Fitness consequences of seasonal reproduction: Experiments on the polychaete Nereis virens Sars

A substantial component of the marine invertebrate fauna exhibits highly seasonal and synchronized reproduction (Babcock et al., 1992; Babcock et al., 1986; Giese and Kanatani, 1987; Harrison et al., 1984; Watson et al., 2000). In temperate waters this reproductive strategy delivers offspring into specific conditions because of the inherent seasonality of the environment. The environmental conditions that vary seasonally are very complex; they include relatively simple signals such as the seasonal change in the duration of scotophase and photophase (the photoperiodic signal) and the more derived annual temperature signal, both of which approximate to a sine wave form. The annual temperature cycle, however, has a much lesser degree of predictability due to the greater component of between season and spatial variability or ‘noise’. The seasonal signals also include more complex physical factors that affect the movements, mechanical energy and hydrodynamics of the water masses and which may have important consequences for the dispersal of pelagic offspring (Bhaud and Cha, 1992; Denny and Shibata, 1989). Seasonally variable environmental signals also include a host of secondary biotic factors such as food availability, the presence and abundance of competitors, predators and the presence and concentration of biologically active substances. Biotics include toxins, such as those produced by dinoflagellates (Southgate et al., 1984; Dutz, 1998; Ianora et al., 1999) and diatoms (Ianora et al., 1996; Buttino et al., 1999; Miralto et al., 1999), the excretory products of metabolically active organisms and pheromones released to modify the behaviour of con-specifics, as demonstrated for a number of Nereidae (Hardege and Bartels-Hardege, 1995; Hardege et al., 1994, 1996, 1998; Zeeck et al., 1990). In this sense, the seasonally changing environment is both influenced by, and partially constructed by, the interacting organisms. Many of the seasonally variable physical environmental signals increase in amplitude with latitude and are strongest in polar regions. Seasonal environmental fluctuations become attenuated with depth such that the deep sea environment is typically thought of as a quasi-aseasonal environment at least in its physical components (Tyler, 1988; Tyler et al., 1982; Tyler and Young, 1992). Contrary to earlier concepts and predictions, however, seasonal reproduction is not restricted to the coastal and shelf regions of temperate and polar regions and is observed in such environments.