The Mexican Tetra, Astyanax mexicanus, is a freshwater fish also known as the Mexican Blind Cavefish or Blind Cave Tetra. These fish come in two forms: with eyes and without eyes. The blind variant is a popular fish among freshwater aquarists.
Mexican Tetras are the only characid species native to the United States. Mexican Tetras have a typical teardrop body shape found in most characins. Both forms have triangle-shaped dorsal fins, pointed pectoral and anal fins, and a forked caudal fin. In the blind form, they have no eyes and no pigment which makes them pinkish-white in color.
Mexican Tetras are native to Texas and Mexico living in the Lower Rio Grande and the Nueces and Pecos Rivers. They are a peaceful species. They spend most of their time in midlevel water above rocky, sandy riverbeds. They come from a subtropical climate and prefer warmer waters.
Mexican Tetras are valued for their peaceful nature and intriguing look.
Why is the Mexican Tetra Blind?
The Mexican Tetra is blind as an adaptation to where they live. There are two types of Mexican Tetra fish. One lives in waters that get access to sunlight and the other lives in dark caves. The Mexican Tetra that gets access to sunlight has eyes, while the fish that does not see the sun is born without them.
This is an adaptation that has evolved over many generations of these fish living in completely dark environments. Research shows this is a combination of cellular degeneration in the lens of the eye and natural selection.
Mexican Tetra Evolution
Researchers have debated if the blind tetra and the surface-dwelling tetra are two different species, but have settled on them being the same.
The blind cave tetra does not let its lack of sight keep it from getting food. They are hardy, adaptable fish. They prefer tanks that mimic their natural habitat. They are happiest in a tank with a rocky substrate and low lighting.
They are schooling fish by nature and become semi-aggressive as they age.
The surface-dwelling Mexican Tetra is now being seen in some non-native areas across the United States. Some accidental introductions have meant schools of these fish can be found in Arizona, Oklahoma, and Louisiana. They are in severe decline in New Mexico and are considered endangered in that state.
Mexican Tetras do not do well in cold climates and will migrate to warmer climates during the winter months. The surface-dwelling Mexican Tetra live in a wide range of sunny rivers, tributaries, and reservoirs throughout the southern part of the United States and down into Mexico. The blind tetra is only found in caves or other dark areas. Many places see a school of surface-dwelling Mexican Tetras right outside of the blind tetras’ caves.
The surface-dwelling tetra lives in large schools of several hundred fish while the blind tetra prefers to be more solitary. Surface tetras want swift-moving water with significant outflow while the blind tetra lives in calmer waters.
These fish have developed large jaws, additional teeth, and an increased number of taste buds to help them feed. They also have an amazing memory. They can navigate and map out their habitat.
The two forms have evolved to hunt differently too. The surface-dweller is an aggressive predator. They are seen as potential problems in areas where they are not native. The tetra may target other species’ young. The surface-dwellers live in large groups that tackle predators collectively.
The blind form is the calmer, more peaceful sibling. It prefers to swim alone unless they are breeding. They use highly developed lateral lines along their body to detect movement. The blind form also has an increased olfactory bulb size which increases the fish’s sense of smell.
Mexican Tetra Care
Mexican Tetras prefer warmer waters with temperatures between 68 and 77 degrees Fahrenheit. The tank pH level should be between 6.5 and 8.0.
Mexican Tetras need to be in a 20-gallon tank or larger and 3 or more tetras per tank is recommended. They grow to be around 3 or 4 inches long.
Mexican Tetra Food & Diet
The Mexican Tetra is an adaptable fish. In the wild, their diet varies depending on their location, but they prefer to be mostly carnivorous. Their diet consists of eating crustaceans, insects, annelids, and other smaller fish. In captivity, and if they have no other option in the wild, these fish will become omnivorous. They will eat commercial flake food. They will munch on supplements of live or frozen foods, insect larvae, or algae wafers too.
The Blind Cave Tetra will probably take more time getting to the food than the other fish. Add some foods that sink to ensure they have enough to eat.
Mexican Tetra Lifespan
Mexican Tetra will live for 3 to 5 years in captivity with proper care. In the wild, they will only live about 2 years.
Mexican Tetra Tank Setup
A dark sand or gravel substrate with a cave or shelter will help the Mexican Tetra settle into a new tank. The Mexican Tetra will not harm plants in the tank, any plants that thrive in those water conditions are good to go.
These fish can be a little shy when they first get into a new tank. Once they figure out the space they will swim around without any problems. They can usually be caught hanging out in the middle of the tank.
Mexican Tetra Breeding
The breeding season for the Mexican Tetra is in the late spring through the summer, but in warmer climates, they will breed all year long. They reach sexual maturity around 6 months old and will lay a large group of adhesive eggs.
Mexican Tetras can be bred in captivity fairly simply. They should be set up in a separate breeding tank with similar water conditions as the community tank. The adults are known to eat the eggs, so adding a mesh lining will let the eggs fall through to the bottom of the tank while keeping the adults away from them. Adding a sponge filter is also recommended as they have the filtration power needed to clean the water in the tank without sucking up any newborn fry.
Before breeding, males and females should be identified and moved to separate tanks. Males and females are difficult to tell apart. Males have an orange or yellow tinge to their fins. They also have a small hook on their anal fin.
The male and female pair should be conditioned with high-quality foods for at least a week. Then at the end of the week, the healthiest looking male and the healthiest looking female should be taken out and placed in a breeding tank.
If the spawning was successful, there will be eggs in the tank the next morning. If no eggs are in the tank, take the pair out and try with a different set of fish. Mexican Tetras can lay up to 1000 eggs per female. They will be white and will hatch in about 24 hours. Newborn fry should be fed infusoria for about a week. They may also munch on their egg sacs during this time. After that, they can be given baby brine shrimp, micro worms, or other fry foods. Powdered foods can be introduced around the 1-month mark.
One interesting feature to note: The fry may look like they are born with functioning eyes, but they will eventually get covered up by a layer of skin and will be absorbed into the fish. Keep the fry in the spawning tank until they are big enough to introduce to the community tank.
Mexican Tetra Disease
The Mexican Tetra is a hardy fish not prone to any unique diseases. Ich will be the main illness to look out for in these fish and it may be hard to spot on the light-colored fish. Treatment for Ich can be treated by fixing water parameters, quarantining the infected fish, and adding medication to the water.
Mexican Tetra Tank Mates
These fish can be paired with other fish of similar size and temperaments. Good tank mates for the Mexican Tetra are other Mexican Tetras. A group of 3 or more is suggested. They also get along with other tropical fish who are not aggressive and will let them have their space.
Incompatible tank mates for the Mexican Tetra will be anything that prefers cold water and highly aggressive fish.
Where Can I Find Mexican Tetra for Sale?
Not many online places have Mexican Tetras listed for sale, but some shops have them on sale for about $6 each.
Mexican Tetras have evolved to have two different forms. The surface-dwelling tetra is a schooling fish that prefers to be in big groups and is very active. The blind form is a calmer fish that prefers a slower, more solitary life. They do not like to be completely alone, but a group of 3 to 6 is great.
Blind Tetras have become an increasingly popular fish in the aquarium niche. They are peaceful, fish who live well in a community tank, despite it being blind.