Sea Urchin Eggs Play Hard to Get

Photo credit:  jkirkhart35

Photo credit: jkirkhart35

There is a battle of the sexes raging beneath the waves and the outcome could lead to breakthroughs in our own sexual success. But, instead of combat between adults, this war is a match of molecules, a deft dance between the two most fundamental aspects of male and female: the sperm and the egg. And although it all plays out among the strange sex life of sea urchins, the scenario is all too familiar: guy (sperm) chases gal (egg), while she plays hard to get.

In the cold dark waters of the deep, a male sea urchin starts to spawn. His 10 to 100 billion sperm erupt forth in a milky cloud and begin their impossible race to find a tiny egg floating somewhere in the abyss. (Compared to the mere 280 million sperm per ejaculation of a human, sea urchin males seem pretty studly. Of course, they have to compete with dozens to hundreds of other males also releasing sperm into a very, very large ocean. Most men don't tend to have that kind of intense competition, nor the relatively infinite space in which to find an egg, so no need for the enormous sperm count. So cheer up, chaps, it's simply a situational thing). Of course, such spawning only occurs when other sea urchins are also close by (yes, in the sea, sex can be triggered by the proximity of your nearest neighbors. More on that phenomena, later). Female sea urchins sense the sperm and release about a million eggs each into the black waters.

When sperm bump into the egg, Viola! Fertilization occurs...sometimes.


Close up of sea urchin spines and tube feet. Photo credit:  julia_koefende

Close up of sea urchin spines and tube feet. Photo credit: julia_koefende

However, if there are too many sperm around, more than one may sneak in before the barrier is up, and that is bad news for the egg. One sperm is all she can take—more than one and she dies.  So, eggs are constantly evolving to be highly selective, to keep down the number of sperm that could possibly match and enter her delicate sphere. Sperm on the other hand, are constantly evolving to fit the latest lock eggs configures—kind of like trying to keep up with never-ending software updates.  But this is a battle of the sexes at the most fundamental level.

And from this tantalizing tango comes our most thorough understanding of how fertilization actually works—from proteins to the influence of sperm number on egg health, sea urchins have provided the peep show into the minute and marvelous world of egg-sperm coupling. Because the same proteins are found on human and other mammal eggs, understanding egg-sperm compatibility in sea urchins may pave the way to better understanding of human fertilization down the road.

Uni sushi. Photo credit:  coolinsights

Uni sushi. Photo credit: coolinsights

So the next time you are glomming down some uni (sea urchin roe) in your favorite sushi joint, take a minute to thank those little eggs not only for their taste, but for what they've taught us about sex.