was caught in Nova Scotia, Canada and weighed 20.14 kg (44.4 lb).


Being arthropods, lobsters are largely bilaterally symmetrical; clawed lobsters often possess unequal, specialized claws, like the king crab. A freshly caught lobster will have a claw that is full and fleshy, not atrophied. The anatomy of the lobster includes the cephalothorax which is the head fused with the thorax, both of which are covered by the carapace, of chitinous composition, and the abdomen. The lobster's head consists of antennae, antennules, mandibles, the first and second maxillae, and the first, second, and third maxillipeds. Because a lobster lives in a murky environment at the bottom of the ocean, its vision is poor and it mostly uses its antennae as sensors. Studies have shown that the lobster eye is formed with a reflective structure atop a convex retina. In contrast, most complex eyes use refractive ray concentrators (lenses) and a concave retina. The abdomen of the lobster includes swimmerets and its tail is composed of uropods and the telson.


In general, lobsters move slowly by walking on the bottom of the sea floor. However, when they are in danger and need to flee, they swim backwards quickly by curling and uncurling their abdomen. A speed of 5 meters per second has been recorded.



Crabs are found in all of the world's oceans; there are also many freshwater and terrestrial crabs, particularly in tropical regions. Crabs vary in size from the pea crab, only a few millimetres wide, to the Japanese spider crab, with a leg span of up to 4m.


The earliest unambiguous crab fossils date from the Jurassic, although the Carboniferous Imocaris, known only from its carapace is thought to be a primitive crab. The radiation of crabs in the Cretaceous and afterwards may be linked either to the break-up of Gondwana or to the concurrent radiation of bony fish, the main predators of crabs.


Horseshoe Crabs

Not decapods, or even crustaceans, but fit for inclusion nonetheless.


Horseshoe crabs possess five pairs of book gills located just behind their appendages that allow them to breathe underwater, and can also allow them to breathe on land for short periods of time, provided the gills remain moist. The outer shell of these animals consists of three parts. The carapace is the smooth frontmost part of the crab which contains the eyes, the walking legs, the chelicera (pincers), the mouth, the brain, and the heart. The abdomen is the middle portion where the gills are attached as well as the genital operculum. The last section is the "telson" (i.e., "tail" or "caudal spine") which is used to flip itself over if stuck upside down.


They can grow up to 20 inches (51 cm), on a diet of molluscs, annelid worms, and other benthic invertebrates. Its mouth is located in the middle of the underside of the cephalothorax. A pair of pincers (chelicerae) for seizing food are found on each side of the mouth.


Limulus has been extensively used in research into the physiology of vision. It has four compound eyes, and each ommatidium feeds into a single nerve fibre. Furthermore the nerves are large and relatively accessible. This made it possible for electrophysiologists to record the nervous response to light stimulation easily, and to observe visual phenomena like lateral inhibition working at the cellular level. More recently, behavioral experiments have investigated the functions of visual perception in Limulus. Habituation and classical conditioning to light stimuli have been demonstrated, as has the use of brightness and shape information by male Limuli when recognizing potential mates. It has also been said that it is able to see ultra-violet light.


Among other senses, they have a small sense organ on the triangular area formed by the exoskeleton beneath the body near the ventral eyes.


The mouth opening is between the legs, the gills are visible belowHorseshoe crabs can live for 20-25 years. They migrate into the shore in late spring, with the males arriving first. The females then arrive and make nests at a depth of 15-20 cm in the sand. In the nests, females deposit eggs which are subsequently fertilized by the male. Egg quantity is dependent on female body size and ranges from 15,000-64,000 eggs per female 6. "Development begins when the first egg cover splits and new membrane, secreted by the embryo, forms a transparent spherical capsule" (Sturtevant). The larvae form and then swim for about five to seven days. After swimming they settle, and begin the first molt. This occurs approximately twenty days after the formation of the egg capsule. As young horseshoe crabs grow, they move to deeper waters, where molting continues. They reach sexual maturity in approximately eleven years and may live another 10-14 years beyond that.



Branchiopoda is a group of primitive and primarily fresh water crustaceans, mostly resembling shrimp. There are over 900 known species worldwide.



Triops (notostracans) are small crustaceans in the class Branchiopoda. Triops have two internal compound eyes and one naupliar eye in-between, a flattened carapace covering its head and leg-bearing segments of the body. The order contains a single family, with only two extant genera. Their external morphology has apparently not changed since the Triassic appearance of Triops cancriformis around 220 million years ago. Triops cancriformis may therefore be the "oldest living animal species on earth".


These creatures can make a useful addition to a freshwater aquarium, as they keep the substrate very clean and eat almost anything they find. They may, however pose a slight danger to very small fish or other crustaceans, and may in turn be eaten by large fish. Also, they may damage the roots of aquarium plants they encounter if not kept well fed. They are also very short-lived pets, but are easily replaceable.


Fairy Shrimp

Fairy shrimp (Anostraca) are branchiopods that include brine shrimp. They often appear in vernal pools, pot holes and other ephemeral pools. Although they live in fresh or saltwater they do not live in oceans or seas. They are well-adapted to living in arid areas where water is present for only part of the year. Their eggs will survive drought for several years and hatch about 30 hours after rains fill the pools where they live. Some eggs may not hatch until going through several wet/dry cycles, ensuring the animals' survival through times that the pools don't last long enough for the shrimp to reproduce.



Daphnia are small, mostly planktonic, crustaceans, between 0.2 and 5 mm in length. Daphnia are members of the order Cladocera, and are one of the several small aquatic crustaceans commonly called water fleas because of their saltatory swimming style. They live in various aquatic environments ranging from acidic swamps to freshwater lakes, ponds, streams and rivers.


A few Daphnia prey on tiny crustaceans and rotifers, but most are filter feeders, ingesting mainly unicellular algae and various sorts of organic detritus including protists and bacteria. Daphnia can be kept easily on a diet of yeast. Daphnia provide an important source of food for many larger aquatic organisms including various fish species (e.g. lake trout) and the immature stages of many insects including the Odonata- dragonflies and dameselflies. They are also prey for hydras.


Daphnia are sold by aquatic retailers in both live and freeze-dried form as food for aquarium fish. Their tiny size renders them edible in live form even tiny fish. Daphnia may also be used to clear unwanted algae from fish tanks, provided they are not eaten by the fish. Daphnia is considered an indicator species or sentinel species which is an indicator of ecosystem health.



Isopods are one of the most diverse orders of crustaceans,living in every habitat from terrestrial to deep sea. They are an ancient group with fossils known from the Carboniferous


Gammarus Pulex (Scuds)

Gammarus pulex, sometimes called "freshwater shrimp", is a freshwater amphipod. The adult Gammarus pulex is typically around 11 mm long (though males can be up to 20 mm), with a curved, brown-yellow body.


They prefer flowing water, such as in streams, but they are also found in lakes and ponds, normally at the water's edge; typically residing under stones and between the bases of plant stems and roots, though they tend to feed on the upper reaches of the plants. They swim in bursts when they are caught in the current or disturbed. It has been found that they are pushed further by the current when it is slow-flowing than when it is fast flowing.


Giant Isopod

A giant isopod may be one of approximately nine species of large isopods (crustaceans related to the shrimps and crabs) in the genus Bathynomus. They are thought to be abundant in cold, deep waters of the Atlantic and Pacific Ocean. Bathynomus giganteus, the species upon which the generitype is based, is the largest known isopod and is the one most often referred to by the common name "giant isopod".



Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Many species are planktonic, but more are benthic.



Cyclops is one of the most common genera of freshwater copepods, comprising over 100 species. Cyclops individuals may range from ½–5 mm long and are clearly divided into two sections. The long first antennae are used by the males for gripping the females during mating. Afterwards, the female carries the eggs in two small sacs on her body.

It lives along the plant-covered banks of stagnant and slow-flowing bodies of water, where it feeds on small fragments of plant material, animals or carrion. Cyclops has the capacity to survive unsuitable conditions by forming a cloak of slime.




Canadapsis was a Cambrian crustaceaomorph. A benthic feeder that moved mainly by walking, it was possibly one of the first malacostracans.



Waptia fieldensis was a small, shrimp-like stem group crustacean. Many Cambrian crustaceomorphs such as Waptia lack the mouth parts to be classified as crown group crustaceans that lived during the Middle Cambrian about 510 million years ago.



Anomalocarids were super-predators, living in the Cambrian seas. While currently classed as separate from crustaceans, they bear a startling similarity to modern branchiopod crustaceans (especially anostracans). As such, it should be considered that they may be a form of proto-crustacean.



Trilobites flourished between the Cambrian and Permian periods. Classed as a distinct phyllum of arthropods, they bear some similarities to more primitive crustaceans, such as branchiopods.



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