EVOLUTION OF THE GILLS IN THE OCTOPODIFORMES

摘要

The gills of cirrate octopods are known to be unusual with terms 'half orange' and 'sepioid' commonly used to describe them. The structure and relationships of these gills to other cephalopod gills have not been adequately investigated. In this paper we investigate the evolution of the gills of cirrates and of octopodiforms in general. Octopodiform gills differ from the primitive cephalopod gill, as exemplified by the gills of Nautilus and decapodiforms, by the presence of septa along the axes of the primary and secondary lamellae. The septa apparently constrain the respiratory surfaces to form tree-like folds rather than the fan-like folds of other cephalopods. In members of the Vampyromorpha, the sister taxon of the Octopoda, gills have a peculiar circulation pattern that seems to be a unique adaptation to its deep-sea habitat. The arrangement of blood vessels in the cirrates involves the repositioning of the primary efferent vessels deep within the gill. In addition, an axial anastomosis of superficial afferent vessels resulted in an appearance similar to a decapodiform gill but with afferent rather than efferent vessels on the 'top' of the gill. This, combined with the lack of a branchial canal and the presence of bilaterally symmetrical lamellae, has resulted in the appearance of a 'sepioid' gill. The 'half-orange' gill appears to result from a foreshortening and rotation of the gill to give the impression of a nearly radial arrangement of equal-sized primary lamellae rather than the typical serial arrangement of primary lamellae that decrease in size distally. Apparently, the adaptations of the octopodiform gill resulted from a need to increase the efficiency of oxygen uptake. We suggest that a major factor in the evolution of the Octopodiformes was the adaptation to a habitat low in oxygen.