Reproductive patterns and secondary production of Gammaropsis japonicus (Crustacea, Amphipoda) on the seagrass Zostera marina of Korea

Seagrass beds are diverse and productive components of coastal ecosystems. Amphipods often comprise a major component of their invertebrate fauna and may be among the most important secondary producers. The life history and production of the amphipod Gammaropsis japonicus were investigated monthly for 1 year on a seagrass bed (Zostera marina L.) in Gwangyang Bay, southern Korea. Population density of G. japonicus was closely correlated with seagrass standing crop (P < 0.05), suggesting that there is a biological interaction between these two groups. The life-history pattern was annual and iteroparous with two principal periods of juvenile recruitment during the year; in spring (May to July) and fall (November to December). The mean body length of adults including ovigerous females was larger in the spring breeding period than in the fall and overall sex ratio was female-biased. The brood size and egg volume were positively related to body length of ovigerous females. At any given female size, egg volume was significantly larger in the fall breeding than in the spring, whereas brood size in the spring breeding was significantly larger than that in the fall. The brood volume of females between the two breeding periods was much greater in the fall than in the spring, suggesting higher reproductive effort during the fall breeding period. Annual biomass of G. japonicus was 2.16 g DW m⁻² yr⁻¹ and was higher in the spring than in the fall breeding periods. The annual secondary production (14.59 g DW m⁻² yr⁻¹) was higher than those observed for other amphipods inhabiting seagrass beds. However, annual productivity to biomass (P/B) ratio, at 6.74, was lower than those recorded previously in both temperate and tropical seagrass beds because the number of generations decreases the P/B ratio. The combination of high abundance and production suggests an important role for G. japonicus in the seagrass beds ecosystem as a trophic link from primary producers to higher consumers.