A Long-Term Mark-Recapture Study Documenting Annual Growth Bands within the Shells of Northern Quahogs (= Hard Clams) Mercenaria Mercenaria

In the late 1940s and early 1950s Thurlow C. Nelson and Harold H. Haskin conducted a series of mark-recapture experiments to assess the growth of hard clams (Mercenaria mercenaria) on Delaware Bay mud flats adjacent to the Rutgers University Oyster Research Laboratory in Cape May County, NJ. Two specimens from these experiments were recovered alive on September 7, 1980. The growing shell margin of one of these clams (Specimen #1) had been notched with a hack saw between March and December 1947. The growing shell margin of the second clam (Specimen #2) had been notched with a small triangular file between March 1948 and December 1951. Examination of polished shell sections (cut along the axis of maximum growth) of specimens #1 and #2 revealed, respectively, 33 and 30 dark (translucent) bands within the postnotch regions of the outer and middle shell layer of each specimen. The presence of light (opaque) shell material in the outer and middle shell layers at the growing margin of each of these specimens is consistent with the observations of other workers that indicate the formation of dark (translucent) bands in this species during the winter months in certain coastal environments from the MidAtlantic region northward along the east coast of North America. The present study provides convincing evidence that alternating light (opaque) and dark (translucent) bands within the outer and middle shell layers of Mercenaria mercenaria from Delaware Bay, NJ mud flats reflect annual cycles of growth and can be used for estimating the age of hard clams from this region. It is believed that the present study represents the longest mark-recapture study of growth patterns within the shells on any bivalve mollusc.