Two beaching events of the calycophoran siphonophore Abylopsis tetragona (Otto, 1823) were observed in two different areas of the Strait of Gibraltar during the cold season. The first was discovered on November 2014, on Getares Beach (Algeciras Bay, Mediterranean part of the Strait of Gibraltar), where more than 700 colonies were found deposited along the tideline. The second event was discovered on January 2015, on Paloma Baja Beach (Tarifa, Atlantic part of the Strait of Gibraltar) where an average density of 170 colonies m−2 was spread along the seashore. Both events seemed to be promoted by strong easterly winds, preceded by upwelling episodes that may have concentrated high densities of the siphonophore in superficial layers. This study represents the first report of a calycophoran siphonophore mass stranding. The records were made thanks to citizen science and jellyfish outreach at secondary schools (PERSEUS@school initiative), illustrating the importance of citizen science projects in observing natural phenomena. We consider the monitoring and recording of cnidarian stranding events especially important in highly productive and biologically active areas such as the Strait of Gibraltar

KK, LP and GN were funded by PERSEUS European Project (FP7-287600). KK was also funded by the Rotary Club Genève Lac, Switzerland

7 pages, 6 figures

Peer Reviewed