By Jessie M. Sanders, DVM, CertAqV
Every once in a while, it is not uncommon for a fish to be born without an eye. It is a natural occurrence of genetic chance. But what about a species of fish that has evolved and adapted to having no eyes at all?
Meet Astyanax mexicanus, also known as the Mexican Blind Cavefish or Blind Cave Tetra. These fish are unique within the expansive tetra family, and come in two distinct forms: one with eyes and one without any eyes.
How did this genetic anomaly occur in this species of fish? It is related to where they live. Although the Astyanax mexicanus is one species, there are two types: those that live in waters that get access to sunlight have sighted eyes. Their eyeless relatives live in dark caves, where they wouldn’t be able to see much even if they did had eyes. These remarkable fish adapted to living in completely dark environments, and over many generations evolved to become an eyeless variety. Blind Tetras have become increasingly popular in the aquarium trade and are a good addition to almost any tropical community tank, even though they cannot see.
Evolution of the Mexican Tetra
Where in their lineage did this subspecies lose the need to have eyes and gradually develop to have no eyes at all? Experiments have shown that cellular degeneration in the eye lens itself is key to the lack of eyes (Jeffery, et al, 2003). This phenomenon combined with natural selection over many generations, has, theoretically, led to the development of these eyeless fish. With the lack of sun exposure, these fish also lost skin pigment over time, developing a pink-white skin coloration similar to albinism.
Despite their adaptations, these unique fish live well with others in a community tank; they are peaceful, easygoing fish.
Care for the Mexican Tetra
Tetras are hardy tropical fish. They eat a varied diet, and care for blind tetras is similar to caring for any other tetra species. Because tropical fish require a heated tank environment, keeping a thermometer in the tank to make sure your heater is working appropriately is always recommended.
Mexican Blind Cavefish like to be kept in groups of three or more. They can grow up to 3 inches, and since they do best in groups, it is recommended that they are kept in a 20 gallon or larger tank, especially if they are kept with other species.
They may be a little shy right after you add them to your tank and if you have a cave, they will find it. Just give them a little time to adapt to their new home and before you know it, they’ll be adventuring about your tank. In fact, Cave tetras are able to maneuver about their tank just as well as a fish with eyes. After a few weeks, they will be able to remember their way around different décor in their tank. (Yes, fish do have great memories!)
They use their nares to sniff out food and their sensitive lateral line system can feel vibrations in the water around them. However, keep in mind that the Cave Tetra may be a little slower to their food than their sighted compatriots. You may need to distract other more voracious eaters in your tank or keep your blind tetras with calmer fish.
Overall, the Blind Cave Tetra is a fascinating and unique addition to any community tank. Tropical community fish tanks work best with a diverse range of fish, and the Blind Cave Tetra makes a hardy and easygoing tank-mate. These exceptional fish may need a little extra TLC, but with time, they will become valuable members of your fish world.
Jeffery WR, Strickler AG, Yamamoto Y. 2003. To See or Not to See: Evolution of Eye Degeneration in Mexican Blind Cavefish.
Integrative and Comparative Biology; 43(4):531-541.
http://fishbase.org – Astyanax mexicanus