INVESTIGATION OF GROWTH-RATE-RELATED ALLOMETRY IN STROMBID GASTROPODS USING OXYGEN AND CARBON STABLE-ISOTOPE PROFILES

Marine gastropods of the genus Strombus represent an excellent example of the inherent difficulties of distinguishing ecophenotypic from genotypic variation in fossil organisms. Members of this genus typically display a high degree of intraspecific variability. Strombus proximus, a small species from the Neogene of the Caribbean, occurs in an elongate, low shouldered morph with reduced spines in parts of the Yaque group of the Dominican Republic; this morph may represent a different species or may simply be the result of ecophenotypic variation. Workers in the modern (Martin-Mora et al., 1995) have demonstrated a correlation between morphology and growth rate in the modern Caribbean S. gigas; faster growth correlates with proportionately higher spires and less prominent spines. Growth rate itself is positively correlated with food supply and inversely correlated with population density. The pattern of variation seen in the Dominican specimens seems similar and may also represent differential growth rates rather than genotypic variation. If growth rates in S. proximus can be determined, this hypothesis can be tested. S. proximus specimens were microsampled along growth lines and examined for seasonal cycles in d¹⁸O and d¹³C values. Pronounced seasonal cycles would enable the inference of growth rates and allow a comparison between these rates and morphologic characteristics. d¹⁸O data show a pattern of variation similar to, but of lower amplitude than, seasonal cycles seen in modern Caribbean strombids. Unfortunately, interpretation of this d¹⁸O pattern is not entirely straightforward. In some instances, fluctuations in the “warm” part of cycle make it uncertain whether we are looking at a single season or an entire year with a warm winter. d¹³C data are also ambiguous, and vary in their phase relation to d¹⁸O. Preliminary interpretations of the data allow the possibility of a correlation between growth rate and morphology, but do not support it unequivocally due to alternate possible interpretations of seasonal length. Stable-isotope and minor-element (Mg,Sr/Ca) analyses of additional specimens along this fossil S. proximus morphological gradient should provide a more robust test of a growth-rate:morphology correlation.