My fair larvae
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MOST PEOPLE ARE AWARE THAT FROGS start out life as tadpoles, butterflies as caterpillars and flies as maggots. They are aware that the larval forms reach adulthood by a remarkable process known as metamorphosis. But few are aware of just how all-pervasive is the phenomenon of transformation from larva to adult in the animal kingdom. About 80% of all invertebrate marine creatures start off life as larvae and become adults by some variation on the metamorphosis theme.
When it comes to life cycles involving alternating larval and adult phases, practically any variation on the theme you can imagine occurs. Many lifetimes could be spent familiarizing oneself with all the subtle variants superimposed on the macro-variations. Some marine invertebrates that alternate between larval phase and adult phase spend both stages in open water — they never settle down on the bottom. The majority of transformers spend their larval phase in open waters as members of the plankton and their adult lives either crawling across, or attached to, the bottom or, alternatively, as parasites on or inside other animals. Further illustrating the infinite ingenuity of the Creator, some that live as parasites as adults have a bet both ways when it comes to their larvae, living part of their larval life in the plankton and another part as parasites. Enriching the variations even further, others live independently as adults but spend one or more larval stages as parasites!
Copepod crustaceans of the genus Monstrilla illustrate the last-mentioned pattern. Adults are free-swimmers, as is the first member of its larval stages, known as a nauplius larva. The nauplius tracks down and enters a polychaete worm. Having gained entry by some marvelous means, it breaks into a blood vessel and there grows two or four "arms" that are designed to absorb nutrients from the blood. This phase alone takes in nourishment; it has to build a store of food to last it through all other stages of its life cycle. Neither the adult nor free-swimming nauplius larva eats anything.
When found under a rock, ribbon worms, also known as proboscis worms, look like little balls of tangled thread. Put them in an aquarium and, depending on the species, you might see them stretch out into an amazingly long, extremely thin worm (60 centimeters or more long and only a couple of millimeters thick), often beautifully colored. The pilidium larva of those that have a larval stage undergoes an ingrowth of four or five pockets which eventually fuse around the gut forming a continuous cavity around an inner kernel, separating it from a surrounding husk. The inner mass undergoes a rather complicated metamorphosis into a tiny dense worm which lives like a parasite inside its larval covering. At length the worm is released and drops like a bomb to the sea floor, while the ciliated husk swims off and dies.
The intricate design of many larvae renders them worthy of study in their own right. Researchers have found that sea urchin embryos develop "an elaborate crystalline skeleton" (Inoué & Okazaki, April 1977, Biocrystals, Scientific American, p. 83) by a complicated mechanism. The same authors exuberantly declare that, "The simple elegance of the triradiate spicules and the sculptured beauty of the skeletal spicules were wonderful to behold" (p. 86). All hail the power of natural selection!
A universal truth of larval life is that mortality rates are extremely high, particularly during early larval life and later upon settling. To compensate for the staggering losses, huge numbers are produced; some starfish release up to 200 million eggs in one laying episode. The Creator of all figured out the specific vicissitudes that particular kinds of larvae would undergo during all phases of their existence, calculated the numbers precisely, and got it right every time when it came to determining the balance between the number that must be produced to keep the species going but not for it to rule the world.
How do larvae that have been subject to the whims of currents and washed many miles from home return to their place of "birth"? Blue crabs of Chesapeake Bay leave the shallow waters of estuaries and muddy shores for the ocean to release their eggs (James Cameron, May, 1985, Molting in the Blue Crab, Scientific American, p. 103). Obviously, tiny larvae produced by inshore creatures may be swept way out to sea or drift long distances from home. How do blue crab larvae get back to the Bay, and shoreline larvae back to the shore? Well, it seems as if survival depends upon caprice. Most never make it to a suitable location; flukes of current and turbulence, however, always ensure enough get to where they can make a home. Enough is enough. Many barnacles illustrate convincingly the role played by chance in the survival of their larvae. To survive, mature larvae must find themselves in the wave swash "when the tide is high, and only when it is high" (Thomas Carefoot, Pacific Seashores, p. 32). Tough luck for those ready to settle when the tide is out!
For most species, the challenge of finding a suitable home is far more complex than merely "heading down", requiring far more discriminatory capabilities than reacting to light or gravity. This author often visits a richly-inhabited intertidal rock platform jutting out from the shore for about 200 meters at Penguin, Tasmania. You don't have to be a Sherlock Holmes to note that many creatures are almost always found in a given location or zone. Some are found at the high tide line only. At Penguin, feather stars are invariably found on the extreme west side at or below low tide mark; from memory, I have never found one on the eastern side. Their behavior is probably governed by factors similar to those that influences site selection in Atlantic barnacles: "…larvae of [Atlantic] barnacles are not stimulated to settle in conditions of slow current" (Carefoot, p. 31).
A staggering array of cues and factors are used by larvae of different species to find those spots conducive to — nay, essential to — the good life. What is more, larvae somehow know to space themselves out according to their adult size! Oh the wisdom of the supreme Intelligent Designer. Who could possibly believe that such sophistication came about because a tiny speck of infinite mass exploded?