Effects of temperature and age on the swimming behaviour of a fish parasite, Sparicotyle chrysophrii

Transmission is a key process affecting the stability and dynamics of host-parasite systems and thus driving infections. Efficient management of potentially harmful parasites, such as monogeneans, is essential in aquaculture and requires an understanding of their transmission in changing environments. This study analysed the effect of larval age and water temperature on the swimming behaviour and transmission of Sparicotyle chrysophrii, a pathogenic monogenean commonly found in gilthead seabream, Sparus aurata, from the western Mediterranean Sea. Three experiments were designed to assess the effects of both factors on (1) irregular (i.e. forward, irregular, conspicuous and fast movements) and vertical swimming periods, (2) swimming speed in a thin water layer and (3) upward and downward swimming behaviour. Increasing age reduced the vertical swimming speed and the number of larvae swimming vertically, thus increasing the number of larvae at the bottom of the test tube, when oncomiracidia were added at the top. Increasing water temperature also minimized the number of vertical movements per larva as well as the vertical swimming speed, while shortening the duration of irregular and vertical swimming patterns. The loss of vertical swimming performance of S. chrysophrii with increasing age suggests that active dispersal processes can be compromised after 24 h, especially at higher water temperatures where the infection window is even shorter. Therefore, passive dispersion of the infective stages is likely to contribute substantially to parasite transmission, especially when hosts are not nearby.