Cyprinodontiformes

Many small fish live in fresh water.

Some of these fish are very tiny. Others are a bit bigger. Some fish lay eggs. Other fish give birth to live babies.

These fish often live near the top of the water. They like to eat bugs and worms. Some even eat tiny bits of green plants. They are very good at living in tough places. It is fun to watch them swim!

Many small fish belong to a group called Cyprinodontiformes.

Most of these fish live in fresh water. You may know some of them as killifish or live-bearers. These fish have small mouths and large eyes. They also have one fin on their back.

Some species live in very tough places. They can live in very warm or salty water. Some live in pools with very little minerals. These fish often live near the surface of the water. This helps them get more oxygen. Most of them are carnivores. This means they eat meat like insects and worms. Some fish, like the Flagfish, eat algae. Algae are tiny green plants. Killifish also eat pollen from insects. This helps them make bright colors on their bodies. Some fish in this group lay eggs. Other fish give live birth. This can happen in two ways. Some babies grow inside an eggshell. Other babies grow without an eggshell. The smallest is the least killifish. The largest is the cuatro ojos.

Cyprinodontiformes is a large group of small, ray-finned fish.

Most of these fish live in fresh water. You might know them as killifish or live-bearers. People often call them toothcarps. However, they are not close relatives of true carps. These fish have very specific bodies. They usually have small mouths and large eyes. Most also have one fin on their back. They typically have a rounded caudal fin, which is a tail fin.

These fish have different ways of having babies. Some species are oviparous, which means they lay eggs. Other species are live-bearers. There are two ways to be a live-bearer. In viviparous species, the babies grow without an eggshell. In ovoviviparous species, the babies stay inside an eggshell.

Scientists think live-bearing evolved on its own. It likely happened twice from egg-laying ancestors. One suborder, the Aplocheiloidei, only has egg-laying species. Other groups, like the Poeciliidae, are viviparous. The Anablepidae are mostly ovoviviparous.

Many of these fish live in very tough places. They can survive in very warm or salty water. Some live in pools that are very acidic. This happens when decaying plants make the water change. They can even live in polluted water.

To survive, they often stay near the water surface. This helps them get enough oxygen from the water. Most of these fish are carnivores. They eat things like insects, worms, and tiny crustaceans. Some, like the Flagfish, even eat algae. Killifish also eat pollen from insects. This pollen helps them make bright body colors.

We have known about these fish for a long time. The oldest fossil is called *Cyprinodon primulus*.

These fossils were found in Argentina. They come from the Late Paleocene period. Scientists found these fossils as isolated scales. They are not exactly sure what the fish was. However, they think it was a true cyprinodontiform. Other fish families, like the Kenyaichthyidae, lived during the Miocene in Kenya. This shows the group has a very long history.

This group is very diverse with many different sizes. The smallest fish is the least killifish. It is very tiny when it is an adult. The largest fish is the cuatro ojos, or *Anableps dowei*.

You might see these fish in an aquarium at home. Many popular aquarium fish belong to this order. For example, guppies are very common pets. These fish connect the wild world to our own homes. They show how life can thrive in many different ways.

Cyprinodontiformes is a diverse order of ray-finned fish. Most species in this group are small to medium-sized. They are widely known as toothcarps. However, they are not closely related to true carps. True carps belong to the superorder Ostariophysi. In contrast, toothcarps belong to the group Acanthopterygii. Many people recognize these fish because they are popular in aquariums. This includes well-known groups like killifish and live-bearers.

Physically, these fish share several common traits. Most have small mouths and large eyes. They typically possess a single dorsal fin on their backs. Their caudal fin, which is the tail fin, is usually rounded. The size variety within the order is quite large. The smallest is the least killifish, known as Heterandria formosa. The largest is the cuatro ojos, or Anableps dowei.

One of the most interesting aspects of this order is how they reproduce. Scientists divide the families into three reproductive strategies. Some species are oviparous, which means they lay eggs. Others are live-bearers. Live-bearers are divided into two specific types. Viviparous species give birth to young that develop without an enclosing eggshell. Ovoviviparous species carry the young inside an eggshell until they are born.

Evolutionary history shows that live-bearing is not a single event. Scientists believe vivipary and ovovivipary evolved independently from egg-laying ancestors. This likely happened at least twice. The suborder Aplocheiloidei contains only oviparous species. Within the other suborder, Cyprinodontoidei, certain groups are also exclusively egg-laying. These include the superfamilies Cyprinodontoidea and Valencioidea. This shows how different lineages reached different reproductive solutions.

The fossil record provides clues about the ancient history of these fish. The oldest known fossil is a species called ?Cyprinodon primulus. These fossils were found in Argentina. They consist of isolated scales from the Late Paleocene period. While scientists are not certain of its exact identity, they believe it was a true cyprinodontiform. Other ancient groups include the extinct Kenyaichthyidae. These lived during the Miocene period in Kenya.

Many species in this order are experts at surviving in extreme environments. They can live in very salty or very warm waters. Some inhabit heavily polluted areas. Others live in rain water pools that lack minerals. These pools can become acidic due to decaying vegetation. To cope with these tough conditions, many fish live near the water surface. This allows them to access oxygen-rich water.

Dietary habits also vary across the order. Most members are carnivores. They often eat insects, worms, and aquatic crustaceans. Some researchers, such as Scheel in 1968, observed that gut contents often included ants. Killifish also consume pollen grains from insects. They use carotenoids and other chemicals from this pollen to create body pigments. While most are carnivores, some exceptions exist. The American flagfish is a known algae eater.

Systematics helps scientists organize these many families. The suborder Aplocheiloidei includes families like Aplocheilidae and Rivulidae. The suborder Cyprinodontoidei is much larger. It contains families such as Poeciliidae, which includes the viviparous guppies. It also includes Anablepidae, which are mostly ovoviviparous. This complex classification shows how much variety exists within a single order of fish.