New Records of Magallana gigas and Callinectes sapidus from the Romanian Black Sea Coast Based on Citizen-Reported Observations: Contributions to Marine Biodiversity Monitoring

The number of alien species recorded in the Black Sea has increased over recent decades, raising concerns regarding their long-term ecological consequences. Among the taxa currently extending their distribution within the basin are the Pacific oyster, Magallana gigas, and the Chesapeake blue crab, Callinectes sapidus. This study presents new records of both species from the Romanian Black Sea coast obtained through citizen-reported observations. Five specimens of M. gigas collected from Constanța and Mangalia harbours were examined, together with one ovigerous female specimen of C. sapidus collected from Vadu Beach. In addition, two male C. sapidus records from Sinoe Lake were documented and validated based on photographic evidence. The finding of M. gigas in Mangalia Harbour extends the known distribution of the species along the Romanian coastline and suggests that the species is more widespread than previously documented. The occurrence of an ovigerous C. sapidus female confirms that reproduction is taking place in Romanian Black Sea waters. Morphometric measurements and reproductive traits were within the ranges reported from other Black Sea and Mediterranean populations. While none of these species can currently be regarded as invasive in Romanian coastal waters due to the absence of documented ecological impacts, the increasing frequency of records suggests that both warrant continued monitoring. The results further indicate that citizen-reported observations, when validated by specialists, can complement conventional surveys and provide valuable information on the occurrence and spread of conspicuous alien marine species, thereby supporting sustainable marine biodiversity monitoring and informed ecosystem management.