Metabolic rate of embryonic little skate,Raja erinacea (Chondrichthyes: Batoidea): The cost of active pumping

Abstract

Near-hatching embryonic little skates, Raja erinacea, are highly active within their egg capsules, displaying a characteristic tail beating, which pumps water through the capsule. We measured the metabolic rate of late-stage embryos to determine whether oxygen sufficient for the embryo's needs will diffuse through the egg capsule, and to assess the energetic cost of tail beating. Metabolic rate was inferred from oxygen consumption rates while embryos were in the capsules, unencapsulated, and anesthetized and unencapsulated. Anesthesia inhibited voluntary movements, including tail wagging, allowing an estimate of the standard metabolic rate (SMR). Averaged over five embryos, the SMR was 0.032 ± 0.004 ml O2 g–1 hr–1. There was no significant difference in metabolic rate between encapsulated (0.058 ± 0.009 ml O2 g–1 hr–1) and unencapsulated (0.049 ± 0.009 ml O2 g–1 hr–1) skates. Tail beating was found to be energetically expensive, requiring a 53%–81% increase over the SMR. From literature values for the oxygen permeability of the egg capsule we conclude that tail beating is required to supply sufficient oxygen to the embryonic skate. This observation is consistent with the proposal that actively pumping water through the capsule, by tail beating, has played an evolutionary role in the shape of the capsule. J. Exp. Zool. 283:13–18, 1999. © 1999 Wiley-Liss, Inc.