Annual reproductive cycle and reproductive efforts of the Manila clam Ruditapes philippinarum in Incheon Bay off the west coast of Korea using a histology-ELISA combined assay

We investigated the reproductive effort of the Manila clam Ruditapes philippinarum during different gametogenic stages with combining histology and immune assay techniques. To determine the level of gonad maturation, 1- to 2-mm-thick slices of the dorso-ventral section were cut from the middle of clams for histology. The quantity of eggs in each clam was determined from the remaining tissue using rabbit anti-clam egg protein IgG in an indirect enzyme-linked immunosorbent assay (ELISA). At the Begmiri tidal flat in Incheon Bay off the west coast of Korea, clams commenced gametogenesis in February and the first spawning female was observed in July. Clams continued to spawn into October (18.5 °C). The monthly mean gonad-somatic index (GSI), the ratio of egg mass to body weight, ranged from 4.27 (April) to 20.63 (July). The GSI increased rapidly from April (4.3) to May (16.8), peaked in July (20.6), then dropped dramatically from August (14.0) to September (5.5), indicating that clams at the Begmiri tidal flat have a major spawning pulse during August and September. The histology–ELISA combined technique enabled us to assess gametogenic stepwise reproductive efforts of clams because we could simultaneously determine the reproductive stage and quantity of eggs. The GSI of mature females ranged from 20.9 (May) to 26.6 (July), while that of partially spawned clams ranged from 12.8 (June) to 7.2 (September), suggesting that clams discharge as much as 50% of their eggs during the major spawning pulse. Histology revealed that residual eggs in spent clams were resorbed, and that clams may restore approximately 4.6% of their body weight by this energy-recycling process. The histology–ELISA combined technique provided both quantitative and qualitative information about clam reproduction, which is crucial for clam fisheries and aquaculture.

► We investigated reproductive effort (RE)/annual reproductive cycle of Manila clam. ► RE and reproductive stage (RS) was simultaneously measured from each clam. ► ELISA-Histology combined assay enabled to assess quantity of eggs and RS. ► Gonad somatic index (GSI) of ripe clam ranged 20.9-29.6, or 2.5-13.8 million eggs. ► Spring plankton blooming coincided with the rapid gonad maturation in late spring.