Hepatopancreatic endosymbionts in coastal isopods (Crustacea : Isopoda), and their contribution to digestion

Three isopod species (Crustacea: Isopoda), commonly found in the intertidal and supratidal zones of the North American Pacific coast, were studied wit h respect to symbiotic microbiota in their midgut glands (hepatopancreas). Ligia pallasii (Oniscidea: Ligiidae) contained high numbers of microbial sy mbionts in its hepatopancreatic caeca. Numbers of endosymbionts were strong ly reduced by ingestion of antibiotics. By contrast, the hepatopancreas of Idotea wosnesenskii (Valvifera: Idoteidae) and Gnorimosphaeroma oregonense (Sphaeromatidea: Sphaeromatidae) did not contain any microbiota. Results of feeding experiments suggest that microbial endosymbionts contribute to dig estive processes in L, pallasii, the most terrestrial of the three isopods that we studied. The acquisition of digestion-enhancing endosymbionts may h ave been an important evolutionary step allowing isopods to colonize terres trial habitats where relatively indigestible leaf litter is the primary foo d source. By contrast, the ability to digest phenolic compounds was most de veloped in one of the more marine species, suggesting that this trait may h ave evolved independently in isopod species that consume a phenolic-rich di et, whether in marine habitats or on land.