Owing to the absence of light at the bottom of the ocean, scientists previously believed that the deeper a fish lives, the less complex their visual system would be. But according to a new study published on 9 May in Science, this may not be so accurate. It appears that deep-sea fish species living in the near-total darkness have evolved highly-sensitive vision.
In vertebrates, vision is achieved through light-induced changes detected by photopigments found in the rods and cones of the retina at the back of the eye. But photopigments only react to certain wavelengths of light, which is why humans can only see colour in daylight and are colourblind at night.
Deep-sea creatures have adapted in many ways to the dark ocean environment, including increased eye or pupil sizes, extremely telescopic-shaped eyes, and modifications to the microscopic retina structure itself.
As the new findings suggest, one of these evolutionary changes is an extreme sensitivity to a different source of light – the faint bioluminescence emitted by bacteria, shellfish, octopuses, and other fish in the deep sea. These subtle flashes of light are barely perceptible to most vertebrates but are the primary source of light in the deepest parts of the ocean.
To determine whether fish living in the darkest depths of the ocean can actually see, the international team of researchers, led by Prof Walter Salzburger from the University of Basel in Switzerland, analysed the genomes of 100 deep-sea fish.
Using gene sequencing technology and computer simulations, they discovered an abundance of rod opsin genes, which generate the rod opsin photopigments, or simply opsins – retinal proteins that detect dim light. In fact, it seems certain deep-sea fish produce multiple rod opsin proteins capable of capturing every photon of light at multiple wavelengths. Whereas, the light-sensitive rod cells in humans contain only one type of opsin.
Importantly, the rod opsin genes discovered by the scientists cover a range of wavelengths associated with the bioluminescence of light-emitting organs of deep-sea organisms.
The superb vision of these deep-sea creatures may actually enable them to see colour even in the darkest depths of the sea, where the sunlight cannot reach. This is because opsins produce amino acid sequences that can detect the different wavelength of light, therefore, the presence of multiple opsins make colour vision possible – even in near darkness.
For example, one species that stands out is the silver spiny fin (Diretmus argenteus), now known to be the vertebrate with the most photopigment genes – 38 copies of the rod opsin gene.
The results “reveal a previously undescribed visual system that allows for colour vision in the dark”, the authors write. This multiple rod opsin-based vision may allow deep-sea fish to detect bioluminescent signals throughout the deep-sea environment, providing them with an evolutionary advantage by allowing them to spot prey and avoid predators.
(1) Musilova, Z. et al. Vision using multiple distinct rod opsins in deep-sea fishes. Science (2019). DOI: 10.1126/science.aav4632