Chasing annual killifish in Africa

Killifish: Fascinating Fish Surviving in Extreme Conditions

Killifish appear annually in places like rain-filled puddles, ditches, and floodplains that emerge during the rainy season and disappear during the dry season. Annual killifish, sometimes called seasonal, are divided into summer killifish, which occur in Africa, and winter killifish from South America.

A lecture by RNDr. Radim Blažek, Ph.D., about killifish continued the journey to explore fish in another part of the event organized by the Brno association Cyperus.

The Life Cycle of Killifish

They lay incredibly drought-resistant eggs in vertisol, which can survive in dry mud for months or even years. Vertisol is a clay-rich soil that shrinks and swells with changes in moisture. During dry periods, it is characterized by deep cracks. It is further characterized by the spontaneous loosening of the substrate. Embryos inside the eggs can pause their development (diapause) during the dry season. When the rains return and the puddles refill, the eggs hatch, and in as little as two weeks, for example in Mozambique, adult individuals can be observed in the monitored puddle. The speed of growth and maturation of the fish is breathtaking. Before the water dries up again, the killifish reach sexual maturity, reproduce, and lay eggs. The adult fish then die.

In Africa, the expedition also encountered lungfish (Protopterus). This is a genus of four species of African lungfish. They are characterized by the ability to breathe atmospheric oxygen using lungs, similar to axolotls. They lose this ability at around 30 cm in size. During the dry season, they burrow into the mud and enter a state of estivation (a state of torpor), where they survive until the next rainy season. The enemy of killifish is clearly a bug that can suck a killifish dry in two hours, leaving only a shell.

Coloration of Killifish

The team also studied populations that were colored differently and compared their locations and the reason for the coloration. They focused primarily on the coloration of male N. furzeri in the Mozambique region.

Allopatric Speciation

This occurs when a population is divided by a geographical barrier (e.g., mountains, river). Isolated populations evolve independently, and due to genetic drift, mutations, and selection pressures, they genetically diverge.

Sympatric Speciation

This occurs within a single geographical area without a physical barrier. It can be caused, for example, by different food specialization, sexual selection, or polyploidy.

This was followed by enthusiastic talks about Gambia, Kenya, Sudan, Zimbabwe, and finally Tanzania and Zambia.

Tanzania

Tanzania offers enormous diversity and an amazing location for studying killifish. The team explored 185 puddles, 129 killifish. In one puddle, they identified as many as 6 different species of fish that lived there together. The most common species was N. melanospilus or the black-spotted killifish. Furthermore, the endemic species N. ocellatus, which grows up to 8 cm, occurred there. N. ocellatus is an annual fish that completes its entire life cycle within a single rainy season. Their eggs are incredibly resilient and are able to survive in dry mud for even several years until the next rains arrive.

Conclusion - The Incredible Abilities of Killifish

Thanks to their often incredible adaptations, killifish are fascinating fish because they thrive in an environment that would be uninhabitable for most other fish.

Thank you for the photos: Zdeněk Dočekal