A comparison of water loss and gill areas in two supralittoral amphipods from New Zealand

Total gill area and gill distribution were measured for the sandhopper Talorchestia quoyana (Milne-Edwards) and the beach flea Transorchestia chiliensis (Milne-Edwards). For both species the gill structure and proportional area contributed by individual gills was similar. Gill 6 (G6) was the largest, providing 36% of the gill area in Tal. quoyana and 30% in Tr. chiliensis. The gill area/total dry weight relationships were similar, Y = 1.3 X^0.79 for Tal. quoyana and 1.4 X^0.78 for Tr. chiliensis. Small, medium and large amphipods survived >24 h in aerial conditions close to 100% RH at 15 °C. Rates of water loss in desiccating conditions increased with decreasing RH. Lethal water loss exceeded 30% weight loss for both species. Rate of water loss, (R) mg water loss. mg wet wt tissue. h^–1 exposed to 75% RH for Tr. chiliensis was 0.21, resulting in total mortality within 2 h. Medium Tal. quoyana were the most resistant group surviving 4 h exposure to 75% RH with R = 0.08. Differences in desiccation tolerances of the two amphipods are not explained by body water content, gill area relationships or the larger maximal size of T. quoyana. Results were combined with those from other talitrids to examine the relationship between gill area, water content, desiccation habitat and oxygen consumption in aerial and aquatic conditions. There were no consistent relationship between gill area, O₂ uptake and desiccation resistance. Amphipods show compensatory respiratory adaptation with individuals from all habitats, showing similar rates of oxygen uptake, either in air or in water, whichever was their most usual respiratory medium. Q¹⁰ values close to 2.0 were found in all ecomorphological groups. Sandhoppers, including Tal. quoyana, are best able to survive terrestrial conditions associated with a low humidity environment. It is concluded that the water loss characteristics of Tr. chiliensis limit its distribution on sand beaches to areas of high relative humidities.