Although a significant body of literature has been devoted to the chronobiology of aquatic animals, how biological rhythms function in molluscan bivalves has been poorly studied. The first objective of this study was to determine whether an endogenous circadian rhythm does exist in the oyster, Crassostrea gigas. The second objective was to characterize it in terms of robustness. To answer these questions, the valve activity of 15 oysters was continuously recorded for 2 mo in the laboratory under different entrainment and free-running regimes using a high-frequency noninvasive valvometer. The present work demonstrates the presence of a circadian rhythm in the oyster Crassostrea gigas. First, oysters were entrained by 12 L:12 D conditions. Then, free-running conditions (D:D and L:L) indicated that the most frequently observed period ranged from 20 to 28 h, the circadian range. That endogenous circadian rhythm was characterized as weak. Indeed, the period (τ) of the individual animals exhibited high plasticity in D:D and L:L, and the animals immediately followed a 4-h phase advance or delay. Additionally, C. gigas appeared as a dual organism: all oysters were nocturnal at the beginning of the laboratory experiment (January), whereas they were diurnal at the end (March). That shift was progressive. Comparison with a full-year in situ record showed the same behavioral duality as observed in the laboratory: the animals were nocturnal in autumn–winter and diurnal in spring–summer. The significant advantage of a plastic and dual circadian rhythm in terms of adaptability in a highly changing environment is discussed. (Author correspondence: d. tran@epoc. u-bordeaux1. fr )

Marine coastal habitats are complex cyclic environments as a result of sun and moon interactions. In contrast with the well-known circadian orchestration of the terrestrial animal rhythmicity (approx. 24 h), the mechanism responsible for the circatidal rhythm (approx. 12.4 h) remains largely elusive in marine organisms. We revealed in subtidal field conditions that the oyster Crassostrea gigas exhibits tidal rhythmicity of circadian clock genes and clock-associated genes. A free-running (FR) experiment showed an endogenous circatidal rhythm. In parallel, we showed in the field that oysters' valve behaviour exhibited a strong tidal rhythm combined with a daily rhythm. In the FR experiment, all behavioural rhythms were circatidal, and half of them were also circadian. Our results fuel the debate on endogenous circatidal mechanisms. In contrast with the current hypothesis on the existence of an independent tidal clock, we suggest that a single ‘circadian/circatidal’ clock in bivalves is sufficient to entrain behavioural patterns at tidal and daily frequencies.

The objective of the present study was to monitor water‐quality assessment by a biological method. Optimum dissolved inorganic mercury sensitivity in the freshwater bivalve Corbicula fluminea was estimated using a combined approach to determine their potentials and limits in detecting contaminants. Detection by bivalves is based on shell closure, a protective strategy when exposed to a water contaminant. To take the rate of spontaneous closures into account, stress associated with fixation by one valve in common valvometers was integrated, and the spontaneous rhythm was associated with daily activity. The response in conditions where the probability of spontaneous closing is the lowest was thus taken into account. To develop dose–response curves, impedance valvometry, in which lightweight impedance electrodes are applied to study free‐ranging animals in low‐stress conditions, also was used combined with a new analytical approach. The logistic regression dose–response curves take into account variations in both response time and metal concentration in water to significantly improve the methods aiming at determining the optimal sensitivity threshold response. This approach demonstrates that in C. fluminea, inorganic mercury concentrations under the range of 2.0 to 5.1 μg/L (95% confidence interval) cannot be detected within 5 h of addition.

The precise environmental conditions under which broadcast spawners spawn in the field remain largely unknown. We investigated this issue in the oyster Crassostrea gigas using three different methods at different time scales in two traditional oyster farming areas of the French Atlantic Coast, the Bay of Arcachon and Marennes‐Oléron. We directly recorded spawning at high temporal resolution using high‐frequency non‐invasive (HFNI) valvometry from 2007–2014 and measured the dry mass and oyster larvae abundance in 2008 and 2009. We analyzed a 29‐yr series of oyster D‐larvae numbers in the Bay of Arcachon (1982–2010). By combining these three approaches, we demonstrated that during the summer months at both sites, spawning in C. gigas occurs in the morning or during the evening, essentially at high tide of perigean spring tides, independent of the positions of these oysters, above or below the lowest water level. We characterized the associated water currents at the spawning location in the Bay of Arcachon and observed that spawning systematically occurs during the early phase of a water current peak, at the beginning of ebbing. We propose that this water current peak acts as a final trigger for spawning. These results have ecological consequences associated with gamete encounters and the dispersal of fertilized eggs (zygotes).

Although the prevailing paradigm has held that the polar night is a period of biological quiescence, recent studies have detected noticeable activity levels in marine organisms. In this study, we investigated the circadian rhythm of the scallop Chlamys islandica by continuously recording the animal’s behaviour over 3 years in the Arctic (Svalbard). Our results showed that a circadian rhythm persists throughout the polar night and lasts for at least 4 months. Based on observations across three polar nights, we showed that the robustness and synchronicity of the rhythm depends on the angle of the sun below the horizon. The weakest rhythm occurred at the onset of the polar night during the nautical twilight. Surprisingly, the circadian behaviour began to recover during the darkest part of the polar night. Because active rhythms optimize the fitness of an organism, our study brings out that the scallops C. islandica remain active even during the polar night.