A CLOCKWORK MOLLUSK: INVESTIGATING THE RELATIONSHIP BETWEEN SHELL ACTIVITY AND SUBDAILY GROWTH INCREMENTS IN THE FRESHWATER BIVALVE ANODONTA CYGNEA

Growth increment analysis has proven to be a powerful and versatile tool for geobiologists and paleontologists investigating problems as diverse as paleoclimate reconstruction, watershed dynamics, evolutionary trends and the length of astronomical cycles in deep time. Annual, fortnightly and daily banding patterns are well documented and have been subject to extensive analysis. In contrast, the existence, biological causes, and sclerochronological utility of subdaily (ultradian) growth increments have received very little attention.

Digital time-lapse photographic records of activity provide a non-invasive technique for investigating the behavior of laboratory bivalves, and image archives obtained in this way can be subjected to image analysis. Long term records (exceeding six months) have been compiled for laboratory specimens of the freshwater bivalve Anodonta cygnea and subjected to image analysis. Several subdaily periods in shell activity have been identified, with cycle lengths of approximately four minutes and one hour, and similar periods have been observed in species of marine bivalves. By comparing these activity records with analysis of shell growth microincrements, we hope to provide greater insight into the biological drivers of shell formation and to delineate the ultimate temporal resolution of sclerochronological data.