Graphic Science

An Improbable Life Cycle

A parasite manipulates the behavior of at least one of three hosts to travel from water to air—and back

Parasites give new meaning to the cliché “eat or be eaten.” Often their life cycle can be completed only if they are ingested by a host—multiple times for some—making the odds of their survival seemingly minuscule. To improve their chances, certain parasites manipulate their hosts’ behavior to make it more likely the eater will get eaten.

The parasitic cestode Schistocephalus solidus requires a much larger host—specifically, a three-spined stickleback fish— to grow in and then a bird to breed in. But the parasite’s larvae, less than a millimeter long, are too small to be eaten by the fish. Instead a larva must first be ingested by a copepod, a crustacean akin to a tiny shrimp. When ready for its next host, the larva makes the copepod twitch. If all goes well (for the parasite), a threespined stickleback then eats the copepod. Inside the fish, the larva grows enormously, making the poor stickleback gasp at the water’s surface, where it is likely to get snacked on by a bird. Inside the bird, the parasite matures and mates, sending its eggs back to the water through the bird’s poop. And so the cycle begins again.

7 A water bird consumes the infected fish. The parasite matures into an adult tapeworm inside the bird’s intestine, mates and produces eggs. THIRD HOST Fish-eating water bird (Kingfisher shown)

Stomach

Intestine

6 The mature parasite larva causes the bloated fish to swim close to the surface and slow down, making it an easy target for predators.

Mature larva

SECOND HOST Three-spined stickleback fish (Gasterosteus aculeatus) Infective parasite larva

5 The infected copepod is ingested by the parasite’s second host, a three-spined stickleback fish. The parasite grows quickly in the fish’s abdomen.

A two-gram fish may have several 300-milligram worms inside. 4 About two weeks later the parasite has matured into a fully infective larval stage. It is now primed to be consumed by its next host. The parasite increases the activity of its copepod host, causing it to attract the attention of predators. Parasite adult worm (Schistocephalus solidus)

Noninfective parasite larva Parasite egg

Immature larva 1 START HERE Parasite eggs pass into the water in the feces of their parents’ bird host. They hatch about three weeks later into free-swimming larvae.

2

FIRST HOST

The tiny larva is ingested by its first host, a copepod. Copepod (magnified view of Macrocyclops albidus shown here)

3 At first, while the parasite is still developing and not yet ready for its second host, the larva reduces the activity level of its host. The sluggish, floating behavior of the copepod makes it less likely to be detected by predators. Parasitologist Nina HaferHahmann studied the cestode S. solidus at the Max Planck Institute for Evolutionary Biology in Plön, Germany. She showed that the parasite directly manipulates its copepod host’s behavior and that this ability is heritable.