Behavioural maintenance of highly localised jellyfish (Copula sivickisi, class Cubozoa) populations

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The medusae of cubozoan jellyfishes have sophisticated behaviours and are strong swimmers. Therefore, they have the potential to influence their distribution and connectivity among populations. We used ecological and behavioural data in combination with local oceanography to estimate the potential of medusae in the cubozoan Copula sivickisi to disperse from local populations at scales of hundreds of meters to kilometres. The distribution of C. sivickisi was mapped on a fringing reef at Magnetic Island, Queensland, Australia, with underwater jellyfish camera units (JCams). The availability of reef habitat, dominated by Sargassum sp. algae and coral, had a significant effect on the abundance of medusae. Medusae were 11–7 times more abundant at shallow (≤ 4.1 m) and mid-depth (4.2–7 m) sites with high to moderate habitat availability, compared to deep sites (≥ 7.1 m) where habitat availability was low. Further, medusae were absent at sites far from suitable habitat, both alongshore and in deeper waters. Medusae displayed preferential habitat selection. They were found low in the water column near reefs in depth stratified plankton tows and they preferentially attached to Sargassum in a habitat choice experiment. The swimming speeds of C. sivickisi medusae were determined experimentally and were equivalent to or faster than most of the current speeds measured where the populations occured. The results suggested that medusae can attach to habitat and swim against currents to maintain positions on reefs, thereby restricting dispersal. Incipient speciation is highly likely within the species’ cosmopolitan distribution.