Breathing underwater is tough. Fish get by thanks to their elaborate gills, which have a large surface area to efficiently extract oxygen from the water. However, eggs don't have this luxury and must rely entirely on whatever oxygen seeps in through their jelly-like shells. The situation worsens the larger the egg, where the influx of oxygen must supply a greater volume. Benjamin Martin at the University of Amsterdam in The Netherlands wondered whether the largest aquatic eggs, like those of Chinook salmon, may struggle at times to get the oxygen they need.
With an international team of collaborators, Martin set about investigating how developing salmon eggs respond to an increase in oxygen demand due to higher temperatures, or a limited supply when oxygen in the water is depleted; both of these scenarios afflict the river beds where wild salmon spawn and are becoming increasingly frequent as our planet warms. In a super-sized experiment, the team reared thousands of salmon eggs in the lab at nine different combinations of temperature and oxygen levels and monitored how much oxygen the embryos consumed throughout development.
It turns out that for an embryo encased in jelly, everything depends on oxygen: the rate at which they develop, their tolerance of higher temperatures, the size of the hatchlings and, importantly, survival. A decrease in the amount of oxygen in the water has dire consequences, increasing mortality of the brood, but it was especially devastating when it occurred late in development, when the embryos’ need for oxygen was highest. An increase in temperature generally led to faster embryo growth, but also a greater need for oxygen, making them even more vulnerable to a depletion of oxygen in the water. The team then fed these results into a mathematical model that illuminated the egg's inner oxygen supply. Salmon embryos were surprisingly tough and tolerated an 80% cut in oxygen within the egg. However, in late development, the embryos were always oxygen starved, which constrained their growth, and even a small decrease in the oxygen level in the water or an increase in temperature could push them over the edge and decrease survival.
But nature is messy in ways that lab experiments and mathematical models cannot capture. Therefore, the team reared additional eggs in artificial gravel beds that naturally mimicked the salmons’ spawning grounds. The slow water flow through the pebbles meant that the oxygen consumed by the eggs wasn't replenished as quickly as in the lab, making them even more vulnerable to higher temperatures. In addition, the hapless eggs sitting downstream of the bunch suffered most, as their greedy siblings upstream had already gobbled up all the oxygen in the water, resulting in smaller hatchlings and lower survival. Even eggs sitting right next to one another would experience vastly different levels of oxygen, because the water flow through the egg–gravel mix was random. As if braving low oxygen and high temperatures wasn't enough, in the wild, the survival of a salmon egg is also a lottery.
While some adult fish seem to handle the whims of climate change quite well, their embryos typically do not. In the long term, a species is only as resilient as its most sensitive life stage. The high vulnerability of fish embryos is rooted in their precarious oxygen supply within the egg. Even under optimal conditions, growing fish embryos become starved of oxygen, which leaves only razor-thin margins to tolerate any increase in temperature or decrease in oxygen level. The life of a fish embryo is like a reckless tightrope act where survival depends on walking a delicate balance between oxygen supply and demand.