Epigean and hypogean Palaemonetes sp. (Decapoda, Palaemonidae) from Edwards Aquifer: An examination of trophic structure and metabolism

This study addresses the causes of the metabolic depression observed when examining the metabolism of hypogean versus epigean organisms. We examined the two current hypotheses regarding the cause of metabolic cave adaptation, a paucity of food and low oxygen availability, both necessary for ATP production, by first determining if the hypogean environment examined, Edwards Aquifer, was resource limited. Stable isotope analyses indicate that there is extensive microbial chemolithoautotrophic production providing resources for the hypogean organisms. δ¹³C values ( ≤30% ) were well below that of terrestrial biome indicating that C in the aquifer originates from chemolithoautotrophic inorganic carbon fixation, not photosynthetically derived material resulting from terrigenous sources. Data suggest the artesian system is a complex geochemical ecosystem providing inorganic energy sources from both methane and sulfates. Metabolism, examined via key aerobic and anaerobic proxies, and organismal proximate composition indicated there was no difference between metabolic rates and energy storage of Palaemonetes antrorum (stygobitic) and Palaemonetes kadiakensis (epigean). This indicates that resources within the oxic aquifer are not limited. We demonstrate that it is necessary for one, or both, of these selective pressures to be present for metabolic cave adaptation to occur.