In situ observations reveal a link between association behaviour, camouflage and eye complexity in midwater amphipods

In the largest habitat on earth-the deep open ocean-water depth is typically linked to adaptations for camouflage and vision. Recently, it was hypothesized that association behaviour is a primary evolutionary driver of the great eye diversity in a group of crustaceans, the hyperiid amphipods (suborder Hyperiidea). Here, we test that hypothesis by investigating the link between association behaviour (symbiotic or free swimming), depth, eye complexity and camouflage across a broad range of hyperiid diversity. We collated a unique dataset containing observations of 48 of the 80 hyperiid genera from a comprehensive literature review, blackwater scuba photographs and nearly 30 years of submersible observations. By scoring association behaviour in 5759 in situ observations, we show that approximately one third of hyperiid genera have adopted a free-swimming lifestyle. This supports a shift from the current assumption that all hyperiids live in close association with gelatinous animals, to one allowing a free-swimming, predatory lifestyle in many genera. Using phylogenetic generalized linear mixed model (PGLMM) analyses, we show that a free-swimming lifestyle correlates with body transparency and more complex eyes at upper mesopelagic depths. Our results support the hypothesis that a free-swimming lifestyle drives eye complexity while increasing the need for transparency.