Chapter 13 Parasitism Key ConCepts Enslaver Parasites: A Case Study In science fiction books and movies, villains sometimes use mind con- ConCept 13.1 Parasites typically feed trol or physical devices to break the will and control the actions of their on only one or a few host individuals. ‣ victims. In these stories, a person may be forced to perform strange or ConCept 13.2 Hosts have adaptations grotesque actions, or to harm themselves or others—all against their will. ‣ for defending themselves against Real life can be just as strange. Consider the hapless cricket shown in the parasites, and parasites have adaptations video found in Web extension 13.1. This cricket does something that a cricket for overcoming host defenses. ordinarily would never do: it walks to the edge of a body of water, jumps in, ConCept 13.3 Host and parasite and drowns. Shortly afterward, a hairworm begins to emerge from the body populations can evolve together, each in of the cricket (Figure 13.1). For the worm, this is the final step in a journey ‣ response to selection pressure imposed that begins when a terrestrial arthropod—such as a cricket—drinks water by the other. in which a hairworm larva swims. The larva enters the cricket’s body and feeds on its tissues, growing from microscopic size into an adult that fills ConCept 13.4 Parasites can reduce all of the cricket’s body cavity except its head and legs. When fully grown, the sizes of host populations and alter the adult hairworms must return to the water to mate. After the adults mate, ‣ outcomes of species interactions, thereby the next generation of hairworm larvae are released to the water, where they causing communities to change. will die unless they are ingested by a terrestrial arthropod host. ConCept 13.5 Simple models of Has the hairworm “enslaved” its cricket host, forcing it to jump into ‣ host–pathogen dynamics suggest ways to the water—an act that kills the cricket, but is essential for the hairworm control the establishment and spread of to complete its life cycle? The answer appears to be yes. Observations diseases. have shown that when crickets infected with hairworms are near water, they are much more likely to enter the water than are uninfected crickets (Thomas et al. 2002). Furthermore, in ten out of ten trials, when infected crickets were rescued from the water, they immediately jumped back in. Uninfected crickets do not do this. Hairworms are not the only parasites that enslave their hosts. Maitland (1994) coined the term “enslaver parasites” for several fungal species that alter the perching behavior of their fly hosts in such a way that fungal spores can be dispersed more easily after the fly dies (Figure 13.2). The fungus Ophiocordyceps unilateralis also manipulates the final actions of its host, the ant Camponotus leonardi. First, an infected ant climbs down from its home in the upper branches of trees and selects a leaf in a protected environment about 25 cm above the soil (Andersen et al. 2009). Then, just before the fungus kills it, the ant bites into the selected leaf with a “death grip” that will hold its body in place after it is dead. The fungus grows well in such protected environments but cannot survive where the ant usually lives—at the tops of trees, where the temperature and humidity are more variable. Thus, while the ant’s final actions do not benefit the ant, they do allow the fungus to complete its life cycle in a favorable environment. Figure 13.1 Driven to Suicide the behavior of this wood cricket (Nemobius sylvestris) was manipulated by the hairworm (Paragordius tricus- pidatus) emerging from its body. By causing the cricket to jump into water (where it drowns), the hairworm is able to continue its life cycle. Flies infected by the fungus perch in an atypical position, from which fungal spores can easily spread to healthy flies. own bodies (Figure 13.3). Our faces are home to mites that feed on exudates from the pores of our Wind skin and on secretions at the base of our eyelashes. There are bacteria and fungi that grow on our skin and under our toenails. Arthropods such as lice Spores may live on our heads, pubic regions, and other Healthy flies typically parts of our bodies. Moving inward, our tissues, perch on the upper organs, and body cavities can be infested with a surfaces of low-lying rich variety of organisms, from bacteria to worms vegetation—where to fungi to protists. they are exposed to fungal spores. Although some symbionts are mutualists (as we will see in Chapter 14), the majority of them are parasites. A parasite consumes the tissues or body fluids of the organism on or within which it lives, its host; some parasites, called pathogens, cause diseases. Unlike predators, parasites usually have a higher reproductive rate than their hosts. Also unlike predators, Head Head louse Pediculus Mouth Bacterium Actinomyces Heart and blood Protist Plasmodium Lungs (malaria) Lung worm Bacterium Yersinia Angiostrongylus Figure 13.2 Enslaved by a Fungus Shortly before they die pestis (plague) Fungus Coccidioides from the infection, yellow dungflies infected by the fungus Ento- Bacterium mophthora muscae move to the downwind side of a relatively tall Liver Mycobacterium Liver fluke Fasciola plant and perch on the underside of one of its leaves. This position (tuberculosis) increases the chance that fungal spores released by Entomophthora Small intestine will land on healthy yellow dungflies. (after Maitland 1994.) Bacterium Vibrio Kidneys (cholera) Fungus Candida spp. Amoeba Entamoeba Even vertebrates can be enslaved by parasites. Tapeworm Taenia Rats typically engage in predator avoidance behav- Large intestine iors in areas that show signs of cats. However, rats Pinworm Enterobius Arms and legs infected with the protist parasite Toxoplasma gondii Tick Ixodes behave abnormally: they do not avoid cats, and in (Lyme disease) some cases they are actually attracted to cats. While such a behavioral change can be a fatal attraction for the rat, it benefits the parasite because it increases the chance that the parasite will be transmitted to the Body surface Skin Body louse Pediculus next host in its complex life cycle—a cat. Itch mite Ecology Cain How do some parasites enslave their hosts? Can Sarcoptes scabiei (scabies) Sinauer Associates the hosts fight back? More generally, what can these Body tissues Elizabeth Morales Illustration Services Nematode Wuchereria Figure Ecology 13.02.eps Date 12-17-07 remarkable interactions tell us about host–parasite relationships? (elephantiasis) Feet Fungus Trichophyton rubrum Introduction (athlete’s foot) More than half of the millions of species that live on Figure 13.3 The Human Body as Habitat Different parts Earth are symbionts, meaning that they live in or on of our bodies provide suitable habitat for a wide range of symbi- other organisms. To begin to understand how many onts, many of which are parasites; only a few examples are shown symbionts there are, we need look no further than our here. Some of these organisms are pathogens that cause disease. © 2011 Sinauer Associates, Inc. This material cannot be copied, reproduced, manufactured or disseminated in any form without express written permission from the publisher. Ecology Cain Sinauer Associates Elizabeth Morales Illustration Services Figure Ecology 13.03.eps Date 01-22-08 parasitism 285 parasites typically harm, but do not immediately kill, the Fishes organisms they eat. The negative effects of parasites on their hosts vary widely, from mild to lethal. We see this varia- Birds tion in our own species, for which some parasites, such as Mammals the fungus that causes athlete’s foot, are little more than a nuisance. Others, such as the protist Leishmania tropica, can True bugs cause disfigurement, and still others, such as Yersinia pestis, the bacterium that causes the plague, can kill. There is simi- Beetles lar variation in the degree of harm caused by parasites that Flies infect other species. Parasites vary in many other ways, as we’ll see next as we examine their basic biology. Wasps Butterflies and moths ConCept 13.1 Parasites typically feed on only one or a few host individuals. Trees 0 2 4 6 8 10 12 14 16 95 Parasite Natural History Average number of parasite species per host Parasites vary in size from relatively large species (macro- Figure 13.4 Many Species Are Host to More Than One parasites), such as arthropods and worms, to species too Parasite Species In a study conducted in Britain, most host spe- small to be seen with the naked eye (microparasites), such cies were found to harbor more than one parasite species. the as bacteria, protists, and unicellular fungi. But whether number of parasite species shown here for fishes, birds, and mam- they are large or small, parasites typically feed on only mals includes only helminth worm parasites and hence is likely to underestimate the actual number of parasite species found in one or a few host individuals over the course of their these vertebrates. lives. Thus, defined broadly, parasites include herbivores, Averaging across the six groups of organisms other than ver- such as aphids or nematodes, that feed on only one or a tebrates (which we exclude because the data underestimate few host plants, as well as parasitoids (see p. 264), insects the true number of parasites), what is the average number whose larvae feed on a single host, almost always killing it. of parasite species per host? Suppose the number of para- Most species are attacked by more than one parasite site species were determined for a previously unstudied (Figure 13.4), and even parasites have parasites.