Life in the hypoxic and anoxic zones: Metabolism and proximate composition of Caribbean troglobitic crustaceans with observations on the water chemistry of two anchialine caves

The interaction of freshwater, seawater, and limestone creates the unique environment of the anchialine cave. Anchialine cave systems are characterized by the presence of several strong density interfaces that isolate the cave system into components: regions of freshwater at the surface, a hydrogen sulfide layer, and a warm oceanic intrusion that is devoid of oxygen. These unusual physical parameters provide a challenging environment for the organisms that reside there. Within the hypoxic and anoxic regions of the cave system, a unique community of macrofauna has adapted to cope with constant low oxygen conditions and a depauperate food supply. We examined respiration rates and proximate compositions of individuals representing seven orders of troglobitic crustaceans collected from two Bahamian anchialine caves. In addition, we examined the physical conditions of both caves to determine ambient oxygen concentrations. Mass-specific respiration rates of the troglobites were more than an order of magnitude lower than those of comparably sized pelagic crustaceans. When compared to organisms residing in oxygen minimum zones, respiration rates of the troglobites were similar, or in some cases, lower. Enzymes of the troglobites were anaerobically poised with lactate dehydrogenase activities exceeding those of citrate synthase in all species. Malate dehydrogenase activities were greater than lactate dehydrogenase activities, indicating further evolutionary adaptations to an anaerobic environment.