Foreword

In 1974, Jim Green published his excellent review of the "Parasites and Epibionts of Cladocera" (Green 1974). Until now, this has been the key reference in this field, not only for the taxonomy of parasites and epibionts but also for their natural history. The strength of Green's review is that it is a comprehensive account of what was known about parasites and epibionts of Cladocerans at the time. Historically this meant, however, that parasites, and in particular microparasites, were poorly documented, because little was known about these tiny organisms. One of my aims here is to concentrate particularly on the parasites, because their roles in the ecology and the evolution of their hosts have been neglected for a long time. Although my original plan was to keep the framework of this book similar to Green's paper, I soon realized that this was not possible because too much new material is available. As my interest centers more on parasitism, I focus here on parasites, leaving aside epibionts. There is, moreover, so much information about parasitism in the Cladocera that I have decided to split the work into two parts and publish it as two independent books. The present book deals with the ecology, epidemiology, and evolution of the parasites of Daphnia and other Cladocerans. The second part will deal with the taxonomy and natural history of all parasites known to Daphnia.

Setting the Stage

The ecology of members of the genus Daphnia has possibly been more closely investigated than any other taxon. For centuries, researchers studied Daphnia ecology not only for its key role as a primary consumer in the food chain of freshwater ponds and lakes but also as a model species for phenotypic plasticity (e.g., cyclomorphosis and predator-induced defense), behavior (e.g., vertical migration), toxicology, and the evolution of sexual and asexual reproduction (e.g., geographic parthenogenesis). In recent years, a burst of genetic research has addressed a number of evolutionary questions, resulting in a well-rounded picture of the evolutionary ecology of the genus.

For a long time, research focused on predators as the main enemies of Daphnia in their natural habitat. Predatory fish, phantom midge larvae, and water boatmen were among the key culprits and received a lot of attention, in particular after predator-induced defenses were described. This interest in predators existed in sharp contrast to the lack of attention paid to another class of Daphnia’s natural enemies, parasites and epibionts. Despite numerous taxonomic studies on the epibionts and parasites of Daphnia, there was less than a handful of ecological studies on them up until about 15 years ago. The growing awareness that parasites are ubiquitous and may play an important role in most natural ecosystems has changed this, as the increasing number of publications about Daphnia parasites and epibionts confirms (Figure 1.1).

Figure

The hard life of Daphnia . Drawing by Daniela Brunner, Basel.

Defining Parasites

Although parasites have traditionally been defined by a combination of conceptual and taxonomic features, I use an entirely conceptual definition here. I consider a parasite to be any small organism (including viruses) that lives in close association with a host organism and for which it seems reasonable to assume that the host carries some cost. These costs may be clearly visible, in the form of reduced fecundity or survival, but may in some cases be subtle. For example, reduced sexual attractiveness (leading to reduced mating success) or reduced competitive ability may not be very visible. I devote an entire chapter to discussing the fitness costs caused by parasites. This conceptual definition of a parasite includes members of various taxa, such as viruses, bacteria, fungi, and protozoa, but also includes functional categories (not taxonomically defined), such as pathogens and helminths. In contrast to typical predators, parasites do not always kill their hosts, and if they do, it may take a considerable amount of time, during which the parasite may be transmitted to other hosts, and the host remains in the community competing with other organisms for space, food, and mating partners.

In the literature on Cladocera and more specifically on Daphnia, parasites are often distinguished from epibionts. Whereas the former are usually endoparasites, i.e., located within the body of the host, the latter are located on the body surface and may therefore be labeled as ectoparasites. In the main part of this book, I concentrate on endoparasites and exclude epibionts. However, this is not to say that epibionts are not parasites or are not important. In fact, I believe that most epibionts fulfill the definition of parasites used here, because they are often closely associated with their hosts and cause harm to their hosts. This harm may not be directly visible, but there are certainly increased costs for swimming, which may have consequences for other fitness components, such as fecundity, survival, competition, and mate finding (Threlkeld et al. 1993). It has also been suggested that epibiontic filter feeders compete with their hosts for food (Kankaala and Eloranta 1987). On the other hand, it has been suggested that under certain conditions, high loads of algal epibionts may provide additional food for the host and thus result in a net benefit (Barea-Arco et al. 2001). However, this form of a food supplementation is certainly not the typical effect of epibionts.

I do not include epibionts in this book, because I feel that there is less need to discuss the epidemiology of this functional group than for endoparasites. However, I will refer to them whenever it might further our understanding of Daphnia–parasite interactions.

Host–Parasite Interactions

Parasites may be directly or indirectly involved in the ecology and evolution of a broad range of phenomena: host population dynamics and extinctions, maintenance of genetic diversity, sexual selection, evolution of genetic systems, and evolution of sexual recombination, to name just a few. Certainly, parasites possess features that make them very attractive as explanatory factors in the evolution and ecology of their hosts. These features include their high abundance in nearly every ecosystem, their typically narrow host range (compared with typical predators), their adverse effects on their hosts (e.g., reduced fecundity and survival), and density dependence during horizontal transmission (Anderson 1979, 1993; Anderson and May 1978; May and Anderson 1979; Price 1980).

On the other hand, hosts are the environment for the parasites and thus define their niche. Most parasites are not viable outside of their hosts for extended periods (not considering resting stages) and therefore—from the parasite's point of view—parasite and host form an inseparable biological unit. Thus, parasite ecology is closely linked to the ecology of its hosts, and the parasite's natural history is best seen in the light of its host's biology. In this book, I focus largely on members of the genus Daphnia as hosts. Whenever possible, I include information on other Cladocerans.

Outline of This Book

Following this introduction, Chapter 2 gives a general summary of Daphnia biology, highlighting at the same time those aspects that may be relevant for the study of parasitism. This summary is followed by Chapter 3, which introduces the reader to certain parasite species that are frequently mentioned in the book. This chapter is short, however, because the parasites will be dealt with in detail in the second book.

The next chapters describe the interactions between parasites and their Daphnia hosts. Much of the conceptual parts are derived from general principles of epidemiology but with special reference to the biology of zooplankton and especially Daphnia. Chapter 4 summarizes what we know from parasitological field studies of Cladocerans. Chapters 5 and 6 deal with the sometimes severe fitness consequences of parasitism. In Chapter 5, I review what we know about the negative effect of parasites on the fitness of individual hosts, and in Chapter 6, I review the little we know about how hosts fight parasites. Chapter 7 is on host ranges and discusses what we know about the specificity of Daphnia parasites.

Chapters 8 and 9 address aspects of parasitism at the population level. A central chapter of this book is "Epidemiology." Its two parts deal with transmission processes and with the actual epidemiology of Daphnia parasites. Chapter 9 introduces the important question of whether parasites regulate their host populations or even drive them to extinction.

Chapter 10 introduces a number of experiments that one may do with Daphnia and its parasites within the framework of a student course or for research purposes. These simple experiments may be used to illustrate principles of host–parasite interactions. Experiments are suggested at the individual level as well as at the population level.

From Chapter 4 onward, I end each chapter by posing open questions and highlighting major gaps in our knowledge.

A Glossary provides definitions of terms from Daphnia biology and parasitology used throughout the book.

Updates and Corrections

I will maintain a Web site on my home institution's server to report updates and correct errors. If you find errors, disagree with certain statements, or find that I neglected important information, I would be happy to read your comments. Please send me an email: hc.sabinu@trebe.reteid

References

1. Anderson RM . Parasite pathogenicity and the depression of host population equilibria. Nature. 1979;279:150–152.
2. Anderson RM, 1993. Epidemiology. In: Cox FEG, editor. Modern parasitology. Oxford: Blackwell. p. 75-116.
3. Anderson RM , May RM . Regulation and stability of host-parasite population interactions. I. Regulatory processes. J Anim Ecol. 1978;47:219–247.
4. Barea-Arco J , Perez-Martinez C , Morales-Baquero R . Evidence for mutualistic relationship between an algal epibiont and its host Daphnia pulicaria. Limno Oceanogr. 2001;46:871–881.
5. Green J . Parasites and epibionts of Cladocera. Trans Zool Soc Lond. 1974;32:417–515.
6. Kankaala P , Eloranta P . Epizooic ciliates (Vorticella sp.) compete for food with their host Daphnia longispina in a small polyhumic lake. Oecologia. 1987;73:203–206. [PubMed: 28312288]
7. May RM , Anderson RM . Population biology of infectious diseases: Part II. Nature. 1979;280:455–461. [PubMed: 460424]
8. Price PW, 1980. Evolutionary biology of parasitism. Princeton: Princeton University Press.
9. Threlkeld ST , Chiavelli DA , Willey RL . The organisation of zooplankton epibiont communities. Trends Ecol Evol. 1993;8:317–321. [PubMed: 21236181]