Bivalve hemocyanins--a comparison with other molluscan hemocyanins.

1. The hemocyanins of the protobranch bivalves Yoldia thraciaeformis, Yoldia limatula and Acila castrensis have absorption spectra similar to other hemocyanins. 2. Hemocyanins from all three bivalves appear as six-tiered cylinders in the electron microscope (30-32 nm in diameter by 34-38 nm in height). Yoldia thraciaeformis and A. castrensis hemocyanins tend to dissociate to three-tiered half molecules with polar images and also to associate into long tubular polymers. 3. Yoldia thraciaeformis and A. castrensis hemocyanins chromatograph on Sepharose 4B gel close to gastropod hemocyanin (Mr = 9 x 10(6] rather than chiton hemocyanin (Mr = 4 x 10(6]. 4. Hemocyanins from all three vivalves have subunits with electrophoretic mobilities similar to gastropod and polyplacophoran hemocyanin subunits and slower than octopodan hemocyanin subunits. 5. These similarities between bivalve and gastropod hemocyanins are consistent with the hypothesis that bivalves and gastropods have shared a common ancestor.

BIVALVE HEMOCYANIN: STRUCTURAL, FUNCTIONAL, AND PHYLOGENETIC RELATIONSHIPS.

The hemocyanin-like molecule found in the blood of the most primitive bivalves (protobranchs) reversibly binds O2. Its respiratory properties and its sedimentation behavior are both distinctive. Although its electron-dense image looks like that of the gastropod hemocyanins, its molecular weight differs from those of all other molluscan Hcs and is more consistent with the concept of bivalve hemocyanin as a pair of octopod hemocyanins. Bivalve hemocyanin occurs in the solemyoids as well as the nuculoids, which argues for the integrity of the Protobranchia as a natural taxon. The ancestral bivalve O2 carrier was previously believed to be a simple intracellular hemoglobin, which is found in the less primitive Pteriomorpha. The most obvious interpretation of the present results, however, is that hemocyanin is the primitive bivalve O2 cannier and that it was replaced by the red blood cell, which originated at least twice: once in the pteriomorph bivalves and at least once in other taxa.

Hemocyanin respiratory pigment in bivalve mollusks.

Hemocyanins, high molecular weight oxygen-binding proteins, were identified in two species of protobranch bivalve mollusks, Acila castrensis and Yoldia limatula. Although hemocyanins have been reported in chitons, gastropods, and cephalopods, they have not been observed in the Class Bivalvia. In A. castrensis the dissociation products of hemocyanin, characterized by gel electrophoresis, had a subunit molecular weight of approximately 250K. Negatively stained preparations of extracted hemocyanin formed protein aggregates in the shape of cylinders measuring 35 by 38 nanometers. X-ray microanalysis of hemocyanin aggregates in thin sections of Y. limatula demonstrated the presence of copper in the molecules. The discovery of hemocyanin in the protobranchs reinforces the primitive nature of the taxon and is further evidence that the major molluscan classes have a common ancestry.