New insights into oyster high-resolution hinge growth patterns

While oyster shells are one of the most common mollusks used for the analysis of (paleo)environmental and (paleo)climatic records based on geochemical proxies, high-resolution growth rate changes still need to be determined. Promising previous works are restricted to small portions of shell sections due to difficulties in continuous growth increment revelation. Based on a mark and recapture experiment of Magallana gigas specimens reared in an intertidal area of Normandy (France) for 22months, and a sclerochronological approach using cathodoluminescence microscopy, this study provides the longest high-resolution record of growth increments in oyster shells to date. Different growth patterns were identified likely related to the oyster age. After age 1year, the formation of growth increments follows an expected tide-related model, leading to the mineralization of 2 calcitic increments per day, together with growth rate changes at lunar and semi-lunar periodicities, and a seasonal trend with occasional growth breaks during winter when temperatures fall below 6 degrees C. However, for oysters younger than 1year, i.e., before reaching their sexual maturity, the growth increment analysis reveals unconventional patterns. In this case, oysters' growth is associated with either a large number (5) or less than one increment per day depending on the period. This pattern is also associated with frequent growth cessations, although the growth rate of the shell is high at this period. Our results illustrate that the high-resolution sclerochronological approach is required for accurate paleoenvironmental reconstructions based on oyster shells.