Giant clam growth and isotope data

The health of reef-building corals has declined due to climate change and
pollution. However, less is known about whether giant clams, reef-dwelling
bivalves with a photosymbiotic partnership similar to that found in
reef-building corals, are also threatened by environmental degradation. To
compare giant clam health against a prehistoric baseline, we collected
fossil and modern Tridacna shells from the Gulf of Aqaba, Northern Red
Sea. After calibrating daily/twice-daily growth lines from the outer shell
layer, we determined that modern individuals of all three species
(Tridacna maxima, T. squamosa and T. squamosina) grew faster than Holocene
and Pleistocene specimens. Modern specimens also show median shell organic
δ15N values 4.2‰ lower than fossil specimens, which we propose is most
likely due to increased deposition of isotopically light nitrate aerosols
in the modern era. Nitrate fertilization accelerates growth in cultured
Tridacna, so nitrate aerosol deposition may contribute to faster growth in
modern wild populations. Furthermore, colder winter temperatures and past
summer monsoons may have depressed fossil giant clam growth. Giant clams
can serve as sentinels of reef environmental change, both to determine
their individual health and the health of the reefs they inhabit.