Irvy R. Quitmyer

Marking time with bivalve shells; oxygen isotopes and season of annual increment formation

Most bivalve shells contain macroscopic growth-increment patterns which form with an annual periodicity. These are best observed in radial shell cross-sections and consist of alternating dark (translucent in transmitted light) and light (opaque in transmitted light) increments. A couplet of dark and light increments constitutes one years shell growth and subdivides the continuous shell growth record into successive, annual components. Conventional wisdom suggests that dark increments represent slow shell growth associated with cold winter temperatures. Such a pattern characterizes shells of the much-studied hard clam, Mercenaria mercenaria, from coastal New England. Investigations of other species indicate, however, that not all dark increments form in response to cold temperatures. Yearly cycles in oxygen isotopic data from fossil and modern bivalves demonstrate considerable variability in the season of growth increment formation. Isotopic data suggest Jurassic Gryphaea arcuata from Great Britain formed dark increments during the warmest season, as do many other bivalves. In fact, oxygen isotope profiles across the shells of modern and fossil Mercenaria spp. from the US. east coast and the Atlantic and Gulf Coastal Plains indicate that in northern latitudes the dark increment typically forms during the winter season whereas in lower latitudes it forms during the summer to early fall. A transition zone occurs in the Middle Atlantic region where aspects of both patterns are observed. Water temperature extremes (high or low) appear to be the primary factor controlling the timing of dark (translucent) increment formation. Because of this dichotomy in the season of growth-increment formation, high-resolution isotopic sampling is invaluable when interpreting shell records in sclerochronological contexts, particularly for fossil bivalves.

Habitat-specific growth of hard clams Mercenaria mercenaria (L.) from the Indian River, Florida

Abstract Microgeographic variation in shell growth rate of Mercenaria mercenaria (L.) among habitats within the Indian River, Florida, was studied to develop a baseline against which the significance of M. mercenaria shell growth rate differences observed over a larger geographic scale could be compared. Hard clams of the genus Mercenaria were collected from 525 randomly located stations within the Indian River during July and August, 1986. Clams ( n = 922) were returned to the laboratory and taxonomically identified using diagnostic protein loci. Clams identified as M. mercenaria ( n = 465) were isolated and their shell growth rate determined by internal shell growth-increment analysis. The annual pattern of internal shell growth-increment formation in genetically identified M. mercenaria from the Indian River was documented using specimens collected monthly from September 1987 through August 1988. The translucent (slow growth) increment was deposited during summer and fall, whereas the remainder of the year was characterized by formation of the opaque (rapid growth) increment. This pattern is similar to that described for other hard clam populations from the southeastern United States. To compare shell growth rates of M. mercenaria among habitats within the Indian River, we used the ω parameter of Gallucci and Quinn. The range of variation in ω among habitats within the Indian River was similar to that observed for hard clams in Narragansett Bay, Rhode Island, but less than that observed for hard clams from various sites throughout the state of Florida. Shell growth rates of M. mercenaria in the Indian River increased with decreasing depth, and shell growth rates were more rapid in areas characterized by the presence of submerged aquatic vegetation than in areas devoid of such vegetation. Results of this study indicate that comparisons of shell growth rates among hard clam populations over a wide geographic range must be tempered by a consideration of shell growth rate variation among habitats within each site.

What Are We Measuring in the Zooarchaeological Record of Prehispanic Fishing Strategies in the Georgia Bight, USA?

ABSTRACT People promote environmental change through behaviors such as habitat alteration and over-exploitation; they adapt to environmental changes for which they are not responsible; and they implement cultural changes that may be either causes or consequences of local environmental conditions. Thus, communities and populations could be impacted by interactions and feedbacks to stimuli and responses of both anthropogenic and non-anthropogenic origin. Given the complexity of these phenomena, many archaeologists and ecologists focus on one or two variables. Due to growing interest in the long-term historical record offered by archaeological data, it is timely to pose the question: “What are we measuring in the zooarchaeological record of fishing strategies?” Our answer is that we are measuring interrelated social and ecological patterns and processes influencing long-term social and ecological dynamics. We demonstrate the complexity of these measurements with a case study focusing on zooarchaeological data that show a trend for increasing diversity and decreasing mean trophic level in fishes between 2500 BC to AD 1565 in the Georgia Bight (USA). The conceptual framework we follow identifies gaps in our knowledge that limit our ability to distinguish between correlations and causal factors underlying this trend; but the framework itself enables us to clearly identify weaknesses that need to be resolved in order to explain the trends themselves.

Contemporaneous deposition of annual growth bands in Mercenaria mercenaria (Linnaeus), Mercenaria campechiensis (Gmelin), and their natural hybrid forms

Abstract We analyzed the periodicity of annual band deposition in the shells of each of three hard clam genotype classes ( Mercenaria mercenaria (Linnaeus), Mercenaria campechiensis (Gmelin), and hybrid forms of the two species) collected from the Indian River lagoon, Florida. Terminal growth bands from each of 396 genetically identified specimens were studied, using the translucent–opaque staging method, to determine the seasonality of rapid vs. slow shell growth. The pattern of terminal growth-band formation that we describe for M. mercenaria from the Indian River is similar to patterns observed for conspecifics collected from Georgia and North Carolina; the translucent growth band (indicative of relatively slow shell growth) is formed during summer and fall, and the opaque growth band (indicative of relatively rapid shell growth) is formed during winter and spring. During summer and fall, growth-band formation in M. campechiensis and hybrid forms is similar to that observed for M . mercenaria . In contrast, during winter and spring subtle but significant deviations from the M . mercenaria growth pattern were noted. For M . campechiensis , relatively equal proportions of the translucent and opaque growth stages were recorded among specimens collected during both the winter and spring seasons. Hybrid forms exhibited a seasonal pattern of terminal growth-band formation reflecting characteristics of each of the two parental species. During winter, translucent and opaque terminal growth bands were equally represented among hybrids (as in M . campechiensis ), whereas in spring significantly more hybrids were in the opaque growth stage (as in M . mercenaria ). The consistency that we describe among genotype classes in the seasonal pattern of growth-band formation suggests that previously described geographic differences in those patterns are primarily mediated by environmental factors rather than by inherent genotypic differences among populations. Nevertheless, subtle differences in the proportions of opaque vs. translucent growth bands observed between species during the rapid-growth stage and the intermediate nature of the hybrid growth pattern at that time suggest that some genetically mediated divergence in the seasonality of growth has occurred during the evolution of these two species.