AbstractsBiology & Animal Science

Callianassa californiensis Dana and Upogebia pugettensis (Dana) inhabit estuarine mudflats and are subjected to the hypoxic conditions that prevail during tidal exposure. Metabolic responses for both these species, as indicated by critical oxygen tension, metabolic rate, and tolerance to anoxia were determined. An oxygen macro-electrode and a physiological gas analyzer in conjunction with the sealed jar method were used in all metabolic experiments. The survival time under anoxia was measured. A procedure is also described for counting heart rates of Callianassa. Field samples were analyzed for oxygen content by a micro-Winkler procedure. The ghost shrimp, Callianassa, and the blue mud shrimp, Upogebia, show metabolic adaptations for living in the mudflat biotope. Both species are good metabolic regulators. Callianassa) however, has a considerably lower critical oxygen tension (10-20 mmHg) than Upogebia (45-50 mmHg). The mean metabolic rates, within the independent range of respiration, are significantly different at the one percent level of probability. Callianassa has a metabolic rate of 0.024 mls 0???/gm wet wt/hr compared to 0.050 mis 0???/gm wet wt/hr for Upogebia. Data are also presented which suggest that postmolt Upogebia are relatively more oxygen dependent than intermolt shrimp and hence temporarily lose their regulatory ability. Heart rates of Callianassa subjected to diminishing oxygen concentrations show a regulatory pattern similar to that of the metabolic rate, with bradycardia occurring at low oxygen tensions. Both species survive anoxia for astonishing periods of time. The mean survival time for Callianassa is approximately 5.7 days and for Upogebia 3.3 days. It is proposed that the quantitative differences in the metabolic requirements of the two species reflect the availability of oxygen in their respective niches. Despite the fact that muds are generally more anoxic than sandy substrates Upogebia, in fact, has more oxygen available owing to a firmly constructed burrow system. Callianassa burrows are not firmly constructed and the upper reaches of the burrows tend to collapse during ebb tide. Hence, the ghost shrimps are most likely exposed directly to hypoxic interstitial waters. Preliminary field data and ecological observations support this conclusion. Upogebia burrow water has a mean concentration of 0.58 mis 0???/l and interstitial water probably less.