| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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| Wild Lives Love On the Rocks Amidst an exciting sea of swimmers, stingers, and stunning beauties, barnacles may seem boring. But within their limestone homes, these tiny crustaceans hide an extraordinary sexual endowment and enviable masonry skills that have guided them down a long and successful evolutionary path. Emerging more than 520 million years ago, barnacles even survived the Permian extinction, which decimated up to 96 percent of all marine species. Today, more than 1,000 species of barnacles are found from the equator to the poles. Their habitats range from intertidal zones to depths of over 4,000 meters. A cubic meter of seawater may hold as many as 2,000 barnacle larvae, and a typical half-mile swath of coastline may be home to over a million adult barnacles. A newly hatched baby barnacle, or nauplius, has a dorsal skeleton and a large central eye, making it look like a tiny crab masquerading as a Cyclops. After undergoing six molts and traveling up to a thousand miles with the currents, these weak swimmers don clamlike bivalve shells and start searching for a hard surface on which to spend the rest of their lives. These cyprids are unable to eat, so they have limited time to find an appropriate site and metamorphose into filter-feeding adults. Even so, many barnacle species are surprisingly selective about where they settle down. One species cements itself only to the lips of humpback whales, while others will settle only on the gill filaments of crabs and lobsters. One particularly picky species prefers to grace the toes of penguins with nature’s own formula for long-lasting—albeit spiky—toenail polish. Once a cyprid has found a suitable spot to spend the remaining 1 to 30 years of its life, it begins to strut around on its first pair of limbs, which ooze a clear cement that affixes the animal into a permanent headstand. This protein-rich cement is more than twice as strong as NASA’s spacecraft epoxy. Ocean aficionados from Navy Seals to sport fishermen bemoan its tenacity, for it costs several hundred million dollars each year to scrape the animals off ships’ hulls. A six-month accumulation of hitchhiking barnacles on the bottom of a vessel can weigh more than 100 tons and increase fuel consumption by 50 percent. Because it doesn’t dissolve in any fluid except water-free acetic acid and is impervious to everything from bacteria to 440¼F temperatures, barnacle cement may one day be synthesized to fill cavities and mend broken bones. Within 16 hours of gluing itself down, a barnacle has typically finished constructing its limestone house and transformed into an adult. It looks something like a headless, feathery-legged shrimp inside a tiny volcano. It eats by swiping its jointed legs, called cirri, through the opening in its shell to snag passing plankton. Barnacles are unique among crustaceans for encasing themselves in heavy, permanent shells that mandate a sedentary lifestyle-a condition that may provide an explanation for their improbably large male sexual organs. Most barnacles are hermaphrodites that possess both male and female reproductive systems. However, they cannot usually fertilize themselves, making it necessary for them to extend a long, snakelike penis into neighboring barnacle shells in order to reproduce. Relative to body size, barnacles have the longest genitalia of any organism on the planet and are able to inseminate a neighbor up to seven shells away. Although this remarkable appendage makes normal cross-fertilization possible, a number of species secure the benefits of genetic variation in a different way-with tiny live-in males. Live-ins settle into limestone fortresses built by members of their own species, receiving protection and sometimes nourishment in exchange for their fertilization services. These mini-males lack all of the female sexual organs, and in some cases are reduced to little more than sacks of sperm. Darwin was the first to identify these small males, which he termed "dwarf males" when paired with a female lacking all male organs and "complemental males" when housed within a hermaphrodite. After filling his house with nearly 10,000 barnacles-many of them collected on his Beagle voyage-he dedicated eight years of painstaking and sometimes frustrating research to these tiny animals before harnessing this knowledge to write The Origin of Species. Darwin found the variety of barnacle reproductive startegies eminently curious, and was particularly astonished by hermaphrodites that received fertilization assistance from neighbors as well as resident males. In 1851, he offered up the first theory to address this phenomenon. He noted that barnacle live-ins servicing females are smaller and more simplified than those paired with hermaphrodites, suggesting that all barnacles were originally hermaphroditic, and unnecessary organs were gradually lost as the sexual divergence occurred. In this scenario, species with both hermaphrodites and complemental males were evolving from combined to separate sexes. However, recent DNA-sequencing work suggests that the progression may have happened in reverse, with originally separate sexes converging to create hermaphrodites in more recently evolved species. As the debate over the barnacle's original sexual strategy continues, scientists are trying to understand why such dual reproductive methods evolved in the first place. Many believe the answer has to do with population density. Lower population densities, caused by reduced food availability, overly deep water, or a species' need for very specific attachment surfaces, can make cross-fertilization much more difficult. In these situations, live-in males would help to ensure reproductive success. However, species living among crowds would acheive more genetic diversity through hermaphroditic cross-fertilization. For example, California's sea whip barnacle (Conopea galeata) prefers to settle on the narrow stalks of sea whips and sea fans, which often results in low population densities. Neighboring species such as Tetraclita rubescens take a different approach. They tend to aggregate close together, filling a wide area with closely packed shells, irrespective of the surface beneath them. Thus, while a T. rubescens may be probed by a dozen neighbors in a day, C. galeata might have more luck achieving fertilization by keeping a roommate on call. Of course, the question of how a dwarf or complemental male finds an appropriate host to house him still remains. As scientists investigate the pheromones and chemicals that may regulate this process, it has become increasingly clear that the seemingly banal barnacle is indeed impressive for far more than its sexual prowess. Stephanie Greenman is a writer living in the Bay Area. |
Summer 2001
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