Aquatic and terrestrial Styloniscidae (Crustacea Isopoda Oniscidea) from Chinese caves: a morphological and molecular approach

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 Crinocheta, e.g. genera such as Paradoniscus (Olibrinidae) and Haloniscus (Philosciidae), and Synocheta, particularly members of the family Trichoniscidae. Among the Styloniscidae, the other large family of Synocheta closely related to the Trichoniscidae, Thailandoniscus annae, from a cave in Thailand, was previously the only species having an aquatic way of life. During recent investigations in some caves from cone karst areas in western Guangxi Province (southern China) four new species of Styloniscidae have been found: two aquatic forms and two terrestrial forms, all members of the same genus Sinoniscus. All the species show troglomorphic traits, such as lack of eyes and pigment. The two aquatic species represent the first records of terrestrial isopods inhabiting subterranean waters in China. They share some apomorphic morphological characters (i.e. presence of frontal lobes on the cephalon and subchelate anterior pereopods) which distinguish them from the terrestrial forms of Sinoniscus. A molecular analysis of the four species of Styloniscidae was performed using two mitochondrial gene fragments (COI and 16S). The genetic results suggest the close phylogenetic relationships between the aquatic and terrestrial forms and their congeneric status.