Growth rates of the Critically Endangered fan mussel Pinna nobilis in the Slovenian Sea (northern Adriatic)

The fan mussel Pinna nobilis is a bivalve endemic to the Mediterranean Sea that is listed as Critically Endangered on the IUCN Red List. Its decline is due to a disease that has led to mass mortalities throughout the Mediterranean. The Gulf of Trieste (northern Adriatic) used to harbour one of the densest populations of P. nobilis , but since 2019, it has also been affected by this disease. The aim of this work was to analyse the growth of P. nobili s in the Slovenian part of the Gulf, with 3 case studies: (1) in situ measurement of adults before the mass mortality event (MME), (2) in situ measurement of juveniles after the MME and (3) ex situ measurement of juveniles kept in the laboratory. The pre-MME growth rate was comparable to that in other areas of the Gulf. In the first years of life, the growth rate of P. nobilis is higher than the growth rate reported in the southern Adriatic, and comparable to the growth rate in the Spanish lagoons. This is probably related to the physico-chemical conditions and confirms the potential of the area for restoration actions. Captive growth of juveniles could be an effective conservation measure, although juveniles kept in the laboratory grow more slowly than those in the sea, probably because of their lower immune response. Temperature plays an ambivalent role, as it increases the growth rate but also triggers the parasites responsible for the MME, which is of concern, given the recent rise of seawater temperatures and heatwaves.