The term ‘a fish out of water’ generally invokes the image of a fish floundering about on the shore, struggling to return to the life-giving water. However, for Hart's killifish (Anablepsoides hartii), being out of the water is no problem. Unlike most fish, Hart's killifish can live in air for hours at a time by breathing through their skin! This land-loving fish leaves the water for a variety of reasons, including hunting for tasty insects or escaping from predators. In fact, when predators are present in their habitat, Hart's killifish will often lay their eggs out of water to hide them from danger. But what does being laid on land do to the developing embryo inside the egg? Matthew Walsh and Christopher Roden from the University of Texas at Arlington, USA, set out to study whether laying eggs on land changes the embryonic development of Hart's killifish and whether embryos from populations where predators are prevalent do better in air.
To find out, the duo first went to the island of Trinidad, which is home to many Hart's killifish populations, some of which are where predators abound while others live predator-free. Walsh and Roden collected killifish from areas with and without predators, then brought the fish back to the lab to breed before collecting their eggs. In the summer of 2023, the researchers reared eggs from each population in air or in water. They proceeded to measure how long it took the embryos to hatch from their eggs and assess the growth of hatchlings after leaving their egg. Embryos from predator-facing parents hatched from their eggs 9% faster in air than in water, while eggs from predator-free parents hatched at the same time whether reared in air or in water. This means that when predators abound, fish can lay their eggs on land to make their embryos develop faster. This got Walsh and Roden thinking; does developing faster really help fish survive better in predator-filled waters?
To explore their query, Walsh and Roden tested whether fast-developing fish continue to grow faster after hatching from their egg. When they measured the size of young fish 1 month after hatching, the duo found the hatchlings whose eggs were reared in air grew to be 14% larger than the ones reared in water. What's more, air-reared hatchlings grew larger than water-reared hatchlings regardless of whether their parents faced predators. This indicates that going through embryonic development on land benefits Hart's killifish whether or not they might live among predators when they return to the water. The team suspects that Hart's killifish grow bigger when they develop on land because the embryos get more oxygen from air than from water, which gives them a boost of energy, allowing them to grow bigger. The ability to breathe through their skin when in air seems to be improved in populations that face predators. Laying eggs on land caused embryos whose parents faced predators to hatch faster, suggesting that these embryos are better able to harness oxygen from the air and use it to develop faster.
Undergoing early development in air, rather than water, makes these fish grow bigger and helps them to stay safe from predators during this vulnerable stage. For predator-facing populations, laying eggs on land helps their offspring develop faster and hatch sooner, presumably because the embryos are better at using oxygen from the air. More research is needed to understand how facing predators leads to improved oxygen use when on land; however, it is clear that the next generation gets a helping hand when Hart's killifish are faced with predators. So, while most fish struggle if forced ashore, the Hart's killifish thrives instead of behaving like, well, a fish out of water.
