Shell growth in Tivela stultorum (Mawe, 1823) and Callista chione (Linnaeus, 1758) (Bivalvia): annual periodicity, latitudinal differences, and diminution with age

Abstract

During the growth of the shell of the California surf clam Tivela stultorum (Mawe, 1823) and the subtidal bivalve Callista chione (Linnaeus, 1758) from the Adriatic Sea, a period of slower growth occurs annually. This period of growth is reflected by a macroscopic growth band consisting of relatively thin and closely spaced shell-growth increments. Within shells of C. chione collected from the northern part of the Adriatic Sea, the dark colored seasonal band is slow growth is deposited between July and October. The season when the slow-growth band is deposited in T. stultorum apparently varies with latitude. Near the northern limit of the range, the median date of deposition of the seasonal slow-growth band is November; near the southern limit of the geographic range of the species, it is January or February. There is a significant difference in the number of daily growth increments deposited during the seasonal period of slower growth in individuals of T. stultorum collected at different latitudes along the west coast of North America. For example, near 35° N lat. there is an average of 82 daily growth increments in the fourth annual biocheck, while near 26° 30′ N lat. there is an average of 111 daily growth increments; all individuals are of the same age- and year-class. With the possible exception of the first to third or fourth year, when a growth increment is deposited on nearly a daily basis, growth of the shell is apparently not continuous throughout the year in the two species of molluscs studied. For example, there is an average of 353 daily growth increments in 3-year-old individuals of C. chione and only an average of 155 present in 12-year-old individuals. Further, there is an average of 13 mm of growth during one year in 2-year-old C. chione and 2.2 mm in 13-year-old C. chione. Also in C. chione, relatively more shell is deposited during the months of December through February than during spring and summer months. Once shell-growth diversity gradients are determined in modern organisms, it may be possible to follow the development of these gradients through time, to note significant differences in shellgrowth patterns that reflect broad-scale environmental factors, and to delimit present-day and past molluscan provinces.