Implications of Alternative Reproductive Modes for Seasonality and Demography in an Estuarine Polychaete

The consequences of alternative developmental patterns for the seasonal dynamics and demography of benthic marine invertebrates were examined by comparing two local demes (1.5 km apart) of the polychaete Streblospio benedicti Webster (Spionidae). One was dominated by individuals exhibiting planktotrophy, where large numbers of offspring develop from small eggs as feeding larvae, and the other was dominated by individuals exhibiting lecithotrophy, in which fewer numbers of larvae develop from large eggs without feeding. Over a 2.5—yr period we studied development mode, benthic abundances, recruitment, recolonization of disturbed areas, size structure, and reproductive output at the two sites, to evaluate possible effects of larval development mode on: (a) level of population fluctuation, (b) colonization ability, (c) secondary production, and (d) demographic properties. Size—structured population models were used to evaluate seasonal and annual population growth rates (‐l). Sensitivity and life—table response analyses were carried out to examine the relative contribution of fecundity and survivorship to differences in ‐l between demes and among seasons. Strong similarity was observed between planktotroph— and lecithotroph—dominated demes in magnitude and timing of fluctuations in recruitment and total abundance, in benthic population size structure, and in secondary production, average production: biomass ratio, and annual population growth rates. S. benedicti data and a review of the literature provide no support for Thorson's (1950) hypothesis that species with planktotrophic development experience much greater levels of population fluctuation than those with lecithotropic development. However, in this study the planktotrophs exhibited superior colonization ability. They also exhibited stronger seasonality and more variability in production and some demographic properties. These factors combined suggest that populations with planktotrophic development should exhibit different patch dynamics than those with lecithotrophic development. The lecithotroph—dominated deme exhibited a consistent survivorship advantage in larval and juvenile stages that was balanced almost precisely in the planktotroph—dominated deme by a fecundity advantage in early adult stages. This led to similar population growth rates (‐l wk — 1 ), calculated from annual projection matrices, at both sites in both years (range: 1.004 to 1.078 wk — 1 ). At both sites seasonal (cohort) population growth rates were lowest in spring (0.46—0.76) and highest in early summer and fall (usually ≥1.10). Population growth potential for the plankotroph—dominated deme was always greater than for the lecithotroph—dominated deme in early summer; the reverse was true for the fall, overwintering cohort. Comparison of this study with an age—structured analysis of laboratory—reared S. benedicti populations from the same location, but subject to excess food and no predation or seasonality (Levin et al. 1987), revealed greatest demographic similarity between the lab populations and the fall, overwintering cohort from the field. The similar demographic consequences of planktotrophy and lecithotrophy have significance for efforts to model the evolution of life—history patterns, and for understanding the intraspecific and interspecific occurrence of contrasting developmental modes.