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.
The monomeric subunit composition and oxygen binding properties of hemocyanins were examined in 9 taxonomic groups of 43 crustacean species and 1 hybrid. In native polyacrylamide electrophoresis the banding pattern was highly species specific, even in closely related congeners. In less closely related taxa, there was little apparent relationship between phylogenetic affinity and banding pattern. Within a taxonomic group, pH dependence was the most highly conserved and O2 affinity was the most diverse of the O2 binding properties investigated. In congeneric but not sibling species, O2 affinity was more highly correlated with an environmental variable such as temperature than with phylogenetic affinity. Only in very closely related groups found in similar environments were different O2 binding properties correlated with differences in particular electrophoretic bands.
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