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Audience: Advanced undergraduates, graduate students, researchers, faculty and staff at universiities and consulting companies interested in freshwater biology and ecology, limnology, envionmental science, invertebrate zoology and related fields.
“Thank you for the opportunity to comment on the latest edition of Thorp and Covich. I have admired prior editions of this superb book for its comprehensive coverage of freshwater invertebrates. The current edition improves upon the high standard set by prior editions through the use of color and greater taxonomic specificity. Authored by an outstanding collection of experts, individual chapters provide comprehensive coverage of morphology, physiology and ecology, as well as methods for collecting, rearing and preserving freshwater invertebrates. Together with chapters on ecology and habitats of inland waters, this carefully edited volume provides the central knowledge of freshwater invertebrates that every student and researcher will find invaluable. I highly recommend this superb new edition of Thorp and Covich - it is a must-own volume that every student and researcher of freshwater invertebrates will find invaluable.” J. David Allan, Ph.D. Professor and Acting Dean School of Natural Resources and Environment The University of Michigan *** “This 3rd edition contains a wealth of information, which has expanded its utility beyond the earlier editions. Thorp and Covich gathered the recognized experts in North America to compile the full extent of current knowledge on this diverse group of aquatic fauna. The color plates are amazing and add tremendous value to both the learner and learned of the invertebrate biologists.” Michael T. Barbour, PhD Director, Center for Ecological Studies Tetra Tech, Owings Mills, Maryland *** "At last, after half a century, this new edition of Thorp and Covich is a worthy successor to Edmondson’s (1957) classic second edition of Ward and Whipple’s Freshwater Biology. It brings us up to date on the amazing advances in the biology of freshwater invertebrates, the keys are detailed, and the illustrations as beautiful as they are useful." Nelson G. Hairston, Jr. Frank H.T. Rhodes Professor of Environmental Science Department of Ecology and Evolutionary Biology Cornell University *** “The 3rd edition of Thorp and Covich has been extensively revised. The chapters are written by experts who present up-to-date reviews on the structure, function, ecology, and systematics of each invertebrate group. The biggest change from the 2nd edition is an expansion of the taxonomic keys to allow identifying many of the taxa to the species level. References to more-detailed monographs and web sites allow users to quickly gain a fuller perspective on particular groups of interest. The book should continue to be a vital resource for research labs and as a classroom text.” John E. Havel, Ph.D. Professor of Biology Missouri State University
"The 3rd edition of Ecology and Classification of North American Freshwater Invertebrates continues the tradition of in-depth coverage of the biology, ecology, phylogeny, and identification of freshwater invertebrates from the USA and Canada. This edition is in color for the first time and includes greatly expanded classification of many phyla and a downloadable set of references for all chapters."--GrrlScientist’s Maniraptora blog on Nature.com
"This third edition ensures that this work will remain the most up-to-date and comprehensive information source on freshwater invertebrate animals in the US and Canada. Numerous color photographs and some diagrams now brighten more than half of the new chapters. Fifty coauthors contributed, a 35 percent increase from the second edition…. Highly recommended."--CHOICE
James H. Thorp Kansas Biological Survey and Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, Kansas
Alan P. Covich Institute of Ecology, Odum School of Ecology, University of Georgia, Athens, Georgia
Walter W. Dimmick Kansas Biological Survey, and Department of Ecology and Evolutionary Biology, University of Kansas, Larence, Kansas
II. Species and Phylogenies A. Species and Concepts of Species B. Demographic Exchangeability C. The Role of Phylogenies in Studies of Ecology and Behavior D. Phylogenetic Trees
III. Taxonomy and Classification
IV. Synopses of the Invertebrates of North American Inland Waters A. Protozoa (Chapter 3) B. Porifera (Chapter 4) C. Cnidaria (Chapter 5) D. Flatworms: Turbellaria and Nemertea (Chapter 6) E. Gastrotricha (Chapter 7) F. Rotifera (Chapter 8) G. Nematoda and Nematomorpha (Chapter 9) H. Mollusca (Chapters 10–11) I. Annelida (Chapter 12 J. Bryozoa (Chapter 13) K. Tardigrada (Chapter 14) L. Arthropoda (Chapters 15–22)
V. Taxonomic Key to Major Invertebrate Fauna of Inland Waters
VI. Selected References
At some point in almost every biologist's life, an invertebrate stimulated their curiosity. It might have started when they peered through a microscope at pond water and saw a protozoan, hydra, rotifer, or some other creature. Or perhaps they watched a butterfly or bee pollinating a flower, a rolly-polly pill bug curled in a ball within their palm, ants marching rapidly past while carrying heavy loads, waving anemones within a tide pool, or a crayfish scurrying from under a stream rock. For many authors contributing to this book, this childhood fascination led progressively, if not inevitably, to a life-long career in science.
Aquatic scientists focusing on invertebrates conduct research in a wide diversity of fields. Some are fascinated by invertebrate variety and the relationships among species over evolutionary history, or they may be involved in conducting environmental assessments where knowledge of invertebrate identification is typically essential. Others seek to control human diseases by understanding and controlling invertebrate vectors of water-borne diseases, or perhaps they study the population biology or culturing of invertebrates involved in the food chains from which we derive many items, including freshwater crayfish and the zooplankton prey of trout and largemouth bass. Many more investigate the ecology of invertebrates to satisfy their scientific curiosity or to help abate environmental crises facing the natural and human world from future global climate change to more immediate and local environmental perturbations.
Next to their more colorful, and often larger, marine relatives, freshwater invertebrates may initially seem drab and uninspiring. Yet once the curious observer penetrates beyond superficial appearances and thoroughly examines the diverse structural, physiological, behavioral, and general ecological adaptations of freshwater invertebrates, few fail to be impressed by these fascinating animals. In this book, students and professionals alike will be exposed to new aspects of the biology, ecology, and classification of freshwater and other inland aquatic invertebrates. The authors of this book have reviewed the recent literature and drawn together some of the most important highlights associated with these diverse organisms. In this edition, we have also included new images and color plates to illustrate the diversity of forms and function.
This chapter, in particular, serves as a very brief introduction to freshwater invertebrates of Canada and the United States. Following a short discussion of procedures for naming and classifying organisms, we briefly describe the taxonomic groups covered in Chapters 3–22. We also include a taxonomic key to help the reader begin the process of classifying freshwater invertebrates. This dichotomous key will lead to the appropriate chapter for more detailed biological and taxonomic information. Additional information on freshwater invertebrates can be found in various texts on both invertebrate zoology and aquatic ecology.
II. SPECIES AND PHYLOGENIES
The identification of natural units is one of the major guiding principles for biologists that endeavor to study, discover, and document the world's biodiversity. A natural unit of biodiversity is defined here as a unit that results from evolution. Examples of natural units of biodiversity are populations, species, and monophyletic groups of species. Natural units that are given a formal name (e.g., species) are called natural taxa. In contrast, subspecies is an example of a taxonomic unit that is often not a natural unit but rather a convenient partition of geographic variation. Because the number of animal species is enormous and the number of biologists studying them is minuscule in comparison, it is not surprising that our poor understanding of many genera, tribes, and families would frequently result in groups that are not natural taxa.
The principal intellectual tools needed to discover natural taxa have emerged from the disciplines of systematic biology and population genetics. Species concepts and methods for constructing and evaluating phylogenetic trees are central to systematic biology. The discipline of population genetics is important because it permits direct study of the degree of connectivity between populations. Studies of molecular variation enable us to discover the existence of species that are undetectable when we restrict our data to morphological characters. This has resulted in the recognition of multiple species that were formerly thought to be a single wide-ranging species.
Many biologists disagree about the best species concept and which methods of phylogenetic construction are most reliable. While this may seem to be an inconvenient situation for a new student or a researcher who is primarily interested in ecological or behavioral studies, an informed viewpoint about species concepts and phylogenetics is prerequisite for any serious student of freshwater invertebrates. Because of the extensive loss of habitat due to anthropogenic disturbances and the impending wide-scale ecological disruptions from global climate change, extinction is likely for an enormous number of species. Freshwater species could be at the leading edge of the forthcoming storm of extinction, and thus it is more important than ever to underpin ecological studies with a robust understanding of the natural units of biodiversity which we label, or sometimes mislabel, with taxonomic names. The following two sections present a brief introduction to some of the important implications of using different species concepts and the critical importance of phylogenetic trees.
A. Species and Concepts of Species
Students of invertebrate zoology are occasionally confronted with the daunting prospect of identifying specimens to the level of species. Unfortunately, the intellectual basis for taxonomic decisions at the species level is sometimes questionable. Freshwater invertebrates are an interesting and difficult group of organisms because of their great biological diversity and a dearth of genetic studies of wide-ranging species. Informed decisions about the validity of a species requires an understanding of which species concept was used to label the group of organisms collected at a particular place and time.
Species are the result of historical natural processes. Anagenesis is the modification of lineages through mutation, gene flow, natural selection, and random genetic drift. Speciation is the creation of a new lineage by splitting a preexisting lineage or the result of hybridization between two preexisting lineages. Clearly, evolution is a phenomenon of lineages, and these lineages are called species by evolutionary biologists. Therefore, the first logical step to understanding current and past biodiversity is the identification of species and other natural units that result from evolution.
Because species are the products of evolution, their importance as units of biodiversity is paramount. Scientists often need to identify units of biodiversity below the species level, but ultimately definitions of these units must be cast in the light of an appropriate species concept because species serve as the ultimate frame of reference for studies of population variation. Species concepts play a critical role in the interpretation of intraspecific variation because species as natural taxa provide an upper boundary for comparisons among different populations. Whether a particular population or group of populations is distinctive enough to be recognized as a species has been a key issue in many taxonomic and conservation disputes. An understanding of what species are and how they can be identified is required to interpret studies of intraspecific variation and formulate biologically meaningful policies for identifying natural units of biodiversity.
The scientific literature about the nature of species and the best method of identifying species is enormous and contentious. Perhaps, the large quantity and legitimate controversies can even be considered a barrier to understanding for students, professional biologists, and policymakers. As an intellectual framework for understanding and organizing the many different concepts of species, it is useful to consider that they almost always fall into one of two different categories: operational species concepts or ontological species concepts. Operational species concepts provide a researcher with specific criteria, that is, an algorithm to implement in order to determine whether or not two different populations belong to the same species. The biological species concept of Mayr is a classic example of an operational species concept. The operation used by Mayr's biological species concept is the discovery of barriers to sexual reproduction that are assumed to define the boundaries of species. Ontological species concepts provide a theoretical definition of what a species is but do not specify a methodology for the identification of species. The evolutionary species concept of Wiley is an example of an ontological species concept. In this case, species are defined as lineages expected to persist through time, but no method (operation) is prescribed for the discovery of these lineages. In more common parlance, ontological concepts can be understood as theoretical definitions of what species are. Operational species concepts provide only a method of how to discover species and are, by definition, limited by their particular methodology.
Below you will find brief definitions of the biological, phylogenetic, cohesion, and evolutionary species concepts along with comments on their strengths and limitations. These concepts have all been influential and provide a good basis for an entry-level understanding of the many important issues regarding concepts of species. For a more thorough discussion and evaluation of species concepts, we recommend a careful reading of an article by Mayden and Wood.
1. Biological Species Concept
Species are groups of actually or potentially interbreeding populations which are reproductively isolated from other such groups.
The biological species concept has been prevalent in the evolutionary literature for the last several decades and is emphasized in many college-level biology courses. It is probably the species concept most familiar to biologists in diverse fields, such as conservation biology, forestry, fisheries, and wildlife management. Species defined by the biological species concept have also been championed as units of conservation.
Theodosius Dobzhansky, a prominent evolutionary geneticist and an important contributor to the modern evolutionary synthesis, characterized the concept of a biological species as a system of populations. The gene exchange between these systems (species) is limited or prevented by reproductive isolating mechanisms, such as species-specific breeding behaviors, hybrid sterility, and gametic incompatibility. Thus, under the biological species concept, species are simultaneously a reproductive community, a gene pool, and a genetic system. The study of reproductive isolating mechanisms is central to the biological species concept because these mechanisms provide barriers to gene flow that define the boundaries of the reproductive community and gene pool, and preserve the integrity of the genetic system of the species. In practice, however, isolating mechanisms are rarely studied and species are usually diagnosed by differences in phenotypic (morphological) features.
Despite the long historical acceptance of the biological species concept, it has become controversial because a growing number of evolutionary biologists have found the biological species concept unworkable in a wide variety of situations. Critics of the biological species concept come from the fields of both botany and zoology. A fundamental drawback to this concept is that it is exclusively defined in terms of sexual reproduction. Asexual taxa are obviously excluded from this concept, but it is also true that many species capable of sexual reproduction cannot be easily accommodated within the framework of the biological species concept. From the viewpoint of population genetics, species capable of self-fertilization (e.g., parasitic tapeworms and some plants) and those with mandatory sibling mating are more similar to asexual than to sexually outcrossing species. Species that freely hybridize (open mating systems) with one or more other species yet maintain their evolutionary identity as species also provide a serious challenge to the validity of the biological species concept. Groups of freely hybridizing species are known from plants, insects, and vertebrates.
Another important limitation of the biological species concept concerns speciation. The most widely accepted model of speciation is the allopatric model. Generally speaking, the allopatric model entails the geographic subdivision of a single population followed by the differentiation of the isolated subpopulations into new species. Historically, the notion of a correlation between geographic subdivision of populations and speciation grew out of the observation that the closest relatives tend to occupy separate but contiguous geographic areas. Thus, in allopatric speciation lineage independence is achieved when two or more lineages are geographically disjunct. Therefore, isolating mechanisms which are fundamental to the biological species concept have very little, if anything, to do with the process of speciation because the populations undergoing speciation are geographically disjunct from one another. Hence, the evolutionary forces responsible for allopatric speciation have nothing to do with the isolating mechanisms that are a fundamental aspect of the biological species concept. A species concept that fundamentally fails to illuminate the process of speciation cannot provide the intellectual framework for the identification of units of biodiversity. Because it is impossible to study gene flow and reproductive behavior of species known only from fossil remains, the biological species concept cannot be applied to the thousands of species known only from their fossils.
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