Abstract
In this work, we study if ploidy (i.e. number of copies of chromosomes) in the oyster Crassostrea gigas may introduce differences in behavior and in its synchronization by the annual photoperiod. To answer to the question about the effect of the seasonal course of the photoperiod on the behavior of C. gigas according to its ploidy, we quantified valve activity by HFNI valvometry in situ for 1 year in both diploid and triploid oysters. Chronobiological analyses of daily, tidal and lunar rhythms were performed according the annual change of the photoperiod. In parallel, growth and gametogenesis status were measured and spawning events were detected by valvometry. The results showed that triploids had reduced gametogenesis, without spawning events, and approximately three times more growth than diploids. These differences in physiological efforts could explain the result that photoperiod (daylength and/or direction of daylength) differentially drives and modulates seasonal behavior of diploid and triploid oysters. Most differences were observed during long days (spring and summer), where triploids showed longer valve opening duration but lower opening amplitude, stronger daily rhythm and weaker tidal rhythm. During this period, diploids did major gametogenesis and spawning whereas triploids did maximal growth. Differences were also observed in terms of moonlight rhythmicity and neap-spring tidal cycle rhythmicity. We suggest that the seasonal change of photoperiod differentially synchronizes oyster behavior and biological rhythms according to physiological needs based on ploidy.
Highlights
Valve opening activity in many bivalves species is closely related to physiological processes such as respiration, nutrition, and reproduction, which are modulated by environmental parameters [1,2]
Seasonal variations were observed in level of activity and rhythmic patterns of C. gigas valve behavior, as well as in growth and gametogenesis
Variations based on annual photoperiod and lunar rhythms exhibited significant differences according to ploidy
Summary
Valve opening activity in many bivalves species is closely related to physiological processes such as respiration, nutrition, and reproduction, which are modulated by environmental parameters [1,2]. Valve behavior of the oyster Crassostrea gigas is driven in its biotope by the multiple environmental cycles linked to periodical recurrence of sunearth-moon orbital positions [3]. Valve opening duration of permanently immersed diploid oysters follows a strong tidal cycle, modulated by synodic moon cycles. A circadian clock was demonstrated in C. gigas, synchronizing with the daily solar cycle to run at 24h [3,4,5]. This circadian clock allows the anticipation of light / dark alternation, and an internal temporal organization of biological processes [5,6]. The daily rhythm of subtidal oyster behavior was less expressed than the circatidal rhythm [3]
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