This, almost entirely marine, subphylum is perhaps the insectan analogue for the oceans. Whereas insects have become vitally influential in terrestrial ecosystems, so the crustaceans have become in the oceans. Some have moved onto land (e.g., Isopoda), and others into freshwater (e.g., crayfish, some copepods). The group includes the familiar, living Crustacea as well as numerous extinct taxa that are considered to be stem groups to either constituent lineages of crustaceans or to the Crustacea as a whole. There are approximately 50,000 living crustacean species, which range in size from miniscule (less than a millimeter) to enormous (355 cm). Six extant classes are recognized in the Crustacea (Schram, 1986; Martin and Davis, 2001): Branchiopoda (water fleas, brine, tadpole shrimp), Remipedia (remipedes), Cephalocarida (cephalocarids), Malacostraca (crabs, lobsters, isopods, crayfish, shrimp), Ostracoda (seed shrimp, ostracods), and Maxillopoda (barnacles, branchiurans, pen-tastomids, copepods), but these are not all monophyletic (e.g., Schram and Hof, 1998). They are immediately recognizable for their five pairs of head appendages: one set of mandibles, two sets of maxillae, and two pairs of antennae. Most species belong to the Malacostraca (which includes the familiar amphipods, isopods, and crabs) and are benthic creatures.

Like the insects, their biology is incredibly diverse with species ranging from detritivorous to predatory to parasitic, and from solitary to social. A complete treatment of the diversity of both form and biology in the Crustacea is beyond the scope of this volume. Major accounts include Abele (1982), Schram (1983a, 1986), Gore and Heck (1986), Bauer and Martin (1991), Jones and Depledge (1997), Schram and Hof (1998), and Martin and Davis (2001).

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