Giant clams of the genus Tridacna are popular mollusks for reef aquariums. Oftentimes, people unfamiliar with the ocean are familiar only with the shells of clams. Their true beauty, however, can only be realized when they are living and healthy, as they have a thin mantle that is stunning to behold. Tridacna clam mantles have intricate patterns and come in a variety of colors. I’ve seen clams that are purple, blue, green, and yellow, and some even have iridescent qualities. It is not difficult to understand why these clams are so popular in the reef-keeping hobby.
Tridacna clams are found all throughout the Pacific. The four most readily available species are T. derasa, T. squamosa, T. maxima, and T. crocea. In rare instances, one might find the giant clam, T. gigas. There are actually two other species that are not in the aquarium trade, namely T. tevoroa and T. costata, which is a new species discovered in the Red Sea.
Anatomy
Without getting too much into an anatomy lesson, there are some key features of Tridacna clams that should be noted:
As mentioned before, they have brilliantly colored mantles. The mantles are filled with zooxanthellae, much like photosynthetic coral. Zooxanthellae are dinoflagellates of the genus Symbiodinium. They collect inorganic compounds from both the host mollusk and sea water to produce organic molecules (carbohydrates, amino acids, and glycerol) by way of photosynthesis. The organic molecules are then passed back to the host and provide nutrition to the clam, allowing for rapid growth. Early in a clam’s life, the quantity of zooxanthellae is limited so they rely more on filter feeding. It is estimated that smaller clams get 65% of total carbon from filter feeding. Larger clams get only 34% of their total carbon from filter feeding, indicating a much greater reliance on photosynthesis.
Clams have two siphons that move water in and out of their body. There is a large Inhalent siphon lined with fine tentacles. When it opens wide, you can catch a glimpse of the clam’s gills. The water then is expelled through a smaller exhalent siphon. The gills of the clams are interesting in that they are used for both respiration and feeding.
At the base of the clam is a foot by which it attaches to the substrate. There is also a byssal organ that excretes thin byssal threads to anchor it in place. It is critically important that the aquarist avoid damaging the foot or the byssal gland when moving a clam from one place to another. It is easy to damage the byssal organ, and if that happens, chances are the clam will die. Once a clam is anchored to a rock, it is best to move the clam and rock together.
Telling clams apart
Now that we’ve covered the common anatomical features of Tridacna clams, how do we tell the different clams apart?
T. maxima and T. crocea
Maxima and crocea clams are the most popular Tridacna varieties. They have dazzling-colored mantles with intricate patterns. Maximas and croceas are very similar in appearance and often confused too. The difference in the two is that maxima clams are slightly elongated in shape while croceas are more stout and stubby.
Crocea clams are called boring clams because they are able to dissolve the nearby coral substrate, leaving only their mantle visible. At their base, they can secrete a mild acid that, over the years, erodes the calcium carbonate substrate, kind of like how a calcium reactor dissolves media.
Like just about every sessile organism on the reefs, clams are looking to clear out real estate for their personal growth. Some have observed that not only are clams capable of releasing a mild acid at their base to bore down into rock, but they are also capable of releasing acid through their extended mantle to kill off nearby coral.
T. derasa
Derasa clams typically have a golden-colored mantle with streaks of teal and a vibrant blue rim. In comparison to maxima and crocea clams, derasa clams get massive. It is not uncommon to have a derasa grow to over 20 inches in a home aquarium. They are among the more hardy clams, making them an ideal choice for first-time clam enthusiasts.
T. squamosa
While less common than the other three clams, T. squamosa can be found occasionally. Squamosa clams have muted coloration in comparison to maximas and croceas, but they have a distinctive fluted shell. While maximas, croceas, and derasas have somewhat smooth shells, the squamosa has pronounced scales that are quite attractive in their own right.
Clam care requirements
Care requirements for these clams are similar to those demanded by corals in that they require good light, good water quality, and good water flow. If there were one area to focus on, however, it would be light. Tridacna clams are found in shallow areas and require high light, especially as they get older and rely less and less on feeding. As a general guideline, clams smaller than 3 inches would benefit from phytoplankton supplementation.
Second to lighting is the need to maintain tip-top water chemistry. High calcium and alkalinity are a must. Clams are known for fast growth rates and deplete calcium and alkalinity aggressively to power this growth (related: learn techniques for maintaining both of these parameters in a reef aquarium).
As for flow, these clams prefer moderate flow despite originating from shallow areas on the reef that receive heavy wave action. The concern with high flow in the aquarium is the risk of introducing bubbles into the body that the clam cannot effectively eliminate.
Conservation
This genus of clams is listed as vulnerable by the International Union for Conservation of Nature (IUCN). In years past, overfishing brought these clams to the brink of extinction, but this is one of the best cases for aquaculture efforts. Clams are every bit as popular now for food and the ornamental trade, but because they are being aquacultured, they are much less at risk of going extinct.
We have not attempted any sort of aquaculture of clams because we are not set up for it, but I would love to build a facility down the line that specializes in this. I think clams are just amazing.
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