STRESS IS STRESS: MERCENARIA MERCENARIA EXPRESSES SAME MICROSTRUCTURAL SHIFTS WHETHER TOO HOT OR TOO COLD

The shells of bivalve mollusks often have indentations parallel to the shell margin, generally related to severe reductions in growth rates. Apparently these 'growth checks' are related to environmental events, although evidence has been slim. In the bivalve Mercenaria mercenaria, both summer heat and winter cold can produce growth checks, with the most prominent checks forming in winter in the northern part of the range, as in Maine, and in summer in the southern part, as in Georgia.

Examination of growth checks with scanning electron microscopy, using polished, etched, and critical-point dried sections to expose the organic sheaths surrounding crystallites, enables the determination of the shell microstructures, even in somewhat chaotic examples. Within Mercenaria, at least, growth checks are not simply variations in the rate of growth, but commonly involve shifts in microstructures.

Certainly the growth checks are related to environmental stress, either the temperature itself or related effects such as dissolved oxygen or food availability. The highest levels of stress result in actual growth halts, often visible as prominent crevices paralleling earlier shell margins. Slightly less stress is expressed as growth increments of irregular simple prismatic microstructure, preceeding growth halts and sometimes following them as well. Still lower stress can be expressed by simple crossed lamellar (in the inner shell layer) or complex crossed lamellar (in the outer shell layer) microstructure. Such increments usually precede the higher stresses, but sometimes follow. The lowest degrees of stress are expressed by very narrow increments of increased organic matter, where the matrix apparently continued to be formed despite a decrease, cessation, or even reversal (dissolution) of formation of the mineral component. This is quite apparent in the composite prismatic microstructure of the unstressed outer shell layer, and can also be observed in the intersected crossed platy microstructure of the inner shell layer.

Observations on specimens from Maine, Georgia, and New Jersey demonstrate that the nature of the stress has little effect on the pattern of the microstructure response; the same transitions occur in stress from high temperatures as from low temperatures.