Reduced pH affects pulsing behaviour and body size in ephyrae of the moon jellyfish, Aurelia aurita

• At reduced pH the size of ephyra-stage Aurelia aurita was decreased.
• Pulsation rate was reduced by culture in reduced pH seawater.
• Pulsation behaviour was more variable in reduced pH seawater.
• Ephyrae retained capability for rapid pulsation behaviour at reduced pH.

Our understanding of how reduced seawater pH affects the behaviour and growth of scyphozoan jellyfish is poor. Here, we investigated the effects of simulated Ocean Acidification (OA) (pH = 7.6 for 7 d) on pulsing behaviour (as an index of swimming behaviour) and aspects of the morphology of ephyrae of the moon jellyfish Aurelia aurita. Ephyrae exposed to reduced pH had a significantly smaller surface area, central disc area, and lappet length and width than controls. Pulsation rate was significantly lower, and the mean pulse-to-pulse period shorter, in the reduced pH treatment. There was, however, no significant treatment effect on either the maximum or minimum pulse-to-pulse period, suggesting that the ability for rapid pulsations was maintained. Ephyrae from the reduced pH treatment displayed a more variable pulsation behaviour, with an elevated standard deviation and root mean square of successive difference (RMSSD) in pulse-to-pulse period. In summary, reduced pH simulating future predicted Ocean Acidification conditions, had important effects on aspects of swimming behaviour and size of A. aurita ephyra, which may have consequences for survival and the population dynamics of field populations.