Resource Allocation in the Intertidal Salt-Marsh Mussel Geukensia demissa in Relation to Shore Level

Abstract

Ribbed mussel populations at two levels of the intertidal shore were studied for 3 yr at a salt marsh site in Jamaica Bay (New York). Length-specific dry body weight was determined monthly and shell growth was monitored between April and November of each year. Production (Pi) was measured as monthly changes in body dry weight. Annual reproductive output (PR) was estimated indirectly for mussels between 10 mm and 80 mm by determining the gain in body weight prior to spawning (Weight Gained Method). Using a second method (Basal Weight Method), monthly rates of somatic production (Pg) were determined by assuming that, as mussels grow in length, a fraction of monthly Pt is allocated to increasing body weight. The monthly production allocated to growth (Pg) was estimated based on the length-specific body weight (basal weight) in March, when body weights in these populations begin to increase due to gametogenesis. Monthly rates of reproductive output (Pr) were then estimated be substraction as the difference between monthly Pt and Pg. Total PR determined by both methods were very similar. Shell and body growth were concurrent in sexually immature mussels, but body weight preceeded shell growth in older, larger animals. Annual shell increments were correlated with annual PG. In sexually mature mussels, Pr preceeds Pg in larger mussels, that is, allocations of net production to reproduction and somatic growth diverge in time with increasing shell length. Between April and November, shell growth rates were correlated with water temperature. At the higher shore level, mussels allocated a much smaller proportion of total production to growth, exceeding the reproductive effort of the lower-shore population. Variation in mussel production between years probably is related to variability in nannoplankton biomass. Although mussels higher on the shore can reduce somatic growth in order to increase fecundity and reproductive effort, total reproductive output is much reduced as compared to the marsh edge.