The corals of Australia’s Great Barrier Reef are having their annual orgy. Although some corals brood their eggs in their bodies, or bud off clones, most of the Great Barrier Reef’s corals — 140 species worth — release clouds of eggs and sperm into the water en masse.
Tossing precious gametes into the vast vast ocean creates a physics problem with critical consequences: how can the sperm find the eggs to fertilize them before they die or get beaten to pieces by the surf? Corals have evolved at least two strategies to improve their chances: that massive synchronous release of sex cells is one. Buoyant gametes are another.
The mass spawn starts at nearly the same time each year: just after sunset on a day during the week that follows the full moon in late spring. Biologists don’t yet understand exactly how animals that are little more than two layers of cells separated by some jello manage to pick the same day each year. But environmental cues like ocean temperature, the timing of moonlight, and tidal patterns may all play roles in pinpointing the big day.
Whatever the signal, all the corals let go at once, turning the water around them into a gamete soup. The sperm and eggs will only live for a few hours once they hit the water, but releasing so many at once improves the odds that they will find one another before it’s too late.
Still, dilution is a serious danger. A volume of water the size of a six-sided die seems pretty small to us, but to a coral sperm trying to bump into an egg, it’s like searching midtown Manhattan for a friend’s hotel room without knowing the address. Eggs in some coral species may release a sperm-attracting chemical to help draw them in, but the corals’ main strategy seems to be keeping sperm concentrated. Corals that spawn on ‘off days’ do far worse in the fertilization lottery than corals that spawn together.
Spawning success on the Great Barrier Reef: Data from Oliver and Babcock 1992.
Which brings us to the second way corals concentrate their gametes: their eggs are filled with fat. Up to 70% of a coral egg’s weight is made up of fat droplets, and because fat is less dense than seawater, that makes the eggs float.
Those eggs may be spat out of the corals near the seabed, but they’re buoyant enough to shoot up to the surface of the ocean. Some species of coral go a step further and pack sperm and eggs into a millimetre-wide mucous sac. The eggs in the bundle act like water wings for the sperm, dragging them to the water’s surface before the entire package breaks apart and the gametes spread out, usually within 30 minutes of the sac’s release.
Floating concentrates all the sperm and eggs from the reef into one layer only 1 centimeter thick. It’s like cramming all the inhabitants of a high-rise apartment building into a party on the rooftop. There are a lot more sperm near a lot more eggs, and that improves the odds that they will find one another.
Plus, the corals pick an excellent day for their party. They spawn during the neap tide, the time of the month when ocean levels shift the least. The smaller tides mean there’s less churn in the water: less turbulence to break up the thin slick of eggs, sperm, and mucus.
The result? A miles-long slick of viscous pink spung along the Australian coast. The eggs that get fertilised become tiny planula larvae which eventually drop out of the slick and settle to the bottom, where — with luck — they will become the founders of brand-new coral colonies.
[Oliver and Willis 1987 [Denny and Shibata 1989 [Oliver and Babcock 1992 [Arai et al. 1993 [Coll et al 1994 [Moore 2003 [Hagedorn et al 2006 [Sweeney et al. 2011 [Boch et al 2011 [Woodley and Porter 2016]]
Video of 2012 spawn event on the Great Barrier Reef by Stuart Ireland via Vimeo