Energetics of the Deep-Water Bluntnose Sixgill Shark, Hexanchus griseus: Enzyme Activity as an Indicator for Metabolic Rate

Abstract

A species’ metabolic rate can be used to elucidate life history characteristics, such as locomotive capacity, growth rate, sexual maturity, and ecological role. Utilizing metabolic rates is vital in instances where direct observations of these traits are not easily obtained, such as with deep-water species. Measuring species’ enzyme activity levels provides a proxy for metabolic rate. We determined metabolic enzyme activity levels in the white muscle (WM) of the deep-water bluntnose sixgill shark (Hexanchus griseus) , which is believed to be an apex predator and scavenger in deep-sea ecosystems, as well as 5 other Hawai’ i shark species, from both shallow and deep environments (Galeocerdo cuvier, Isurus oxyrinchus, Squalus mitsukurii, Echinorhinus cookei, and Carcharhinus plumbeus). Animals were caught off of Oahu and biopsied for white muscle, which is responsible for burst locomotion used during predator-prey interactions. Assays were conducted on four key metabolic enzymes: Citrate Synthase, Pyruvate Kinase, Lactate Dehydrogenase, and Malate Dehydrogenase. Enzyme activities derived from this study were compared with WM metabolic data from previous studies in order to evaluate similarities and differences between H. griseus and other elasmobranch species, as well as trends in WM metabolic data for elasmobranchs. Our results showed that H. griseus exhibits significantly lower WM metabolic rates than the other species in this study, but appears similar to the WM metabolic rates of Somniosus pacificus , indicating similarities in ecological strategies between these two large deep-water species. Furthermore, we noted a general decrease in WM metabolic activity with depth, which further indicates differences between shallow- and deep-water elasmobranch burst locomotive capacity and ecological strategies