Intraspecific density effects in Nereis (Neanthes) arenaceodentata Moore (Polychaeta: Nereidae)

Effects of intraspecific density on Neanthes arenaceodentata Moore were examined under laboratory conditions by rearing animals for 28 days, in the presence of sediment, at densities of 230, 575, 1150, and 1730 worms/m(2). The lowest density examined in this study was similar to the mean reported field density for N. arenaceodentata. Food was allocated to each beaker on a per worm basis to focus the experiment on competition for space. Individual worm growth increased with increasing density. This result contrasts with other studies of intraspecific density effects in this species conducted in the absence of sediment where negative effects on growth were observed. Estimated individual dry weight and ash free dry weight of worms in the highest density treatment were significantly greater (13 and 18%, respectively) than worms in the lowest density treatment at the end of 28 days of growth. This increase in growth was associated with a decrease in the number of ovigerous females, suggesting a delay in reproductive development, and an increase in the number of worms regenerating posterior segments, suggesting an increase in the number of aggressive encounters. At the end of the 28 day growth experiment, mated pairs were established in separate beakers. Intraspecific density did not have a lasting effect on time to egg deposition and juvenile emergence or fecundity. The benefits of larger body size at higher densities may include the fact that larger worms are more successful at defending territories and finding mates. The results of this study emphasize the importance of sediment as a structuring agent in soft substratum habitats and the potential for complex density dependent effects.