Cryptic Biodiversity

By Butler, Robert S.; Mayden, Richard L. | Endangered Species Update, May-June 2003 | Go to article overview

Cryptic Biodiversity


Butler, Robert S., Mayden, Richard L., Endangered Species Update


Just how many aquatic animal species do we have in the Southeast? We don't have a final answer to this question yet, but through the collaborative efforts of agency biologists and scientists specializing in taxonomic classification, we are moving in the right direction. We do know that the southeastern United States harbors the greatest level of temperate zone aquatic biodiversity in the world. For creatures such as freshwater mussels, the most highly imperiled animal group in the U.S., the level of diversity exceeds that of any other region globally.

Researchers in systematic biology continue to learn about the biological diversity of this region, some of which is referred to as "cryptic" or unknown biodiversity. The discovery of formerly unrecognized species has been made possible by the development of sophisticated analytical tools. Advances in both technology and theory in systematic and evolutionary biology have permitted scientists to detect additional species diversity around the world on the basis of divergences in genetic, morphological (body form), ecological, and behavioral traits that were previously unknown.

Most biologists with a knowledge of aquatic organisms are familiar with species "complexes." Generally, a species complex describes a relatively wide-ranging species that has an unusual distribution or is highly variable across its range in morphological characters, habitat preferences, or other aspects of its anatomy, life history, or ecology. This understanding is rapidly changing. Species are now considered evolutionary lineages, and these lineages are identified using a variety of heritable traits that can have differing degrees of differentiation within and between groups. Scientists are incorporating various types of data into their studies to reveal naturally occurring patterns of diversity. Accurate knowledge of diversity is not only critical to scientists seeking to understand processes responsible for biological diversification, but is equally critical to resource managers hoping to develop effective conservation programs.

Over the past decade or two, thorough studies of the morphological, behavioral, ecological, and genetic variation in species have resulted in the "splitting" of several fairly well-known polytypic (containing populations that have different morphological traits) species into numerous "new" species. Two recent changes that have been important in refining this science include the technological advances in molecular genetics and a more pronounced emphasis on field studies with the examination of live organisms.

Historically, morphological data obtained from preserved museum specimens was the primary source of characters used in differentiating new taxa. Museum materials are essential, but often they do not readily reveal important characteristics found in cryptic species complexes. These types of traits are best found by examining live specimens or by using other methods useful in identifying evolutionary lineages, such as genetic traits. Following the identification of new taxa, all of the evidence, whether it is genetic, morphological, behavioral, ecological, or combinations thereof, is compiled into a formal species description for publication in a peer-reviewed scientific journal.

Several recent examples among the fishes show how the use of genetic and morphological data, combined with phylogenetics (genetic relationships of related organisms) systematics and a revolutionized theory of how species are determined, has improved our understanding of diverse biological lineages. The greenbreast darter (Etheostoma jordani) was long thought to consist of a single species found in most of the major rivers of the highly rich Mobile Basin. A thorough evaluation of the variation in this species based on live and museum specimens revealed four distinct species, one of which, the Etowah darter (Etheostoma etowahae), is now listed as endangered.

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