Inter-annual variation in the reproductive pattern of Manila clam Ruditapes philippinarum and impacts of Perkinsus olseni infection on the reproduction observed from the west coast of Korea

Abstract

We monitored inter-annual variation in gametogenesis, egg production and the level of Perkinsus olseni in Manila clams distributed on the west coast of Korea over a four year period. For the analysis, clams were collected monthly from January 2007 to December 2010 from high P. olseni infection area (Hwangdo). Gonad development and quantity of the eggs in individual clams were determined simultaneously using histology-ELISA combined assay. P. olseni infection intensity in each clam was determined using RFTM and 2M NaOH digestion assay. P. olseni level in clams varied year to year remarkably. The infection intensity increased markedly from June 2008 (1.5 million cells/g tissue) to January 2009 (4.6 million cells/g tissue). In 2009, the infection level stayed in high ranging 1.92 (September) to 4.6 million (January) cells/g tissue. Mass mortality of Manila clam was observed (ca 40%) during the post spawning season in 2009. In spring 2010, P. olseni infection level dropped markedly, ranging from 0.6 to 1.3 million cells/g tissue. Monthly mean P. olseni density in clams in 2010 was significantly lower than the previous year, possibly due to the mass mortality which might remove the heavily infected clams. During 2007 and 2009, female clams in the area showed comparatively longer spawning period, from June to October, with the pulses in August and September. GSI of the females during the spawning peaks from 2007 to 2009 vary 2007 to December 2010 from high P. olseni infection area (Hwangdo). Gonad development and quantity of the eggs in individual clams were determined simultaneously using histology-ELISA combined assay. P. olseni infection intensity in each clam was determined using RFTM and 2M NaOH digestion assay. P. olseni level in clams varied year to year remarkably. The infection intensity increased markedly from June 2008 (1.5 million cells/g tissue) to January 2009 (4.6 million cells/g tissue). In 2009, the infection level stayed in high ranging 1.92 (September) to 4.6 million (January) cells/g tissue. Mass mortality of Manila clam was observed (ca 40%) during the post spawning season in 2009. In spring 2010, P. olseni infection level dropped markedly, ranging from 0.6 to 1.3 million cells/g tissue. Monthly mean P. olseni density in clams in 2010 was significantly lower than the previous year, possibly due to the mass mortality which might remove the heavily infected clams. During 2007 and 2009, female clams in the area showed comparatively longer spawning period, from June to October, with the pulses in August and September. GSI of the females during the spawning peaks from 2007 to 2009 va