Project Synopsis: The Health of Red Sea Giant Clams

Picture of T. squamosa. The colors in the "lips" are produced by the photosynthetic pigments of the clam's symbionts. From Wikipedia.

The Northern Red Sea is a unique place to study coral reef ecosystems. I am traveling there this summer not to study reef-building corals, however, but instead will focus on a different group of massive reef inhabitants: the giant clams (genus Tridacna). The three species of giant clams in the area represent the northernmost known population of the genus in the world. I intend to compare the growth of fossil and modern populations to determine whether their modern growth is inhibited relative to their ancient counterparts, and whether that decline is due to human pollution and influence on their environment.

Giant clams grow to huge sizes with the help of symbiotic algae that live within their tissue. The algae get nutrients and a safe environment while the clams are free to harvest the sugars produced by photosynthesis, allowing them to hypercalcify and build massive, thick shells at a rapid rate. The three species of the Red Sea vary in their degree of reliance on these symbionts as opposed to the filter feeding which they also use for some nutrition. The most photosynthetic species, T. squamosina, is also the most threatened. It is only able to live at the reef top, where it has access to sunlight but is also vulnerable to human over-harvesting. Shells are highly sought after as souvenirs, which has severely impacted populations of giant clams around the world.

T. squamosina shell from California Academy of Science Collection. Photo by Dan Killam.

I hypothesize that the giant clams of the region are also impacted by pollution from agricultural runoff, fish farming and urban sewage outflow. All of these factors reduce water clarity and promote algal blooms, which reduce the photosynthetic rate of the symbiotic algae that the clams rely on. I propose that this distinction is what has caused T. squamosina to lose most of its population. Previous studies have shown that it was over 80% of pre-human fossil assemblages, but it is now reduced to a small remnant population.

To test this idea, I will be collecting fossil clam shells and comparing their shells to modern individuals obtained from the beach at the Eilat Coral Reserve. These shells can be cut to observe internal growth bands, which are a record of the rate of growth, length of life and metabolism of the organism throughout its lifespan. If clams in the past grew faster, they should have wider bands on average. If longevity was longer, they should have more total bands across their shells. The carbonate of each band is a record of the local environment at the time. If clams were subject to different temperatures, we will be able to see that signal in the oxygen isotopes of their shells. Lastly, we intend to extract amino acids protected within the shell crystals to determine whether the clams' tissue makeup has been impacted by human-sourced nitrogen from fertilizers and sewage. If we find that slow-growing, short-lived clams predominantly seem to be influenced by human pollution, we will have created a link between the suppression of their growth and pollution, which is valuable information to explain their decline in this region.

I am a paleobiologist and a conservationist, and so hope that being able to compare long-dead and more recently dead individuals of these amazing organisms will help us understand what must be done to protect and increase their populations in the future.