Passage IV

NATURAL SCIENCE: This passage is adapted from David Quammen's Natural Acts: A Sidelong View of Science and Nature (©1985 by David Quammen). In this selection, Quammen describes the unusual attributes of the sea cucumber.

Sea cucumbers are not vegetables. They only look and act that way. In fact they are marine animals of the echinoderm phylum, a primitive group that also includes starfish, sea urchins, and two other star-shaped members called the feather-stars and the brittle-stars. Echinoderms are distinct from almost all other animal groups in being radially, rather than bilaterally, symmetrical. In other words they know top from bottom but not front from back nor left side from right side: They all share a pentamerous anatomical organization, with most of their features occurring in fives: five axes of symmetry, five sets of each organ, five major arteries, and for those like the starfish and the brittle-stars, five legs. They have a mouth hidden under the belly, and an anus that generally marks the center of their back. The skin of an echinoderm is often described as "leathery" or "rubbery" but think instead of the texture of imperfectly cooked tripe. Imbedded in that skin are calcareous plates, in some cases quite small and with no interconnections, constituting a minimal skeleton. Echinoderms have been known to stay in one spot, without moving, for up to two years. They have never heard of eyes. They developed all these eccentric proclivities, back in the Cambrian period a half billion years ago, before any consensus arose as to how an animal is supposed to behave. But just as the echinoderms are exceptional among animals, so the sea cucumbers are exceptional among echinoderms.

They retain the five-sided symmetry on the inside but don't give much hint of it externally. Sometime in the dim past they grew so tall and top-heavy that they have tipped over permanently onto one flank. The radial symmetry is now 90 degrees off kilter. Consequently they do have a discernible front: the end with the mouth, around which have been added a ring of tentacles like the leaf ends of celery. . . . They shuffle across the sea bottom in worm-like fashion, by means of muscular contractions and elongations that roll down their soft bodies in waves. Moving deliberately, they swallow the rich benthic mixture of sand and muck, strain the organic debris from it in their long simple gut, and pass the sterile sand out behind. Theoretically at least, they glide along like an open pipe while the sand, rippling faintly as it is cleaned, remains stationary.

In sea cucumbers (again, uniquely among all echinoderms) the skeletal plates are reduced to microscopic size and come in delicate patterns like snowflakes, but serve who knows what use. In overall body shape, some species resemble Italian sausages, some are more faithful to their garden namesake, some display the distinguished profile of a balloon overfilled precariously with tapioca. They range from the size of a baby gherkin to the size of a huge zucchini, one of those monstrous country-fair winners that gets its photo sent out on the AP wire. They are variously decorated in swirls and mottles and stripes of lavender, orange, yellow, parakeet green. Truly these guys are out in left field.

But it bothers them not. In the deepest trenches of the ocean they carry on blithely and quite successfully, working a zone that few other animals are equipped to explore. Researchers on the ocean abyss have discovered that, at a depth of 13,000 feet, sea cucumbers account for half of all the living organisms. Down at 28,000 feet, the sea cucumber majority rises to 90 percent. And at the ocean's bottomest bottom, 33,000 feet down in the Philippine Trench, almost no living creatures are to be found—except sea cucumbers.

In shallower waters, like those coral formations off the west coast of Mexico, they also get along well. This is in part because sea cucumbers have few natural predators, owing presumably to the various nasty poisons contained in the mucous secretions of their skin. Additionally, some species have developed the useful trick of self-mutilation: If a lobster or an otter or a snoopy human lays hold of one of this group, the sea cucumber constricts itself drastically at certain points along the body, and breaks into several pieces. The predator, ideally, will be satisfied with a middle or a posterior section. All the sections are destined to die except the front end, with the mouth and tentacles. If this chunk is left in peace, from it will regenerate a new entire cucumber.