Intraspecific variations in shell calcification across thermal window and within constant temperatures: Experimental study on an intertidal gastropod Monetaria annulus

The relationship between temperature and shell growth was examined by rearing an intertidal cowrie species, Monetaria annulus (Gastropoda, Cypraeidae), at seven constant temperatures between 21 degrees C and 34 degrees C over four experimental years. Data on shell growth were collected during the callus-building stage, in which calcification exclusively occurs around the spiral shell constructed in the juvenile stage. Our experiments elucidated some clear trends among temperatures, although careful consideration is required to interpret the results obtained under the potential confounding factors between temperature and experimental year. Calcification was accelerated by increasing temperature in the range between 21 degrees C and 33 degrees C, and drastically slowed down at 34 degrees C probably due to heat stress. No relationship was found between body size and callus growth rates. The duration of callus-building indicated a negative exponential relationship with rearing temperature. However, no simple relationship was found between temperature and final shell thickness; the final shell thickness described two peaks around 21 degrees C and 30 degrees C. This finding strongly suggests that a physiological control by calcifiers cannot be ignored to explain the thermal response of calcification rates. Our results also suggest that considering the thermal seasonality as well as the difference in mean temperature, is required to explain the negative latitudinal dines in shell thickness frequently observed in molluscan species. We also focused on the variation among the individuals reared at the same temperatures. Statistical analyses suggested that the final shell thickness and the duration of calcification were better described by the normal and lognormal distributions, respectively. The distribution of shell growth rates did not considerably deviate from the normal distribution with respect to skewness and kurtosis. However, their heteroscedasticity among temperatures was inherent and significant; its variance positively depended not on the mean but on temperature. The observed pattern might be an evidence that possible stabilization selection on calcification rates is more relaxed at warmer temperatures. (C) 2016 Elsevier B.V. All rights reserved.