An annual killifish (Austrofundulus limnaeus) egg. Photo credit: Daniel Zajic.

An annual killifish (Austrofundulus limnaeus) egg. Photo credit: Daniel Zajic.

Fish are usually synonymous with water. Take away the water and frankly the future is bleak. But not for annual killifish (Austrofundulus limnaeus). Although the adults don't fare well when their temporary homes dry up, they leave behind eggs in a form of suspended animation – known as diapause – as the seeds of the next generation, ready to resume developing as soon as the rains return. And these eggs are extraordinarily tough. ‘Annual killifish can survive for months to years in dry mud’, says Daniel Zajic, now at Linfield University, USA. The dormant eggs seem to seal themselves shut about a week after their ponds have vanished, to prevent themselves from losing further water, but this could also affect basic life support by preventing them from absorbing oxygen. Zajic, Jonathon Nicholson and Jason Podrabsky at Portland State University, USA, wondered how the resilient fish eggs deal with dehydration.

But first the trio wanted to know just how long the embryos – some in suspended animation and others that had resumed developing – could survive in dry air (85% relative humidity). Zajic collected eggs at different life stages and placed them in a hermetically sealed sterile box for more than 18 months, patiently checking their survival every day and then 2–3 times a week after a few months. Amazingly, over 10% of the fish embryos in suspended animation survived more than 16.5 months out of water, with one particularly hardy individual surviving 19.4 months (587 days). However, the embryos that had emerged from suspended animation were less resilient than their dormant cousins, although they were still able to endure the dry conditions better than other fish, with around half surviving a whole month and some even making it over 3 months.

Nicholson then sealed some of the dormant embryos and some developing embryos in sterilised individual vials with normal air to measure their oxygen consumption, in order to find out how much energy they were using in the dry conditions. ‘Keeping everything clean and sterile is very important when measuring oxygen consumption … microbes can quickly deplete available oxygen and affect the measurements’, says Zajic. However, the team was surprised to see that the dormant embryos – which are meant to switch off oxygen consumption and use anaerobic respiration instead – were still consuming oxygen after more than a month. Their ability to prevent further water loss didn't seem to affect their ability to consume oxygen, even though they must have somehow sealed their eggs to prevent themselves from drying out completely. The team suspects that the dormant embryos may have increased their oxygen consumption to run metabolic processes that protect them from the effects of dehydration. However, when the researchers analysed the oxygen consumption of embryos that had kick started development in dry air, their oxygen consumption plummeted by 60% after 7 days, probably due to the general stress.

Most intriguingly, almost 20% of the reactivated embryos seemed to stall their development 7 days after resuming and the team suspects that this pause could help the fish population to re-establish itself quickly when their ponds eventually refill. ‘Having embryos at different stages of development in the soil that respond differently guarantees survival of at least some embryos, even if others are lost if early rainfall turns out to be a false start’, says Zajic.

References

Zajic
,
D. E.
,
Nicholson
,
J. P.
and
Podrabsky
,
J. E.
(
2020
).
No water, no problem: stage-specific metabolic responses to dehydration stress in annual killifish embryos
.
J. Exp. Biol.
223
,
jeb231985
. doi:10.1242/jeb.231985