Effects of ichthyotoxic algae, <i>Heterosigma akashiwo</i>, on the locomotor behavior of marine medaka (<i>Oryzias melastigma</i>) larvae

Abstract Fish larvae face a significant threat from harmful algae blooms (HABs). There are still gaps in knowledge regarding the effects of ichthyotoxin-producing HABs on the locomotor behavior of fish larvae and their subsequent ecological implications. In this research, the effects of an ichthyotoxin-producing HAB species, Heterosigma akashiwo, on fish larvae were investigated, and compared with those of a non-toxic diatom Skeletonema costatum. Five locomotor parameters were measured, i.e. the average swimming speed (AS), maximum swimming speed (MS), swimming distance (SD), proportion of large turn angle (LTA), and relative swimming time (RT). Results showed that all locomotor parameters exposed to S. costatum had similar trends with those of the control group (filtered seawater) during 12-h experiment, both showing quadratic curves. Comparatively, three parameters (AS, SD, and LTA) showed quartic-curve trends. Meanwhile, significant increases were observed in the other two parameters (MS and RT) although they exhibited similar quadratic curves to the control group. These results illustrate the short-term effects and relatively long-term cumulative effects on the locomotor behavior caused by H. akashiwo, but not by non-toxic S. costatum. This finding indicates the negative effects of the ichthyotoxin-producing HAB species, which could affect feeding behavior and predation avoidance of fish larvae.