The tapeworms begin manipulating the beetles even before they are inside them. Beetles are lured to egg-laden droppings by an aroma that’s apparently irresistible to the insects. If a beetle should come across droppings from a healthy rat and droppings from a parasitized one, it’s more likely to choose the pile with the tapeworm eggs. If you trap the fragrance of infected dung and preserve it in liquid, a drop of this perfume will bring beetles scurrying. No one knows if the eggs themselves produce the scent, or if it’s one of the chemicals produced by the adult tapeworms inside the rats, or if the parasites somehow change that rat’s digestion so that the host itself makes it. Whichever is the case, it’s enough to seduce the beetles into eating a tapeworm, perhaps into being eaten by a rat.
Once inside the beetle, the tapeworm then uses more chemicals to sterilize it. Like most other insects, a beetle stores up reserves of energy in a structure called the fat body that runs along its back. Female beetles use some of this material to build the yolks for their eggs. To get the reserves to the eggs, they have to send a hormone signal to the fat body. The fat body cells respond to it by making a yolk ingredient called vitellogenin. The vitellogenin leaves the fat body and flows through the beetle until it reaches the eggs in the ovaries. A beetle egg is surrounded by a retinue of helper cells that leave only a few cracks between them. The cracks are so few and so small, in fact, that it’s hard for anything to get through them and to the egg itself. But when the right hormones latch onto these helper cells, they make them shrink, opening up the spaces. With enough of these hormones, the vitellogenin can reach the egg itself and turn into yolk.
The tapeworm can destroy this chain of events at several links. It makes a molecule that gets into the fat body and slows down the cells as they make vitellogenin. Some vitellogenin still gets out of the fat body, but little of it seems to reach an egg. It appears that the tapeworm makes yet another molecule that can lock into the receptors on the helper cells in the ovaries. It plugs up the receptors to stop the hormone from latching on and making the helper cells shrink. The helper cells stay swollen, so the vitellogenin can’t get into the egg. The effect of these molecules is to stop the beetle from diverting what could be perfectly good tapeworm food into its own eggs.
Once it has matured inside the beetle, the tapeworm is ready to find itself a rat. The beetle certainly wouldn’t agree, so the parasite has to pull open another drawer of drugs. Some of them—probably opiates that blunt feelings of pain and fear—make the beetle less conscientious about concealing itself. Put it on a pile of flour, and the beetle will be likely to wander the surface instead of burrowing out of sight. The tapeworm makes it sluggish, slow to escape from an attack. Still, an infected beetle does its best to defend itself if a rat should take it in its jaws. A flour beetle comes equipped with a pair of glands on its abdomen that it uses to release a foul-tasting chemical, and a rat that grabs the beetle in its mouth is likely to spit it out. But once the tapeworm reaches maturity, it blocks the gland from making its poison. When the infected beetle tries to defend itself, it doesn’t taste all that bad to the rat; it is thus far more likely to be eaten than a healthy counterpart. From beginning to end, the beetle is guided and tugged by its parasite.
If you turn off the Ventura Freeway at the town of Carpinteria, California, and drive a short way toward the ocean, passing a teddy bear warehouse and a set of train tracks, you come to a chain link fence. Beyond it lies a low expanse covering hundreds of acres of lush low plants like pickleweed. This is the Carpinteria salt marsh. One clear summer day, an ecologist named Kevin Lafferty unlocked the fence gate and led me inside. He wanted to show me how a salt marsh works. Lafferty was dressed in a pair of bathing trunks and a fraying T-shirt with fluorescent lion fish on it; he shuffled along the dirt path in flip-flops, with a pair of scuba booties in one hand. I spent a few days all told in the company of Lafferty, and during my entire visit I saw him in nothing more formal. His face was young and his hair was wheat-colored. He has surfed along these beaches since he came to the University of California at Santa Barbara in 1981. It would be hard now to pick him out on a wave as a biology professor instead of a sophomore.
He talked about the marsh as we walked toward the sea on a raised dirt path. “You need some sort of interior space below sea level to get a salt marsh. You can have a river cut a channel and the sea is able to intrude upon it at high tide. That’s the standard East Coast version. Or you could have tectonic activity that leads to subsidence.” He gestured back inland, up toward the San Ynez Mountains, which loomed over the freeway, fog draped on them like a scarf. “The whole California coast line is a complicated mix of tectonic activity, plus changes in sea level. The basin here is thought to have been flooded by the ocean because it has subsided.” The area is now about a foot below sea level, so that the sediments carried by the Santa Monica and Franklin Creeks are dumped in this basin rather than reaching the sea. Each day the high tide pushes its way into the marsh, spilling over the creek banks and flooding this place all the way back to the chain link fence. “If the sea level stayed the same and there was no tectonic activity, this might be dry land in a hundred years. But if the land is continually subsiding, then the sediment can’t catch up,” says Lafferty. The opposing forces of accumulating sediment, incoming freshwater, and the ebb and flow of sea water have all reached a compromise in the form of this broad, water-logged expanse cut through by channels.
Each day at low tide, the soil bakes in the sun, evaporating its water while holding on to the brine. The soil is actually saltier in places than sea water. In these conditions no trees can survive. Instead, there is a low carpet of tough plants adapted to the salt. Pickleweed, for example, pumps the briny water out of the ground and stocks away the salt in its fruits, using the fresh water left behind. Along the bare mud flats that line the marsh channels, algae grow in dull green varnishes. The algae may look subdued, but they’re actually reveling in almost perfect conditions. The mud is packed with nitrogen, phosphorus, and other nutrients carried down from the mountains. Because the bare flats are exposed every time the tide drops, the algae get far more sunlight than they would if they were always submerged. Today at low tide the algae are photosynthesizing merrily. Scattered along the banks are thousands of miniature birthday hats: the conical shells of California horn snails that graze the algae. “They’re mowing a fast-growing lawn,” Lafferty says.
The many invertebrates here, such as littleneck clams and sand dollars, make good meals for vertebrates. Some fish, like the arrow gobies and the killifish, live in the estuaries year round, huddling in the low water when the tide ebbs and then feeding at high tide, when they’re joined by curious stingrays and sharks wandering in from the sea. Today the killifish are the only fish to be seen. They dart around, every now and then turning to one side to expose the brilliant glint of their bellies. Along the banks of the channels are bigger holes, these the size of fists rather than fingers. When the morning sun hits them, crabs slowly crawl out—lined shore crabs, which crack open the snails like walnuts, and fiddler crabs, which slowly raise their giant claws as if saluting the newborn day. There aren’t many mammal predators here—the growth of towns like Carpinteria has driven away the mountain lions and bears, leaving only raccoons, weasels, and house cats. But the salt marsh is still a carnival for birds—for Caspian terns, willets, plovers, yellowleg sandpipers, curlews, dowitchers—all picking their way through the feast.