Because the world under water does not conform to the world above the waterline, we humans try to make connections no matter how ludicrous. For example, a sea cucumber is not a garden variety vegetable because it is an animal. Further, one variety of sea cucumber is called a sweet potato, though a tuber it is not. The sea cucumber Caudina arenicola came upon its sweet nickname because its shape and coloration resemble the Thanksgiving dinner staple. Recently, I came upon a bounty of these invertebrates in the La Jolla Ecological Reserve’s submarine canyon. With smooth, shiny, mottled-orange skin, the plump relation to sea stars and sea urchins lacks tube feet — but not mobility. Using its muscular body, it can inch along, though it won’t win any races, even against a garden snail. And despite a dearth of protruding sensory tentacles, the potato has no problem finding food as it ingests massive amounts of sand, which are passed through the digestive tract on a continual basis. Like separating the wheat from the chaff, nutritious particles like diatoms are sieved and digested. So, despite the quantity, not quality, strategy, it can still be said that the animal is a picky eater. In fact, sand accounts for most of the weight and bulk. Remove the sand, and the chubby cucumber deflates like a balloon. Aside from the load of sand particles, the sweet potato is typically host to a foreigner within its walls. A symbiotic pea crab enjoys a commensal relationship with its host cucumber, meaning that the crab benefits while the host reaps zilch. Though not considered parasitic (the crab comes and goes as it pleases), the potato isn’t always unharmed because the pea crab must enter and exit the “safe house” through the host’s narrow rectum, which is found at the end of a slender, siphonlike “tail.” A disconcerting visual to be sure. Adding to the dicey maneuver, the opening is not especially elastic, because tiny bones are embedded in the skin. That the bones are fused make for a relatively rigid opening, and as a result, a crab forcing its way in may damage or kill the sea cucumber. Fortunately, the host isn’t a flophouse because only one sexually mature crab holes up at any given time. Considering the host’s risk without benefit, it’s understandable why the cucumber acts to thwart a crab from entering. The potato either blocks entry by digging its tail end into the sediment or tries to dislodge the crab by sticking out the rear end and shaking it from side to side. Sometimes the sweet potato is successful. Otherwise, the pea crab makes a beeline for the intestine to help itself to a meal painstakingly collected and digested by the host. For the pea crab, the end truly justifies the means. Another symbiont, a tiny, eulimid snail sequestered in a pointy white shell, is not as benign as the pea crab. The minute-sized tyrant takes up residence on the sweet potato’s smooth surface by drilling its noselike organ through the rubbery skin, then siphoning off the host’s nutritious body fluids. Once the snail attaches, it is a permanent arrangement. The life of a sea cucumber that is a sweet potato is spent in sediment familiar to the plantings of its terrestrial alter egos. Though there are differences in environment (under water or topside) and being animal or vegetable, common ground exists. All are periodically plagued by snails and insect pests (insects and crabs are arthropods). Thus, it is fitting (not necessarily for the pea crab) that humans look to for ways to connect our air and water relations because in the end (not only for the pea crab), all of us are citizens of the same planet. — Judith Lea Garfield, biologist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores.
