A sectioned shell of Tridacna maxima showing growth bands (numbered 1-10) that help reconstruct past climate events. The white dotted line marks a notable decline in growth rate, likely corresponding to the 2010 El Niño event [FG: First Growth, LG: Later Growth, OL: Outer Layer, IL: Inner Layer]. (PC: AA Fousiya, IIT Kanpur)
A close-up of a giant clam’s mantle tissue showing its iridescent blue edges and characteristic spots. These patterns are created by specialized cells containing crystals that help protect the clam from excess sunlight.
AA Fousiya, working in IIT Kanpur, is one of the very first women doctorates from Lakshadweep, and has been working for several years now, looking at Lakshadweep’s recent climatic past. With her team she has focused on the ability of giant clams to tell us how ocean temperatures have varied in Lakshadweep waters. The team sectioned a 10-year old giant clam from the Minicoy lagoon for this study and used a series of spectrographic and petrographic analyses to examine growth rates.
It turns out that giant clams are very much like old-growth trees whose tree rings allow us to reconstruct so much about the past. Under the microscope, the clam showed clear growth bands that grew at approximately 3 mm a year on average. What was even more important was that the clam showed a marked decline in growth rates in 2010 (to around 2.5 mm), possibly marking a response to the large 2010 El Niño event that affected so much of the reefs of Lakshadweep.
The intricate folds of a giant clam’s mantle tissue reveal tiny green dots – these are zooxanthellae, the microscopic algae that live symbiotically within the clam’s tissues, providing it with nutrients through photosynthesis.
Why do giant clams make such good markers of these events? Very much like hard coral, giant clams are photosynthesising animals, and form symbiotic partnerships with microscopic algae. Under conditions of temperature stress, these bivalves bleach in exactly the same way that corals do, getting rid of their photosynthetic partners. Because the algae are so critical to their feeding, their loss is associated with a period of starvation, leading inevitably to reduced growth rates for that year. These get recorded as a much thinner band in the clam’s shell. This is what Fousiya’s team found when they looked at the shell under the microscope. The giant clam managed to survive the 2010 El Niño event, but it clearly was a hard year for the animal.
But that’s not all. Giant clams also track past temperature in other, more subtle ways, The research team also evaluated δ18O isotope records in the clam. These are chemical signatures that get incorporated in the bands of the shell. Every time water temperatures fall by 1 °C, these isotope values increase by around 0.23 parts per thousand. What the team was able to show was that the clam had peaks in isotope values that appeared to track cyclones recorded in the area, when water temperatures drop suddenly. Based on these studies, Fousiya and colleagues conclude that T. maxima is a robust record-keeper of past climate in tropical waters.
Giant clam specimens showing different growth patterns. The complex ridges and surfaces of these clams contain vital information about past environmental conditions, including temperature changes and extreme weather events.
Giant clams are among the longest-lived bivalves on the reef – they can live to over a 100 years. Over that lifespan, they bear silent witness to every storm, every heating event, every change that has occurred in Lakshadweep’s waters, as we continue to change the earth’s climate with overconsumption and pollution. It is a sobering thought.
[If you want to read more about this remarkable set of studies, you can find the papers here:
https://link.springer.com/article/10.1007/s00338-023-02455-8
and
https://doi.org/10.1007/s00338-022-02263-6
or you can write directly to the team to get a pdf.]