By Danielle Torrent
Sea cucumbers are among the most abundant animals on the planet, occupying waters from the tropical reefs to the Antarctic and dominating the greatest habitat – the deep sea floor. But their diversity and ecological roles are poorly understood.
To make matters worse, these loaf-like, bottom-dwelling echinoderms have been fished for at least 1,000 years. Dried sea cucumber, or bêche-de-mer, is an Asian delicacy, with some species selling for nearly $120 per pound. Those in the genus Bohadschia have been heavily fished for generations, yet the species within the group have been indistinguishable, said Gustav Paulay, invertebrate zoology curator at the Florida Museum of Natural History on the University of Florida campus.
“They’re getting heavily exploited around the world, so knowing who the species are is very important,” Paulay said. “If you can’t tell the species apart, it’s hard to tell if the ones you’re fishing here are the same as the ones they’re fishing there – if you exploit one population, is it a widespread or local one?”
Researchers have traditionally classified sea cucumbers based on differences in their ossicles, tiny bone-like structures within their body wall. But for Bohadschia, it is impossible to differentiate species by their ossicles, Paulay said. In the first comprehensive classification of the group, Paulay and a team of scientists at the University of Guam used a combination of DNA data and field observations to sort the 12 members of the genus, identifying one new species. Their findings were published online in the Zoological Journal of the Linnean Society April 8 and in the May 2013 print edition.
“There was just chaos in the literature because everybody thought the ossicles should tell the story, but they don’t,” Paulay said. “The wonder of DNA taxonomy is that you can basically figure out who the species are using DNA and then go back and see how you can tell them apart in life. Because much of the past taxonomy was done by people in museums, they couldn’t see them in real life, so we did a lot of field work and discovered that we could tell them apart based on their live appearance.”
Bohadschia contains some of the largest and most common animals found on coral reefs that belong to the group of echinoderms, which includes starfishes, sea urchins, brittle stars and crinoids. They range from the Red Sea to South Africa and east to Hawaii and southeastern Polynesia. Because of their large size – some reach a foot in length – and conspicuous colors, recreational divers often sight sea cucumbers in this group. Their conspicuousness also makes them easy prey for commercial fishermen, said David Pawson, a sea cucumber expert at the Smithsonian Institution who was not involved with the study.
“They are very easy to catch – they don’t move around very much, so you can just put your hand on top of one and pick it up, and you’ve got it,” Pawson said. “Because of the ease with which you can catch them and the high price you can get for them, they’ve been collected everywhere and they have literally become almost extinct in many areas.”
When provoked, Bohadschia sea cucumbers discharge Cuvierian tubules as a defense mechanism.
“These long, spaghetti-like strings are branches of their gut that they can push out of their body, inflate and make extremely sticky at the same time,” Paulay said. If you handle them, you end up totally covered with these sticky filaments. It’s really a mess.”
While this is mostly an inconvenience to humans, it can be life threatening to potential predators such as fish or crabs, as they can entangle gills or immobilize appendages.
“If you’re a predator, you’re going to be covered in these tough strings that stick all over your face and body, persist for days and you can’t get them off,” Paulay said. “Among the sea cucumbers, these guys are the worst, in terms of the copious amounts of these really sticky filaments they can deploy.”
Sea cucumbers are like earthworms of the sea, Paulay said. As deposit feeders, they consume and burrow in sediment, a process that helps maintain the ecology of the ocean floor.
“If they weren’t there, the ecosystem would be different – they are important because they’re so abundant,” Paulay said. “Clearly, if you take them out, there would be big changes in the sediment.”
In their analyses, researchers also discovered at least one hybrid form of sea cucumber.
“These hybrids had been alluded to before, but we found evidence for a lot of hybridization between two species,” Paulay said. “It’s biologically interesting – it’s not common that you find hybrids in nature.”
The more information scientists gather about the biology of sea cucumbers, the greater the likelihood of preserving their existence in nature, since the data may be used to rear the animals in captivity, Pawson said.
“You can bet your life that the more these animals are collected – and they are protected in many parts of the world now as threatened or endangered species – there’s going to be an ever-increasing demand for them as food, as a delicacy,” Pawson said. “So the only way to deal with that is to breed them in captivity, and the more you know, the better chance you’ve got of doing it.”
Paulay and his team recently collected two additional species in the Red Sea, one of which may be new, so the work on this group is unfinished. But the study provides a much-needed foundation for understanding this confusing group of marine organisms, Paulay said.
“It’s just satisfying to go after a problem that has baffled scientists, since there’s probably 50 papers in the literature discussing these animals and they all confuse these species,” Paulay said. “Finally, we know what’s going on for the first time. Imagine if there were all these birds around you and you’d call a hummingbird a crow, and an eagle a vulture, and no one could agree, then finally someone came around and said, OK, that’s a hummingbird, that’s an eagle and that’s a vulture. That’s the state the world was in before we sorted this out.”
Study co-authors include graduate student Sun Kim and Alexander Kerr of the University of Guam.