Interspecific and Intrashell Stable Isotope Variation Among the Red Sea Giant Clams

Abstract

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 39 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.

Giant clam growth in the Gulf of Aqaba is accelerated compared to fossil populations

“…Among our specimens, we found a positive correlation between growth rate and temperature, corroborating other observations of tridacnid growth [37]. These differences in mean temperature between individuals likely result from microenvironmental variability at different depths along the reefs of the Gulf of Aqaba, which can experience greater than 3°C differences between the shallow lagoon and the fore-reef [23]. Depressed growth above 27°C aligns with past observations of tridacnid thermal tolerance thresholds [8].…”

“…We milled a subset of nine shells (six modern and three fossil) sequentially at 3-4 mm resolution to reconstruct seasonal δ 18 O oscillations for palaeothermometry to calibrate growth lines (electronic supplementary material, figure S2) and collected bulk samples using a Dremel tool from the outer growth layer for determination of mean palaeotemperature. Bulk sampling averages the δ 18 O values of increments across the multiple years of growth displayed by the majority of our studied individuals [23]. δ 18 O values were converted to temperatures using the equation of Grossman & Ku [24], previously used in multiple Tridacna studies [23][24][25][26][27] and a seawater δ 18 O value of 1.8‰ relative to VSMOW previously measured from the Northern Red Sea [28,29].…”

“…Bulk sampling averages the δ 18 O values of increments across the multiple years of growth displayed by the majority of our studied individuals [23]. δ 18 O values were converted to temperatures using the equation of Grossman & Ku [24], previously used in multiple Tridacna studies [23][24][25][26][27] and a seawater δ 18 O value of 1.8‰ relative to VSMOW previously measured from the Northern Red Sea [28,29]. To measure nitrogen isotope ratios of shell organic matrix, we collected chips from near the shell margin and ground them for δ 15 N measurement with a CE Instruments NC2500 elemental analyser interfaced to a ThermoFinnegan Delta Plus XP Isotope Ratio Mass Spectrometer.…”