Temperature regulates the switch between light-synchronized and unsynchronized activity patterns in the subtidal bivalve Pinna nobilis

Abstract

This study provides new information on the biological rhythms of subtidal bivalves, using the fan mussel Pinna nobilis as a model. The objective was to determine which factor(s) provoke the change between two different patterns observed in the annual cycle of this species: P1, which is characterized by the individuals opening and closing their valves according to the presence or absence of ambient light and P2, which is characterised by behaviour that is not dependent on the presence of light. Magnetoresistive sensors were installed on 8 fan mussels to record gaping activity in laboratory conditions. Different temperature and light treatments were applied. The results showed that temperature was the factor modulating the change between behavioural patterns. The individuals switched to P1 when temperature reached 24.5°C. In this pattern, individuals are entrained by light, displaying a circadian rhythm linked to the daily light treatments. During P2, the circadian rhythm was missing or very weak, contrary to in situ observations. The results of this study contribute to the understanding of the biology of the species and planning new conservation strategies. Furthermore, the observed relationship between temperature and P1 is of interest within the framework of fan mussel captive breeding. Altogether, this information is especially relevant in view of the recent mass mortality of P. nobilis in the western Mediterranean.