An innovative investigation into the ocular structures of species known for their remarkable lifespans may unlock the biochemical secrets behind enduring visual acuity.
For a considerable period, the prevailing scientific assumption was that Greenland sharks (Somniosus microcephalus) possessed, at best, exceptionally deficient eyesight. Their existence is predominantly spent in the subdued illumination of the ocean’s abyssal zones, where they frequently host parasites affixed to their corneas.
However, a collaborative scientific contingent from the United States, Switzerland, and Denmark has now demonstrated that Greenland sharks are indeed capable of sight, maintaining a functional visual apparatus throughout their extensive lifespans.
This revelation posits that certain elements of the extraordinary DNA repair mechanisms associated with the sharks’ exceptional longevity might contribute to the sustained vitality of their retinas. Nevertheless, the potential ramifications for human ocular health remain indeterminate.
The study was meticulously performed on the ocular tissues of Greenland sharks procured and humanely euthanized between the years 2020 and 2024, under governmental research authorization from the relevant territory. Specimens utilized in this research all bore an estimated age exceeding one hundred years, with the eldest individual assessed at over 130 years of age.
Greenland sharks inhabit profound aquatic environments, descending to depths of up to 1,200 meters (3,940 feet), though they are capable of venturing even deeper. In these environments, the availability of light is drastically limited; beginning at approximately 100 meters, the intensity of light penetrating the water column diminishes precipitously. Below roughly 1,000 meters, solar illumination is virtually nonexistent.
This profound absence of light, coupled with the common occurrence of ocular parasites, had previously led numerous researchers to infer that these marine creatures could subsist with minimal or no visual capability. The comprehensive molecular, genomic, and histological analyses conducted by the research team on the Greenland sharks’ eyes have contravened this long-held belief.
Typical vertebrate retinas comprise rods and cones – distinct categories of photoreceptor cells. Rods are specialized for visual perception under low-light conditions, whereas cones facilitate color discrimination in brighter environments. Notably, the retina of the Greenland shark is devoid of cones entirely, an adaptation observed in a limited array of organisms that predominantly inhabit dimly lit surroundings.
Furthermore, the researchers rigorously evaluated their rhodopsin, a highly photosensitive protein resident within rod cells, which is indispensable for scotopic vision. The light-capturing efficacy of rhodopsin in Greenland sharks exhibits a peak sensitivity at approximately 458 nanometers, corresponding to the blue wavelength where the faint ambient light in the sharks’ environment is most concentrated.
The retinal tissues examined by the researchers were found to be in a state of excellent health, exhibiting no indicators of degenerative processes, even in individuals exceeding a century in age. However, the specific DNA repair mechanisms potentially responsible for this preservation have yet to be elucidated. Measurements of the cornea indicate that these structures continue to effectively transmit the majority of incident light, irrespective of the presence of an attached parasite.
Collectively, these findings strongly suggest that Greenland sharks possess an exceptional evolutionary adaptation for perpetual nocturnal vision within the sunless depths they inhabit.
“Our empirical findings corroborate the existence of a conserved and fully operational visual system in adult Greenland sharks, which appears exquisitely suited for extreme low-light environments,” the research team has articulated in their publication.
“The Greenland shark serves as a compelling exemplar of enduring retinal integrity, supported by both biochemical and histological evidence, and underscores the remarkable capacity for neuronal upkeep in the context of extreme longevity.”
