A World Apart

The larval lifestyle may seem alien to us terrestrial bipeds, but it comes quite naturally to most creatures--especially inhabitants of the world's oceans.

A tiny larva, not much larger than a speck of dust, swims through the swirling soup of plankton in the cool waters of Puget Sound. Rows of minute cilia along the sides of its body pulsate continuously, pulling single-celled algae near before flicking them into its mouth. Fifty feet below the larva, an adult of the same species creeps across the rocky seafloor in search of a meal. Looking nothing like the larva and colossal by comparison (weighing about a million times more), this animal--a Pisaster ochraceus sea star, or starfish--is an active predator, searching out clams and mussels to pry open with its powerful arms. The larva and the adult lead lives that differ in almost every conceivable way: what they eat, how they move, what predators they must avoid, and the physical world they must negotiate.

From a human perspective, this may seem an odd arrangement. Even as embryos, we possess many anatomical features of our future adult bodies, albeit often in rudimentary form. Furthermore, only a few temporary structures appear during human development, most notably the transient gill slits that close when we are still early embryos, the placenta that feeds us in the womb, and the baby teeth that erupt soon after birth. Human development is quite direct, involving a fairly steady progression toward adult form.

Not so for most animals. The vast majority begin life as larvae that differ drastically from the corresponding adults. Many familiar animals have a larval form: caterpillars turn into butterflies, and tadpoles into frogs. But it is among the ocean's marine invertebrates that the larval lifestyle is most dramatically displayed. By one estimate, about 170,000 species of marine invertebrates exist worldwide, including not only sea stars but also sea urchins, sea cucumbers, sea slugs, and sea lilies, as well as corals, clams, barnacles, and feather-duster worms. These animals typically spend days, weeks, or even months in larval form, mostly swimming in the top ten to twenty feet of water in the company of myriad other creatures. (One bucketful of seawater might contain the larvae of a dozen or so species of marine invertebrates.) At the end of the larval stage, the animals drop down to the seafloor and metamorphose into adults. There they live, grow, and eventually reproduce, releasing sperm and eggs into the water and beginning the cycle again.

The marine invertebrate larvae are so small that their discovery came only in the late 1700s, with the development of good microscopes. Samples of seawater examined through these new instruments revealed a world teeming with unfamiliar organisms. Early observers believed that these tiny creatures must be the adults of previously unknown species, and they named them according to the animals' often bizarre shapes--such as pilidium (from the Greek word for "hat") or auricularia (from the Latin for "ear").

Barnacle larvae found crawling on adults, for instance, were thought to be parasites--a misconception not corrected until the 1820s, when Irish surgeon and amateur naturalist John Vaughn Thompson observed them metamorphosing into immature barnacles. Zoologists initially responded to his findings with disbelief; for centuries, many people had believed that goose barnacles were the young of real geese (hence the barnacle's common name), and zoologists were understandably wary of this new and seemingly equally fantastic claim.

Two decades after the publication of Thompson's findings, German physiologist Johannes Muller accidentally discovered a second example while studying a microscopic creature to which he had earlier given the scientific name Pluteus paradoxus, or "strange easel"--an apt name for a creature whose triangular profile and projecting "legs" gave it the general appearance of an artist's easel, albeit a nearly transparent one flecked with bright red spots. Muller was surprised to observe a miniature brittle star (a slender relative of sea stars) growing inside the body of this minute animal. His continuing patient observations revealed that the two creatures were in fact one and the same: the adult develops inside the swimming larva, whose body is cast away when the adult takes up residence on the seabed.

One by one, nearly all the creatures in the peculiar microscopic bestiary of ocean water were found to be the larval stages of familiar animals. By the beginning of the twentieth century, scientists could confidently assert that a complex life cycle with an extended larval "detour" is in fact the most common method of development in the animal kingdom. This newly discovered complexity raised several questions: Why is the larval stage such a widespread feature of animal life, and if it is advantageous, why don't all animals go through one? And why do larvae look so bizarre?