Citizen science detects the undetected: the case of Abudefduf saxatilis from the Mediterranean Sea

Volunteers are increasingly involved in environmental research and their role in monitoring biological invasions is rapidly expanding in the Internet Era. On November 2012, a recreational diver reported the sighting of an unidentified and unfamiliar fish to an interactive web project ‘Seawatchers’ (http://www.observadoresdelmar.es) that aims to acquire geo-referenced observations of exotic fish species in the Mediterranean Sea. The individual was spotted on August 2009 along the coast of Tarragona, Spain and it was later identified as the Sergeant major Abudefduf saxatilis (Teleost, Pomacentridae), a tropical Atlantic species previously unrecorded in the Mediterranean Sea. This finding highlights the potential of citizen scientists in revealing the occurrence of new exotic fishes in the marine environment, providing a small but concrete advancement towards early detection, which is important for management of biological invasions.