As someone who studies seal whiskers, I am all about sensory biology that allows aquatic animals to navigate in dark or murky water. I came across a recent popular article about blind Mexican cavefish: How do blind cavefish find their way? The answer could be in their bones. The full scientific publication can be found here: Cranial asymmetry arises later in the life history of the blind Mexican cavefish, Astyanax mexicanus. Mexican cavefish are born with symmetrical eyes, but their eyes gradually regress due to the extreme darkness that they live in. The eyes are completely lost as the cavefish become adults, and the remaining eye orbits collapse and are subsequently filled with fat deposits. The asymmetrical body shape of the cavefish gives them a type of “handedness,” like left- or right-handed traits in humans. This handedness could also contribute to their swimming pattern/direction because cavefish tend to exhibit a “wall-following” swim pattern, where they keep one side of their body close to the tank/cave wall. The blind fish are able to feel where they are thanks to their lateral line system, a sensory system comprised primarily of neuromasts. Cavefish utilize their asymmetrical bone structure, and consequently asymmetrical neuromasts, to thrive in their pitch black environment. In this article (Blind Cavefish Navigate Their Way in the Dark: Here’s How), there is a video showing cavefish swimming behavior. In addition, interestingly, despite being blind, cavefish can still detect light differences thanks to a specialized gland.
After looking into the aforementioned publication, I thought about flounders because they also have some crazy eye biology going on. However, their eyes migrate rather than disappear. It was the asymmetry aspect that tied cavefish to flounders in my mind. Lots of asymmetries exist in animals, but I wanted to stay on the topic of fish today. My favorite flounder/flatfish article that I found was The Improbable-but True-Evolutionary Tale of Flatfishes. Flounders are grouped together with other flatfish (e.g. halibut), which are “the most asymmetrically-shaped vertebrates to ever live on earth.” Just like cavefish, flounders start out symmetrical when they are born. As the fish mature, one eye begins to migrate to the other side of the head, thereby also moving/rearranging bones. I thought that the most interesting part was that a flounder’s swim pattern could be affected by light levels. The article explains that both vision and internal balance receptors are involved in the flounder’s swimming pattern and development. It is only when the lights are turned off that flounders get confused and begin swimming erratically (article also includes video of this behavior). This is because the brain tries to keep a balance of brightness on either side of the flounder while it is swimming, but, without light, their subpar internal balance compass takes over. Consequently, most flounders sink to the bottom…a place where having another eye on top of your head does not sound like a horrible idea. I found it fascinating that darkness could have such a huge impact on the development of these fish. I would highly recommend watching the videos in the articles! Also, try checking out these clips:
How Odd Bone Structure of Blind Cavefish Helps in Navigation: short clip that summarizes the scientific publication listed above
Flatfish Metamorphosis: image compilation of a flatfish maturing/eye migration
Flounder is Master of Disguise (Nat Geo Wild): talks about flatfish eye migration/camouflage
Other Resources: Flounders’ Eyes Face Skyward. How Do They See the Ocean Floor?
Lastly, Happy World Environment Day!