Gleaners of the Sea
by Phil Lambert, Curator of Invertebrates
They sprout tentacles like orange carnations from beneath rocks, ooze along the bottom at the speed of an hour-hand, or slurp their way through rich, organic mud. They exist everywhere from the intertidal zone to the ocean depths. Appearing like a miniature herd of gnus trundling across the abyssal plains, they graze on the rich, organic "snow" that drifts down from above.
They are sea cucumbers. It may surprise some readers to know that there are 32 kinds found in British Columbia, and we are still discovering new ones. I have described six new species from BC since 1986.
Along with their relatives the sea stars, sea urchins and brittle stars, sea cucumbers are known as Echinoderms (spiny-skinned animals). Echinoderms evolved over 500 million years ago and rapidly diverged in form. Some became extinct but five main classes survived the mass extinctions that decimated other species such as the dinosaurs.
Most of British Columbia's 32 species are quite secretive. A tuft of feeding tentacles protruding from beneath a rock or crevice or a tell-tale mound of mud at its back door usually gives away its location.
Figure 1: White Sea Cucumber, Eupentacta quinquesemita, with tube feet extended. Photo: Jim Cosgrove, RBCM
Whether it's a small Tar-Spot Cucumber hiding amongst a bed of mussels or a White Cucumber clinging to a current-swept rock, most species depend on plankton for their food supply, either directly or indirectly. The common Orange Sea Cucumber,
Cucumaria miniata, spreads its bush of sticky, orange tentacles to capture small particles of food floating by on the currents. Whatever sticks gets licked off as the tentacle is thrust into the gaping mouth.
Figure 2: Orange Sea Cucumber, Cucumaria miniata, draws a feeding tentacle into its mouth. Photo: Brent Cooke, RBCM.
About a dozen species of sea cucumbers feed on dead plankton and other particulate matter that settles to the bottom of the ocean and creates mud. These sea cucumbers burrow like earthworms, aerating the bottom and recycling nutrients. In fact, many could be mistaken for worms. The burrowing forms have lost the rows of tube feet that are characteristic of most other sea cucumbers.
For the most part, reproduction by sea cumbers is a hit or miss affair. The sexes are separate. When the time is right, vast quantities of eggs or sperm are shed into the water and, depending on the current and the proximity of the opposite sex, fertilization may take place.
Figure 3: California Sea Cucumber, Parastichopus californicus. Photo: Jim Cosgrove, RBCM
For example, the California Sea Cucumber raises the front half of its body like a cobra ready to strike, then releases its eggs or sperm from a pore in the top of its "head". Despite the vagaries of this method, it does work. Every spring the tiny larval forms, shaped like bright orange teardrops, appear with the other planktonic animals and plants, looking for a suitable place to settle. Ironically, many are snared by the sticky tentacles of adult sea cucumbers and devoured.
To avoid this fate, some sea cucumbers have evolved a different strategy. Confined to small species less than four centimetres in length, these brooders produce fewer and larger eggs, but more time is invested in protecting them.
One cold winter's night, I witnessed one of the warm domestic scenes that occurs every year in the brood of the Tar-Spot Sea Cucumber. When I carefully lifted the body of the mother and reached underneath, her babies were cold and wet but seemed quite happy. She clutched them between her body and the cold rock. Soon they would be venturing out on their own from the parent's protection.
Figure 4: Cucumaria pallida, a new species described by the author. Photo: Brent Cooke, RBCM.
The sea abounds with glamorous animal life like sharks and whales, yet small creatures like sea cucumbers, without eyes or a brain, have extraordinary adaptations of their own. At a recent conference, a type of tissue was reported resembling human ligaments or tendons that are unique to Echinoderms. The stiffness of this "catch connective tissue" can be modified by direct nervous control. For example, a spine can be locked in place without the involvement of muscles; therefore, very little energy is expended.
Why is this such a big deal? Extend the feeding tentacles, lock everything in place and relax. Just think if you could stand and hold your arms in the air without using any muscles. Dr. Motokawa from Japan suggests this ability is one reason for the long-term success of Echinoderms.
Native people have harvested sea cucumbers for generations. In the tropics, they call the dried bodies "beche de mer" and use it in their diet and as a trade item. As long as they were harvested for local consumption using primitive techniques, the stocks were sustainable. But now, industrialized countries offer top prices and huge quantities are sucked up. The sea cucumbers in British Columbia have not escaped the notice of our entrepreneurs either. In 1994, the sea cucumber fishery in British Columbia totalled 196 tonnes with a landed value of $ 976,000. This fishery is seventh in value among shellfish species.
Figure 5: Scanning electron micrograph of a table ossicle (length about .05 mm) from Parastichopus leukothele. Found in the skin of most sea cucumbers, ossicles are made of a crystalline form of calcium carbonate. The characteristic shape for each species helps with identification. Photo: P. Lambert, RBCM.
Local harvesters collect the California Sea Cucumber, Parastichopus californicus. Using mop-like tentacles, this species act like a miniature vacuum cleaner, crawling along the bottom slurping up the thin film of sediment that settles there. Because they crawl around in the open, harvesting is as simple as picking cherries. The only constraint is that scuba divers can only stay down for a limited time each day. At present, each of the five fishery areas have an arbitrary quota on a three-year rotation so that the area is left to recover for two years between openings.
Our knowledge of this species' growth rate, its age to maturity and recruitment of juveniles is poorly known. Some studies suggest that this species grows slowly. In all the 26 years that I have been scuba diving, I have seldom seen juveniles of this species. This implies that new individuals arc not frequently added to the population, and the species is unlikely to rebound quickly if over-harvested.
Although only one kind of sea cucumber is commercially important, all play a significant role in the ecosystem. In the spring, masses of eggs and larvae become food for all sorts of animals. Imagine juvenile herring gorging on these nutritious yolky eggs, later to become a meal for growing salmon. Many other connections could be made to our food species. After talking to an audience about sea cucumbers I have occasionally been asked, "What good are they?." It is questions like this that raise my hackles and make me realize how human-centred we can be. All species are just trying to survive in this world and are intrinsically valuable. Hopefully, research on sea cucumbers will raise the profile of this lesser known—yet fascinating—group of animals living in our coastal waters.
First published in Discovery: News and events from the Royal British Columbia Museum 24: 1-2 (1996)