Annual cycle of shell growth increment formation in two continental shelf bivalves and its paleoecologic significance

• Published in: Paleobiology
• Main Author: Jones, Douglas S.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 1980
• Subjects: Arctica islandica; Ocean quahog
• Online Access: http://dx.doi.org/10.1017/s0094837300006837; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0094837300006837

The bivalves Spisula solidissima , the Atlantic surf clam, and Arctica islandica , the ocean quahog, from the continental shelf off New Jersey, contain repeating structures in their shells. By analyzing the growing shell margins in living specimens at bi-weekly (or sometimes monthly) intervals throughout two consecutive years, it was possible to define an annual cycle of shell growth increment formation in both species. The shell increments in each species are microstructurally distinct units that form over a period of several months at select seasons of the year. Each species has two alternating shell growth increments, GI I and GI II. GI I (the annual growth line of previous studies) is formed annually in the late summer-fall in S. solidissima and in the fall-early winter in A. islandica. These periods correspond to the spawning phase of the reproductive cycle in both species. No winter rings were found. The annual increments were used to determine age and growth rate in both Recent and Pleistocene specimens. They may also be useful in determining season of death. Because shell growth increments are formed in synchrony among living populations in these species, mass mortalities may be distinguished in the fossil record. Accurate age and growth rate determinations in fossils are important in many paleobiologic contexts, such as deciding between increased longevity or growth rate in cases of phyletic size increase.