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

Near-hatching embryonic little skates, Raja erinacea, are highly active wit hin their egg capsules, displaying a characteristic tail beating, which pum ps 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 b eating. Metabolic rate was inferred from oxygen consumption rates while emb ryos were in the capsules, unencapsulated, and anesthetized and unencapsula ted. Anesthesia inhibited voluntary movements, including tail wagging, allo wing an estimate of the standard metabolic rate (SMR). Averaged over five e mbryos, the SMR was 0.032 +/- 0.004 ml O-2 g(-1) h(-1). There was no signif icant difference in metabolic rate between encapsulated (0.058 +/- 0.009 ml O-2 g(-1) hr(-1)) and unencapsulated (0.049 +/- 0.009 ml O-2 g(-1) h(-1)) skates. Tail beating was found to be energetically expensive, requiring a 5 3%-81% increase over the SMR. From literature values far the oxygen permeab ility of the egg capsule we conclude that tail beating is required to suppl y 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. Ex p. Zool. 283.13-18, 1999. (C) 1999 Wiley-Liss, Inc.