UC Santa Cruz

Bivalve shells contain growth lines which are formed as a result of periodic environmental or physiological stress, analogous to tree rings. The study of these regular growth increments in the hard parts of bivalves and other calcifying organisms is called sclerochronology. In this thesis, I have examined the ways that sclerochronology can be useful in fields where it is underutilized. The first chapter involves aggregating hundreds of past bivalve seasons of growth worldwide to help answer the question: why do bivalves stop growing in certain seasons and where does it happen? We discuss the primacy of temperature over seasonal food supply as the major determinant of seasonal growth in bivalves, and the latitudinal gradients of likelihood of shutdown that result from this. The second chapter is an investigation of bivalve growth in deep time, looking at the rate of growth of an enigmatic Early Jurassic group called the Lithiotids. For the first time, we have isotopically calibrated their growth, determining that it is quite fast in comparison to other giant bivalves through time but does not corroborate prior hypotheses that they harbored photosymbiotic algae in their tissue. Finally, in chapters three and four we report on investigations of carbon, oxygen and nitrogen stable isotopes of giant clam shells in the Red Sea and relate their growth rate to those environmental proxies. In chapter three, we report interspecific and intrashell differences in carbon and oxygen isotopes for different Tridacna species, corroborating the proposed shallow life habit of the rare endemic T. squamosina, and discuss how the outer shell layer of the giant clams records higher formation temperatures than the interior. Chapter four reports on the unexpected acceleration of growth of modern giant clams in the Northern Red Sea, and we propose that fertilization by anthropogenic nitrate aerosols is recorded in the nitrogen isotopes of their shell organic material. Together, these chapters represent applications of sclerochronology to understand bivalve physiology in the deep and near past, the present and potentially the future.