Tidal Rhythms in the Shell Secretion of Living Bivalves

Abstract

The periodicity of microgrowth patterns in the shells of living bivalve molluscs has been the subject of considerable research during the past two decades. The early literature relating to research during the 1960’s and 1970’s has been extensively reviewed by among others Clark (1974), Scrutton (1978) and Lutz and Rhoads (1980). I do not therefore propose to go over in detail this by now familiar work, other than to draw briefly upon examples to illustrate specific points. For the first meeting in 1977 Scrutton (1978) reviewed current knowledge of how growth increments form in corals, bivalves and stromatolites and how faithfully they record elapsed time. At the second meeting Ohno (1983) presented a paper on the variability of the formation of growth increments in cockles Cerastoderma (= Cardium) edule shells. It is my intention to present to this meeting a review of the current knowledge of tidal microgrowth patterns in living bivalve molluscs, using illustrations from my own work as well as other researches carried out during the past decade. The key to understanding the periodicity of the microgrowth patterns in acetate peels of polished and etched shell sections has been to study the way in which the organism reacts to environmental fluctuations. The only way to do this is to work with living material. Once the patterns are fully understood, then perhaps the results of the work can be extrapolated to fossil relatives from the geological past.