Nov. 14, 2011 -
Coral reefs are often called the rainforests of the sea because they are so packed full of diversity. Yet, scientists still don't know how many species actually live among reefs, even though the answer would be a useful way to know what’s at risk from environmental threats. Now, a new study on shrimp, crabs and other crustaceans suggests that reefs are even more astoundingly diverse than previously studies have proposed. And many species live in a very limited range. The findings emphasize the need to put more effort into both understanding and protecting reef-dwelling communities. "We have no idea what lives in coral reefs in any kind of real way," said Nancy Knowlton, a coral reef biologist at the National Museum of Natural History in Washington, D.C. "We don't even know how much we're losing."
The extent of overall diversity in the seas is still up for debate, with estimates ranging from 600,000 to more than 9 million species, not including bacteria. Of those species, somewhere between a third and a quarter live in and among reefs. To get a better count on the variety of underwater life, Knowlton and colleagues chose to focus on crustaceans. This group -- which includes lobsters, crabs and shrimps -- is the second most diverse coral-dwelling community after mollusks. Crustaceans are well described by science. And they come in a surprising array of knockout styles. "They're gorgeous, they're just beautiful," Knowlton said. "A lot of them have spectacular colors and cool shapes. Some are iridescent. That's my favorite part of the work, just looking at them one by one."
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For the new study, Knowlton and colleagues visited seven locations in the Indian Ocean, the western and central Pacific, the northwestern Hawaiian Islands, and the Caribbean. In some places, the researchers collected dead heads of coral, which house just as many crustaceans as living corals do, if not more. In other places, they planted little condominiums and waited for crustaceans to crawl in and get settled. The team came back a year later and picked up the apartment buildings. They ended up with a total of 93 separate crustacean communities.
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Carefully, the scientists broke apart the true corals and the imitation versions. They collected all the crustaceans that had settled in. And they put the animals into cups. It would have been too difficult to try to identify by name everything that crawled out. Instead, the researchers grouped the creatures and took DNA samples for analysis. "I didn't recognize most of them," Knowlton said. "Very few people can look at them and say, 'Oh, I know what that is.'"
Analyses turned up 525 species of crustaceans in a total of just 6.3 square meters (68 square feet). The set, which included mostly shrimp and crabs (including the hermit crab shown here), represents 80 percent of known crustacean species in all the seas of Europe. Since the study area was so small, there are probably many more out there.
Of all the crustaceans collected in the study (including the shrimp shown here), 38 percent of species occurred only once. And 81 percent showed up in just one location. That suggests that wiping out just one reef in one place could destroy multiple crustacean species that live nowhere else. "This reinforces something we already knew to a degree, which is that there are a lot of species out there that nobody has ever seen or named," said Emmett Duffy, a marine ecologist at the Virginia Institute of Marine Science at the College of William and Mary in Gloucester Point. "There was so little overlap among the different sites. If you only live in one place and there is some sort of environmental impact, you can't recolonize."
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Rather than bringing scientists closer to an absolute number for how many species live in the seas, the new study suggests that crustaceans (such as the crab shown here) -- and likely worms, sponges and other invertebrates -- are vastly and unexpectedly diverse. That should inspire new appreciation for these often overlooked creatures, said Duffy, who has studied the crustacean colonies and the complex social behaviors they can display. "The focus in marine biodiversity studies has been on big, charismatic animals like fish, whales and sharks," Duffy said. "This study shows that those are only the tip of the iceberg. There's really a huge treasure trove of species out there that are largely completely unknown."
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