Swimming ability of the Carybdea marsupialis (Cnidaria: Cubozoa: Carybdeidae): implications for its spatial distribution

Although usually considered part of the plankton, cubozoans are strong swimmers. The aim of this study was to determine the influence of the active swimming ability of the box jellyfish Carybdea marsupialis on the spatial distribution of a well-studied population off Dénia (NW Mediterranean) where adults and juveniles do not overlap geographically. To achieve this aim, we analyzed the swimming speed, proficiency, effective velocity, and effective displacement index (EDI) of 27 individuals with diagonal bell widths (DBW) ranging from 1.1 to 36 mm. The laboratory analysis utilized conventional video recordings and the video analysis tool Tracker. Mean swimming speed for small juveniles (DBW ≤ 5 mm), medium juveniles (5 mm < DBW< 15 mm) and adults (DBW ≥ 15 mm) were 9.65 ± 0.76 mm-1, 21.91 ± 2.29 mm-1 and 43.10 ± 1.78 mm-1 (mean ± s.e.m.), respectively. Comparing these results with the local currents obtained from drifting buoys analyzed in the area over the course of three years, adults would be able to swim strongly enough to overcome almost 70% of the currents, whereas the small juveniles would not reach 17%. This allows larger individuals to select their habitat, while smaller individuals are left dependent on advection. Although experiments adding currents in aquaria would be necessary to confirm these theoretical results, the data obtained would be useful in improving the performance of bio-mathematical models used to predict jellyfish blooms since, even though the sting of C. marsupialis is non-fatal, it may produce systemic effects in sensitive swimmers. Summary statement The analysis of the swimming ability of C. marsupialis elucidates its key role in the spatial distribution of a northwestern Mediterranean population. ### Competing Interest Statement The authors have declared no competing interest.