Holothuria edulis is better known as the burnt hot dog sea cucumber. Besides having a terrific name, this creature is fascinating—one that scientists insist is an underrated but valuable member of ocean ecosystems. A team of researchers recently published a report on the sea cucumber and its importance in the journal Conservation Genetics.
Like all sea cucumbers, H. edulis is essentially a squishy tube with a mouth at one end and an anus at the other. It rests during the day and emerges to hunt at night, using its little tentacles to shove seafloor garbage and sediment into its mouth. It then poops out clean, freshly oxygenated sand. Think of it as the ocean's very own carpet steamer. Also much like other sea cucumbers, when H. edulis feels threatened, it can literally puke its guts out, ejecting its organs into the surrounding water.
Believe it or not, H. edulis and some of its relatives sometimes have a hard time finding mates or ideal mating conditions. Fortunately, they’ve got a backup plan: asexual reproduction.
The problem with solo baby-making is that it effectively dries up the gene pool. An increase in asexual reproduction (the most concentrated form of inbreeding) means a decrease in genetic diversity, which is bad for the species' long-term prospects.
Reduced genetic diversity in animals like sea cucumbers may indicate that they’ve started going it alone. Previous studies have shown that other sea cucumber species are more likely to reproduce asexually in contaminated waters. Because of this, a sea cucumber’s genes can tell biologists a lot about how the animals and their environment are doing.
These days, unfortunately, many sea cucumbers are not doing so well. Ocean pollution and habitat destruction are shrinking available safe space. Sea cucumbers are also valued as both food and medicine in some parts of the world, and some of those areas have only recently implemented regulations to prevent overfishing.
One such area is Okinawa, Japan, home to our friend the burnt hot dog sea cucumber. To check up on H. edulis populations, a team of scientists dove to find them at six points around the main island. The researchers then just snipped a small bit of tissue from each animal’s outer body wall before returning it to its home. Back on land, the researchers sequenced each animal’s DNA and compared the results.
"The data tell a story," study co-author Iria Fernandez-Silva said in a press statement. "We saw low genetic diversity in some sea cucumber populations along Okinawa's eastern coastline, where water is polluted by nearby industry, runoff, and coastal development. In contrast, populations in more pristine sites on the island's west coast were more genetically diverse. Since populations appeared disconnected from one another, we can predict that overfishing might be the last straw for vulnerable sea cucumber populations ill-equipped for a comeback."
Why does this matter? Think back to H. edulis's nighttime seafloor cleanups and their role as the ocean's carpet steamer.
"It's easy to underestimate the sea cucumber," Fernandez-Silva said. "Sea cucumbers look goofy, move slowly, and barf up their guts when startled, but these invertebrates are superstar ocean cleaners that are hugely important to marine ecosystems.”
Fernandez-Silva and her colleagues say it’s up to Japan to set an example of good marine stewardship. Tighter environmental and fishing restrictions may be the only way to preserve this vital part of the ecosystem.
"Where ocean life is concerned, looks aren't everything," says Fernandez-Silva. "We urge global communities to speak up in defense of important marine species, even those that look like burned hot dogs.”