Metabolism of pelagic cephalopods as a function of habitat depth: A reanalysis using phylogenetically independent contrasts

Metabolic rates of deep-living animals have been intensely studied (1). Wit hin pelagic fishes, crustaceans, and cephalopods, a strong decline in rates of mass-specific metabolism with depth has been observed. Childress and Mi ckel (2) put forward the visual interactions hypothesis to explain this gen eral pattern. Their hypothesis states that reduced metabolic rates among ma ny deep-sea pelagic taxonomic groups result from relaxed selection for stro ng locomotory abilities for visual predator-prey interactions in the light- limited deep sea. This pattern has, however, been tested using mean metabol ic rates for species as individual data points. Felsenstein (3) warned that , because species are descended in a hierarchical fashion from common ances tors, they generally cannot be considered as independent data points in sta tistical analyses. Statistical methods have recently been developed that in corporate phylogenetic information into comparative studies to create phylo genetically independent values that can then be used in statistical analyse s. Reliable independent phylogenetic information has only recently become a vailable for some deep-sea organisms. The present contribution reanalyzed t he metabolic rates (4, 5) of pelagic cephalopods as a function of, for cons istency with previous studies, MDO (minimum depth of occurrence) using phyl ogenetic independent contrasts derived from a recent molecular phylogeny (6 ). This analysis confirms the existence of a significant negative relations hip between metabolism and minimum habitat depth in pelagic cephalopods but suggests that phylogenetic history also has considerable influence on the metabolic rates of individual species.