When morphology, molecular phylogeny and ecology do not match: some examples among aquatic and terrestrial Oniscidea

Most species of terrestrial Isopoda require a high degree of humidity to survive, but only a small number of species are known to live strictly or temporarily in an aquatic environment. Many of the aquatic Oniscidea belong to the sections Synocheta (members of the families Trichoniscidae and Styloniscidae) and Crinocheta (members of Olibrinidae and Philosciidae), most of which inhabit subterranean waters. In most cases, the aquatic species show peculiar morphological characters that resulted in the separation of these forms in distinct genera, often considering them as primitive taxa compared with related terrestrial ones. In some of these cases molecular analyses have shown that the phylogenetic relationships do not agree with morphology and that adaptation to the aquatic and subterranean environment has obscured the evolution in morphological characters. Groundwater calcrete aquifers of the Australian arid zone are inhabited by a highly diverse stygofauna, including many species of Oniscidea. According to a morphological analysis, the stygobitic Oniscidea belong to the genus Haloniscus and to the genus Andricophiloscia. The genus Haloniscus includes also H. searlei, an aquatic species widely distributed in the salt lakes of southern Australia and Tasmania, and H. anophthalmus from anchialine waters in New Caledonia. In Australia the genus Andricophiloscia includes also the epigean A. stepheni from Western Australia. All stygobytic oniscidean species are endemic to single calcretes, which can be viewed as "subterranean islands". A molecular analysis of mitochondrial DNA (CO1) of several populations of the stygobitic and epigean species of Haloniscus and Andricophiloscia showed that the Australian species form a monophyletic group and their separation in distinct genera on the basis of morphology is not supported. Another example which is now under investigation is that of Alpioniscus and Utopioniscus (Trichoniscidae) species from Sardinia. At present Alpioniscus includes two species in Sardinia: the subterranean A. fragilis and the endogean A. thanit. The genus Utopioniscus is presently known with one aquatic species (U. kuehni) from two anchialine caves off the eastern coast of central Sardinia. For its morphology and aquatic life the species was considered to be the most primitive representative of the family Trichoniscidae. Recent investigations in many Sardinian caves revealed the presence of U. kuehni also in subterranean freshwater lakes, and of a new aquatic species in Su Palu Cave, which shows intermediate morphological characters between Alpioniscus spp. and Utopioniscus kuehni. The new hypothesis that we are working on with a molecular approach is that U. kuehni might be closely related to Alpioniscus species, secondarily returned and morphologically adapted to an aquatic way of life.