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 compared 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. philippinarum were immersed in strontium chloride solutions of varying concentrations, and the effects of immersion time (17 or 24 h) and concentration (0.72, 1.44 or 2.88 g l -1) were investigated. 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 1200x. Using the FMM, growth increaments of only tens of micrometers were detectable under fluorescence optical microscopy up to a magnification of 600x. 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 methodologies. 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 bivalves.