National Geographic : 1952 Jul
Strange Babies of the Sea For yet another relative of the lobsters the study of plankton babies has helped solve a problem which defeated earlier zoologists. The acorn barnacle, which grows limpetlike on the rocks (page 48), and the related ship's barnacle, were first classified as mollusks. After all, they had shells, so where better could you put them? The French naturalist Georges Cuvier (1769-1832), apparently with some misgivings, so classified them in his Animal Kingdom, although he no longer accepted the medieval myth that they would later drop off and grow into barnacle geese! Babies Aid Study of Parents However, when young barnacles were exam ined as they first swam away into the plankton from the shelter of the parent shells, they were found to have quite unmollusklike jointed legs. In fact, they belonged to the same order as the crabs and lobsters, albeit as hum ble relatives. This is just one of many cases where a study of young animals has led to a better under standing of the parents, a principle surely ap plicable to man himself. Sea urchins and starfish are among the most colorful inhabitants of the reefs. The unwary diver is liable to make an all-too-close ac quaintance with the spines of the black urchin shown on page 49. Each female urchin sets free thousands of minute eggs into the plank ton; as they drift there, they grow into animal cules even stranger than the young lobsters and crabs. Page 51 shows the young of both sea urchins and brittle stars-frameworks of slender arms connected by a small body in the middle. These arms are stiffened by the most delicate of supporting skeletons, sometimes built with three lengthwise rods and connecting struts like a radio mast. These are joined in the middle to a basketlike cage which protects the body. When these babies, looking like long-legged stools, are ready to leave the plankton some six weeks later, they change to the grownup shape in an even weirder way than the lob sters. In the case of the starfish, a bud begins to grow out of the back of the body, and from it tiny starfish arms emerge. The bud grows away from the rest of the baby on a slender stalk, which finally breaks off. Then the little starfish settles to the bottom of the sea, while the old body and arms swim away and eventu ally die. A somewhat similar trick is played by some of the planktonic baby snails. The massive shells which their parents can drag about on the bottom would be far too heavy for the babies to swim with, even in a miniature edition. Usually the young snails get around this difficulty by forming an early shell which is exceedingly thin and delicate. But this is none too strong a foundation on which to start building the adult shell; so some relatives of the well-known cowries have two shells. The baby uses one of these, a crystal-clear spiral with rows of sawteeth around the edge and completely unlike the adult shell. Then a second shell is formed inside this one, and, as the baby grows up, it throws off its old house and starts building on the new founda tions. The squids and octopuses may look far re moved from cowrie shells, but they are really quite close relations. Though their young (pages 46, 52) do not usually differ as widely from the parents as do some that we have been considering, they deserve mention, notably for their beautiful coloring. The skin which cov ers their transparent bodies contains sacs of various colored pigments. The animals can expand or contract these, and so produce con stantly changing color effects. Not content with this, many of these crea tures are equipped with light-producing or gans, also in assorted hues, so that they can put on a most spectacular fireworks display in the deep, dark waters in which they live.* "Searchlight" Shines into Squid's Body As usual, the more we study these animals the more problems they present. Why, for in stance, should some of these little squids have elaborate searchlights, complete with lens and reflector, arranged so as to shine into the body instead of outward? And what strange design formed another kind which always has one eye large and the other small? Another curious specimen is the little Cranchia on page 52. When we brought it up in the net, it swam gently around a dish until it bumped into anything solid. Then it would hide its head, like the legendary ostrich, by pulling it inside its mantle. It would lie there looking out at us through the transparent mantle walls, for all the world as if it felt safe behind a glass window. We have been fortunate to catch the young stages of another very interesting octopus, the argonaut, or paper nautilus (page 49). The adults probably live somewhere outside the reefs, because they are occasionally found washed into shallow water after a gale. Their shell, which delights the collector fortunate enough to find one, does not house the body of the animal, as it does in snails, but rather serves as a baby carriage in which the female * See, in the NATIONAL GEOGRAPHIC MAGAZINE: "Round Trip to Davy Jones's Locker," June, 1931, "Depths of the Sea," January, 1932, and "Half Mile Down," December, 1934, all by William Beebe.