Deep-sea fish larvae rewrite the rules of how eyes can be built

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 fishes, 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 fishes, 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 fishes 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 dragonfishes, and a member of the lanternfishes, 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 fishes 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 fishes 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.