I remember getting about six months to a year into this hobby, thinking I knew everything about proper water parameters, lighting, dosing, and keeping a reef tank in general. Or, at least enough to keep most coral alive. I had zoanthid, leather, and ricordea colonies thriving and my rose Bubble Tips had never looked better, so I must be doing everything perfectly, right? And then, my focus settled onto clams. Specifically, a bright blue Maxima Clam at my old LFS. He was about two inches, and the most beautiful clam I had seen them get in. We quickly purchased him, took him home, and added him to my tank. A few days later, I woke up to find my clam gaping open, and bristleworms eating the dead tissue. Unfortunately, I was still a beginner at the time that had gotten way too confident and didn’t do the proper research into clam care requirements until it was too late. From talking with other hobbyists, this seems to be a mistake nearly all of us made at some point with certain corals or invertebrates.
A Brief Rundown of Each Species
Four common species are kept in home aquaria. Here is a quick description of each:
Tridacna Derasa is one of the larger clams we keep in tanks, and typically regarded as the hardiest. These can reach 24 inches in diameter. Due to their natural wide spread habitat all throughout the reefs within the Pacific Ocean and Coral Sea, most Derasas at your LFS are going to be wild specimens that were farmed in the ocean instead of aquaculture facilities. Their mantles are usually gold, brown, or yellow with electric blue or green trim around the edges. Fast growers, they need to have ample space within the tank as they can triple in size within the span of one year.
Tridacna Squamosa has seen increasing popularity within recent years. Reaching about 18 inches maximum, this clam is known to be a bit less demanding than the other species in the lighting and flow department. Scutes, or scale/leaf like shelf growths on the outer side of the shell of Squamosas is the easiest way to identify them. Small fish or fry my use these scutes as shelter from larger tankmates. Like Derasas, this clam is very widespread and can be found on nearly any reef substrate. Mantles are usually brown, with black, tan, gold, yellow, orange, white, green, and/or blue markings. Although rare, some have a completely blue mantle with darker colored markings. This color morph is highly sought after, and thus very expensive.
Tridacna Maxima are the most popular clam species. They are the second smallest of the four, only reaching sizes of 12 inches. Their shells are less symmetrical than the other three, and grow small, close together scutes on the outside of their shell. Maximas are slightly pickier than Squamosa and Derasa clams in that they require a hard, rocky surface to attach to and will move around until they find a spot they approve of. Inhabiting rocky reefs in great numbers, congregations of this clam can be found from Africa, Australia, Japan, to Polynesia, along with in the Red Sea. They are the most widespread of all tridacnid clams. And, because they often live in shallower waters, they can be pickier about flow. Maximas typically will not do well with simple linear flow. Mantles are diverse, and beautiful. Patterns may include stripes, spots, marbling, splotches, or other intricate displays. Colors include bright blue, dark blue, purple, gold, green, pink, black, red, orange, and more.
Tridacna Crocea is the slowest growing and smallest giant clam species, only reaching 6 inches in size. Much like Maximas, they can be found in large congregations and come in striped/spotted/blotched patterns consisting of blue, green, gold, tan, purple, brown, and orange colors. Unlike the others, though, Crocea clams will burrow into your rockwork by boring a hole into it over time. Unfortunately, most of these clams in the aquarium trade are wild specimens. To distinguish between these and Maximas, remember that Croceas have smaller and less defined scutes, along with a longer and wider byssus gland.
Body Structure of Giant Clams
Although clams seem like a simple animal, they have complex organs. Each have gills, a stomach, a heart, kidneys, gonads, siphons, mantles, byssus gland, byssal threads, hyaline organs, eyes, and more. Corals do not possess these, so they require far fewer calories than clams do to survive. That is why you may have enough bioload and light for corals, but not nearly enough to support a clam. For example, my 48 inch, four bulb T5HO fixture was enough for LPS and softies, but not nearly enough to keep a clam happy.
This is the first place many aquarists will fail. Clams need a tank that has stable salinity (SG of 1.023-1.025) , alkalinity (9-12 dKH), calcium (380-450 PPM), pH (8.1-8.3), nitrates (2-20), strontium (6-10 PPM, utilized in shell growth), and iodine (0.04-0.08 PPM). If any of these parameters falls out of place, the clam will suffer and likely die. Nitrates are usually a problem for SPS keepers, who tend to keep them close to zero. But, clams require nitrates of at least 2 ppm, or they will starve. Calcium and Alkalinity, and the other elements to a lesser degree, are used very quickly by clams and must be tested regularly to ensure you or your doser is dosing enough back into the system to keep up with the clams demand and usage.
This is the second place where most fail. Clams need intense lights, and typically high quality, powerful LEDs or halides are recommended. But, young clams under 2 inches are easily damaged by light. All sized clams need to be properly acclimated to intense lighting. Starting your Tridacnid low in the tank and lowering the intensity of your lights is a good way to start. You can ramp your lights up and move the clam higher little by little every week until it is fully acclimated. Unless you are feeding older clams multiple times a week, most recommend a PAR rating of 250+ is typically recommended, and scientific experiments have shown they are much more productive at PARs of 700-1200. That being said, some do have success keeping clams in 100-250 PAR areas if they are fed well and often. But, I would highly recommend shooting for 250-500 PAR given how shallow the natural environment is for these animals.
Most clams available are under 4 inches. Under 4 inches, most still require supplemental feeding of zooplankton, phytoplankton and bacterioplankton (e.g. PNS Probio™) about twice a week. Unfortunately, I also see mostly 1-2 inch clams in fish stores. At this size, they require these feedings daily for healthy growth and survival. While getting a baby clam can be much cheaper, and you get to watch it grow, remember you will need to dedicate time every day to feed it. Over 4 inches, you can relax on feeding. Many aquarists don’t feed their clams at all as the tanks waste and the clams zooxanthellae are plenty to keep it happy. But, because they do require some nitrates and dissolved wastes in the water column to consume, fish keepers with low bioload tanks should expect to still feed once or twice a week to make up for having fewer fish. This is one area where having a heavily stocked tank is actually extremely beneficial and directly benefits your inhabitants. Clams filter tons and tons of water daily, so they will act as a nutrient sink in even the most overcrowded tank. Obviously, if you are going to have a heavy bioload, do the proper research and stay responsible!
Water movement is crucial. Because they come from shallow reefs, clams are used to strong current and waves. But, most home aquariums rely on plain linear flow. While clams do not prefer this, they can adapt to live with it as long as you set it up correctly. You need to ensure that the water flow doesn’t cause your clams mantle to fold upwards much, and that the clam doesn’t retract its mantle too much. If these things occur, your giant clam will ultimately die. Be sure his mantle is fully extended during the day, and appears to be full, happy, and actively pumping water through its siphons.
Final Words- Aquacultured Clams
Finally, if you can find them or request your LFS order one specifically, purchase aquacultured clams instead of wild farmed/harvested ones. Some of these species are so over fished that they have gone nearly or completely locally extinct in some areas. They are relatively easy to breed in aquaria, so most species are easily found aquacultured. Wild clams are also much more difficult to acclimate and maintain in tanks, while their aquacultured counterparts are much more forgiving. In addition, they won’t (or shouldn’t) come in with hitchhikers. Many small snails, crabs, and bacteria consume clams and will ride in on them when collected from the ocean. To ensure your investment doesn’t have pests that will quickly eat it (and possibly tank mates) on it, aquacultured is the way to purchase. If you can get your hands on one, and are able to provide for the clams needs, Tridacnids make beautiful centerpieces to any reef tank!