Does abundance of spawners matter?: Referring to the reproduction in dominant cohort of asari clam in Sanbanze, Tokyo Bay

Abstract

To examine the population dynamics, and particularly the reproductive ecology, of asari (Manila) clams in Sanbanze, Tokyo Bay, we monitored in detail the biweekly temporal variations in spatial density of clams through their growth stages from larva to spawner in 2001–2005, and the long-term bimonthly variation in overall density in the benthic clams in 1988–2014. In the four autumn cohorts in 2001–2004, there was a high density of the dominant cohort in 2002. The distribution in growth stages from larva to spawner indicated an abundance of spawner clams due to migrated larvae. Long-term variation of juvenile density in spring (June) did not correlate significantly with that of spawners in the previous autumn (October) (spawner–juvenile relation), although the density of spawners in autumn correlated with that of juveniles in the previous spring (juvenile–spawner relation). Particularly, the abundant 2002 autumn cohort, driven by a small spawner cohort in 2001, could generate extremely poor numbers in the 2003 cohort despite its high abundance. Non-significant correlations between the densities of spawner and offspring juvenile could be attributed to reported reproductive characteristics of the asari clam. Specifically, plasticity in gametogenesis, i.e., unsynchronized development of gametes within a gonad and among individuals along with frequent resorption of fully developed gametes, may play an important role in the fluctuation of the abundance of released eggs. Moreover, variability in the strength and frequency of key stimulants for clam spawning (temperature, physical disturbance, and other factors) could cause additional fluctuation. Successful spawning of the previous generation may be one of the determinant factors for the abundance of spawner stock in Sanbanze.