TEMPERATURE EFFECTS ON SHELL GROWTH PATTERNS IN MERCENARIA MERCENARIA AT VARIOUS LATITUDES ALONG THE U.S. ATLANTIC COAST

Although Mercenaria mercenaria is among the best-studied bivalved mollusks along the U.S. Atlantic coast, questions remain about the relationships between its growth habits and its environment. For example, it is understood that the ‘annual' growth checks on shells may represent winter or summer stresses, but the actual relationships to temperature or latitude, and the age of the clam, are unclear.

We examined three shells from each of four locations along the Atlantic coast using thin sections, etched sections, and high-resolution sampling for stable isotope analysis. By locating the analyses directly on the sections, we could interpret the shell growth patterns in terms of relative temperatures, from youth through maturity.

All shells examined had growth checks, sometimes very prominent, and nearly all seemed to mark changes in temperature. As a rule, the temperature differences were greater for checks in the more mature parts of the shells, suggesting longer growth halts as the clams aged.

Shells from Georgia, North Carolina, and New Jersey had their most prominent growth checks between the highest temperature growth and the lowest, suggesting growth halts during the late summer and fall. In contrast, the shells from Maine had growth checks following their lowest temperatures, suggesting growth halts in the winter.

All the shells had additional, less prominent growth checks in the more mature portions of the shell. These typically occurred in the spring, often following closely upon a rise in temperature. These have been suggested to represent spawning stresses, although certainly other explanations are available. In the Maine clams some of these occur in the fall as well.

The inner layer of this species will commonly display zones of relatively white, opaque microstructure alternating with relatively dark, transparent, and sometimes pigmented microstructure. In the shells from Georgia, North Carolina, and New Jersey the transparent zones correlated well with the higher temperatures; in the Maine shells the transparent zones tended to be infrequent and narrow, and showed little correlation.