Lampreys are jawless fishes known for their parasitic lifestyles. Young lampreys move from freshwater streams to find a host to latch onto in the ocean. After more than a year of feeding on a host, the lampreys move back to freshwater streams to breed and eventually die. These very different phases of their lifestyle come with major differences in their environments, moving from saltwater to freshwater, and going from eating continuously to living off energy reserves in the body. It is not surprising that these big differences in the environment might result in changes to the lamprey's body, including how they sense the world around them. Previous work has suggested that the sizes of different parts of the brain responsible for the senses change with age. So how might vision change when a lamprey moves from a well-fed state in a marine environment to starving in a freshwater stream? Hermann Collin from the University of New South Wales, Australia, and colleagues from La Trobe University, Australia, and the University of Western Australia studied lamprey eye anatomy in the early and late stages of the life cycle to understand how vision might change across time.

The researchers collected Australian lampreys (Mordacia mordax) from several streams in New South Wales, Australia, at two points in the animals’ life cycle – the early phase when they were younger and smaller, moving downstream towards the sea; and the late phase, when they were older, larger and moving upstream away from the sea and back to their freshwater breeding grounds. The researchers used powerful microscopes to compare the anatomy of the eyes, including the cornea (the clear part of the eye that allows light in) at the two different life stages.

They discovered that there were several differences between the eyes of lampreys in the early life phase and those in the late phase. The outer layers of the cornea were thicker in the late life stage lampreys than the younger lampreys that were headed out to sea. These thicker layers of the cornea in older lampreys have a similar refractive index to sea water, so they do not bend light as it enters the eye, giving the fish a clear view; the cornea is optimized for vision in the saltwater that the older lampreys migrated from. The team also investigated the mucoid layer of the cornea, which is responsible for keeping the eye moist. They discovered that it is attached to the tissue that surrounds the iris in the late phase of the lamprey's life, but is detached in the young lampreys, which may allow extra room for the eye to grow when fish are young, similar to how human babies are born with unfused skulls to allow for brain growth. The young lampreys also had less organized collagen bundles in the center of the cornea compared with the older lampreys. As the lamprey grows, these collagen bundles likely become more organized to keep the cornea as transparent as possible, which helps the lamprey to see which fishes may serve as their next buffet.

While lampreys aren't particularly charismatic, they offer some amazing insights into the eyes and vision of some of the earliest vertebrate ancestors, as they are so closely related to these ancient animals. Understanding how their vision works and how it changes when exposed to different environments can help us to uncover how modern vertebrate vision originated and how vision may be affected by aging or the environment. This knowledge can help researchers to better understand how perception may have evolved and how senses such as vision can respond to changes in challenging environments.

Collin
,
H. B.
,
Ratcliffe
,
J.
and
Collin
,
S. P.
(
2023
).
The functional anatomy of the cornea of the shorthead lamprey, Mordacia mordax (Mordaciidae, Agnatha): a comparison between downstream and upstream migrants
.
J. Morphology
284
,
e21552
.