The deep sea is cold, dark, and under immense pressure. Yet life has found a way to prevail there, in the form of some of Earth’s strangest creatures.
Since deep-sea critters have adapted to near darkness, their eyes are particularly unique — pitch-black and fearsome in dragonfish, enormous in giant squid, barrel-shaped in telescope fish. This helps them catch the remaining rays of sunlight penetrating to depth and see the faint glow of bioluminescence.
Deep-sea fish, however, typically start life in shallower waters in the twilight zone of the ocean (roughly 50-200 metres deep). This is a safe refuge to feed on plankton and grow while avoiding becoming a snack for larger predators.
Our new study, published in Science Advances, shows deep-sea fish larvae have evolved a unique way to maximise their vision in this dusky environment — a finding that challenges scientific understanding of vertebrate vision.
The nightmare of seeing in the twilight zone
The vertebrate retina, located at the back of the eye, has two main types of light-sensitive photoreceptor cells: rod-shaped for dim light and cone-shaped for bright light.
The rods and cones slowly change position inside the retina when moving between dim and bright conditions, which is why you temporarily go blind when you flick on the light switch on your way to the bathroom at night.
While vertebrates that are active during the daytime and predominantly inhabit bright light environments favour cone-dominated vision, animals that live in dim conditions, such as the deep sea or caves, have lost or reduced their cone cells in favour of more rods.
However, vision in twilight is a bit of a nightmare — neither rods nor cones are working at their best. This raises the question of how some animals, such as larval deep-sea fish, can overcome the limitations of the cone-and-rod retina not only to survive but even to thrive in twilight conditions.
Starting where the fish start
To understand how newly born deep-sea fish see, we had to start where they do: in the twilight zone of the ocean.
We caught larval fish from the Red Sea using fine-meshed nets towed from near the surface to a depth of around 200m. This way, we got hold of three different species — the lightfish (Vinciguerria mabahiss) and the hatchetfish (Maurolicus mucronatus), both members of the dragonfish, and a member of the lanternfish, the skinnycheek lanternfish (Benthosema pterotum). Next, we studied what their photoreceptor cells looked like on the outside and how they were wired on the inside.
First, we used high-resolution microscopy to examine the cells’ shape in great detail. Then we investigated retinal gene expression to identify which vision genes were activated as the fish grew. Finally, we got some experts in computational modelling of visual proteins on board to simulate which wavelengths of light these tiny fish may perceive.
By combining all the approaches, we were able to piece together a picture of how these animals see their world. This sounds relatively simple, but working with deep-sea fish is anything but easy.
While these animals are generally thought of as monsters of the deep, in reality, most reach only about the size of a thumb — even when fully grown. They are also very fragile and difficult to get.
Working with larval specimens that are only a few millimetres long is even more difficult. However, by leveraging support from the deep-sea research community, we were fortunate enough to combine specimens from multiple research expeditions to piece together an unusually complete picture of visual development in these elusive animals.
So what did we discover?
For decades, scientists have thought that, as vertebrates grow, the development of their retina follows a predictable pattern: cones form first, then rods. But the deep-sea fish we studied do not follow this rule.
We found that, as larvae, they mostly use a mix-and-match type of hybrid photoreceptor. The cells they are using early on look like rods but use the molecular machinery of cones, making them rod-like cones.
In some of the species we studied, these hybrid cells were a temporary solution, replaced by “normal” rods as the fish grew and migrated into deeper, darker waters.
However, in the hatchetfish, which spends its whole life in twilight, the adults keep their rod-like cone cells throughout life, essentially building their entire visual system around this extra type of cell.
Our research shows this is not a minor tweak to the system. Instead, it represents a fundamentally different developmental pathway for vertebrate vision.
Biology doesn’t fit into neat boxes
So why bother with these hybrid cells?
It seems that to overcome the visual limitations of the twilight zone, rod-like cones offer the best of both worlds: the light-capturing ability of rods combined with the faster, less bright-light sensitive properties of cones. For a tiny fish trying to survive in the murky midwater, this could mean the difference between spotting dinner or becoming it.
For more than a century, biology textbooks have taught that vertebrate vision is built from two clearly defined cell types. Our findings show these tidy categories are much more blurred.
Deep-sea fish larvae combine features of both rods and cones into a single, highly specialised cell optimised for life in between light and darkness. In the murky depths of the ocean, deep-sea fish larvae have quietly rewritten the rules of how eyes can be built, and in doing so, remind us that biology rarely fits into neat boxes.
Fabio Cortesi, ARC Future Fellow, Faculty of Science, The University of Queensland, and Lily Fogg, Postdoctoral Researcher, Helsinki Institute of Life Science, University of Helsinki
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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