Survival on a semi-arid island: submersion and desiccation tolerances of fiddler crabs from the Galapagos Archipelago

During tidal cycles, semi-terrestrial fiddler crabs are subject to alternating periods of submersion and desiccation. Here, we compare physiological and biochemical adjustments to forced submersion and desiccation in two fiddler crabs from the Galapagos archipelago: the indigenous Leptuca helleri, and Minuca galapagensis. We examine ecological distributions and habitat characteristics using transect analysis; survival after 6 h forced submersion at different salinities (0, 21 and 42 {per thousand}S), and after 6 or 12 h desiccation challenge, including alterations in hemolymph osmolality; and, oxidative stress responses in the gills and hepatopancreas, accompanying glutathione peroxidase (GPx), glutathione S-transferase (GST) and glutathione reductase (GR) activities, and lipid peroxidase (LPO). We provide an integrated biomarker response (IBR) index for each species based on oxidative stress in each tissue and condition. Our transect study revealed that L. helleri occupies an intertidal niche while M. galapagensis is supralittoral, L. helleri being less resistant to submersion and desiccation. After 6 h submersion, L. helleri survived only at 21 {per thousand}S while M. galapagensis survived at all salinities. Hemolymph osmolality decreased at 0 {per thousand}S in M. galapagensis. After 6 h desiccation, osmolality decreased markedly in L. helleri but increased in M. galapagensis. Enzyme assays were not performed in L. helleri owing to high mortality on submersion/desiccation challenge. After submersion in M. galapagensis, hepatopancreas GPx activities decreased in 0 and 21 {per thousand}S while GR activity was strongly inhibited at all salinities. Gill LPO decreased in 42 {per thousand}S. On desiccation in L. helleri, GPx activity was inhibited in the hepatopancreas but increased in the gills. GST activity increased while LPO decreased in both tissues. After desiccation in M. galapagensis, hepatopancreas GPx activity increased. Both hepatopancreas and gill GST and GR activities and LPO were strongly inhibited. The IBR indexes for L. helleri were highest in fresh caught crabs, driven by gill and hepatopancreas LPO. For M. galapagensis, submersion at 21 {per thousand}S contributed most to IBR, LPO in both tissues responding markedly. Leptuca helleri appears to be a habitat specialist adapted to a narrow set of niche dimensions while M. galapagensis survives over a much wider range, exhibiting little oxidative stress. The species physiological flexibilities and limitations provide insights into how fiddler crabs might respond to global environmental change on semi-arid islands.