Massive arrivals of egg capsules of the gastropod Pachycymbiola brasiliana and the hydroid Amphisbetia operculata in the beaches of the Southwestern Atlantic Ocean

The phenomenon of subtidal organisms arriving, accumulating, and depositing onto the beach is not unusual, although they have been seldom documented in South America, especially regarding marine invertebrates. In this study twelve major events that happened at the Atlantic South American shorelines between 35° and 38°S during the austral summer of 2022–2023 were investigated. The arrivals were mainly composed by egg capsules of the gastropod Pachycymbiola brasiliana and by colonies of the hydroid Amphisbetia operculata. Prior to these events, persistent winds from the north/northeast quadrant that promoted moderate to strong currents toward the coastline were recorded. The compilation of other historic events reveals that the 2022–2023 are probably the largest and best documented events involving these species. Great mortality of recruits of the gastropod Pachycymbiola brasiliana may have consequences for the species as well as other members of the local assemblages and food webs. Contrarily, to established populations of the hydroid Amphisbetia operculata, these events pose minimal biological or ecological threats. Unpleasant subsequent conditions for tourists at the beaches are caused by the wrack. This contribution also draws attention to the lack of legislation in Argentina regarding how this wrack should be removed from the beaches.