Daily growth lines in some living Pectens (Mollusca: Bivalvia), and some applications in a fossil relative: Time and tide will tell

Abstract

Laboratory experiments on two species from the Pecten (Pecten) subgroup of pectinids indicated that they form growth lines with a true (solar) daily periodicity. Experimental subjects that formed growth lines in numbers equal to their days of growth were consistently those with uniform, apparently continuous shell growth, and subjects that formed fewer lines than days were those with stress marks, or disturbance lines. No subjects formed more growth lines than days. Furthermore, those specimens forming stress marks had those marks spaced out in very similar patterns, apparently in response to stressful events in their aquarium, such that the patterns could be correlated, not only between the specimens with interrupted growth, but with the others as well. In the original experiments, a natural light–dark stimulus had been present (transparent aquaria near an outside window). To test whether the subjects were responding to this stimulus, another experiment was conducted in which several individuals were subjected to an accelerated light–dark regime. These subjects responded by increasing production of their growth lines to approach the number of cycles to which they had been exposed. An opportunity to extend this work into the fossil record came with the collection of a number of Pliocene pectinids, closely related to one of the species used in the experiments, from a thin horizon that almost certainly represents a catastrophic event. Measurements of the intervals between growth lines were made across hundreds of increments on many paired (and presumably untransported) valves. Comparison of these growth rate records suggested that many, and perhaps all of these individuals died at the same time. Moreover, frequency analysis of these records revealed a regular fluctuation of about 15 days, suggesting a tidal cycle marked off by solar increments. When the growth rate patterns were compared with nearby tidal regimes, the best fit was for a regime matching the currently accepted paleogeography of the collecting site in Pliocene time. This strongly supports the experiments on the living species, for tidal patterns in the fossil group could not have been revealed without an accurate solar marker.