From youth to death of old age: the 50-year story of a Pinna nobilis fan mussel population at Port-Cros Island (Port-cros National Park, Provence, Mediterranean Sea)

A population of the fan mussel Pinna nobilis Linnaeus, 1758 (molluscan, Bivalvia) has been studied since 1969 at Port-Cros Island (Port-Cros National Park, Provence, France, Mediterranean Sea). This population dwelt on a substrate constituted of dead rhizomes of the seagrass Posidonia oceanica (‘dead matte’) covered by coarse sand, between 10 and 38 m depth. 122 individuals were studied (census and biometry) within the framework of a long term monitoring programme. The population probably originated in a single cohort that settled ~5 years before 1969. Since the first survey, no juvenile was observed in the studied area. The mortality rate peaked between 1978 and 1984, when individuals were 15-20 years old. In 2009, the two last living individuals were observed. They died between 2009 and 2014, at an age between ~45 and 50 years. The putative longevity of the species is therefore far above the 10-20 years previously assumed. The total shell length of the last two survivors was 73 cm and 75 cm, respectively.