| Summary: | Here we evaluate the so-called Thorson's rule, which posits that direct-development and larger eggs are favored towards the poles in marine organisms and whose validity been the subject of considerable debate in the literature, combining an expanded phenotypic dataset encompassing 60 species of benthic octopuses with a new molecular phylogeny. Phylogenetic reconstruction shows two clades: clade 1 including species of the families Eledonidae, Megaleledonidae, Bathypolypodidae and Enteroctopodidae, and clade 2 including species of Octopodidae. Egg size, development mode and all environmental variables exhibited phylogenetic signal, partly due to differences between the two clades: whereas most species in clade 1 inhabit cold and deep waters, exhibit large eggs and hatchling with holobenthic development, species from clade 2 inhabit tropical-temperate and shallow waters, evolved small eggs and generally exhibit merobenthic development. Phylogenetic regressions show that egg size exhibits a conspicuous latitudinal cline, and that both egg size and development mode vary with water temperature. Additionally, analyses suggest that egg size is constrained by body size in lineages with holobenthic development. Taken together, results suggest that the variation in egg size and development mode across benthic octopuses is adaptive and associated with water temperature, supporting Thorson's rule in these organisms. Octopodoidea phylogeny The Octopodoidea phylogeny was inferred using three mitochondrial genes – 16S rRNA (16S), Cytochrome Oxidase I (COI) and Cytochrome Oxidase III (COIII) – and one nuclear gene – Rhodopsin (RHO) – for a total of 61 species. The phylogenetic trees based on a Bayesian framework using Mr. Bayes v3.2 (Ronquist et al. 2012) were performed by means of MCMCMC (Metropolis Coupling Monte Carlo Markov Chains), four chains were run using 10,000,000. The trees were rooted using Vampyroteuthis infernalis as an outgroup. The phylogeny from the partitioned data set is consistent with the current octopus ...
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