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). Within pelagic fishes, crustaceans, and cephalopods, a strong decline in rates of mass-specific metabolism with depth has been observed. Childress and Mickel (2) put forward the visual interactions hypothesis to explain this general pattern. Their hypothesis states that reduced metabolic rates among many deep-sea pelagic taxonomic groups result from relaxed selection for strong locomotory abilities for visual predator-prey interactions in the light-limited deep sea. This pattern has, however, been tested using mean metabolic rates for species as individual data points. Felsenstein (3) warned that, because species are descended in a hierarchical fashion from common ancestors, they generally cannot be considered as independent data points in statistical analyses. Statistical methods have recently been developed that incorporate phylogenetic information into comparative studies to create phylogenetically independent values that can then be used in statistical analyses. Reliable independent phylogenetic information has only recently become available for some deep-sea organisms. The present contribution reanalyzed the metabolic rates (4,5) of pelagic cephalopods as a function of, for consistency with previous studies, MDO (minimum depth of occurrence) using phylogenetic independent contrasts derived from a recent molecular phylogeny (6). This analysis confirms the existence of a significant negative relationship 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.