Killifish

Killifish are a diverse group of egg-laying fish belonging to the order Cyprinodontiformes. They are not just one single family, but include several groups like Aplocheilidae, Fundulidae, and Rivulidae. There are approximately 1,270 known species of killifish. The largest family among them is Rivulidae, which contains more than 320 species.

These fish are important because they have developed incredible ways to survive in changing environments. They can live in permanent lakes or in tiny pools that disappear quickly.

Many killifish have developed a specialized survival mechanism called diapause. This process allows their eggs to survive periods of partial dehydration. This is a vital adaptation for living in ephemeral waters, which are bodies of water that do not last long. If these eggs remained fully submerged in water for more than a few weeks, they might not survive. Instead, the eggs can wait for the right conditions to hatch. Some adults, such as the species Kryptolebias marmoratus, can even survive out of the water for several weeks.

Killifish live in many different types of habitats across the globe. Most species inhabit fresh or brackish water. You can find them in the Americas, ranging from Newfoundland and Labrador down to Argentina. They also live in southern Europe, the Middle East, Africa, and Asia. Some species live in very extreme locations. For example, the Devils Hole pupfish lives only in a single cavern in Death Valley National Park. This cavern is at least 150 meters deep and branches out from a small opening.

Scientists divide killifish into different groups based on their lifespans. Most killifish live in permanent streams or rivers and survive for two to three years. However, there are specialized forms known as "annuals." These fish live in temporary ponds and flood plains. Annuals have a much shorter lifespan, often living no longer than nine months. Because they grow and age so quickly, they are excellent models for biogerontological studies, which is the study of the biology of aging.

One specific species, the African turquoise killifish (Nothobranchius furzeri), is a major focus for researchers. It is the shortest-living vertebrate that can be bred in captivity. These fish live only between three and nine months. They reach sexual maturity in just three to four weeks. Their reproductive success, or fecundity, peaks between eight and ten weeks. Researchers use tools like the CRISPR/Cas9 system to edit their genomes. This helps scientists study aging and diseases like Dyskeratosis congenita.

Killifish are also famous for their ability to adapt to extreme pollution. In some areas, they have evolved resistance to dangerous industrial chemicals. Over just 50 to 60 years, some populations developed a tolerance for toxins like mercury and PCBs. They can survive levels of these chemicals that are 8,000 times higher than a normal lethal dose. This happened because of mutations that turn off molecular pathways responsible for cellular damage. This genetic diversity allows them to survive in Superfund sites where other animals cannot.

In the wild, killifish play many roles in their food webs. Most feed on aquatic arthropods like mosquito larvae, worms, and crustaceans. Some species, like those in the genus Orestias, are planktonic filter feeders. Others are predators that eat other fish. Some, like the American Flagfish, eat algae and plant matter. Their behavior can also be altered by parasites. When infected by a certain type of fluke, killifish swim near the surface or upside down. This exposes them to birds, which complete the fluke's life cycle by eating the fish.

Many people keep killifish as pets because of their bright colors. Species like the Striped panchax are common in pet shops. However, keepers must be careful with certain species. The Striped panchax has a mouth as wide as its head and will eat smaller fish. The golden topminnow is another popular species, but it is known to jump out of tanks. For these fish, a firm cover and floating plants are recommended. Killifish remain one of the most interesting subjects for both hobbyists and serious scientists alike.