One morning, scientists from the University of California-Davis' Bodega Marine Laboratory came into work to find that nearly all of their purple sea urchins—plus all of their six-armed sea stars—were dead or dying. "The sea stars picked up their little tube feet and died," says Laura Jurgens, a doctoral student in the lab. "The urchins were basically just dissolving."
This was in August 2011, two years before marine biologists began seeing mass deaths from sea star wasting, the widely reported disease that makes sea stars "melt." And the culprit here wasn't disease, as the Bodega scientists would eventually discover: The killer was a temporarily elevated population of algae in the Pacific Ocean, from which the lab draws water to fill its sea-life tanks. A team from the lab would ultimately determine that the algae bloom—which lasted perhaps 24 hours—killed more than 99.99 percent of the millions of purple sea urchins and six-armed sea stars living on a 60-mile stretch of rocky California tideline. This was one of the most unusual and dramatic die-offs marine biologists have ever recorded. The Bodega scientists reported their research on the phenomenon this week, in the journal PLoS One.
To be sure, many of California's coast species survived the algae bloom. Some species, including fish and marine mammals, showed no signs of harm at all. Still, many of these critters will feel the effects of the vanished urchins and starfish for years to come. "The tidepools are essentially going to work differently without these big species in them," Jurgens says.
"The tidepools are essentially going to work differently without these big species in them."
Urchins are important to cleaning the water of floating vegetation, while six-armed sea stars are important predators. Both animals' absence is likely reverberating throughout the food chain. Plus, both are fairly slow-moving, so it may be decades before they re-colonize. So far, the Bodega scientists have noted some returning urchins, but no six-armed sea stars.
Outside of the 60-mile affected area, purple sea urchin and six-armed sea star populations seem fine, but scientists generally don't like to fix a problem like this by moving animals. Instead, they're watching what happens next. Whatever scientists learn may help predict the consequences should a die-off like this happen again (which is a possibility). The algae species that populated the 2011 bloom are able to survive, dormant, in ocean-floor sediment for years, before emerging again when conditions are right. Meanwhile, scientists have noted harmful algae blooms have increased in frequency and severity over the past 15 years because of human activity such as pollution, overfishing, and boat travel.
If you're not a tidepool animal or a marine biologist, the effects of the 2011 die-off may not seem immediately obvious. But if you're able to reach some of the remote shores where the urchins once lived, you'll be able to tell because the rocks are dimpled with shallow, bowl-like depressions. Those are burrows the urchins excavated for themselves and in which they spent nearly all their lives. Some grew so big, they couldn't leave—at least, not until they died.
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