Distribution patterns of gregarine parasitism of wild marine bivalve, Anadara cornea (reeve, 1844) concerning seasonality and water quality

Gregarines are groups of protozoan parasites that ubiquitously infect arthropods worldwide including host from terrestrial and aquatic environment. To date, 1,600 gregarine species have been described, but less study for invertebrates have been recorded for these apicomplexan parasites. Wild marine bivalve species Anadara cornea (Reeve, 1844) populations were surveyed for gregarines at Setiu Lagoon in Terengganu, Malaysia for a year. This species of bivalve was previously unlisted as hosts. Malaysia is known as a country that experience with rainy and dry season. The trends of rainfall regime in Peninsular Malaysia is mainly affected by the seasonal monsoon i.e. Southwest (SW) monsoon (May to October) and Northeast (NE) monsoon in November and ends in April the following year. The objective of this study was to associate the incidence of gregarine in common wild bivalves, A. cornea of Setiu Lagoon, Terengganu in relation to water quality and seasons. Gregarine prevalence and mean intensity of infections were compared between seasons. Samples were collected at seagrass beds which contain sandy muddy area which is a suitable habitat for this bivalve. The study was conducted from January to December 2018 with a total of 480 sample bivalve collected ( n =40 per month). Total numbers of parasites were counts during this study were 33,239 phagocytes. The highest intensity of the infection of the parasite was found in November ( n =5014) and the lowest number of infection was in January ( n =767). The confidence interval for proportions normal approximation in the prevalence of dry season was 48.9% with 95% of CI=45.3%–54.3%. The confidence interval for proportions normal approximation in the prevalence of rainy season was 47.7% with 95% of CI=43.2%– 52.2%. Risk estimation for odds ratio (OR) was 0.087 with 95% CI=0.01-0.68 Pearson Chi-Square showed the significant value (p≤0.05) cases via season. Most of the site infections of this parasite were on gill and digestive tract. Prevalence of parasites was considerably the same for both seasons. The mean intensity levels of infections were slightly high during the rainy season, which was assumed characterized by low surface seawater temperature, low salinity, and high levels of rainfall. Regression analyses revealed that season was significantly related to gregarine prevalence and intensity. This result emphasizes the importance of considering season and habitat as a part of the consideration factor while studying gregarine in bivalve ecology.