TO MOST DIVERS, SEA CUCUMBERS WOULD SEEM LIKE THE INACTION FIGURES of the oceans. Mainly, they come off as inert, sausage-shaped lumps lying randomly on the sandy bottom and perhaps the least interesting obects on the reef.
In fact, some of them have real Captain Echinoderm moves in them. For one thing, they’re nocturnal so what you see in the daytime isn’t what you’d get at night, when they creep around on their little tube feet.
And they have some defensive moves we humans might find useful, providing we didn’t mind expelling our internal organs as diversionary weapons. Also, it’s estimated that, as detritus feeders, they recycle up to 90 percent of the ocean’s benthic biomass.
ABUNDANT AND DIVERSE
There are some 1,400 known species of sea cucumbers and, it’s projected, as many as 3,000 still unidentified. They’re found on the seafloor around the world, not just in tropical regions but in cold waters, in coastal shallows and the deep ocean plain (and, there are a few species that swim, or at least float). It’s estimated that in the sparsely inhabited depths of the abyssal plain they constitute 90 percent of the life found there.
This broad range means that their shapes, traits, capabilities and survival tricks can vary considerably. In such a large group of species, size varies from a few inches to (in one species) to 5 meters/16 feet in length. Most you’ll encounter on the reefs are likely to be a foot or so long and 2 or 3 inches in diameter.
They share Phylum Echinodermata with sea stars, brittle stars and sea urchins. Taken from the Greek words for hedgehog and skin, echinodermata refers to the bumpy surfaces left by skeletons made up of small calcareous plates that lurk just under their tough skins. Echinoderms are also characterized by five body sections laid out around a central axis, most clearly exhibited in sea stars.
Sea cucumbers – in Class Holothuroida have this arrangement but they’re like sea stars put together for a Picasso painting. Where the flattened bodies of sea stars have their mouths on their lower surfaces and their cloacal openings on their upper sides, sea cucumbers have evolved to elongated, bulky bodies with their mouths at one end and cloacal openings at the other.
SEA STARS, LYING ON THEIR SIDES
Cloacal openings serve as anuses but also have other functions – for one thing, sea cucumbers have a pair of branched “respiratory trees” extending from the cloaca through their elongated bodies, keeping their internal organs aerated. For another, their reproductive organs are close by.
It’s true: Essentially, sea cucumbers are traditional echinoderms evolved to lie on their sides, and they breathe through their rear ends, drawing water in and out through their cloacal openings. And they mostly reproduce by broadcast spawning through the openings.
There are still five-part structures concealed longitudinally inside their leathery bodies. Most (not all) have little tube feet arranged along five rows, three on the lower surface and two on the upper. Mostly they still rely on echinoderms’ fluid-based vascular systems that use hydraulic pressure to power their feet, although sea cucumbers most often utilize coelomic fluid rather than seawater .
Members of Genuses Holothuria and Actinopyga have extremely dramatic defensive moves, worthy of comic book hero stature.
- When disturbed by predatory fishes, lobsters or crabs, they turn their bodies cloaca-end towards the molester and eject their lengthy respiratory tubes, which are nearly always quite sticky and which emerge as darting, adhesive threads that enmesh and immobilize their attacker. Leaving the threads behind, the sea cukes go on their way.
- Really pressed, they may eject not only their respiratory trees but their reproductive organs and perhaps part of the intestines, entangling their predator. The sea cukes survive and regenerate the missing organs in about six weeks.
- Some sea cucumbers discourage attack by predators with a toxin (called, appropriately, holothurin) present in their skins, sometimes ejected along with their insides during the aforementioned defensive maneuvers. While there are several large predatory mollusks that are unaffected by the poison, this is a good reason to be wary of putting sea cucumbers in your aquarium. Really.
ONE MORE REALLY ODD FACT
Several types of cleaner shrimps and other marine animals are known to have commensal relationships with sea cucumbers. The oddest are perhaps the 6-inch long blenny-like pearl fishes (Periclimenes imperator) that make their homes in sea cucumbers’ cloacal openings, hiding out from predators and taking in nutrition from the passing flow of waters as the sea cucumber breathes.
The fishes have developed techniques for backing up and ensconcing themselves in one of the respiratory tubes without triggering the toxic reaction. Some specialized polychaete worms and crabs also find protection within sea cucumbers’ mouths and cloacal openings.
On the other hand, species in the sea cucumber Genus Actinopyga have developed anal teeth that can prevent entry of shelter-seeking critters.
WHAT GOOD ARE SEA CUCUMBERS?
Essentially, sea cucumbers are good for eating things. Some sea cucumbers are plankton feeders, using their mouth tubes to filter microscopic prey from the water currents. Others fulfill an essential role as large-scale detritus feeders.
At least 500 species are known to live almost exclusively on food found on and in the seafloor, including diatoms and other microscopic life and decaying organic matter. They gather it with as many as 30 specialized tube feet surrounding their mouths, sometimes using them to probe the sediments and extract debris.
In the process, they take a lot of grit into their digestive tract, expelling it through their cloacal openings. In this sense they play the role of earthworms in the garden, breaking down particles into smaller bits for further action by bacteria – and recycling into the oceans. Again, it’s estimated that they cycle up to 90 percent of the ocean’s benthic biomass.
OTHER (POSSIBLE) SUPER HERO TRAITS
As a food for humans, sea cucumbers have long been considered a delicacy among Southeast Asian cultures, used in soups, stews and stir-fries. In Japan, they’re sometimes served with sushi. Naturally, being an odd food from the ocean, some people with too much money consider them an aphrodisiac.
Some sites suggest they’re a superb source of protein, and some suggest that Mediterranean sea cucumbers are higher in protein that those from Asia.
Sea cucumbers have long been incorporated into Chinese traditional medicine, and there are websites that tout the benefits of sea cucumbers as health aids, including relieving joint pain, treating gum disease, aiding in wound healing and (possibly) treating cancer.
The more forthright of these note that the evidence is preliminary, at best.
PRINCIPAL SOURCES: “Holothuroidea,” Encyclopedia of Life; “Sea Cucumbers Have a Secret Power,” excerpt from Adapt by Amina Khan, Popular Science; “Sea Cucumbers,” National Geographic; Reef Creature Identification, Florida, Caribbean, Bahamas & Reef Creature Identification, Tropical Pacific, Paul Humann, Ned DeLoach; Indo-Pacific Coral Reef Field Guide, Gerald Allen, Roger Steene; Marine Biology, Fourth Edition, Peter Castro, Michael Huber; Oxford Dictionary of Zoology, Michael Allaby, Oxford University Press.