Isotope sclerochronology of Mercenaria mercenaria, M. Campechiensis, and their natural hybrid form: Does genotype matter?

Abstract

Isotopic variation in northern and southern hard clam (quahog)shells is used in studies including paleoecology, paleoclimatology, and archaeology. It is unknown, however, whether species-specific isotopic differences exist. Three genotypes-Mercenaria mercenaria, M. campechiensis, and their natural hybrid form-are found in coastal Florida waters and differentiation of genotypes can be difficult to determine morphologically. This issue may be problematic when using archaeological shells as paleoclimate archives, because genetic analysis cannot be done on such specimens. Their co-occurrence in coastal Florida waters provides a unique opportunity to study whether all three genotypes of modern individuals record the same environmental information preserved as variation in oxygen and stable carbon isotope ratios. A random sample of 49 individuals collected alive at the same time and from the same locality in Pine Island Sound were classified to genotype using allozyme electrophoresis. Three juveniles from each genotype were selected for isotopic analysis to control for ontogenetic effects. Timing of growth increment formation inferred from oxygen isotope ratios reveals similar overall patterns wherein dark (slow growth) increments formed in mid-to late spring and light (fast growth) increments formed in late fall. Results of the mixed model ANOVA (analysis of variance) indicate that no significant species-related differences exist in the variation of oxygen and carbon isotope ratios, although the Kolmogorov-Smirnov goodness-of-fit test detected a systematic difference among ␦ 13 C values of M. mercenaria and M. campechiensis comparison and M. mercenaria and the hybrid shell comparison. Any genotype or combination thereof is, thus, suitable for environmental and climate reconstruction using oxygen isotope ratios. The utility of carbon isotope ratios as an environmental proxy, however, remains questionable.

Determining the habitats of mollusk collection at the Sapelo Island shell ring complex, Georgia, USA using oxygen isotope sclerochronology

“…Clark, 1979;Quitmyer et al, 1985Quitmyer et al, , 1997Jones et al, 1989, in press;Jones and Quitmyer, 1996;Jones et al, in press;Elliot et al, 2003;Surge and Walker, 2006;Walker and Surge, 2006;Andrus and Crowe, 2008;O'Brian and Thomas, 2008). Two quahog species (Mercenaria mercenaria and M. campeciensis) and a hybrid exist in this area, but a comparative analysis showed no discernible differences in oxygen isotope fractionation between their shells (Surge et al, 2008). Species identification, which is difficult using hard tissues alone, is therefore not necessary in this clam.…”

New Species Longevity Record for the Northern Quahog (=Hard Clam),Mercenaria mercenaria

“…For M. mercenaria populations on the Atlantic coast, estimates of the VBGF growth constant K normally range between 0.20/y and 0.35/y (Jones et al 1989, Harding 2007). In cooler areas-for example, Southampton water in the United Kingdom and in Snug Harbour, Cape Cod, MA-low growth constants been reported (0.149 (Brown et al 2010) and 0.117 (Surge et al 2008)), and these were in relatively shallow water.…”

“…The annual periodicity of increment formation in the shell of M. mercenaria has been demonstrated by mark-and-recapture experiments (Peterson et al 1983) stable isotope profiles (Jones et al 1990, Jones & Quitmyer 1996, Surge et al 2008, and sequential sampling (Clark 1979, Peterson et al 1985, Jones et al 1990, Arnold et al 1991, Arnold et al 1998. In this article, we report the new maximum age observed for this species as well as growth parameters of a population of M. mercenaria containing the longest lived members of the species.…”

Isotopic evidence for climate change during the Vandal Minimum from Ariopsis felis otoliths and Mercenaria campechiensis shells, southwest Florida, USA

“…Earlier studies reported that oxygen isotope ratios in M. mercenaria and M. campechiensis can be used to reconstruct ambient environmental conditions (Elliot et al, 2003; Surge and Walker, 2006; Surge et al, 2008). However, δ 18 O w of estuarine water is difficult to constrain, but here we use this to our advantage.…”