Evidence of a 2-day periodicity of striae formation in the tropical scallop Comptopallium radula using calcein marking

The periodicity of striae formation in the tropical scallop Comptopallium radula (Indo-West Pacific Ocean) was investigated with an in situ marking technique, using the calcein fluorochrome. To minimize scallop stress caused by excessive handling, in situ benthic chambers were used for marking experiments. Once marked, scallops (shell height range: 38.4-75.8 mm) remained on site in a large benthic enclosure and were collected at regular time intervals to count new striae formed after marking, over a period of 3 months. A 3-hour exposure period with calcein (150 mg L-1) was sufficiently long to create a detectable mark in nearly all shells. It was however impossible to count the striae in 48.2 % of the shells (mainly large specimens) because of a very small growth after marking. Lack of significant mortality during the experiments indicated that tested calcein concentrations were not lethal. A decrease in shell growth rate was observed after marking but the respective impacts of calcein toxicity and changes in environmental conditions could not be discriminated. Our results suggest that in situ calcein marking inside benthic chambers is suitable for shell growth studies of scallops, provided the latter are not too old. After marking, the juvenile C. radula formed an average of one stria every 2.1 days in summer. Reports of 2-day periodicity in biological rhythms are rare. Striae formation in C. radula may be controlled by an endogenous oscillator, synchronized by an environmental cue acting as a zeitgeber, such as seawater temperature or sea level pressure, both of which exhibit 2-day variations in the Pacific Ocean. As in many other scallop species, C. radula forms striae periodically under natural conditions, but this study shows that in pectinid juveniles, this periodicity can deviate from a daily cycle. These results suggest that C. radula shells have tremendous potential for recording environmental conditions during periods ranging from months to a few years and with a resolution of 2 days.