Sexual Reproductive Modes in Polychaetes: Classification and Diversity

A two-factor classification system for types of reproductive modes within the Polychaeta is described. The classification is based on the type ofIarval development and the fate of the female gametes (free-spawned or brooded in a variety of ways). A compilation ofinformation from the literature allowed the classification of 306 species. The Orders Phyllodocida and Spionida show the greatest diversity of reproductive modes. The most common reproductive mode involves the free spawning of gametes and the development of planktotrophic larvae. It is apparent that there has been multiple evolution of many reproductive modes during the course of polychaete evolution. This plasticity is argued to exceed that of the Classes Gastropoda, Bivalvia and Malacostraca. Polychaetes display an extraordinary diversity of reproductive traits (Schroeder and Hermans, 1975). It is not uncommon for congeneric species to possess radically different means of reproduction. For example, the maldanid Axiothella mucosa produces gelatinous egg masses attached to the female tube (Bookhout and Hom, 1949). Sibling species of A. rubrocincta brood their young inside their tube and free spawn demersal eggs, respectively (Wilson, 1983). The plasticity of polychaete life histories has undoubtedly contributed to their success in the marine environment (Knox, 1977). Although a considerable literature exists on the reproductive traits of polychaetes (see references in Table 2), there has been no effort to survey the distribution of reproductive modes across orders and families. Fauchald (1983) divided polyaetes generally into three general reproductive life styles, although intermediate species that are difficult to classify are common. My approach involves a finer classification of reproductive modes. Each class incorporates two aspects of reproduction. The first concerns the fate of the ova, particularly whether they are spawned freely into the seawater or afforded some type of brood protection or encapsulation. The second aspect concerns the type oflarval development: planktotrophic (having planktonic feeding larvae), lecithotrophic (having planktonic, non-feeding larvae) or direct (having no free-swimming larval phase) (Thorson, 1950). The distribution of these modes across the orders and families of the Polychaeta are examined, using data from 306 species. MATERIALS AND METHODS I surveyed the literature for descriptions of the reproductive biology ofpolychaetes. To be classified in my scheme, information had to be provided on the fate ofthe ova (whether they are free-spawned or brooded) and the type of larval development (planktotrophic, lecithotrophic or direct). The classification system that I have developed (Table 1) represents all combinations of six ova fate classes and the three larval development classes. The ova fate classes were developed based on my own knowledge of polychaete reproductive biology and are strictly applicable only to polychaetes. Each class is presumed to require specific morphological and physiological adaptations peculiar to that class, e.g., the ability to produce gelatinous egg capsules or the evolution of structures on the body for brooding. The larval developmental classes (Thorson, 1946; 1950) were selected a priori. These larval developmental classes can be applied to all marine invertebrates. The abbreviations given in Table I are used extensively in the text. The ova fate classes are: 1) free spawning, 2) brooding on the body, 3) brooding within the body (viviparity), 4) brooding in capsules within the tube, 5) brooding along the linings of the tube and 6) encapsulation in a gelatinous mass. The classification incorporates the notion of "mixed" development (Pechenik, 1979) in which embryos are sequestered for some portion