Reproductive ecology and population dynamics of two sympatric species of Macoma (Bivalvia)

Populations of the sympatric intertidal bivalves, Macoma secta and M. nasuta are compared as to reproductive ecology and population dynamics. Histological examination of seasonal gonadal changes indicate that both species undergo dormancy in the winter months and rapid gamete proliferation in the spring. However, the timing of spawning differs. The M. secta population synchronously spawned in August while the M. nasuta population spawned lightly in late spring and more heavily in the fall. Fecundity estimates indicate M. nasuta adds 140% of winter weight in gametes and M. secta only adds 43% of winter weight in gametes. Spat fall was detected in January for both species and was light in intensity, for either species only 24 per m2. Nursery areas were determined for M. nasuta, which corresponds to adult distributions, and for M. secta, which correspond to the upper and lower fringes of the intertidal distribution but not the center. Analysis of variance on distributional data indicate the temporal stability of density and size for M. secta. M. nasuta densities were also temporally stable but sizes were not. Both species demonstrated significant changes in density with tidal height. A measure of seasonal population variability, the Population-Time Mean Square, was found to have zero correlation with tidal height for M. secta, meaning that environmental stress was the same for individuals at each tidal height . This is believed related to the gradient of calm size to tidal height. An argument for density dependence in M. secta is given with emphasis on individual growth and realized population fecundity; the null hypothesis: As density increases, the resources per individual decrease, therefore the growth rate of individuals decreases. Analysis of sympatric populations indicate differing patterns of life history, pointing out that selection has favored these two species coexistence. Analysis of population dynamics indicated that in comparison to M. nasuta (Low Tide Horizon), M. secta (Mid Tide Horizon) was more stable (terminology from Ricketts, Calvin and Hedgpeth, 1969). This supports Castenholz’ (1967) idea that where climates are mild, the intertidal communities are more stable than the subtidal communities

Reproductive ecology and population dynamics of two sympatric species of Macoma (Bivalvia)

Populations of the sympatric intertidal bivalves, Macoma secta and M. nasuta are compared as to reproductive ecology and population dynamics. Histological examination of seasonal gonadal changes indicate that both species undergo dormancy in the winter months and rapid gamete proliferation in the spring. However, the timing of spawning differs. The M. secta population synchronously spawned in August while the M. nasuta population spawned lightly in late spring and more heavily in the fall. Fecundity estimates indicate M. nasuta adds 140% of winter weight in gametes and M. secta only adds 43% of winter weight in gametes. Spat fall was detected in January for both species and was light in intensity, for either species only 24 per m2. Nursery areas were determined for M. nasuta, which corresponds to adult distributions, and for M. secta, which correspond to the upper and lower fringes of the intertidal distribution but not the center. Analysis of variance on distributional data indicate the temporal stability of density and size for M. secta. M. nasuta densities were also temporally stable but sizes were not. Both species demonstrated significant changes in density with tidal height. A measure of seasonal population variability, the Population-Time Mean Square, was found to have zero correlation with tidal height for M. secta, meaning that environmental stress was the same for individuals at each tidal height . This is believed related to the gradient of calm size to tidal height. An argument for density dependence in M. secta is given with emphasis on individual growth and realized population fecundity; the null hypothesis: As density increases, the resources per individual decrease, therefore the growth rate of individuals decreases. Analysis of sympatric populations indicate differing patterns of life history, pointing out that selection has favored these two species coexistence. Analysis of population dynamics indicated that in comparison to M. nasuta (Low Tide Horizon), M. secta (Mid Tide Horizon) was more stable (terminology from Ricketts, Calvin and Hedgpeth, 1969). This supports Castenholz’ (1967) idea that where climates are mild, the intertidal communities are more stable than the subtidal communities