Aquatic Mandibulates Chapter 19

Diversity 1. Over 67,000 living species and probably several times that number belong to the phylum Arthropoda, Subphylum Crustacea. 2. Together, insects and crustacea compose over 80% of all named animal species. 3. Probably the most abundant animals in the world are members of the copepod genus Calanus.

Calanus

Subphylum Crustacea The main distinguishing characteristic of crustaceans is that they have two pairs of antennae. 2. The head also has a pair of mandibles and two pairs of maxillae. 3. There is one pair of appendages on each of the additional segments; some segments may lack appendages. 4. All appendages, except perhaps the first antennae, are biramous (they have two main branches).

Classes Six classes of crustaceans are generally recognised: Branchiopoda — including brine shrimp (Artemia) and Triops (Notostraca) Remipedia — a small class restricted to deep caves connected to salt water, called anchialine caves Cephalocarida — horseshoe shrimp Maxillopoda — various groups, including barnacles and copepods. It contains Mystacocarida and Branchiura, which are sometimes treated as their own classes. Ostracoda — small animals with bivalve shells Malacostraca — the largest class, with the largest and most familiar animals, such as crabs, lobsters, shrimp, krill and woodlice.

Branchiopoda  The Branchiopoda consist of four living groups: the Anostraca ("fairy shrimp" or "sea monkeys"), Notostraca ("tadpole shrimp"), Cladocera ("water fleas"), and Conchostraca ("clam shrimp'). There are also two fossil groups that belong to the Branchiopoda, but it is unresolved whether they are subgroups within the living groups or evolved separately.  Here the fossil groups will be discussed within the living groups to which they are most similar. 

fairy shrimp Daphnia magna

Remipedia Remipedes, a rare group of marine crustacean first described in 19811, occur only in anchialine karst systems in Central America, the Caribbean, the Canary Islands, and western Australia. Anchialine caves are generally dolomite or limestone in composition and contain a marine layer that flows beneath a fresh or slightly brackish layer.

Dorsal view of a remipede from the Bahamas

Cephalocarida These tiny crustaceans make a living by feeding on benthic marine detritus. Only nine species are known. The second pair of maxillae of cephalocarids closely resembles the appendages of the thorax. In this and other characteristics cephalocarids resemble what we imagine primitive crustaceans might have been like.

Cephalocarids also lack abdominal appendages Cephalocarids also lack abdominal appendages. They have small compound eyes that are buried in the exoskeleton, rather than being raised on stalks as in most other crustaceans. As in branchiopods and malacostracans, cephalocarids feed by generating currents with their thoracic appendages. These currents bring in food particles, which are trapped and and passed anteriorally along a ventral groove leading to the mouthparts.

Maxillopoda Maxillopods are generally small animals with barnacles being the exception to this rule.  They commonly have shortened bodies, with a reduced abdomen that typically lacks appendages.  It has been suggested that the ancestor common to all maxillopods developed sexual maturity before it had completely metamorphosed into the adult form.  If this hypothesis is correct, it would explain the small size, shortened body plan and reduced number of appendages typically found in Maxillopoda. 

Barnacles

Ostracoda Ostracods consist of little more than a head.  They have the typical five pairs of appendages on their head but only 1-3 pairs of appendages on the rest of the body. Their bivalved carapace may cause you to mistake them for tiny clams or mussels, thus the common name of "mussel shrimp".  The Ostracoda are one of the most successful crustacean groups with approximately 8000 living species.  Ostracods are generally small, ranging in length from 0.1 to 32 mm (that's smaller than a poppy seed to the size of a meatball). 

Malacostraca  Malacostraca can be divided into two groups, the Phyllocarida, and the Eumalacostraca.  Phyllocarida contains the oldest crustacean known and includes only one living group.  They are also the only Malacostracans with phyllopodus (leaf-like) appendages. 

The Eumalacostraca consists of all Malacostracan groups other than the Phyllocarida. Eumalacostracans generally possess a well-developed carapace and a long, muscular abdomen. Eumalacostraca is the group that contains most of the animals the general public recognize as crustaceans, such as shrimp, crabs, lobsters. Choose from the cladogram below to begin exploring the Malacostraca.

Shrimp Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the sea bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important food source for larger animals from fish to whales. They have a high resistance to toxins in polluted areas, and may contribute to high toxin levels in their predators. Together with prawns, shrimp are widely caught and farmed for human consumption.

Crab Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail“ or where the reduced abdomen is entirely hidden under the thorax. They are generally covered with a thick exoskeleton, and armed with a single pair of chelae (claws). 6,793 species are known. Crabs are found in all of the world's oceans. Additionally, 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 4 m.

Lobster 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. A notable feature of a Lobster's blue blood is that it is made up of haemocyanin, found also in snails and spiders, which contains copper. This is unlike haemoglobin which contains iron. Inside lobsters is a green goopy substance called tomalley, which serves as the hepatopancreas, fulfilling the functions of both liver and pancreas.

Giant Lobster Caught in England

Crayfish http://www.mackers.com/crayfish/ http://www.enchantedlearning.com/subjects/invertebrates/crustacean/Crayfishprintout.shtml http://www.smithlifescience.com/CrayfishDissection.htm