Vision in Deep-Sea Oegopsid Squids

Abstract

Squids of the order Oegopsida are a lineage of coleoid cephalopods with over 300 species that often inhabit the light-limited deep sea (>200m deep). Despite their dim surrounding environments, oegopsids use vision as their primary sense to navigate and possess camera-type eyes that evolved convergently with vertebrates. However, oegopsid vision remains understudied when compared to near-shore and shallow water cephalopod species. Specifically, there is little information available on the basic anatomy and functions of their retinas. Therefore, this thesis aimed to increase understanding of the oegopsid retina and determine if any correlations exist between any findings and the life-histories and behaviours of the studied species. Overall, there were four main aims of this study: 1) Investigate the retinal anatomy of oegopsids and identify any specialisations that may be beneficial for vision in the deep sea; 2) to determine whether screening pigment migrates in oegopsids despite many species living in relatively dark conditions; 3) measure the spectral sensitivity curves of the retinal visual pigments and determine whether any correlations exist between those characteristics and known behaviours of the studied species; and 4) sequence the rhodopsin gene of the studied species and create a rhodopsin phylogenetic tree to identify any traits that may drive its spectral sensitivity specialisation. The results revealed several trends that suggest that certain regions of their retinas typically contain longer photoreceptors that may aid individuals to recognise prey, predators and conspecifics. The photoreceptor lengths observed in adult oegopsids, relative to age-related increased length previously reported in juveniles, suggest a possible slowing or cessation of photoreceptor growth during adulthood. Furthermore, it was experimentally shown that screening pigment within the retina does migrate to the distal ends of the photoreceptors in oegopsids, a feature that reduces the amount of light that contacts the retina, which prevents photoreceptors from becoming oversaturated in bright ambient light conditions. Finally, by characterising the spectral sensitivity curves of the retinal visual pigments and sequencing the opsin gene, the absorbance profiles obtained from retinal pigment extracts suggest differences among species that may be associated with bioluminescent capability. Specifically, the spectral sensitivity curves of Histioteuthis miranda, Taningia spp., and Mesonychoteuthis hamiltoni – all species that are bioluminescent – exhibited broader sensitivity curves, potentially indicating increased sensitivity to additional wavelengths of light when compared to non-bioluminescent species. However, because cephalopod rhodopsin and metarhodopsin possess overlapping absorbance spectra, contributions from both photopigment states cannot be completely excluded. Consequently, the reported absorbance characteristics should be interpreted as estimates of visual pigment spectral sensitivity rather than definitive measurements of rhodopsin alone. Overall, this thesis showcases the complexity of oegopsid vision, and that additional research is required to fully comprehend the mechanisms that drive their success as voracious predators in light limited deep-sea environments.