Fish of the Day - Mexican Tetra
Today's fish of the day is the Mexican Tetra!
(image source: Wikipedia https://en.wikipedia.org/wiki/Mexican_tetra)
The Mexican tetra, Astyanax mexicanus, also known as the blind cave tetra, is a freshwater fish known for their surface dwelling, and blind cavefish forms. These fish are found throughout subtropical America, in Southern Texas and New Mexico, and Northeastern Mexico. Along the Rio Grande, Nueces, and Pecos rivers. Living over gravel and sandy regions in mid to shallow moving waters. Waters must range from 68-77°F or 20-25 °C, and fish will migrate south to warmer waters when temperatures decrease in winter.
(image source: Denver Zoo, https://denverzoo.org/animals/blind-cave-fish/)
Blind cavefish are shaped by the pitch black caves they live in, losing their silver color and turning albino-esc pink, and losing their eyes. The degree of eye loss relating to the amount of light present in their environments. Despite this characteristic eye loss, the blind Mexican tetra can continue to react to light when introduced. This is due to a pineal eye, light sensing nerves that wire into the brain separate from the eyes. However, this pineal eye is primitive, only able to tell shadow from light, and unable to sense finer details. The loss of vision has little effect on the tetras, with the lateral line sensing organs becoming particularly sensitive, and a heightened nervous system. This allows for both an extraordinary sense of smell, as well as slight echolocation. Blind cavefish can be found in 3 major cave regions, around 30 living populations, with DNA testing showing that Mexican tetra surface dwellers have entered and adapted to their cave environments on 5, if not more, separate times. This means that different populations of fish have undergone regressive evolution uniquely of other cavefish populations, making it all the more fascinating that all populations of cavefish and surface dwelling fish can interbreed.
(image source: fish base, https://www.fishbase.se/photos/PicturesSummary.php?resultPage=2&ID=2740&what=species)
Schooling in groups of several thousand fish, in both the wild, and in captivity these fish are fierce hunters. Growing to a grand total size of 12cm, the Mexican tetra primarily consumes invertebrates. Crustaceans, Arthropods, larvae, and water beetles are common. Aquarium fish are known to bite at the fins of other fish, and occasionally engage in cannibalism. When food supplies dwindle, a common event in cavefish, the diet can be supplemented with algae. Despite their efficiency as hunters, these fish are regularly predated by larger predatory fish. Due to this, these fish can emit an alarm substance, which warns other fish to hide.
(image source: wikipedia, https://en.wikipedia.org/wiki/Mexican_tetra#/media/File:Epigean_and_cave-dwelling_Mexican_tetras_Astyanax_mexicanus.jpg)
There is no particular breeding season for the Mexican tetra. Male and female fish signal to another interest with erratic and exaggerated gill and mouth movement, likely to sense water movement of another in blind populations. Breeding takes place as the tetras eject ova and sperm well swimming side by side, hiding eggs in crevices. Within 24 hours fertilized eggs will hatch into a larval form. These fry will sexually mature within 6 months, and can live up to 10 years! Interestingly, fish are born with eyes that will later cloud and then shrink due to lack of light in cave populations.
(2013). Retrieved from https://www.qualitymarine.com/news/blind-cave-tetra-astyanax-mexicanus/
(N.d.-a). Retrieved from https://www.fishbase.se/summary/Astyanax-mexicanus
(N.d.-a). Retrieved from https://denverzoo.org/animals/blind-cave-fish/
