The life cycle and productivity of the amphipod Gammarus mucronatus on a northern Massachusetts salt marsh

Gammarus mucronatus was the dominant amphipod found in the salt marsh pools of the Parker River National Wildlife Refuge. Densities were positively and highly associated with the presence of floating algal mat dominated by Cladophora spp. Gammarus mucronatus is multivoltine with some overlap of cohorts. The first juveniles appeared in April, first adults matured in May at 3.5–4.5‐mm length (77% mature at 4.5 mm). Individuals from the last cohort, which began in November, matured at larger sizes (15% at 4.5 mm, 100% at 6.5 mm). The equation of egg number ( N ) against length of female ( L ) was N = 9.00L – 36.31 for overwintered females and N = 3.05L – 3.38 for the smaller summer females. Egg production was equivalent to 2.5 kcal m −2 yr −1 . The mean cohort interval was 112 days and mean cohort turnover rate ( P : B ) was 4.3. The two midsummer cohorts had the lowest growth efficiencies, 0.34 and 0.37. The first cohort, spanning 77 days (21 April–9 July), contributed the highest daily rates, more than half the annual production (8.7 g m −2 ), and the highest growth efficiency, 0.56. Several approaches to the calculation of annual production yielded a range of 12.4–15.8 g m −2 yr −1 dry wt with turnover rates also from 12.4 to 15.8. Mean annual production was equivalent to 64.1 and respiration to 67.0 kcal m −2 . When coupled with egg production this gave a mean growth efficiency of 0.50. When the values for respiration, growth, and reproduction are summed, the energy flow through G. mucronatus is equivalent to 5.8% of the net annual primary production in these pools.