Respiratory Responses of the Blue Crab Callinectes sapidus to Long-Term Hypoxia

Blue crabs (Callinectes sapidus) were held in hypoxic (50-55 mm Hg) water for 7-25 days. Postbranchial blood PO2 fell by about 80% within 24 h and then remained unchanged. Postbranchial blood total CO2 increased within 24 h and remained elevated for the duration of the experiment. There was no change in postbranchial blood pH, osmolality, or Cl. Lactate, urate, and Ca+2 all raise the O2 affinity of blue crab hemocyanin; by 25 days, blood lactate and urate had risen slightly, but Ca+2 had increased dramatically. Hemocyanin concentration had also increased by 25 days. At both 7 and 25 days there was an intrinsic increase in hemocyanin-O2 affinity and a change in subunit composition. The highly adaptive homotropic change is believed to be due to an attendant shift in the proportions of two of the three variable monomeric hemocyanin subunits. Thus, both heterotropic and homotropic adaptations enhance blood oxygenation at the gill during long-term hypoxia.