(To complete all classifications ETI has added the Kingdom and the Phyla of all the different taxa treated on this DVD-ROM without higher classification descriptions. Texts from Lynn Margulis and Karlene V. Schwartz, Five Kingdoms. CD-ROM. Copyright 2002 ETI / Freeman & Co Publishers).
[Editor's note: Crustacea are also—and widely— considered as a Subphylum of the Phylum Arthropoda].
The defining feature of Phylum Crustacea is the possession of two pairs of antennae on the head, distinguishing crustaceans from all other arthropods (Phyla Chelicerata and Mandibulata). Many familiar animals belong to the Crustacea phylum—among them are crabs, lobsters, brine shrimp, and krill. The two pairs of jointed antennae of the crayfish Cambarus are typical crustacean features; the presence of antennae distinguish crustaceans from chelicerates (Phylum Chelicerata). Biramous (branched) appendages distinguish crustaceans from insects and other members of Phylum Mandibulata with their unbranched appendages. Biramous appendages have two branches to each appendage (limb); the two branches connect to a single base, and each of the two branches consists of several jointed segments. The crustacean exoskeleton may be as thick as a lobster’s or as thin and flexible as the outer covering of a soft-shelled crab larva. Crustaceans—along with chelicerates (Phylum Chelicerata) and mandibulates (Phylum Mandibulata)—are still considered by some to be part of Phylum Arthropoda. But consistent distinguishing features of these three groups imply that they are three independent evolutionary lineages, and we agree with zoologists who raise each group to phylum status. We agree to place pentastomes—formerly Phylum Pentastoma—as Class Pentastomida within Phylum Crustacea. Pentastomes—highly modified parasites—are regarded by specialists to possibly be crustaceans, similar to branchiurans (marine crustaceans that parasitize fish skin). Crustacea number about 45,000 species.
As protostome coelomates, the crustacean body cavity is a coelom and, during embryological development of protostomes, the mouth originates at or near the blastopore, the opening from the outside of the blastula to its interior. The crustacean body plan resembles that of other arthropods in some respects (nervous system, for example), but most crustaceans have three body sections—head, thorax, and abdomen—as do insects; in comparison, chelicerates have two body sections (fused cephalothorax and abdomen). In some crustaceans, the head is fused with various thoracic segments to form a cephalothorax. The crustacean coelom is small and the main body cavity—a hemocoel, or cavity in which blood circulates—is part of the circulatory system, as in other arthropods. The crustacean mode of gas exchange, however, differs from that in other arthropods; gills—evident in crabs and lobsters—exchange gases with the water or air. A tube-shaped heart with ostia is the circulating pump for blood. In lobsters, one of the mouthparts is modified as a gill bailer, which the animal uses to generate a current of water through its gill chamber, enhancing gas exchange through the gills. Crustaceans that are semiterrestrial (for example, the hermit crab Clibanarius) inhabit the intertidal zone of the sea beach, a habitat that provides at least occasional moisture to their respiratory surfaces. Some crustaceans have blood vessels; others have no vessels and pump blood only to the hemocoel. A dorsal brain, ventral nerve cords with ganglia in each body segment, and sensory organs such as the compound eyes of Daphnia and statocysts constitute the nervous system. Krill chemically produce light in luminescent organs on their legs and eyestalks; their light displays may function in mating or in protecting them from predators, such as penguins, that are below them in the sea. All crustacean muscle is striated, typical of arthropods, and quicker to contract and relax than the smooth muscle that predominates in other invertebrates. The one-way digestive tract includes a mouth and stomach in which food is ground extensively; food then passes to a midgut sometimes associated with organs that facilitate digestion, such as the “liver” of a lobster. Crustaceans may be dioecious, often with sexual dimorphism that ranges from a difference in size between females and males to sexual dimorphism of appendages (males may have appendages used in mating) to the extreme sexual dimorphism of some isopods. Barnacles are hermaphrodites; the long penis of one barnacle fertilizes another nearby but permanently attached barnacle. A three-segmented, free-living larva called a nauplius is a characteristic crustacean larva. Some barnacles brood embryos to the nauplius larval stage. Some highly specialized crustacea are so bizarre in form that they can be classified only upon observation of their nauplius larvae. Diverse larvae having forms as exotic as their names—zoea, megalops, phyllosoma, mysis—are present in decapod custaceans, an order of Class Malacostraca that includes crabs, some shrimp, hermit crabs, lobsters, and crayfish. Crustacean larvae are abundant in the ocean’s plankton. Other species in Class Malacostraca lack a larval stage; young hatch as juveniles. Some freshwater copepods and members of Class Branchiopoda are parthenogenetically reproducing females. Dissolved nitrogenous wastes diffuse from the crustacean body through the gills. Freshwater crustaceans’ green glands or antennal glands excrete water and resorb salts by way of nephridial canals in the anterior of the animal, collecting the resulting liquid in a bladder that discharges through an excretory pore near the antenna base. Chelicerate and crustacean excretory organs are similar; from the hemocoel, fluid is collected and eventually excreted as urine.
Crustaceans have evolved an enormous array of appendage diversity: gills, food collection, food shredding, defense, courtship display, walking, swimming, generating respiratory currents, brooding eggs, and clinging to seaweed. The crustacean head [two pairs of antennae, one pair of mandibles (food crushers), two pairs of maxillae (food handlers and current generators)], thorax (eight pairs), and abdomen (six pairs) all bear appendages. Females of one isopod species carry the male of the species on their antennae, the anteriormost appendages.
The covering of crustaceans provides a rigid skeleton that functions in locomotion, support, and protection; in general, the chitin-protein covering is similar in all arthropods, though less rigid in many species. Calcium carbonate is deposited in crustacean cuticle, strengthening the cuticle; insect cuticle lacks this calcification. Joints between sections of this armor are thinner cuticle, affording flexibility. The outermost layer of the cuticle is protein and often water-impermeable wax, rendering the soft body within somewhat resistant to water loss. At the same time, gas exchange across the body surface is severely reduced; oxygen and carbon dioxide diffuse from gills, which have great surface area and blood supply. Beneath the waxy layer lies a blue, red, yellow, or black pigmented chitin and protein layer underlaid by a calcified layer that hardens the cuticle. The epidermis, which secretes the cuticle, is the innermost layer. The crustacean must shed its rigid cuticle to accommodate growth. Enzymes partly break down the old exoskeleton, and new cuticle is secreted before the old cuticle splits. During ecdysis (escape) the crustacean not only slips out of its exoskeleton, but also jettisons the chitinous, internal linings of its fore- and hindgut.
Phylum Crustacea includes the major classes (largest and most diverse of the classes in number of species) Branchiopoda, Ostracoda, Copepoda, Cirripedia, Malacostraca, and Pentastomida (Linguatulida), as well as other classes including rare species or those in restricted habitats such as Remipedia, which inhabits underground saltwater caves in the tropics. In freshwater, including vernal pools, and saline habitats, crustaceans are the most prominant arthropods. Brine shrimp (Artemia), water fleas (Daphnia), fairy shrimp, tadpole shrimp, and others in Class Branchiopoda mainly inhabit freshwater, although the brine shrimp withstand highly saline water. Class Branchiopoda also includes commensals (epibionts) that live on other crustaceans, herbivores, detritus feeders, and filter feeders. Ostracods (Gigantocypris) live mostly in marine and estuary habitats; a few species are abyssal or freshwater. Copepods (Calanus) are enormously abundant in diverse habitats: marine (from ocean surface to 5000 m deep), freshwater, estuarine, interstitial (between sand grains, belonging to the meiofauna); some species are parasitic or commensal (symbiotrophic). Many commensal copepods are color matched to their hosts—crinoid echinoderms. As the base of worldwide food webs that include commercially valuable fishes, copepods affect our own species. Isaacsicalanus is a newly described copepod from a hydrothermal vent. Class Cirripedia comprises all barnacles, such as Balanus. As adults, cirripeds cement themselves permanently to firm marine substrates. Research on this biomaterial reveals a strong new adhesive protein, promising for cementing artificial spare parts, such as artificial hip replacements, into humans. Coronula diadema, a huge planktonic barnacle, settles as an epibiont on Megoptera, the humpback whale. Many barnacles are sessile as adults; their calcium carbonate shelter houses the chitin-covered crustacean within, complete with biramous appendages.
Class Malacostraca comprises more than half of the 45,000 crustacean species: amphipods (sand fleas), isopods (sow bugs, pill bugs, woodlice), and decapods [lobsters (Homarus), crayfish, crabs (Cancer,) some shrimp, hermit crabs (Pagurus)]. Isopods dwell in caves, in all damp soil habitats, under logs and rocks, in the depths of the ocean, as parasites of marine animals including whales, molluscs, fish, and other invertebrates, and as commensals with echinoderms, polychaetes, and bivalve molluscs. The krill (Euphausia) is one of the most ecologically critical malacostracan species; krill is the primary food for many marine fish species, sea birds, seals, filter-feeding whales, and other crustaceans worldwide. Exploitation of Euphausia superba, the krill of Antarctic seas, by humans for food may directly reduce the base of marine food webs that are critical to life on this planet.
Crustaceans may be derived from annelid-like ancestors; the annelid–arthropod connection is currently in a state of flux. All appendages of most primitive arthropods are assumed to be similar with serial repetition of similar segments, resembling the bodies of extinct trilobites and of recently discovered remiped crustaceans. Some believe that pentastomes are descended from early crustacean arthropods (Phylum Crustacea), because both pentastomes and arthropods undergo extensive larval development and molt their cuticles at certain stages in their life cycles; molecular sequence data support this view. Affinities of crustaceans with the two other arthropod phyla Mandibulata (Phylum Mandibulata) and Chelicerata (Phylum Chelicerata) is uncertain. Crustaceans may have an evolutionary origin independent from insects, centipedes, millipedes, and other terrestrial arthropods.
The authors of the chapter Larval Decapoda (Brachyura) (G. Pohle, L.M. Mantelatto, M.L. Negreiros-Fransozo, A. Fransozo) consider the Crustacea as a Superclass and adopted the following general classification for this taxon:
Superclass Crustacea Pennant, 1777
Class Malacostraca Latreille, 1806
Subclass Eumalacostraca Grobben, 1892
Superorder Eucarida Calman, 1904
Order Decapoda Latreille, 1803
Suborder Pleocyemata Burkenroad, 1963
Infraorder Brachyura Latreille, 1803