In our eyes and those of most vertebrates, two different visual sensory cells are responsible for vision – rods and cones. These differ in appearance and function. But some deep-sea fish go their own way with their retinal cells: as larvae, they develop a unique hybrid of the two sensory cell types, as biologists have discovered. The hybrid visual sensory cells of these fish resemble light-sensitive rods in appearance, but genes for cones adapted to brightness and color vision are active in them. This makes these fish an absolute exception among vertebrates. This adaptation could make it easier for them to orient themselves in their juvenile habitat, the twilight zone of the sea.
The eyes of vertebrates are a true miracle of evolution. Its flexible lens, different types of visual sensory cells in the retina and sophisticated signal interconnection make it possible to recognize colors, shapes and structures even in difficult lighting conditions. Some mammals can see UV or infrared light themselves, others have particularly good night vision or visual acuity. Almost all vertebrates have two basic types of visual sensory cells in common. “For more than 150 years, textbooks have told us that the eyes of most vertebrates are made up of cones and rods,” says senior author Fabio Cortesi from the University of Queensland in Brisbane. The rods contain a particularly large number of densely packed visual pigments and are particularly sensitive to light. The cones, on the other hand, are adapted to bright conditions and ensure, for example, color vision in humans.
First cones, then rods – usually
Which visual sensory cells dominate in an animal’s retina depends on the lighting conditions in their habitat: Day-active animals usually have a particularly large number of cones in their retina, while nocturnal animals have more rods. “At one extreme in this regard are species with pure rod retinas, including some nocturnal reptiles and many deep-sea fish,” explain lead author Lily Fogg from the University of Queensland and her colleagues. But despite these differences, the development of the eyes and retina always follows the same pattern – at least that is the common assumption: “Vertebrates begin their lives with a retina dominated by cones, while the rods only develop later in life,” say the biologists. “This ‘cone first’ sequence of retinal development fits well with the ecology of terrestrial vertebrates and also most marine fishes, which initially live in the bright upper zones of the sea.”
However, this does not apply to many deep-sea fish, which also live as larvae in darker ocean layers. These fish species often begin their development in the twilight zone of the ocean before moving into the dark depths as adults. “We know that these deep-sea fish then optimize their sense of vision in order to be able to see in the dark,” says Cortesi. “We therefore wanted to investigate how the sense of vision develops during the juvenile stages of these deep-sea fish, before they dive into one of the darkest and largest habitats on earth.” To do this, the researchers examined the eye development of three species of deep-sea fish from the hatching of the larvae to the adult: the lanternfish Benthosema pterotum, the salmon herring (Maurolicus mucronatus) and the deep-sea fish Vinciguerria mabahiss, which is a member of the mollusk family. The juvenile stages of all three fish species occur in the twilight zone of the Red Sea at water depths between 20 and 200 meters. However, only the salmon herring later stays in this semi-dark zone, the other two move into the pitch-dark deep sea.
Deep-sea fish larvae with hybrid visual sensory cells
When the biologists examined the eyes of these fish species at different life stages, they discovered something astonishing: “From the earliest life stages onwards, photoreceptors that correspond morphologically to rods dominated the retinas of all three species,” report Fogg and her colleagues. Unlike what is typical for vertebrates, these fish do not begin their lives with cones in their retinas. “These three deep-sea fish therefore deviate from the usual cone-to-rod development path of other vertebrates,” explain the researchers. What is even more unusual, however, is what happens in the externally rod-like light sensory cells of these fish larvae. Analyzes of gene activity revealed that numerous genes are read in these visual cells that are actually only active in cones. As larvae, these three fish species have a unique mixed type of the two photoreceptors. “These hybrid cells combine the molecular machinery and genes of the cones with the shape and features of the rods,” says Cortesi.
These deep-sea fish not only break the normal sequence of visual sensory cell development, they have also developed their very own hybrid form of photoreceptors. The biologists suspect that these fish larvae have adapted to the dim lighting conditions of their juvenile habitat. “These photoreceptors optimize vision in dim and dim conditions,” explains Fogg. It is fitting that the salmon herring retains this mixed form of its visual sensory cells even into adult life, as it continues to stay in the twilight zone later on. The other two deep-sea fish migrate as adults into the dark zones of the deep sea and, at the same time, convert their hybrid receptors into normal rods.
Source: Lily Fogg University of Queensland, Brisbane) et al., Science Advances, doi: 10.1126/sciadv.adx2596