N-specific Metabolic Data Are Not Relevant to the ‘visual Interactions’ Hypothesis Concerning the Depth-related Declines in Metabolic Rates: Comment on Ikeda Et Al. (2006)

Ikeda et al. (2006; Mar Ecol Prog Ser 322:199–211) declared that they were testing the ‘visual interactions’ hypothesis for the decline in metabolic rates with depth in midwater animals, but their data and data analysis are irrelevant to this hypothesis for 4 reasons: (1) they do not show changes relative to the live mass of the animals studied; (2) they do not compare species-specific data; (3) they do not use evolutionarily significant descriptors of habitat depth; and (4) there are significant differences in the respirometry methods within their own data sets, and between those data and the data that support the ‘visual interactions’ hypothesis. Live weight mass-specific expression of rates is necessary when considering hypotheses concerning the evolution and function of living animals, although N-specific expressions may be useful in other contexts.