Assessment of temperature effects on the variation of reproductive cycle and parasites infection on Manila clam (Ruditapes philippinarum) in Shi-Heung-Ri beach on the east coast of Jeju Island

Abstract

Post two decades since the first report of seasonal variations of Perkinsus and Cercaria and reproductive cycle of Manila clams distributed in Shi-Heung-Ri beach on the east coast of Jeju island, the climate change draws a query that whether these parameters alter in this area. Accordingly, the physiological condition of clams, P. olseni, Cercaria and other parasite infections were surveyed. From obtaining sea surface water temperature (SST) from KHOA database, the linear trend estimated that the mean SST fluctuation post 2 decades increased 1.07℃. Adult Manila clams were collected monthly from May 2019 to April 2020 and analyzed using gill RFTM (Ray's Fluid Thioglycollate Medium) assay (for P. olseni) and histology. The condition index of clams was highest in July (6.7±1.7) then decrease in August and September that correlated to the spawning peak. For RFTM assay, the dramatic increase in P. olseni infection intensity from October (2.1×105) to November (8.7×105) coincided with post-spawning condition of the clams. The prevalence and infection score of P. olseni was high with the mean of 89.7% and 3.0, respectively, without seasonal changes. The proportion of Rickettsia-like-organisms (RLOs) infection were highest in April (50.0%) and lowest in November (6.7%). Metacercaria Parvatrema duboisi mostly infected in the summer and autumn (16.7-43.3%) whilst C. tapidis and C. pectinata incidentally infested in some months with low prevalence (3.3 and 6.7%, respectively). To compare with the result in 2001-2002, Manila clams in the present study expressed poorer physiological condition (low CI and delay spawning season). Besides, the prevalence and infection score of P. olseni increased while the percentage of C. tapidis infestation decreased after 18 years. Thus, this study suggests that a slight increase in SST may exerted a great influence on the parasite infection dynamics although further investigation needs to be carried out to warrant this hypothesis.