Temperature effects on hemocyanin oxygen binding in an Antarctic cephalopod

The functional relevance of oxygen transport by hemocyanin of the Antarctic octopod Megaleledone senoi and of the eurythermal cuttlefish Sepia officin alis was analyzed by continuous and simultaneous recordings of changes in p H and hemocyanin oxygen saturation in whole blood at various temperatures. These data were compared to literature data on other temperate and cold-wat er cephalopods (octopods and giant squid). In S. officinalis, the oxygen affinity of hemocyanin changed at DeltaP(50)/ degreesC = 0.12 kPa (pH 7.4) with increasing temperatures; this is similar to observations in temperate octopods. In M. senoi, thermal sensitivity was much smaller (<0.01 kPa, pH 7.2). Furthermore, M. senoi hemocyanin display ed one of the highest levels of oxygen affinity (P-50 < 1 kPa, pH 7.6, 0 de greesC) found so far in cephalopods and a rather low cooperativity (n(50) = 1.4 at 0 degreesC). The pH sensitivity of oxygen binding (Delta log P-50/D elta pH) increased with increasing temperature in both the cuttlefish and t he Antarctic octopod. At low Po-2 (1.0 kPa) and pH (7.2), the presence of a large venous oxygen reserve (43% saturation) insensitive to pH reflects re duced pH sensitivity and high oxygen affinity in M. senoi hemocyanin at 0 O C. In S. officinalis, this reserve was 19% at pH 7.4, 20 degreesC, and 1.7 kPa O-2, a level still higher than in squid. These findings suggest that the lower metabolic rate of octopods and cuttle fish compared to squid is reflected in less pH-dependent oxygen transport. Results of the hemocyanin analysis for the Antarctic octopod were similar t o those reported for Vampyroteuthis-an extremely high oxygen affinity suppo rting a very low metabolic rate. In contrast to findings in cold-adapted gi ant squid, the minimized thermal sensitivity of oxygen transport in Antarct ic octopods will reduce metabolic scope and thereby contribute to their ste nothermality.