Strontium as a marker for estimation of microscopic growth rates in a bivalve

Abstract

Microscopic growth increaments in bivalve shells were measured by marking the shells of the short-necked clam Ruditapes philippinarum with the strontium chloride (strontium marking method, SMM). The results were com- pared with those achieved by the fluorescent marking method (FMM) using calcein. Because strontium has a higher atomic mass than calcium, strontium-enriched areas in the shells are visible as a bright band in the back-scattered electron image under the scanning electron microscope (SEM). R. philippi- narum were immersed in strontium chloride solutions of vary- ing concentrations, and the effects of immersion time (17 or 24 h) and concentration (0.72, 1.44 or 2.88 g l -1 ) were investi- gated. These shells exhibited clearly visible marks 19 or 20 d post-immersion. Using the SMM, a microscopic (single-digit µm) increment in shell growth could be detected under SEM with a magnification of at least 1200 × . Using the FMM, growth increaments of only tens of micrometers were de- tectable under fluorescence optical microscopy up to a mag- nification of 600 × . The SMM is thus superior to previous methods for detecting microscopic increases in shell growth. Marking methods should ideally be based on long-lasting markers that are easily detected and involve simple method- ologies. The SMM meets these requirements. It can be used to estimate growth in small bivalve specimens and growth rates in slow-growing species such as deep-sea and polar