Sea cucumber gut slime may be killing the GBR

Sea Cucumbers are like the crappy vacuum cleaners (the ones that spit out half the dirt they take in) of the ocean, and I mean that in the best possible way. They trudge along the sea floor like the gigantic slugs that they are, basically eating a lovely stew of sand and rocks and leaving it up to their digestive tracts to filter out the edible stuff. These cukes then proceed to poop out the not-so-edible stuff (the hence the crappy vacuum analogy) for other organisms to use or munch on. (Yay nutrient cycles!) But, thanks to ocean chemistry getting increasingly¬†unbalanced for a multitude of unsavory reasons, their role as ocean floor recycling bins may be dissolving coral reefs, at least according to a study published back in late December in the Journal of Geophysical Research. In the course of their digestive antics sea cukes secrete acid compounds to dissolve the carbonate-based sand, spilling a whole bunch of soluble carbonate minerals into the water. But, what the heck does that have to do with reef building…

Brief Digression on How to Build a Coral Reef: The primary component of a coral reef is calcium carbonate (CaCO3). That goes for both the coral and the surrounding sand a rubble that falls to the ocean floor and fills up the crevices and holes between rock and coral. Most organisms that live on a reef either add to or absorb CaCO3 from the surrounding environment. In a healthy reef, there’s balance between CaCO3 getting absorbed into the reef and CaCO3 getting released by orgs like sea cucumbers.

….Marine scientists use this to measure how healthy a reef is. So, when there’s not a lot of CaCO3 in the water, the reef is growing aka healthy. And vice versa – too much CaCO3 means something’s out of whack, and in this case, sea cucumbers could be contributing to reef erosion. But, it’s by no fault of their own, increased ocean acidity and warmer waters (aka thermal stress) make it harder for other organisms that live on the reef to turn soluble CaCO3 into solid CaCO3.

Brief Digression on Ocean Acidity: As with most environmental issues these days, the main culprit behind the world’s oceans getting more acidic is the ridiculous amount of excess CO2 in the atmosphere (thanks coal, oil, and gas!). All that carbon dioxide gets absorbed by the world’s oceans; combine CO2, water, and carbonate ions, and (poof!) sea water gets more acidic. Only slightly kidding. Don’t believe me, check out NOAA’s unusually informative website.

In this particular study is that they looked at a specific section of the One Tree Island Reef area of the gigantic Great Barrier Reef creatively title DK13 and collected sea cukes to study in a lab setting. Although it’s name might cause one to think otherwise, One Tree Island does in fact have more than more form of vegetation. DK13 is oddly popular among sea cucumbers, so the research team, lead by Kenneth Schneider of the Carnegie Institution, wanted to find out if they could somehow be contributing to the higher calcium carbonate levels. They collected a swath of animals (Stichopus herrmanni and Holothuria leucospilota) to study how sea cuke digestion changes sea water in the lab and found an alarming amount of CaCO3. One could argue that the reef itself is producing this excess CaCO3, but it that were the case you’d expect to see the reef getting bigger, which it’s not. The same research group previously found that the DK13 was dissolving at night, and they estimate that sea cucumbers produce about half of the carbonate released during the night.

Second cousins of starfish and sea urchins, sea cucumbers enjoy slurping up food with their 8 to 30 tentacle-like feet, ensaring their enemies with slime thread, and breeding both sexually and asexually. Ow ow. CREDIT: Carnegie Institution/Dr. Aya Schneider Mor.

Obviously, scientists at Carnegie aren’t just hating on defenseless sea cucumbers – lead author Schneider said, “Although sea cucumbers may play a part in reef dissolution, they are also an important part of an incredible marine environment.” For the non-chemically inclined, this might sound like bad science (especially since the press release makes it sound like sea cucumbers are making the water more acidic, which would be kind of hard given that CaCO3 is basic). The fact that the water is getting more basic is good because it might stave off ocean acidification in the long term, but at the moment, understanding how sea cucumbers could be damaging Australia’s Great Barrier Reef. As Mr. T would say, I pity da fool who messes with ocean chemistry.

For a more positive take on sea cucumbers: here’s a study from last February that aims to use these “over-sized slugs” to save the world, one fish farm at a time, in addition to transforming British gourmet cuisine. “Next on Jamie Oliver, how to prepare sea cucumber en flambe in the nude…” Cheerio!