Sea cucumbers are quite important to the marine environment. But before getting into why, let's take a couple of minutes to get past all the anal references that are almost inevitable when describing these simple yet intriguing creatures.
The common name is an obvious choice because quite a few of the more than 1,700 species that fall into the class known as Holothuroidea resemble lumpy cucumbers. They are cousins to sea urchins and starfish, and if you slice one open you'll find a similar five-chamber symmetrical growth pattern. Sea cucumbers don't have a brain or eyes and rely on a network of nerve strings to detect light and movement. Like starfish, they have hundreds of tiny pod feet that allow them to move slowly across the seabed.
The Butt of Many Jokes
And now for the part that gets all the attention. Almost everything sea cucumbers do involves their... butts. They suck up sand and silt that is passed through an intestine-like tube that runs the length of their body. Once all the organic matter is removed, the residue is released in long rope-like piles of poop excreted from the rear end. The scientific name for this excretory orifice is a cloaca, but almost no one uses that term. It's a lot more fun to go with the various anal references.
The sphincter muscles of the cloaca/anus pump water into the sea cucumber, where growths known as respiratory trees extract oxygen. So yes, they breathe through their butts. And it gets weirder. While mouths bring in most meals, the sea cucumber's rear-end respiration process captures any suspended food particles in the water, which are kept digested. So add feeding to the list of tasks that happen at the hind end.
Sea cucumbers even use their anal orifices for self-defense. When confronted by a would-be predator a sea cucumber may expel some of their internal organs, coating their would-be attacker in goo. Not to worry, because the 'cukes can regenerate the lost tissue in fairly short order. Some species take things one step further and spew long strands of sticky, toxic goo known as Cuvierian tubules that entangle and sometimes immobilize the assailant.
And finally, there's the whole issue of things taking up residence in the hind end. Small crabs, shrimp and fish take up residence in sea cucumber booties. These squatters don't provide benefits to their host, but the cucumbers are generally tolerant of the intrusion. Not all visitors are welcome, however, as some species of pearlfish will make their way inside to nibble on the sea cucumber's gonads and other internal organisms. As a defense against unwanted intrusions, some species such as the rump-toothed sea cucumber grow anal teeth to deter unwanted entry.
Janitors Of the Reef
With the bathroom humor out of the way, it's time to discuss why sea cucumbers are one of the true unsung heroes of the ocean. Sea cucumbers are nature's version of a rhumba. They feed on the decaying organic matter they filter from the sand and sediment passing through their bodies. This process, known as bioturbation, keeps the seabed in balance and prevents overgrowth of oxygen-sucking algae blooms. In addition, filtered waste is rich in essential nutrients such as nitrogen, ammonia, and calcium carbonate, which is vital for coral growth.
Marine biologists studied sea cucumber cleaning on a 7-square-mile section of Australia's Great Barrier Reef. Based on daily poop counts, they concluded that the average sea cumber put out about 1.3 ounces of filtered sand each day, which works out to about 31 pounds a year. This may not sound like a lot, but there's strength in numbers, as the research team counted more than 3 million sea cucumbers in the study area. Doing the math, they concluded that the combined efforts of these simple “janitors of the sea” filtered more than 60 tons of sand.
More Than A Meal
Sea cumbers have been scouring sea bottoms for at least 450 million years, but now face a new threat from one of the planet's more recent additions to the food chain. In some Asian cultures, these unappealing-looking creatures are considered a delicacy, and they are now popping up on menus across the globe. With some varieties fetching upwards of $100 a pound, 16 species are now on the endangered list.
Fortunately, there is a growing awareness of the importance of sea cucumbers to ocean health. They are also seen as a solution to the problem of fish farming waste. Fish raised in coastal pens put out a lot of waste that can cause fish-killing bacterial infections. Adding sea cucumbers to tanks or under floating pens does a lot to clean up the mess, and as a bonus, they become an added-value crop that takes the pressure off wild-harvested varieties.
On your next dive, take a minute or two to search the sand. Chances are you'll find a sea cucumber going about its never-ending cleaning duties. It may not be much to look at, but few creatures in the ocean do more to keep things tidied up.