Giant clams, a sheltered reputation
By Jarno Verdonk
Giant clams, Tridacna Sp., are not widely known by the every-day tourists. Three different species occur at Koh Tao; Tridacna maxima, Tridacna squamosa and Tridacna crocea. Although giant clams are not widely known, they can easily compete with the rest of the coral reef in a beauty competition due to their stunning camouflaging coloration. Moreover giant clams live up to their name and can reach impressive sizes, the largest species in this genus reaching up to 1.4 meters! At Koh Tao however the largest individuals grow up to a more conventional 40 cm.
Giant Clams get hold of food in two different ways. The first source of food for giant clams is zooplankton, which are obtained through filter-feeding, this is the main food source for giant clams in their early stages of life. Giant clams also have a symbiotic relationship with a unicellular algae (called zooxanthallae), which live in the tissues of the clams. These algae share some of the sugars and carbohydrates they produce, through photosynthesis, with the clam. Not only does the zooxanthallae provide about 90% of the clams daily energy budget, they are also responsible for the clams unique and ornate coloration.
Giant clams are an important key stone species for the reef ecosystems: they provide food for other reef inhabitants, keep the reef nutrient levels low by filtering the water, contribute to the reef structure, and make the reef more resilient to climate change by recycling their zooxanthellae. Making conservation efforts for this species important!
Her Majesty, the Queen of Thailand has implemented laws making all giant clams in Thailand a protected species, and also put the Thai Navy in charge of their protection. Since the 1980’s, the Thai Fisheries Department has also been running a hatching, nursery, and restocking program for Giant Clams. The reef conservation team of the New Heaven Dive School (NHRCP) are actively trying to keep a healthy population of giant clams around Koh Tao by transplanting marine cultured giant clams (Tridacna squamosa) out in the reefs (read the full summary of that program here). But a proper protocol for the transplantation is lacking, leaving many factors unsure and resulting in a high mortality rate shortly after transplantation.
In 2015, I joined the NHRCP as an intern, and also completed a study on the giant clams as part of my degree in biology at the Utrecht University. My study focus was on factors involved in the survival rates during the transplantation of giant clams.
The first question was if there is any effect from caging the giant clam on their behavior. I used video to document simulated attacks of predators on the giant clam, and compared the response behaviors between marine cultured giant clams and wild individuals. Normally, when a large fish swims over a giant clam, or a small fish tries to bite one, the clam will eject a jet of water to deter the would-be predator, and also closes up to protect itself. During the study, I found it interesting although the initiation of the reflex started at the same distance, the marine cultured giant clams close their valves up to ~ 10% less while attacked by a large predator. The cage had no influence on the giant clam’s behavior to small predators probably due to the fact that those predators are able to move through the fencing. Essentially what I found was that the cultured clams had learned not to fear large predators, could this be why survival rates were low?
My second job was to compare density of the shell between wild and marine cultured individuals. A higher density means that the shell is less likely to be crushed by large predators like triggerfish. After measuring the density of many shells, I found a slight decrease of 0.2444 g/cm3, 1% of their total density, in the cultured individuals. This indicates that marine cultured giant clams are more likely to be predated on then their wild relatives.
Taking both those effects in account it could be said that marine cultured giant clams are less fit to survive outside the cages. However, to really understand this process I had to dig deeper. To my surprise, the next part of the study proves that this is not the main cause for the high mortality rates.
The next part of my study looks at the influence of the stress of being moved from the cages to the reef on the survival of the clams. Stress is mostly seen as a negative factor, and the expectations here were that acute stress would have a temporary paralyzing effect, and a main cause for the high mortality rate. Surprisingly, I found that when cultured and wild calms were moved, the marine cultured giant clam’s behavior towards predators was more efficient. The 10% difference in valve closure found earlier vanished, and both wild and stressed marine cultured giant clams showed a similar responds towards all predators. So neither the stress, nor the decreased response appears to be the main reason for the high mortality rates.
A fourth component of the study was the surveying of the current giant clam population at Koh Tao. Out of this section of the study came the positive result that Koh Tao has a healthy population of giant clams! Furthermore, an overview of population densities among different sites came forth out of this part, which comes in handy with determining which sites are most in need of help.
The populations have also been studied at a smaller scale, focusing on distribution patterns. For this part, all giant clams in four different sites have been located and saved on a GPS while swimming over the site. Remarkably, I found that all giant clams form clusters, it is rare not to find another individual within 8 meters of the first one you find. Thinking about the fact that giant clams are mostly dependent on the oceanic currents for their distribution the fact that they are able to locate themselves within 8 meters of a relative really is exceptional! Since giant clams naturally form clusters, the recommendation for transplanting here is thus to make clusters as well.
Moreover the distribution patterns highlights the areas within the reef most suitable for giant clams. Most clams were found in the middle of the reef around 4-8 meters of depth. The outskirts of the reef were not heavily populated, however the shallow outskirts showed a higher density of giant clams than the deeper outskirts. Therefore this paper recommends to transplant the marine cultured giant clams to the middle of the reef only.
So eventually the riddle of why there is a high mortality rate has not been solved. Nevertheless a few factors have been highlighted, which together will hopefully improve the survival rates of the clams in the nursery programs operated by the Thai Department of Fisheries and the NHRCP. I hope that these results will positively affect this program, and also create a base study for future students and interns to build upon.