Carlastyanax facts for kids

Quick facts for kids Carlastyanax aurocaudatus
1917 illustration
Conservation status
Near Threatened (IUCN 3.1)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Characiformes
Family:	Characidae
Genus:	Carlastyanax Géry, 1972
Species:	C. aurocaudatus
Binomial name
Carlastyanax aurocaudatus (C. H. Eigenmann, 1913)
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Synonyms
Astyanax aurocaudatus Eigenmann, 1913

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The Carlastyanax aurocaudatus is a small freshwater fish. It belongs to the Characidae family. This fish lives in the Río Cauca in Colombia. It is the only type of fish in its group, called a monotypic genus. This group is named Carlastyanax.

This fish was first named in 1972 by Jacques Géry. Before that, it was called Astyanax aurocaudatus by Carl H. Eigenmann in 1913. People have discussed if its special group name, Carlastyanax, should be used. But today, scientists agree it is a real and unique species.

This fish has cool colors. Its back is greenish-yellow, and its belly is white. Its fins can be red or yellow. What's neat is that the colors are different for male and female fish. This is called sexual dichromatism. Female fish have yellow-gold tail fins. This is how the fish got its name. Male fish have reddish or pinkish tail fins.

This fish lays its eggs at different times of the year. It likes clear water streams with rocky or natural bottoms. It lives in a small area. Its home also has insects and plants that it eats.

How This Fish Got Its Name

The Carlastyanax aurocaudatus was first named by Carl H. Eigenmann in 1913. Scientists have studied it a lot to figure out its proper group. At first, they thought it belonged to the Astyanax group.

In 1972, a French scientist named Jacques Géry created a new group for it. He looked at things like its teeth and fins. One important feature was a special hooked tooth. He named the new group Carlastyanax.

Later, in 2005 and 2006, some researchers said it should go back into the Astyanax group. They thought it looked more like those fish.

But in 2013, more scientists looked closer. They found other differences that made Carlastyanax unique. For example, it has four teeth in one row, while Astyanax fish have five. It also has eight rays in its dorsal fin, while Astyanax fish have nine.

A study in 2015 used molecular phylogeny (looking at DNA) to confirm this. Today, Carlastyanax is seen as a real and separate group. The C. aurocaudatus is its only species. In 2017, scientists found that Carlastyanax is related to the Creagrutus and Piabina groups.

What's in a Name?

The Carlastyanax aurocaudatus is sometimes called the gold-tailed tetra. Its full name actually means "gold tail." "Auro-" means "gold," and "caudatus" means "tail."

The group name "Carlastyanax" honors Carl H. Eigenmann. It also includes "Astyanax," the original group the fish was thought to be in.

How This Fish Looks

The Carlastyanax aurocaudatus usually grows to about 6.0 cm (2.3 in) long. Some bigger ones can reach 7.4 cm (2.9 in). It has 8 rays in its dorsal fin and 26–28 rays in its anal fin.

Its head is big with a rounded snout. Its body is strong. The main color of its scales is white on the bottom. The top part of its back is green to yellow. It has a silver or silver-blue stripe along its side.

It usually has a blue spot on its shoulder. This spot is long and goes up and down. Its front fins are pink or red with a dark edge. Its other fins have different colors depending on if it's a male or female fish.

There is another fish, Bryconamericus pectinatus, that looks similar. But it stays in its own group, Bryconamericus.

Boy Fish vs. Girl Fish: Color Differences

Male and female C. aurocaudatus fish have very different colors. In males, the anal fin and tail fin are reddish. The tail area is also reddish. The dorsal fin has a dark edge and is white or yellow at its base.

In females, the anal fin and the whole tail area are yellow or yellow-gray. The dorsal fin still has a dark edge. But its base is often a dark yellow instead of pale. The scientist Eigenmann noticed the tail color difference when he first described the fish. But he didn't know it was a difference between males and females.

Besides color, there are other differences. Male C. aurocaudatus don't have hooks on their fins like some other related fish. Instead, their fin-rays are just bigger than the females'. Males are also much larger than females.

Where It Lives

The Carlastyanax aurocaudatus lives in a small area. It is found only in the upper Cauca river basin in Colombia. You can find it in different creeks and small rivers there. It was first found in the Boquia area.

This fish likes places with natural material on the bottom. It also prefers clear water. A mix of natural materials and stones on the riverbed is common where it lives. It tends to stay close to the bottom of the river. It likes water with a pH of 5.8–7.5 and a temperature of 22 °C–25 °C.

What It Eats and How It Lives

More than half of what C. aurocaudatus eats is insects. It especially likes insect larvae that live in water. Insects that live on the bottom are also common in its diet.

The rest of its diet is plant material. Most of this plant material comes from outside the water, like leaves or seeds that fall in. This fish is good at eating different things. So, it doesn't compete much with other fish for food. Even if other fish eat insects, C. aurocaudatus also eats things like seeds and fruits.

This fish lays its eggs during both the wet seasons (April to May, September to October) and the dry seasons (January to February). During these times, the colors of its anal and tail fins become brighter.

Carlastyanax aurocaudatus has been seen living with other fish species. These include Cetopsorhandia boquillae, Trichomycterus caliense, and Poecilia caucae.

Protecting This Fish

The IUCN (International Union for Conservation of Nature) says Carlastyanax aurocaudatus is a "near threatened" species. This means it could become endangered in the future.

It lives in a small area. Farming and development are big problems for it. These activities cause pollution. They also lead to cutting down trees, which harms the areas along the rivers where the fish lives. However, scientists are not sure yet if this is directly affecting the total number of C. aurocaudatus fish.