The shell as a recording device: growth record and shell ultrastructure of Lampsilis radiata radiata (Pelecypoda: Unionidae)

Abstract

Growth records retained in the shell of Lampsilis radiata radiata and shell ultrastructure were examined in a population from Lake Champlain, Vermont, U.S.A. Maximum size was 75 mm as measured from the umbo to the farthest margin. Ages were obtained by counting annual bands, and growth plots were constructed. By knowing the amount grown by individuals of the same age in the same year, the years 1975 and 1977 were identified as poor for growth of younger individuals. Light micrographs of shell thin sections showed macroscopic annual lines, and microscopic and ultrastructural lines. Ultrastructural lines were visible within macroscopic lines. Scanning electron micrographs of etched sections clarified the light micrographs and allowed recognition of two damage-response structures embedded within the nacreous layer in the umbonal region: adventitious conchiolin and prismatic crystals. Theories of growth line formation are presented. Shell growth appears to follow an alternating cycle of protein deposition as organic matrices followed by CaCO3 secretion into the matrices. The Lutz–Rhoads theory of growth line formation, based on alternating periods of aerobic and anaerobic metabolism, was examined in detail and found unsuitable for this species.