Changes in the swimming behavior of Pseudodiaptomus annandalei (Copepoda, Calanoida) adults exposed to the diatom toxin 2-trans, 4-trans decadienal

• We investigated the behavior of copepods in response to a diatom-derived toxin.
• 2-trans, 4-trans decadienal was selected as a model polyunsaturated aldehyde.
• Effects were observable within 30 min of direct exposure.
• Decadienal induced a dose-dependent hyperactivity while reducing swimming speed.
• We observed a trend toward Brownian motion and more vertical displacements.

Diatoms are broadly present in marine habitats and often dominate seasonal phytoplankton blooms in polar and temperate latitudes. Certain species produce polyunsaturated aldehydes upon mechanical wounding caused by mesozooplankton grazing. Ample evidence is available on toxin-induced reproductive failure in copepods, yet their behavioral effects remain unclear. Here we present results of laboratory experiments in which we investigated the immediate effects of the diatom-derived aldehyde 2-trans, 4-trans decadienal on the three-dimensional swimming behavior of the calanoid copepod Pseudodiaptomus annandalei. Short-term direct exposure to the toxin at 3 μM, 6 μM and 12 μM induced hyperactivity in the three adult states, as evidenced by a marked and dose-dependent increase in the number of trajectories. It also caused a higher proportion of vertical movements. In males and ovigerous females exposed to decadienal at 3 μM and 6 μM, hyperactivity came with an equally specific dose-dependent decrease in swimming speed. Males and ovigerous females swam faster at 12 μM than at 6 μM, suggesting a complex mode of action of the toxin. In non-ovigerous females, decadienal had little effects on swimming speed, supporting the assumption that female copepods are less affected by certain environmental stressors. Multifractal analysis revealed differences in the statistical properties of the swimming behavior between experimental conditions. The moment structure function of the displacement appeared to be moderately multifractal in the three adult states swimming in control water. Ethanol as carrier solvent at 200 ppm caused an increase in swimming speed and a switch toward a more ballistic motion in males and ovigerous females. On the opposite, exposure to the toxin reduced or cancelled the effects of ethanol and resulted in a more Brownian motion for high moment values. Decadienal had little effects on the behavior of non-ovigerous females except at the highest concentration. Our results demonstrate that decadienal, a model diatom aldehyde, impairs the behavior of adult copepods. They provide further information on the interaction between diatoms and their main predator.