Adaptation of gill traits in benthic octopuses from shallow waters to the deep sea

Abstract Deep-sea organisms exhibit remarkable adaptations that enable survival under extreme environmental conditions. Among them, benthic octopuses show distinct morphological modifications compared to their shallow-water counterparts, particularly in respiratory structures. This study investigates macroevolutionary adaptations in gill morphology, specifically gill size and lamellae count, across environmental gradients from shallow to deep ocean, using benthic octopuses as a model system. We compiled data on gill traits for 52 species from biological collections and published sources and analysed these within a robust molecular phylogenetic framework. Phylogenetic comparative methods were employed to assess trait-environment correlations and to fit evolutionary models (Brownian Motion, Early Burst, Rate Trend, and Ornstein–Uhlenbeck) to infer adaptive processes. All traits exhibited strong phylogenetic signal, and the Ornstein–Uhlenbeck model provided the best fit, indicating stabilizing selection towards reduced gill size and lamellae count with increasing depth. Both traits were negatively correlated with latitude, depth, and oxygen concentration, but positively correlated with temperature. These findings provide compelling evidence for adaptive evolution in respiratory traits of benthic octopuses, highlighting the role of environmental gradients in shaping functional morphology across global ocean depths.