Periodicity and shell microgrowth pattern formation in intertidal and subtidal areas using shell cross sections of the blood cockle, Anadara granosa

This study aims to investigate the periodicity of shell microgrowth lines and increments of cockles in the intertidal and subtidal areas. A total of 600 marked specimens of the blood cockle Anadara granosa were placed in buried cages that were placed at three sites of differing water levels (Site A: exposed in all low tides, Site B: exposed during spring tides, and Site C: continuously immersed) in Pinang Island, Malaysia. Cockles were collected on a fortnightly basis and the shells were sectioned from the umbo to the ventral margin, before being polished, etched and photographed under a light microscope to observe microgrowth lines and increments. The majority of the cockle shells examined showed that the number of growth lines in shell structures was close to the number of tidal emersions (P > 0.01). The number of microgrowth lines observed in shells during the study period was significantly different (P < 0.01) from the number of days and tidal changes. Similarly, cockles grown in the laboratory in a simulated tidal regime, deposited a number of microgrowth bands which coincided with the number of tidal emersions. This study shows that shell microgrowth lines are typically formed in accordance with tidal emersions, providing a calendar base for high resolution environmental reconstructions. Endogenous activity synchronised by tidal cues may control rhythmic microgrowth line formation in subtidal areas.