Killifish: Masters of survival in extreme conditions

There is still much to discover, as evidenced by new information about Killifish. By this, we mean the growing number of identified species. For instance, if you read the magazine Živa in 2013 (issue 6/2013), you would have learned from the author Martin Reichard that more than 60 species were known within the genus Nothobranchius, which is considered the most species-rich. If you attended R. Blažek’s lecture in Brno in 2024, however, he was already speaking about 97 species.

Yet, this is far from the only fascinating thing about Killifish. Below, we will explore this topic in more detail.

Behavior of Killifish and their reproduction

Killifish have a hierarchical social structure and are most active in the morning hours, whether for foraging or reproduction. Their short life cycle corresponds to a rapid rate of reproduction – females mature their eggs overnight so they are ready for spawning the following day. Males engage in competitive behavior, which includes both intimidating displays and targeted aggression such as tail strikes, biting, and even lethal skirmishes.

Courtship is brief, and the goal is to ensure a large and genetically diverse offspring.

The male initiates courtship with a series of jerky movements as he approaches the female. She may swim away, indicating her disinterest in spawning. If she does not swim away, the male continues and uses pressure from his head on her back or head to guide her towards the bottom of the pond or tank. At the bottom, he encircles her with his dorsal and anal fins and presses her against the substrate.

This behavior ensures that the eggs are securely deposited in case the pond or water body dries up.

Annual Killifish are a fascinating group of fish that have managed to adapt to life in periodically drying ponds. Their remarkable ability to survive in these extreme conditions is due to a number of evolutionary adaptations, including unique embryonic development.

Challenges of drying ponds and adaptation of Killifish

For fish, the desiccation of ponds poses a serious threat. Adult Killifish die without water, and their offspring, concealed in eggs, must withstand drying conditions. Annual Killifish have evolved a unique solution - their embryos can enter a dormant state, so-called diapause, which temporarily halts their development.

These small creatures survive dry periods as embryos in eggs, embedded in sediment. Their development stops.in one of the three developmental phases of rest (diapause), and then continues with the arrival of the rains. This strategy allows them to survive in uncertain conditions, where the ponds can be filled with water for just weeks but also for months.

Three phases of diapause and plasticity of embryonic development

During the dry period, the substrate at the bottom of the pond dynamically changes due to drying out., which places specific demands on developing embryos. Annual Killifish have therefore developed a unique combination of three developmental stages (diapause I, II, and III) that allows them to cope with these changes.

• Diapause I:It occurs after the eggs are laid in mud with low oxygen content.
• Diapause II:During diapause II, the embryo already has a body axis and a basic head structure. It remains in this stage for a long part of its development, and its metabolism is significantly slowed down.
• Diapause III:Diapause III occurs after the end of diapause II and probably serves to wait for a specific external signal, such as repeated flooding of the substrate with water, which will definitively trigger hatching.

The passage through diapause is not necessarily essential for the successful development of the embryo. Annual Killifish therefore develop directly, without a resting stage. The ability to go through, but also skip individual stages of diapause, gives the embryos of annual Killifish a high degree of flexibility and allows them to adapt to uncertain conditions in drying ponds.

Factors influencing embryonic development and rapid life cycle

The embryonic development of Killifish is influenced by a number of factors, including genetic predispositions, maternal influence, and environmental factors such as oxygen, humidity, and temperature. The life cycle of larvae is remarkably fast. After hatching, they grow at an incredible rate and they reach sexual maturity in just 17 days!Then they lay eggs practically every day until the pond dries up again.

Scientific significance and research

Scientists are intensively studying Killifish, especially the genus Nothobranchiusfrom East Africa.

1. Genetic research and evolution:
   Scientists, for example, studied the genetic diversity of pond-dwelling organisms in various ponds and found that even ponds located just a few hundred meters apart can be genetically isolated. This suggests that the Killifish hardly move among them and each population evolves independently.
   Studies have also revealed that the Killifish has specific segments of DNA in its genome that are responsible for triggering and regulating diapause. These genes are activated in response to environmental signals, such as a decrease in oxygen levels or the drying up of ponds.

2. Aging research:
   Nothobranchius furzeri has become a model organism for aging research. Its short life cycle (in laboratory conditions, they live only a few months) allows scientists to study the aging processes on an accelerated scale.
   Research has shown that by Killifish, there are genes that influence lifespan and the rate of aging. By manipulating these genes, scientists were able to extend the lifespan of the Killifish and improve their health in old age.
   Interestingly, the passage through embryonic diapause may influence the aging of Killifish . Studies have shown that individuals who have undergone diapause often have a longer lifespan and are more resistant to age-related diseases.

3. Research on diapause:
   Scientists are investigating the mechanisms that allow the embryos of Killifish to enter diapause and awaken from it again. They are finding out, how embryos survive in extreme conditions of water and oxygen scarcity, and what signals trigger their further development.
   Research on diapause by Killifish may have practical applications in areas such as the preservation of biological material (for example, sperm or eggs) or the development of new treatment methods for diseases related to metabolism and aging.

An interesting fact is the existence of color forms of males in many species. Red and blue forms often occur, which differ so much that a layperson would consider them to be two different species. Why these forms exist and what significance they have remains a mystery to scientists.

Conclusion

Annual Killifish are a fascinating example of viability and adaptability in an unpredictable world. Their unique survival strategy in periodically drying ponds, based on embryonic diapause and a rapid life cycle, allows them to thrive even in extreme conditions. The study of these fish not only reveals the secrets of evolution and developmental biology but also provides valuable insights into the mechanisms of aging and the possibilities of influencing them. In times of climate change and increasing pressure on ecosystems, the Killifish show us that even in the most challenging conditions, life can adapt and find a way to survive. Understanding these adaptive mechanisms may be important for ensuring biodiversity and the sustainability of life on Earth in the future.

Citation:
*Vrtílek, M., Žák, J., Blažek, R., Polačik, M. (2024): Laboratory Fishes from Temporary Pools 2. Behaviour and Interspecies Reproductive Barriers / Laboratorní ryby z vysychajících tůní 2. Chování a mezidruhové reprodukční bariéry. Živa, 4: s. 205.

Source:

• Blažek, R. (2024, October 19). Behind the annual killifish to Africa. Lecture as part of the Cyperus lecture series at the aquarists' meeting Aquarist Autumn, Brno Žebětín.
• Reichard, M. (2013): Annual Killifish – Fish Adapted to Seasonal Desiccation of Their Habitats. Živa, 6: 289.
• Vrtílek, M., Polačik, M., Blažek, R., Žák, J. (2024): Laboratory Fishes from Temporary Pools 3. Surviving Drought and Diapause. Živa, 6: 344.
• Vrtílek, M., Polačik, M., Blažek, R., Žák, J. (2024): Laboratory Fishes from Temporary Pools 1. Introduction. Živa, 3: 141.