ABSTRACT
Elasmobranchs have remained virtually unchanged since their emergence from jawless fishes over 400 million years ago and, in many ways, they are considered ancestral in their design compared to extant vertebrate species (Helfman et a/. 1997). Yet, remarkably, they have evolved diverse, highly complex mechanisms for assuring reproductive success. In contrast to many bony fish species, elasmobranchs are considered K-strategists, exhibiting slow growth, high maternal investment, and production of only a few, often welldeveloped, offspring. Added problems of seasonality and a migratory life style have made the acquisition of successful mating and reproduction strategies crucial to their survival across evolutionary time. Examples of such anatomic and hmctional adaptations include internal fertilization (all species), viviparity (some sharks and all rays), oviparity (some sharks and all skates), and elaborate mechanisms of excessive yolk production for intrauterine cannibalism (some sharks) (see Chapter 8 for references and details). ln addition, sperm storage in males and/or females has evolved in many different elasmobranch species (Pratt and Tanaka 1994). The ultimate success of all such strategies, nonetheless, is predicated on the production of sufficient numbers of hmctionally competent spermatozoa.
