There’s a lot you can learn about human reproduction by spending a little time with shellfish.
Just ask Jeffrey Riffell, an assistant professor of biology at the University of Washington.
Scientists have long known that many species release chemicals to draw sperm to an egg during reproduction. Sponges do it. So do plants. So do humans. And so do shellfish. But much about these chemical attractants is unknown.
Riffell and a colleague took a novel approach to learn more. They studied the chemical cues released by a troubled type of shellfish — the abalone. By choosing to focus on these mollusks they could advance our understanding of human biology — and perhaps find ways to help save some marine creatures.
The colorful shell of a red abalone. Photo by Ignacio Vilchis
Abalone, with their swirling colorful shells and yummy meat, tend to congregate in rocky crevices. But they have become so popular in recent decades that overfishing has caused them to decline. I’ve written about Puget Sound’s vanishing pinto abalone. But it has sad company. California’s green abalone is struggling and white abalone along the West Coast is nearly extinct. The reason, in part: abalone are broadcast spawners — males and females release sperm and eggs into the water in cloudy bursts. When fishermen or poachers take too many abalone, those left behind often wind up too far apart for these reproductive elements to meet up. Attempts at mating prove unsuccessful.
Abalone broadcast sperm and eggs into the water in milky clouds. Photo by Daniel Morse
So how far apart is too far apart? Why? And what does that have to do with human reproduction?
Riffell and Richard Zimmer from the University of California, Los Angeles, noted that when a red abalone egg is ready to be fertilized it releases a chemical attractant, tryptophan. Yes, that’s the same chemical that makes us drowsy after a big meal of turkey. But in these eggs it is an energizer. The egg essentially doubles or triples in size — sometimes it grows by five times — turning itself into a sort of homing beacon. That gives the sperm an easier target to hit. Meanwhile, the same chemical release also causes sperm to increase in speed, “like a bloodhound focusing in on a particular scent,” Riffell said. That’s the recipe for successful reproduction.
But the researchers recently noted in a paper published in the Proceedings of the National Academy of Sciences that the effectiveness of these chemical releases can be dramatically altered by environmental conditions — most notably, water currents. If water flow is too mild, the chemicals don’t travel far enough. If the water is too rough, the chemicals get smeared. “If the flow isn’t just right they won’t spawn,” Riffell said. The same dynamics affect other species, though in different ways. Purple sea urchin, for example, respond well to the pounding surf of breaking waves.
Riffell’s research has many implications. It suggests that fluid dynamics caused by muscle contractions may influence the potential for successful fertilization in other animals, including humans. And it suggests that scientists seeking to restore troubled abalone populations need to do more than simply transplant shellfish close together into marine waters. They need to transplant the shellfish in an area where water conditions are optimal for reproduction. They need, in fact, to go with the flow.