OXYGEN-HEMOCYANIN RELATIONSHIPS IN THE LAND CRAB, CARDISOMA GUANHUMI

Abstract

1. The hemocyanin of the land crab, Cardisoma guanhumi, under physiological conditions, possesses a high oxygen affinity. 2. The oxygen equilibrium curve is sigmoid, with a fairly high degree of interaction occurring between the oxygen-combining sites of the hemocyanin molecule. 3. As in all other crustacean hemocyanins investigated, a normal Bohr effect is present. 4. Van Slyke analyses of the oxygen content of pre-branchial and post-branchial blood samples indicated that nearly all of the oxygen present was in combination with the hemocyanin; consequently the oxygen partial pressures throughout the circulatory system were very low. The hemocyanin did not become saturated with oxygen during passage through the gills. These results are similar to those previously found for three other species of decapod crustaceans. 5. The carbon dioxide content of the blood was very high, averaging almost 50 volumes per cent. 6. It is shown that the low half-saturation value of the hemocyanin confers a stability of loading pressure in the face of changing environmental temperature. It is speculated that the high oxygen affinity may also represent an adaptation tending to counter the effect of water-conserving measures on the rate at which oxygen diffuses through the respiratory surfaces into the blood. 7. Zuckerkandl's studies on the crab, Maia squinado, are discussed and it is tentatively concluded that, though the hemocyanin of the decapod Crustacea may serve several functions and may not always be essential for respiration, its primary function is that of a blood respiratory pigment.