Mutualistic Relationships

The small giant clam is maintaining an intrinsic relationship with zooxanthellae (of the genus symbiodinium but also with others), which are dinoflagellates algae. They use the by-product of the photosynthesis, principally organic carbon, as food, while the zooxanthellae is using the waste of the clams to make more photosynthesis.

Tridacna maxima, as the other symbiotic giant clams, has developed a complex harvesting strategy. The zooxanthellae are acquired from the surrounding, to the mantle cavity, to the stomach, to a specialised organ involved in harvesting them, the zooxanthellal tubular system (Norton, 1992). The zooxanthellae are harvested in a complex of tube that look like lungs. The tubular system is composed of 3 level of tubes, the first one coming from the stomach and finishing in the mantle of both right and left side of the clam. Secondary tubes goes up to the edges of the mantles and tertiary tubes are blind ended and aimed to maximise the surface area for increasing exchange. The tubes are vessels with thin walls allowing nutrients transfer and cilia allowing the generation of a current. Zooxanthellae are regularly flushed back into the stomach, and goes back to the sea water through the whole digestive system without being digested (and we do not know why !), as well as new ones are recruited from the surrounding.

In addition, only 34% of the carbon found in clams bodies comes from filter feeding (Norton, 1992) which means that up to 66% of the carbon comes from harvesting zooxanthellae. This is an example of an efficient carbon sequestration mechanism. However, zooxanthellae seems to be nitrogen and phosphorus limited, according to different studies (Ambariyanto, 1997; Belda, 1993). However, the exchange mechanism occurring between the host and the symbiont is not usual. The exchange are done by translocation from the haemolymphn as shown on the Figure 1.

Figure 1. The translocation mechanism in the clam/zooxanthellae symbiosis

Some of the mechanisms of the symbiosis are not yet known and more research is currently undertaken about this complex symbiosis.

As for corals, the clams can be subjected to bleaching if a breakdown of the symbiosis occurs. This is likely to occur in different events such as the increase of water turbidity for long period (due to flood for examples), climates changes (sea level rise, increase of the sea surface temperature). Clams, as corals, could be heavily impacted because they are relying majorily on zooxanthellae for their food supply.