On the lookout for the endangered, only to find the invasive

Abstract

Among the most worrisome threats to Mediterranean Sea habitats are the range expansions of invasive species and declines of native species. An ongoing (disease-related) mass mortality event has brought the native Mediterranean fan mussel (Pinna nobilis) to the brink of extinction. Using nylon-mesh collectors, we assessed larval P nobilis recruitment at selected sites in the Columbretes and Balearic islands, in the northwest Mediterranean. Before the mortality event, which had begun in 2016, P nobilis juveniles were very common in our samples; in 2020, however, not a single juvenile was collected. Recently, and to our surprise, another bivalve species appeared in our samples: the pearl oyster (Pinctada imbricata radiata), a Lessepsian invader (that is, from the Red Sea via the Suez Canal) that was first reported (as adults) on the mainland coast of Spain in 2019. However, the larval recruits of this invasive oyster were detected in our collectors in the Balearic Islands almost one year earlier in 2018. Although P imbricata radiata adults have yet to be recorded in the Columbretes Islands, which are located between the Spanish mainland and the Balearic Islands, this small 1-cm-long juvenile (pictured) was collected there in 2020. Our findings highlight adverse environmental outcomes from the perspective of larval dispersal. Larval collectors can serve as a means to document the recovery (or in this case, the loss) of native species and at the same time provide an early alert for invasions. Is the establishment of the nonnative P imbricata radiata and the extinction of the imperiled native P nobilis imminent? While the first outcome might be unavoidable, we can only hope to find fan mussel juveniles in our collectors again soon.