An optimistic future for sea urchin sperm

Sea urchins have one job to do: make more sea urchins. They do that by releasing their sperm and eggs into the open ocean. With luck and a bit of hard work, those gametes manage to meet up in the water and set about making new sea urchins. Scientists call this “broadcast spawning,” and it’s the reproductive equivalent of throwing a million darts into the air, blindfolded, and hoping at least a handful of them find a bull’s-eye somewhere. It’s not quite as romantic as the kind of mating we humans do, but it works.

Sea urchin sperm rely upon a pH gradient to power their mitochondria, the so-called “power plants” of the cell. After being shunted out into the great big blue, the sperm cells take in sodium ions and release hydrogen ones. As a result of that molecular swap, the pH level inside the sperm increases. Once the sperm’s interior becomes more acidic (has lower pH) than the surrounding seawater, a cascade of events ensues which ends with the mitochondria being switched on.

When mitochondria work properly, they allow the sperm cells to swim through the ocean with vigor in search of a friendly egg. Strong mitochondria make for better swimmers. If the mitochondria are underpowered, it could prove problematic to the urchins’ long-term viability.

All of that is a preamble to this: as the planet warms, the oceans are slowly becoming more and more acidic. That’s bad news for corals, for fishes, and also for bivalves like mussels. It should be bad news for sea urchins, and in some ways it is, but it turns out the story is a bit more complicated.

That acidified seawater disrupted the swimming abilities of sperm (ranging from urchins to oysters, corals, and fish) was already known, but what was not yet known was the mechanism. So Macquarie University marine ecology graduate student Peter Schlegel, together with colleagues from CSIRO, Sweden’s University of Gothenburg, and Ecotox Services Australasia, set out to understand why acidified seawater seems to disrupt the swimming abilities of urchin sperm.

They collected male Centrostephanus rodgersii sea urchins from rocky shallows near Sydney, Australia, and transported them back to Macquarie University. In the lab, the researchers provoked the release of the urchins’ sperm by injecting them with a small dose of potassium chloride. Once the sperm was broadcast, it was easy to suck them all out of the water. The sperm were dunked into fresh seawater that was modified to fit one of three pH scenarios. One batch of water was held at the acidity of present-day oceans, a second was set to 0.3 pH units below current levels, which is the projected coastal water condition for the year 2100, and the third was set to 0.5 pH units below the current level, which is the estimated condition for 2300.

Then, the researchers stained the sperm so that they would glow green when the mitochondrial membrane potential was low, and orange when it was high. Orange fluorescence meant that the mitochondria were firing at full speed, but green meant that they were underpowered.

After they had been stained and exposed to their seawater treatment, the sperm got a swimming test. That involved placing a single sperm into a drop of seawater and watching it under a microscope.

As the seawater become more acidic, the mitochondria shifted from glowing orange to green. In the year 2300 scenario, their membrane potential decreased on average by more than half, compared with the sperm that were allowed to swim in the present-day seawater. But swimming speed wasn’t adversely affected by acidification at the levels estimated for 2100, they actually swam a bit faster. The sperms’ performance at the acidification projected for 2300, however, did suffer.

Even though the sperms’ mitochondria were impaired under both conditions (and especially in the far-future condition), the researchers remain hopeful. That’s because reductions in mitochondrial activity ought to increase the sperms’ longevity. And “increased sperm longevity,” they say, “increases fertilization success when sperm-egg encounter rates remain high over prolonged periods.” In other words, despite being less adept at swimming, sperm might be able to survive longer in acidified seawater because they use their energy more slowly.

Of course, that all depends on the eggs’ ability to survive in acidified seawater and on the effects of acidification on the eggs’ receptivity to sperm, which are things that the researchers say remain uninvestigated.

Still, the data provide a somewhat hopeful outlook on the future of sea urchins. If natural selection winds up favoring sperm that can maintain decent swimming skills in the face of acidified seawater, the sea otters of the future can look forward to many years snacking on their favorite foods. – Jason G. Goldman | 10 April 2015

Source: Schlegel, P., Binet, M. T., Havenhand, J. N., Doyle, C. J. and Williamson, J. E. (2015). Ocean acidification impacts on sperm mitochondrial membrane potential bring sperm swimming behaviour near its tipping point. J. Exp. Biol. 218, 1084-1090. DOI: 10.1242/jeb.114900.

Header image: Centrostephanus rodgersii urchins via Flickr/John Turnbull.