Antarctic and Sub-Antarctic isopod crustaceans (Peracarida, Malacostraca)

Isopod crustaceans (whose representatives on land are known as woodlice, pill bugs or slaters) are an order of the crustacean supraorder Peracarida. The latter represents a heterogeneous and highly diverse group, comprising more than a third (ˊ21,000 species) of total described crustaceans (Martin & Davis 2001). Isopoda have been proved a popular and useful taxon to assess large-scale patterns in biodiversity and biogeography (e.g. Hessler & Wilson 1983, Brandt 1992, Svavarsson et al. 1993, Wilson 1998) for several reasons. Their dispersal potential is limited to adult movement (such as passive and to a lesser extent active migration). This is because, like all peracarids, isopods brood their offspring in a ventral brooding pouch (marsupium), which is formed by extensions of their thoracic limbs. Thus, isopod distributions have the potential to reflect older centres of origin and radiation and so be valuable for biogeographic analyses. Apart from amphipod crustaceans, Isopoda are the most diverse peracarid order occurring across all major realms including terrestrial, freshwater and marine habitats. Many isopod species inhabit groundwater and caves, and some have even been recorded from deserts. In marine habitats isopods are a rich and ubiquitous element and have been described across all oceans and depths (intertidal to hadal; Schotte et al. 2009 onwards). Of the 10,300 isopod species described to date, more than half occur in the sea (Bruce 2001, G.C.B. Poore pers. comm.). Isopoda show a remarkable morphological diversity (relative to other peracarids). The earliest marine fossil record (a phreatoicidean) dates back to the Carboniferous (ˊ325 Ma, Schram 1970); few more ancient isopod taxa may have even occurred in the Devonian (416 — 359.2 Ma, Schram 1974). For the Asellota, a group particularly diverse in the deep sea, it has been suggested that they have they evolved between the Carboniferous (359.2 ± 2.5 Ma) and Triassic (250–200 Ma), while the Cymothoidea, Limnoridea and Sphaeromatidea (formerly Flabellifera) are more derived and probably experienced a major radiation during the late Mesozoic (145.5–65.5 Ma, Wilson 1998 and citations therein). Most isopod species have become well adapted to a benthic lifestyle and only few taxa have secondarily regained the ability to swim (e.g. family Munnospidae; Hessler & Strömberg 1989). The great variety of morphologies and functional traits is, for example, reflected by a wide range of feeding strategies — from detritus- and filter-feeders to ecto-parasites and active predators. A highly specialised mating system (particularly in asellotes) has probably been key to explain the ability of isopods to have colonised and thrived even in ‘extreme’ habitats such as the deep sea and polar regions (cf. Wilson 1991). In the Southern Ocean, the ecological and evolutionary success of isopod crustaceans has been suggested to be partly due to the Cenozoic extinction of brachyuran decapods, and the subsequent occupation of their niches by isopods and some other peracarid orders (e.g. Brandt 1992). Peracarids have some physiological and morphological adaptations to the highly seasonal and cold Antarctic environment, including the possession of a ventral marsupium. Particularly, the inability of Mg 2+ regulation in combination with general Mg 2+ sensitivity has been thought to be a reason for the near extinction of benthic decapods in the Antarctic (e.g. Thatje et al. 2005). However, recent work by Wittmann et al. (2010) revealed similarly high haemolymph magnesium levels in some Antarctic isopods compared to decapods. Thus different eco-physiological and behavioral features must be responsible for the diversification of isopods at high Southern latitudes (Wittmann et al. 2010).