Crustacea Masters of Marine Arthropod Diversity

There are approximately 52,000 described species of crustaceans, and many of those have successfully colonized freshwater habitats (for example, crayfish, conchostracans) and terrestrial habitats (for example, land crabs, pillbugs). But the majority of the species occur today in the same marine environment as

Lobster (Decapoda) Parribacus perlatus (Courtesy Christopher B. Boyko)

their ancestors in the Cambrian Era. Unlike the case of insects or other invertebrate phyla, it is difficult to speak of a "typical" crustacean form, or bauplan. All crustaceans possess two pairs of antennae, one pair of mandibles, and two pairs of maxillae on a five-segmented head-shield (cephalon), but two or more of these segments are typically fused, depending on the crustacean group. Likewise, the body segments tend toward fusion and a corresponding reduction in appendages. This variation has resulted in a wide diversity of basic forms within the Crustacea, perhaps more than in any other phylum. Many crustaceans possess forms that, although distinctly unalike, are immediately recognizable and familiar: this includes the crabs, shrimp, and lobsters (Decapoda, with more than 10,000 species, 90 percent marine); bivalved ostracods (Ostra-coda, 5,500 species, mostly marine); cope-pods (Copepoda, with more than 8,000 species, mostly marine); and barnacles (Cirripedia, with more than 1,000 species, all marine). The importance of these groups in the marine food web, and as direct or indirect food sources

Crab (Decapoda) Ovalipes sp. (Courtesy Christopher B. Boyko)

for humans, is well documented. However, many other crustacean groups are equally numerous, either in numbers of individuals or species. An example of the former is the pelagic euphausids (Euphausiacea) with only 85 species, all marine, but in population size large enough to form a major food resource for whales.

Members of the Peracarida exemplify taxa with low numbers of individuals (many species are still known from only a single specimen) but with a high diversity of species. Peracarids include isopods (Isopoda, with more than 4,000 species, about two-thirds marine); amphipods (Amphipoda, more than 6,000 species, mostly marine); cumaceans (Cumacea, more than 800 species, all marine); and tanaids (Tanaidacea, 500 species, all marine). All per-acarid groups occur worldwide and at various depths, but they are perhaps most important in that they make up a substantial portion of the deep-sea benthos and may well play important roles in deep-sea ecological processes about which we currently know very little.

The diversity of form that runs through the Crustacea is also likely to have impacted the ability of these animals to exploit almost every type of habitat. Marine Crustacea occur in all oceans and at all latitudes and depths. They also occur in large numbers at some of the most inhospitable regions of the planet, such as at black smoker vents and hydrothermal seeps at abyssal depths. Most marine crustaceans are free-living and tend to be predators, scavengers, or detritivores (for example, shrimp, crabs, cirolanid isopods) or filter feeders (for example, nonrhizocephalan barnacles, porcelain crabs). However, both the Copepoda and Isopoda have numerous parasitic species on a variety of other animals. The amphipods, with the Cyamidae (whale-lice), and the barnacles, with the Rhizocephala, each have one lineage that is exclusively parasitic. Only the Branchiura, or fish-lice, are wholly parasitic and found only on coldblooded vertebrates; this group is found mostly in freshwater, but there are a few marine species.

Some crustaceans have evolved mechanisms that allow them to colonize both land and freshwater environments (for example, pillbugs, land crabs), and these are considered the most derived taxa within their respective groups. This, coupled with the example of the insects, suggests that the evolution of ter-restriality is one-way and that once fully adapted to land, a given lineage cannot return to the sea. However, some Crustacea are more tied to the land than others. Some species, such as all the so-called land crabs (Gecarcinidae), must return to the ocean to release their progeny, as their larval stages require development in the marine environment and thus tie these species to the sea.

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