Nature Journal

14. Sex and the Single Freshwater Mussel

111 ß chapter fourteen Sex and the Single Freshwater Mussel The life cycles of freshwater mussels hold much fascination, especially with their intimate ties to fish hosts. Add to that the subtle variations of their beautiful shells, and you’ve got yourself a lifelong hobby. I broke a cardinal rule of nature writing by giving my mussel an actual voice, but I think it helps to get the major points across. Plus, it allowed me to be slightly outrageous. Put yourself in the place of a freshwater mussel. There you sit, wedged into the gravel of an Alabama river, rhythmically extending and contracting your muscular, hatchetshaped foot to stabilize yourself in the rushing current. With that foot, you clamber a few feet per day in search of clean water, tiny critters to filter, and safety from terrestrial predators, such as raccoons and muskrats. (You probably won’t move more than a few hundred yards during your entire lifetime.) The sun shines brightly down on the 112 ß chapter fourteen water’s surface above you, but having no eyes (and no brain to process visual information), you remain in perpetual darkness. Your simple body plan perfectly matches your simple needs. Incurrent and excurrent siphons protrude between the two valves composing your shell, bringing in and expelling water, respectively. Thin gill membranes remove both oxygen and food (mainly microorganisms and organic bits) from that water, propelling the minute food particles by ciliary action to the tube of your digestive tract. A sheet of glandular tissue, the mantle, surrounds these innards, secreting the glistening nacre (mother-of-pearl) that so beautifully lines your shell. Exquisite colors may decorate your abode—silvery white to deep purple, even orange. Depending on your species, your expected longevity can approach one hundred years. “Okay, fine. I’m happy as a clam. But what about my sex life?” you impatiently ask. Well, a few of you enjoy hermaphroditism, producing both male and female parts with which to impregnate yourself. (Would such be considered a clam-destine affair?) However, most of you “choose” one gender or the other. If male, then your internal organs produce vast quantities of sperm, which are released to the water, then taken into a female’s body through her incurrent aperture and carried to eggs waiting in her gills. Sections of these gills then serve as brood pouches or marsupia , producing parasitic larvae (glochidia) to be discharged into the water column. There they must immediately attach to the skin, gills, or fins of a fish, or die. sex and the single freshwater mussel ß 113 “Hmmmmm,” you astutely ponder. “Aren’t the odds pretty stacked against my precious, precocious glochidia? Won’t they just be washed downstream or gobbled up as chub chow?” True, unless you’re very clever . And clever clams turn the tables on their piscine predators, packing their embryos into explosive conglutinates—mucous-encased bundles resembling flatworms, leeches, or midges. A darter who dares disturb these provocative masses winds up with a face full of bambinos, which happily hitch a ride upstream. Let’s say you’re a female Lampsilis or pocketbook clam, a rather glamorous but cautious creature clamoring for even better odds for her offspring. Why not modify your mantle to mimic a minnow—eyespot, tail, and all? Then use your muscular mussel contractions to wave it seductively in the water. What curious crappie could resist a close-up look (and accompanying cloud of clinging clamlets)? “But how might I ensure the absolute biggest possible payoff?” Then you’d better create a superconglutinate—a transparent, six-foot-long gelatinous tube with all of your glochidia stacked at the distal end, tethered to your body (or attached to a stick or rock) and twirling invitingly in the current, appearing as a sick sucker. When a bigger fish bites this alluring lure, its false eyespot explodes, and the clamsters climb aboard. Your “young-uns” remain embedded in the tissues of the host for one to six weeks, changing little in size but developing all adult organs and structures. (Don’t worry about the fish; it’s more embarrassed than 114 ß chapter fourteen debilitated.) Then these tiny editions, less than half a millimeter in diameter , drop off to begin their independent lives. And, reaching sexual maturity in three to nine years, they’ll (hopefully) repeat their parents’ cycle. “So, will my clam clan be safe at this point?” Not really. Humans consider some...