A Physiological Comparison of Bivalve Mollusc Cerebro-visceral Connectives With and Without Neurohemoglobin. I. Ultrastructural and Electrophysiological Characteristics

The presence of hemoglobin in tissues of a small number of species is puzzling when homologous tissues of closely related species do not possess hemoglobin. Several species of bivalve molluscs possess nervous systems with nerve hemoglobin (neurohemoglobin). These systems were compared to nervous systems from other bivalve molluscs without neurohemoglobin to determine ultrastructural and electrophysiological characteristics under normoxic conditions in an attempt to locate any differences between these two types of nervous systems. Cerebro-visceral connectives from the bivalves Tellina alternata and Spisula solidissima with neurohemoglobin and Tagelus plebeius and Geukensia demissa without neurohemoglobin possess a perineural sheath, a subjacent peripheral layer of glial cells, and glial cell processes that enwrap bundles of 0.2-0.4 µm diameter axons. Neurohemoglobin-containing cerebro-visceral connectives have smaller axon bundles and more dense perineural sheaths than those without neurohemoglobin. These features may be important in oxygen delivery from the neurohemoglobin to the axons. Action potential traces, conduction velocities, refractory periods, strength-duration relationships, and temperature responses of all four connectives are typical of nerves possessing very small axons. There are no obvious electrophysiological differences between cerebro-visceral connectives with and without neurohemoglobin.