Astyanax fasciatus mexicanus in Aquaria and the Wild
By James C. Cokendolpher
reprinted from American Currents, July-Sept. 1978
The characins or tetras have long been a favorite fish among aquarium
hobbyists. Unfortunately, many hobbyists still overlook the less colorful and spectacular
tetras. One group in particular, the genus Astyanax, has often been coined
undesirable. Much to the contrary many species of Astyanax are excellent aquarium
fish. Unlike most tetras, specimens of some Astyanax can be collected by the
hobbyist without an expedition to the Amazon or Congo Rivers.
The genus Astyanax is very rich in species and subspecies. The distribution of Astyanax
is strictly New World and can be found in suitable habitats from southern Arizona, New
Mexico and Texas south to Patagonia (Argentian) on the Atlantic coast. On the Pacific
slope it can be found from Columbia to central Mexico. One species, A. fasciatus, ranges
almost the entire range of the genus and is composed of many subspecies or geographical
races. The northern race, A. fasciatus mexicanus, is to be the subject of this
Body typical for the genus, moderately elongated and strongly compressed laterally; about
equally convex in the upper and lower profiles. Overall the color is a silvery-white to a
pale brassy; ventrally, silvery to olive. The base of the caudal fin with a diamond-shaped
black mark which extends forward, above the lateral line, to behind the eyes and
posteriorly to the notch in the caudal fin as a black bar. Adipose fin present. Caudal fin
deeply emarginate. The fins of the male are colorless except for the pectoral, anal and
caudal fins. The pectorals are pinkish and the caudal fin is a warm yellow-gold with a
black marginal band.
The anal is pinkish to bright red with a black marginal border. Males from the southern
range appear to have less red and more yellow in the anal and pectoral fins. The fins of
the female are similar to the males, but less intense in color. The pectorals are
colorless. Females are larger and more robust, length up to four inches.
The range of Astyanax fasciatus mexicanus is rather large. As presently understood
the native range meets its northern limit in the lower Colorado River drainage of Arizona
and New Mexico as well as the Pecos and Nueces Rivers in Texas. The range then extends
southward along the Gulf coast to an arm near Laguna de Terminos, Campeche, Mexico and
westward in the rivers of the Isthumus of Tehuantepec on the Pacific coast to the Rio
Armeria basin in Jalisco, Mexico. From the Rio Armeria basin the range extends eastward
and northward through eastern San Luis Potos; and Coahuila, Mexico.
As this subspecies readily adapts to clean running waters, bait release introductions
in the southern United States are not uncommon. The introduced subspecies range in Texas
extends from the northern native boundary to the Wichita and Red Rivers in north-central
Texas. The precise introduction ranges in surrounding states are uncertain at the moment.
In the wild, Astyanax fasciatus mexicanus inhabits many varied habitats.
In much of southern Mexico they can be found in flowing jungle rivulets. Further north
they are often captured in rivers created from mountain runoffs. In the extreme northern
areas they are most abundant in running waters of lakes and rivers. No matter where this
tetra is found it seems always to be associated with moving clear waters and a gravel
bottom. It is seldom encountered in deep waters, preferring to stay in the shallows. Even
in rivers and lakes where a large diversity of predatory species are abundant, this
species will seldom seek refuge in the aquatic vegetation. Generally wherever this species
is found it is the dominant fish. In certain rivers and creeks the schools of tetras are
so dense that the water appears black.
CAPTURING LIVE SPECIMENS
Once the hobbyist has located a likely site the job of collecting specimens begins.
Although the fish may seem abundant they are very fast swimmers and are able to jump out
of' the water when disturbed. Dip nets seldom yield specimens unless the fish are located
in a shallow creek not more than a few feet across. Baited minnow traps yield some
specimens but it is best to check the local game laws before attempting this method. Some
minnow traps are 1 illegal without specific scientific collecting permits. The best
all-around method is to use a 20 foot seine. The seine, when dragged through shallow
waters with little vegetation, will usually yield many specimens as well as many other
suitable aquarium fish. Again, the collector should check game laws as seines may be
illegal in some waters. Once the quarry is captured great care must be exercised in
getting the fish home. Never pack more than 2 or 3 fish per carrying bag. These fish are
extremely active and quickly consume all available oxygen. Lowering the water temperature
greatly increases the chances of getting the fish home. A dark container also seems to
quiet the fish. It is much better to arrive home with a few good specimens than a lot of
For proper maintenance of Astyanax a roomy aquarium is a must. For more
than a pair, nothing less than 20 gallons will really work. This species schools actively,
so a community of its own kind is desirable. Although not aggressive, this species has
been known to nip a few fins when crowded; so they are best kept with similar sized fish.
The bottom substrate should be of rocks and small to medium-sized gravel. Plants are not
essential but will help make the inhabitants feel at home. Well circulated, clean water
brings out the brightest colors. Although direct sunlight is suggested the aquarium should
not be too near the window as the water temperature should remain near 75 degrees F.
Occasional fluctuations in water temperature probably won't harm the fish, as the
temperatures in some of its native habitat ranges from slightly above freezing to the
upper 90's. Most commercial flake foods are readily accepted.
For spawning a pair, or even better a trio of 2 males and a single female, is placed
together. Prior to putting the sexes together a period of conditioning should be followed
to allow the female to fill with eggs. The period required for conditioning is not long
(2-4 weeks), but will pay off in the long run. For conditioning plenty of live foods and
clean water are suggested. Once conditioned the fish should be placed together in a roomy
tank. The tank need not be gigantic but should be at least 10 gallons for a single pair. A
photoperiod of 24 hours is very beneficial in promoting spawning. A heavily planted tank
is not essential, but sufficient hiding places must be available for the female to hide
from the aggressive males. The actual spawning is much like other tetras and danios. The
eggs are randomly laid among the plants as well as in open water. Well conditioned parents
seldom eat their eggs. After 2 days together spawning will be completed and the parents
may be separated and reconditioned.
Fertilized ova are 0.77 mm in diameter and have an opaque white center.
Throughout the eggs are many small oil droplets. Since the eggs are slightly adhesive they
tend to stick to plants and gravel. The eggs shell is very soft and will break if handled
roughly. The eggs can not be picked from the substrate as many hobbyists do with killifish
eggs. If it is desired to remove the eggs from the spawning tank, a glass rod can be used
to stir up the substrate, then a fine meshed net can be swirled in the water to collect
the eggs. Although the eggs do not seem sensitive to light it would be wise to treat them
like other tetra eggs and not expose to strong sunlight.
Approximately 24 hours after spawning the eggs will hatch into a small
"tadpole-like" larvae stage. The time for development depends greatly on
temperature and can be delayed up to 3 days with cooler temperatures. The 24 hour
development period is at 75oF. The development of the newly hatched fry depends
solely on nutrients from the yolk sac. No foods need be fed to the fry until the yolk sac
is absorbed. The period of absorption varies but usually by the 3rd day after hatching
foods may be added. At first only the smallest of foods will be taken. Infusoria or green
water is ideal as a first food. In a couple of more days the fry may be shifted over to
microworms and brine shrimp nauplii. Growth is slow, with sexual maturity being reached in
or about the 6th month.
The northern-most race of Astyanax fasciatus, contrary to common belief,
is an ideal aquarium fish. The combination of silvery color and active schooling enhance
desirability of this species. The ease with which breeding occurs and the hardiness of the
adults makes this species a likely candidate for the beginning hobbyist.
Used with permission. Article copyright retained by author.