Blind Cave Tetra

The blind cave tetra is the form of the Mexican tetra that lives in underground caves. As a result of living in total darkness for a long time, its eyes do not develop and become covered with skin; it has also lost its pigment, so its body is pinkish-white. It finds its way with its lateral line organ, which senses vibrations and pressure differences in the water, and it detects food through taste and smell.

The sighted, silvery surface form of the same species is also known among aquarists as the "Mexican tetra." The blind form is a model organism widely used in evolutionary and developmental biology research. It is hardy, has a big appetite and is easy to care for.

The surface form of the species lives in rivers and streams stretching from Texas to eastern Mexico. The blind populations are found in limestone caves in northeastern Mexico, in pools that are completely dark and keep a constant temperature all year round. These waters are usually hard and slightly alkaline.

• Volume: at least 120 liters for a group of 5–8; 80–100 cm of length.
• Water: 20–25°C; pH 6.5–8, medium to hard water. A heater is not essential in most room conditions.
• Decor: because it has no eyes, avoid stones with sharp edges. Sand or rounded gravel, smooth stones and roots are suitable; light intensity does not matter.
• Plants: it usually does not harm plants, but it may uproot newly planted ones while stirring up the substrate.

Omnivorous with a very big appetite. Sinking granules and tablets, flakes, frozen bloodworms, brine shrimp and mysis shrimp are suitable. Because it finds food by smell and taste, sinking foods work better. Avoid overfeeding.

Relatively peaceful but active and greedy. It has minor scuffles with its own kind; it may bite the fins of slow, long-finned fish. It eats small fish and shrimp that fit in its mouth.

Good companions: robust species of similar size: barbs, danios, large corydoras, Ancistrus, peaceful medium-sized cichlids, large tetras.

Avoid: neons and similar small tetras, dwarf shrimp, long-finned fish such as bettas and fancy guppies, overly skittish species.

It lives in a loose group: at least 5, ideally 8 individuals. A mild pecking order forms within the group, but serious fights are rare. Fish kept alone may bother other species more. The sex ratio does not matter.

It is an egg scatterer and breeds easily in the aquarium. In a separate tank it lays a large number of eggs on fine-leaved plants or a spawning grid; the parents must be removed because they eat the eggs. The fry hatch in 1–2 days. As an interesting detail, eye buds begin to form in the embryos but regress in later stages of development and remain under the skin.

• Injury: because it has no eyes, it may bump into sharp decor; choose decor with smooth contours.
• Obesity: because it has a very big appetite, it easily becomes fat; feed in moderation and with a balanced diet.
• General: it is resistant to disease.

Females are larger with a noticeably plumper belly. Males are more slender; the difference becomes clearer especially in mature fish.