Isopoda

Isopods apparently evolved in shallow marine environments by at least the early or mid-Paleozoic. The first isopods were "short-tailed" - with short pleotelsons and terminal styliform uropods. Phylogenetic analyses and the fossil record agree that the earliest isopods (and the most primitive living species) are members of the short-tailed suborder Phreatoicidea. Today, phreatoicids have a strictly freshwater Gondwanan distribution, with most species occurring in the rivers and lakes of Tasmania. The earliest fossil records of isopods are phreatoicids dating from the Pennsylvanian (the Carboniferous Period of the Paleozoic Era), 300 million year ago. However, Paleozoic phreatoicids were marine forms and they had a cosmopolitan distribution; their fossils have been found in marine deposits from Europe and North America. Thus, the present-day Gondwanan freshwater distribution of these primitive crustaceans represents a relic, or refugial biogeographic pattern.

In fact, virtually all of the short-tailed isopod taxa occupy what could be regarded as biogeographical refugia. Primitive Asellota are largely fresh-water or ground-water inhabitants. Higher Asellota live primarily in the deep-sea, an environment nearly uninhabited altogether by other isopod taxa. Microcerberidea inhabit coastal ground waters or are interstitial. Calabozoidea are so far known only from freshwater springs in Venezuela. And Oniscidea have escaped the aquatic world altogether and are the only fully terrestrial crustaceans. In all these cases, it seems that the primitive isopod lineages have found environments that allowed them to escape the challenges of predation by shallow-water marine fishes, their principal predators, which began their major radiation at about the same time the isopods were beginning theirs (in the middle Paleozoic). It may be that the evolution and radiation of the more advanced "long-tailed" isopods, which probably began in the late Triassic (Mesozoic), also helped drive the short-tailed forms into refugia by competition. Unlike short-tailed taxa, species of long-tailed isopods are highly mobile and frequently leave their benthic shelter to swim short distances through the water. Their elongate pleotelsons and broad, laterally-positioned uropods provide hydrodynamic planar surfaces to assist in swimming.

The profound shift from a relatively sedentary, short-tailed morphology to the active, long-tailed form appears to have coincided with the fragmentation of Pangaea. Since the majority of the long-tailed higher taxa are endemic to the Southern Hemisphere, this suggests that they originated on Gondwanan shores shortly after its separation from Laurasia.

Studies of isopod biogeography have been largely restricted to two suborders, the Valvifera and the Asellota. Among the suborder Valvifera, the family Idoteidae is thought to have co-evolved with large brown algae (Phaeophyta; Laminariales) in temperate latitudes (Brusca and Wallerstein 1979). Idoteids are primarily temperate in distribution and closely associated with the cold-water seaweeds upon which they feed and live. The southern range limits of temperate idoteid isopods may be controlled today primarily by biotic factors, such as absence of suitable substrate/food (e.g. laminarians) or predation pressure from tropical shallow-water fishes. Wallerstein and Brusca (1982) documented patterns of idoteid morphology and behavior indicating that when these isopods evolve in warmer waters they are smaller, their bodies are more spinose, their swimming behaviors differ from their temperate cousins, and they also reproduce at smaller sizes; all possible adaptations to avoid increased predation pressure in the tropics. A phylogenetic analysis of the Valvifera suggested that the suborder originated in the temperate Southern Hemisphere, at least by Permian/Triassic times (Brusca 1984); global distribution patterns of some genera can be ascribed to vicariance processes, others to dispersal, ecological phenomena, or a combination of processes.

Of the 10 isopod suborders, 4 have representatives in the deep sea. However, one of these 4 suborders, the Asellota is far and away the predominant deep-sea isopod taxon, and about 90 percent of all described isopods from this environment are asellotans. Wilson (1980) and Wilson and Hessler (1987) argued that the deep sea has been invaded numerous times by asellotans of several families. Here, the Asellota have radiated widely. In polar regions, various asellotan lineages have re-emerged into shallower waters.

Hessler and Wilson (1983) calculated that 32-51% of all benthic species taken in all deep-sea samples, anywhere in the world have been peracarids, and of the peracarids the isopods are by far the most abundant and speciose deep-sea group. Research has shown that most deep-sea genera are cosmopolitan, whereas deep-sea species may be either widespread or highly restricted in their distribution.