[First published at http://excalculus.tumblr.com/post/89487231607/the-annual-fish on June 21 2014. Edited December 2018 – note that many of the image links are broken at this time.]
For the next in my series of odd animals, I think I’ll be sticking with the ludicrously hardcore fish theme. (Previous contender: the icefish. Hemoglobin is for wimps.) Say hello to genus Nothobranchius, the annual killifishes of Africa.
Amazingly beautiful. Tiny and fragile. Thrives in… temporary puddles in the African desert?
First, a bit of background. Nothobranchius are tropical and subtropical fish native to parts of east and southeast Africa known for seasonal rainfall. This sort of area tends to be difficult for any animal due to the extreme dryness for much of the year, and aquatic life in particular has a very hard time. Fish from these kinds of environments tend to be very hardy or have interesting survival mechanisms – the lungfish, for instance, is famously capable of breathing air and sealing itself in a moisture-preserving mucous cocoon. The barramundi of Australia and southeast Asia is so adept at surviving in stagnant, drought-stricken riverbed pools that it is now in high demand for aquaculture. The little killifishes, though, don’t have any of that. When confronted with the question of how to survive the complete drying of the environment, their answer is simple: don’t.
When the unreliable rains stop and the pools they inhabit dry up, all the adult fish die. In fact, they may already be dead of old age – due to the pressure to grow and reproduce quickly these fish have some of the shortest lifespans of any vertebrate. What’s left behind are eggs, buried in mud in watertight casings and suspended in the state known as diapause.
So now we have eggs that can survive prolonged drying and temperatures from -8 to 40 degrees Celsius, but that’s not the only thing they have to do. Waiting for water to trigger development would set them too far behind, so the embryos begin to grow when laid but enter diapause at several discrete stages. Most eggs are timed to reach their final stage of development around when the annual rains would begin, but not all – if there is a drought or a freak early rainstorm any embryos that hatched or developed too far would die. Eggs can wait for months or years at any of these diapause points before continuing to the next one, so that at any given time there will be viable eggs ready to hatch.
The staggered development is amazing enough on its own as an adaptation to a harsh environment, but the closer you look the more incredible it gets. One would expect the embryos to develop most quickly under optimal conditions for fish survival, and for the most part this is true – but in Nothobranchius guentheri (and likely other species as well), the presence of adult fish in the same body of water prevents eggs from developing past the first diapause stage. (Inglima et al. 1981) This effect can be reduced by removing adults or aerating the water, and probably serves to conserve the pool of dormant eggs for future seasons. In the same species, temperature and photoperiod during mating and incubation changes at what stage and for how long eggs enter diapause (Markofsky and Matias, 1977) – meaning the process is tailored depending on the season in which the eggs were produced to maximize their chance of completing development at a good time. And if that weren’t amazing enough, fish from different localities are so adapted to the specific area they live in that they show dramatic differences in lifespan and aging patterns (Terzibasi et al. 2008) even when bred for several generations in a laboratory. The development and hatching of the eggs isn’t a simple staggered timer, it’s an intricate process that takes into account the locality, conditions when the eggs were laid and conditions in the environment during development. And that’s only the mechanisms we know about: it’s entirely possible there are more.
Speaking of N. furzeri, the species from that last study, it holds the dubious distinction of being the shortest-lived vertebrate on the planet that can be bred in captivity with an average lifespan of 9 weeks (maximum 12). The overall crown was stolen from it by a coral goby with a reproductive cycle of 25 days and a lifespan of 59 days, but that’s a story for another time. The rest of the genus ranges from similarly mayfly-esque on the short end to the longest-lived species managing to survive up to a year or so with proper aquarium care. And there is a huge amount of interest in providing proper aquarium care, because although they hail from muddy puddles and spend their short adult lives in a rush of eating and breeding these are some of the most beautiful fish in the hobby, with colors and finnage that would not look out of place on a coral reef.
- Inglima, Kenneth, Alfred Perlmutter, and Jules Markofsky. “Reversible stage‐specific embryonic inhibition mediated by the presence of adults in the annual fish Nothobranchius guentheri.” Journal of Experimental Zoology 215.1 (1981): 23-33.
- Markofsky, Jules, and Jonathan R. Matias. “The effects of temperature and season of collection on the onset and duration of diapause in embryos of the annual fish Nothobranchius guentheri.” Journal of Experimental Zoology202.1 (1977): 49-56.
- Terzibasi, Eva, et al. “Large differences in aging phenotype between strains of the short-lived annual fish Nothobranchius furzeri.” PloS one 3.12 (2008): e3866.
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