Annual variation of reproductive effort, biochemical composition and Perkinsus olseni infection in Manila clam, Ruditapes philippinarum, off the west coast of Korea

Abstract

We investigated the gametogenesis, Perkinsus olseni infection and biochemical composition of Manila clam Ruditapes philippinarum sampled from 4 sites (Bangpo, Gonam, Shinya and Janggok) of Anmyeon Island on the west coast of Korea from April 2005 to September 2006. Gametogenesis of female clams in Shinya, Ganggok and Gonam was firstly observed in January 2006, while in Bangpo was observed in December 2005. Ripe eggs were observed in May of 2005 and 2006 at all sites clams. The spawning of clam in Shinya and Gonam was processed from June to October, while that of Bangpo and Janggok was initiated one month later from July to October. The monthly average of condition index (CI, tissue wet weight/shell dry weight) in a regional clam population was ranged from 0.350 to 0.845, which was declined after showing the maximum value of CI in before spawning. Although CI value of all sites was similar, CI of Shinya population ranged from 0.350 to 0.557 which was relatively lower than that of Gonam, Bangpo and Janggok. The infection prevalence and intensity of P. olseni in clams at Bangpo showed the lowest with 16.8% and 20,373 Perkinsus cell/g tissue wet weight during this study. In contrast, clams from Gonam, Janggok and Shinya were highly infected with P. olseni showing the prevalence 93%, 96.9% and 81.8% and intensity of parasite in three sites ranged from 260,342 to 2,219,304 Perkinsus cell/g tissue wet weight. Seasonal infection intensity of P. olseni were no difference between Bangpo and Shinya populations, it was increased in fall (September-November) and winter (December-February) for Janggok and Gonam populations (p<0.05) after spawning. The exchange of total carbohydrate content among 4 clam populations coincided with CI pattern, whereas protein levels were distinctly different to carbohydrate content. The mean of total carbohydrate content and protein level in a regional clam population varied from 134.8-222.4 mg/g tissue dry weight and 378.8-425.5 mg/g tissue dry weight, respectively. Seasonal changes in reproductive cycle, reproductive effort, Perkinsus infection intensity, condition index (CI) and biochemical composition of the clams, Ruditapes philippinaru, were investigated over 3 years in Sunjae and Sungam and 4 years in Padori and Hwangdo. Based on the Perkinsus infection level, the clam beds grouped into three categories: (1) heavily infection area (HIA: Hwangdo); (2) moderately infection area (MIA: Sunjae and Sungam); and (3) lightly infection area (LIA: Padori). Reproductive cycle of MIA showed regular annual trend, while in HIA and LIA were irregular annual trend. The mean prevalence and infection intensity of Perkinsus ranged from 62-100% and 9,388 (±4,869)-2,063,087 (±917,113) Perkinsus cell/g tissue wet weight (TWWT). Among three clam populations, clams from Padori showed the lowest infection level, the prevalence and intensity were 62% and 9,388 (±4,869) Perkinsus cell/g TWWT, respectively. Hwangdo clams was 100% infected with Perkinsus during this study, and mean infection intensity of Perkinsus was 2,063,087 (±917,113) Perkinsus cell/g TWWT. Clams at Sunjae and Sungam areas showed a high Perkinsus infection prevalence with 96% and 99%, whereas infection intensity of Perkinsus was moderate level were 351,262 (±207,028) and 317,026 (±186,543) Perkinsus cell/g TWWT, respectively. CI and total carbohydrate level are closely associated with annual reproductive cycle of clam. CI and carbohydrate level were higher during spring, when clams were in the developing stage. In spawning season, CI and carbohydrate level was decreased. We found that reproductive effort was negatively correlated with the Perkinsus infection intensity except clams collected from Padori: more heavily infected years produced fewer eggs during the spawning season. Reproductive effort (RE) were measured in Manila clam, Ruditapes philippinarum, collected from 26 sampling sites along the east, west, south and Jeju coasts in Korea 2009 during summer spawning season. To understand spatial pattern of clam RE, an indirect Enzyme-Linked Immunosorbent Assay (ELISA) was applied to quanty the egg production. The gonadosomatic index (GSI) was represented that, a ratio of egg dry weight to tissue dry weight and it varied from 1.76 (±3.30) to 18.91 (±7.98) in all sites. The fecundity of clams estimated from ELISA was ranged from 358,382 to 11,373,973. GSI and fecundity of clams at Duiruini and Wando-subtidal area was much higher than that of other populations. Histological observation also showed mostly full developed in female clams both Duiruini and Wando area. The correlation between fecundity and individual size was significantly positive in the fecundity as increasing egg size of clam. This is the first report on the spatial distribution of clam GSI, and it will be provide a useful information of the broodstock selection for the artificial clam seed production.