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Pearly mussels (Unionoidea) live in lakes, rivers, and streams around the world. These bivalves play important roles in freshwater ecosystems and were once both culturally and economically valuable as sources of food, pearls, and mother-of-pearl. Today, however, hundreds of species of these mussels are extinct or endangered. David L. Strayer provides a critical synthesis of the factors that control the distribution and abundance of pearly mussels. Using empirical analyses and models, he assesses the effects of dispersal, habitat quality, availability of fish hosts, adequate food, predators, and parasites. He also addresses conservation issues that apply to other inhabitants of fresh waters around the globe and other pressing issues in contemporary ecology.

Existence values for obscure resources should not be ruled out a priori because consumers have limited time and resources for information gathering. Furthermore, to the extent that environmental \"good things\" are highly substitutable, existence values must be cautiously interpreted when evaluating public choice alternatives, but similar concerns do not arise in damage assessments. Issues associated with adding up existence values across resources, high per unit values, and property rights in the existence of resources are also addressed.