Global diversity of Isopod crustaceans (Crustacea; Isopoda) in freshwater
George D.F. Wilson
Methodological Considerations. The biodiversity of freshwater isopods is derived from my research on the Phreatoicidea and Asellota, and from the online World List of Isopoda (Kensley, Schotte & Schilling 2005). This list is frequently updated (Schotte personal communication) and currently is the most up-to-date checklist of isopod species. The list was downloaded into spreadsheets (accessed September 2005), checked for completeness, and missing species added where necessary, and the classification simplified for this paper. A diverse group of mostly marine families is loosely referred to as ‘Flabellifera’ sensu lato (cf. Wilson 1999) that refers to a clade of derived isopods. Several competing classifications have been proposed for the flabelliferans and isopod specialists have not reached a consensus, despite the passage of 15 years since Banarescu (1990). The World List uses the older traditional classification of the ‘Flabellifera’ sensu stricto that is known to be paraphyletic (Brusca & Wilson 1991; Wägele 1989; Tabacaru & Danielopol 1999). The use of ‘Flabellifera’ sensu lato has the advantage that the group, often referred to as ‘the higher isopods’ is monophyletic at a deep level in isopod phylogeny, with an origination separate from that of Asellota, Phreatoicidea and Oniscidea. The Microcerberidea are not resolved in this scheme. Wägele (1983, 1989) proposed that this group is nested within the Asellota but they are clearly not, for several reasons (Brusca & Wilson 1991). Wägele (1983), however, did identify a relationship between the Microcerberidae and the Atlantasellidae (which was not treated by Brusca & Wilson 1991), so I include both families in the suborder. The peculiar family Calabozidae is classified as Oniscidea owing to its possession of in-group genitalia characters and other attributes. Tabacaru & Danielopol (1999) provide some support for this classification, although their analysis was conducted at subordinal level.
Marine species, including those from anchialine cave and beach interstitial environments, were filtered out of the downloaded list, either using the type habitat from the list or by consulting the original literature. The data do include, however, a few species from saline or hypersaline continental waters (e.g., species of Haloniscus, Oniscidea). Subspecies were treated as species-level taxa. Most subspecies in the literature could profitably be reclassified as species because genetic studies often find that nominal subspecies are distinct at a fairly high level of divergence. Undescribed species known to me were added to the list where possible, although less than 100 were added. The asellote family Janiridae, which contains the solely freshwater genus Heterias, is a significant exception: Heterias specimens appear in many samples of shallow groundwater, pholeteros and hyporheos in southern Australia and Tasmania, and are common in some habitats such as root mats in caves (Western Australia; Jasinska et al., 1996). Research in freshwater subsurface habitats in other Gondwana continental fragments (Chile, Brazil, New Zealand), have uncovered additional species of this group. Unpublished protein electrophoretic data, identifying over 40 presumptive species-level taxa, implies that Heterias species are short range endemics (P. Horwitz, pers. comm.). This pattern is seen in other subterranean isopods, such as Pygolabis (Keable & Wilson 2006).
An estimate of the unknown species was determined for the Phreatoicidea using data on all Australian genera (Wilson in progress), and information from Gouws et al. (2004, 2005). The estimate was based on possible sites where new species might be found, the rate that new species appeared during our study and the presumed diversity of the genus in question. For example, current research (Wilson & Humphrey in progress) has identified approximately 17 new species of the genus Eophreatoicus that live in springs at the base of the Arnhem Plateau in the Northern Territory, Australia. These species are distinguished both morphologically and genetically (using mtDNA 16S and Cox I sequences), so we are certain of their distinctness. We have surveyed only the western side of the plateau in Kakadu National Park, so less than half of the range of Eophreatoicus is considered known. For this genus, we therefore predict an additional 20 species might be found. On the other hand, the Ponderellidae with only two co-occuring species may have no additional species; ongoing thorough surveys of the Great Artesian Basin in central Australia (W. Ponder, pers. comm.) have found no new sites. Similar estimates were made for other Phreatoicidea (Table). Because isopods in freshwater appear to be short range endemics, this rough estimate may be conservative and dependent on sampling effort. The remainder of the freshwater isopods cannot be treated in this way owing to a lack of direct sampling experience. To obtain an diversity estimate for the other major isopod groups, a simple known to unknown ratio derived from the Phreatoicidea was applied to the remainder of the taxa (Table 1, main document). Although the assumption of similarity of distributions and diversification between Phreatoicidea and the remainder of the isopods is tenuous at best, the former group contains a mixture of epigean and hypogean species, living in situations that resemble most other freshwater isopods, and certainly resembles the most speciose freshwater family Asellidae in this regard. This assumption at least provides a starting hypothesis for further refinement.
Table. Diversity of the Phreatoicidea (several monotypic taxa, Ponderellidae and Crenisopus, not shown). Species column contains described plus known undescribed species. Unknown species are estimated based upon presumed distribution, frequency of encountering new species and number of known species in total. See text for explanation.