A comparative analysis of cone photoreceptor morphology in bowhead and beluga whales

Abstract

The cetacean visual system is a product of selection pressures favoring underwater vision, yet relatively little is known about it across taxa. Previous studies report several mutations in the opsin genetic sequence in cetaceans, suggesting the evolutionary complete or partial loss of retinal cone photoreceptor function in mysticete and odontocete lineages, respectively. Despite this, limited anatomical evidence suggests cone structures are partially maintained but with absent outer and inner segments in the bowhead retina. The functional consequence and anatomical distributions associated with these unique cone morphologies remain unclear. The current study further investigates the morphology and distribution of cone photoreceptors in the bowhead whale and beluga retina and evaluates the potential functional capacity of these cells' alternative to photoreception. Refined histological and advanced microscopic techniques revealed two additional cone morphologies in the bowhead and beluga retina that have not been previously described. Two proteins involved in magnetosensation were present in these cone structures suggesting the possibility for an alternative functional role in responding to changes in geomagnetic fields. These findings highlight a revised understanding of the unique evolution of cone and gross retinal anatomy in cetaceans, and provide prefatory evidence of potential functional reassignment of these cells.

Immunohistochemical Characterisation of the Whale Retina

“…Previous studies demonstrated rod monochromacy in some cetaceans and several mutations in the opsin gene sequence have been reported in cetaceans, suggesting the evolutionary complete or partial loss of cone cell function in the retina, while non-photosensitive cones are maintained (Schweikert et al, 2016). Refined histological and advanced microscopy techniques revealed two cone morphologies in cetacean retinas that are not traditional morphologic cone structures, a consequence of genetic modifications influenced by environmental selection pressure (Smith et al, 2021).…”

“…Rod photoreceptors are more sensitive detectors of light than cones and they are capable of detecting single photons (Field and Rieke, 2002). Thus, rod-based vision provides better underwater vision in conditions where light intensity is low and light is scattered with increasing depth (Smith et al, 2021). In deep-diving species, rod dominance and rapid dark adaptation in particular are traits that indicate these marine mammals use vision primarily in low light levels, where colour vision may be of secondary importance (Peichl et al, 2001).…”