The white empty valves of the deceased clam. Photo by Koh Kwan Siong.

The life of a giant clam in Singapore can be a tough one… Thanks to the vigilant volunteers, I received news that one of the resident giant clam at Sisters Island had died. I really appreciate the feedback from volunteers as I am not always around on the reefs, and highly dependent on citizen scientists to inform us. Thank you for this important record! 🙂

While it is saddening, here’s a summary of possible explanations of how a giant clam may succumb to death:

1) Reduced water visibility?

The reduced water visibility results in lower light penetration to the sea bottom. The availability of natural light is critical for the giant clams, whose symbiont zooxanthellae makes its food with sunlight. The clam obtains the bulk of its nutrients from the zooxanthellae, and in return a safe haven for the micro-algae.

2) Sedimentation?

Sedimentation effects have been hypothesised to possibly 1) block out sunlight, 2) irritate the gills and mantle, and 3) with time, result in mortality. Presently, we do not sufficiently understand the impacts of sedimentation, although preliminary results do indicate that acute sediment pollution can speed up mortality in juvenile clams.

Still alive – the Tridacna squamosa clam found by volunteers in May 2013.

3) Diseases?

Like humans, microbes are omnipresent everywhere… When the clam catches one of these diseases, it retards its growth. With time, the clam will fall ill – if lucky, it can recover and vice versa. Diseases in giant clams are rarely reported and studied, for one, usually by the time symptoms of poor health are detected, the clam goes within the next day or two. This is a common problem that we also face on our hatchery. We attempt to get them to recover by introduction of antibiotics to combat against bacteria, but at times, the effects are limited. It is our hope that in the near future, we can develop rapid assay kits that allow us to assess the health of clams on reefs.

4) Stressors on the reefs?

Drastic changes on the reefs can also have an effect on its inhabitants. The most common phenomenon is the spike in sea surface temperatures, which can cause the expulsion of the important zooxanthellae. In such situation, the clam can recover after the temperatures become cooler, or death due to poor growth and unable to re-acquire zooxanthellae. Other potential stressors include changes in salinity levels and pH.

While we are unable to give a proper assessment on what could have resulted in its death (we need to examine the empty valves closely), this shows the tough life of giant clams on our reefs, and in fact, all reefs! Hopefully, this brings about even more awareness of the plight of giant clams, in light of the increasing extent of anthropogenic impacts on coral reefs.

Start to do your part today! Be informed. Take care of our fragile coral reef ecosystems.