Interspecific and intra-shell stable isotope variation among the Red Sea giant clams

The Gulf of Aqaba is home to three giant clam species with differing
ecological niches and levels of photosymbiotic activity. Giant clams grow
a two-layered shell where the outer layer is precipitated in close
association with photosymbiont-bearing siphonal mantle, and the inner
layer is grown in association with the light-starved inner mantle. We
collected 38 shells of the three species (the cosmopolitan Tridacna maxima
and T. squamosa, as well as the rare endemic T. squamosina), and measured
carbon and oxygen isotope ratios from inner and outer shell layers, to
test for differences among species and between the layers of their shells.
T. squamosina records higher temperatures of shell formation as determined
by oxygen isotope paleothermometry, consistent with its status as an
obligately shallow-dwelling species. However, the known negative
fractionation imparted on tissue carbon isotopes by photosymbiotic algae
did not produce measurable offsets in the carbonate δ13C values of the
more symbiotic T. squamosina and T. maxima compared to the more
heterotrophic T. squamosa. Across all species, outer shell layers recorded
mean growth temperatures 1.8 °C higher than corresponding inner layers,
which we propose is a function of the high insolation, low albedo
microenvironment of the outer mantle, and potentially the activity of the
symbionts themselves. Population-wide isotopic sampling of reef-dwelling
bivalve shells can help constrain the ecological niches of rare taxa and
help reconstruct their internal physiology.