If you are a fan of Batman DC, then you must know one of his famous archenemy: Two-Face! The depiction of Two-Face that I know is Harvey Dent, Gotham City’s former District Attorney, who turned to crime after half of his face became disfigured. His signature is the flipping of his lucky coin that decides whether or not he commits a crime.

Two-Face, a notorious villain in the Batman series. Source: Wikia.

Well, giant clams can also be ‘Two-Face’! During the last coral bleaching event in 2016, Rouzé and Hédouin (2018) reported a single individual with bilateral asymmetry bleaching (Fig. 1b), although the majority of giant clams did not show bleaching signs (Fig. 1a). In this case, warming seawater temperature is the badass here!

Figure 1. Bleaching event observed during the El Niño 2014–2016 event in Tetiaroa atoll, French Polynesia (Pacific Ocean). a) Resistant giant clams Tridacna maxima compared to one sensitive Acropora coral colonies A. tenuis, and b) bilateral asymmetry in bleaching susceptibility of a giant clam Tridacna maxima. Source of photos: Rouzé and Hédouin (2018).

Why is this so?

There are no conclusive explanations to explain this observation yet, but the scientists hypothesised that the different parts of the outer mantle (i.e. clam’s tissue body) may host different species of symbiotic algae (also known as zooxanthellae) (DeBoer et al. 2012). It is known that certain algae species are more resistant to ocean warming than others, thus this specific bleaching pattern could be due to the loss of specific algae species, and vice versa for the side that did not bleach.

Is this bleaching pattern common among giant clams?

The study by Rouzé and Hédouin (2018) revealed only 1 individual of the many other observed giant clams, suggesting that it may not be common. After reading this study, I remembered my snorkelling trip to the Silaqui Ocean Nursery in the Philippines, where I vividly remember several individuals of the Tridacna gigas stocks were two-face (Fig. 2a)! Later in the same year, I had written about the ‘Ghostly Clams‘ and shared my first encounter of mass bleaching in a cohort of cultured Tridacna maxima clams (Fig. 2b).

Bleachy clams
Figure 2. a) Tridacna gigas at the Silaqui Ocean Nursery, Bolinao Marine Laboratory in the Philippines on 21 Jan 2016 and b) cultured Tridacna maxima at the St John’s Island National Marine Laboratory in Singapore on 23 Jun 2016.

When you hear about global bleaching events, you hear a lot about the corals that bleached. But let’s not forget about the other zooxanthellate organisms such as the sea anemones, zoanthids, corallimorphs, and giant clams! Who knows what other interesting observations we may discover by examining the other groups of animals… 🙂

Further reading: