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. 

Sources:

(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

Blind Cavefish Offer Evidence for Alternative Mechanism of Evolutionary Change

In a blind fish that dwells in deep, dark Mexican caves, scientists have found evidence for a long-debated mechanism of evolutionary change that is distinct from natural selection of spontaneously arising mutations, as reported this week in the journal Science. [...]

Using the cavefish, the team demonstrated for the first time in nature how "standing" or "cryptic" genetic variations in an animal, which have been inherited from prior generations without causing any physical changes in the animal, can be "unmasked" by the shock of entering a new environment. Gene variants that improve the animal's ability to adapt to that new environment can then be selected for, and passed on to its progeny. This is distinct from the established evolutionary mechanism of "de novo" genetic mutations that arise by chance after the animal has entered the new environment, which also provide a substrate upon which natural selection can act.

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Rapid Evolution of Novel Forms: Environmental Change Triggers Inborn Capacity for Adaptation

In the classical view of evolution, species experience spontaneous genetic mutations that produce various novel traits -- some helpful, some detrimental. Nature then selects for those most beneficial, passing them along to subsequent generations.

It's an elegant model. It's also an extremely time-consuming process likely to fail organisms needing to cope with sudden, potentially life-threatening changes in their environments. Surely some other mechanism could enable more rapid adaptive response. In this week's edition of the journal Science, a team of researchers from Harvard Medical School and Whitehead Institute report that, at least in the case of one variety of cavefish, that other agent of change is the heat shock protein known as HSP90.