Free The Flight of the Iguana by David Quammen Page B

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    In that sense they belong in company with certain other retiring creatures that go to great lengths to avoid gratuitous violence. I’m thinking especially of the rattlesnake and the black widow spider.

THE SELFHOOD OF A SPOON WORM

    Sex Determination as a Mid-Life Experience
    The study of biology is such a fine antidote to rigid, normative thinking that perhaps all our televised preachers and tin-whistle moralists should be required occasionally to take a dose of it. The experience couldn’t help but be broadening. No general truth emerges more clearly, from even a browser’s tour of the intricacies of the natural world, than this: Chances are, there is more than one right way to do it.
    Flying is a good example. Birds and reptiles and insects and bats and seeds have all mastered that feat, at different times and in their utterly different ways. The arrangement of anatomical support is another. Who is to say that a skeleton should be worn inside the body (as by us vertebrates), when lobsters and other arthropods do so well with their skeletons on the outside, and jellyfish get by with none whatsoever? For a further instance, consider the matter of how gender is determined among those creatures showing two distinct sexes. Boy or girl, cow or bull, colt or mare, goose or gander: The interesting question, biologically, is not which but why. What dictates that a particular individual should turn out to be male or female?
    In mammals the point is decided genetically at the moment of conception. That’s the most familiar sort of sex determination,and we humans are likely to think of it as the norm; but such genetic sex determination (GSD) is just a contingent fact, not a logical or biological necessity. Among certain other animals, known as “sequential hermaphrodites,” sexual identity can change as a stage of growth, as routinely as a human might pass through puberty or menopause. These sequential hermaphrodites, including a number of fish species, begin life in one sexual form (say, as males) and function reproductively in that role for a time; then as they grow older and bigger, they transform at some point to the opposite sex (female), in which role their large size may be more advantageous. If physical magnitude happens to be a more crucial advantage for males than for females, in any such species, then the sequence of sexual identities will be reversed, each individual making its transition from small female to big male.
    There is also a third option for sex-differentiated species, one which has not gotten much scientific attention until the last few years. This option is called “environmental sex determination,” or ESD. The term means, simply, that in certain species the sex of the offspring is determined at a point sometime after conception, by some environmental influence acting upon those unhatched eggs or those sexless young. That environmental influence might be a matter of chemistry or sunlight or temperature or something else. Theoretical ecologists are still struggling to explain just how ESD might have evolved, and just why it might be useful, but in the meantime field and lab studies have shown that the phenomenon is more common than we might expect.
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    This ESD business was probably first recognized in a beast named Bonellia viridis, a benignly grotesque sea animal belonging to the phylum Echiurida, a group casually known as the “spoon worms” and not remotely related to anything you’ve ever heard of. Bonellia itself looks like some sort of bad party joke made out of latex. The adult female of the species consists of a bulbousbody roughly the size of an avocado and with a similar dappled surface, from which extends a long tube-like proboscis ending in a pair of leafy lobes. It lives amid rocks on the bottom of the Mediterranean Sea, where the soft body can find safety by anchoring itself in a hole or a crevice, and