Aquarium Invertebrates: A Look at the Giant Clam Tridacna maxima
There are several species of clam belonging to the family Tridacnidae, which are best known as the tridacnids or giant clams. Of these, one of the most attractive species is Tridacna maxima, which is also one of the most commonly offered species available to hobbyists. I say most attractive because they can come in a wide range of colors, which can be arranged in a variety of unusual patterns, with many specimens being striped, sprinkled, spotted, blotched, marbled, etc. The colors themselves also range from black and white, with essentially everything else in between being seen on some specimen or another. In fact, I'd say it's harder to find a maxima that's unattractive than to find one that is.
To get started, maxima is the most widely distributed species of the tridacnids. They're found in the Red Sea and from East Africa all the way across the Indo-Pacific to Polynesia. They also live as far north as southern Japan, and as far south as the Great Barrier Reef (Rosewater 1965). Maximas can be found in high numbers around many reef areas where waters are relatively shallow and clear, with the majority living at depths less than about 25 feet. Some can be found living as deep as about 50 feet, but their abundance drops off dramatically below about 25 feet, with these deeper-living clams occurring mostly as solitary individuals (Jaubert 1977).
Regardless of their depth of occurrence, essentially all of them are found living on limestone substrates, on top of living corals, or on coral rubble. Supposedly they're occasionally found on sandy bottoms (Pasaribu 1988), but after doing a lot of diving around Japan and Indonesia I have yet to see this. Regardless, on hard bottoms maximas can chemically bore a shallow indentation into the substrate that the bottom of their shells fits into, and they strongly affix themselves in place using a tough structure called a byssus. So, they typically stay in one spot for life, with the bottom third or half of the shell kept out of sight in their burrow. Conversely, on coral rubble bottoms they simply bury themselves amongst the coral chunks and attach to something solid with their byssus if they can. Again, usually only part of the shell rises above the substrate. The odd thing is that they won't do this in aquariums, though. It seems that if they don't start making a burrow while they're relatively tiny, they won't do it at all. So, don't expect a specimen to dig into your live rock. Regardless, they almost always attach to the substrate using their byssus anyway.
Aside from that, the most notable thing to point out here is that, like all the other members of the family, maximas harbor large populations of zooxanthellae. These single-celled photosynthetic algae live in the tissues of a host clam primarily within a specialized system of tubes that permeate the fleshy, colorful, mantle tissue that extends from the top of the shell, and when given enough light, they can make far more food than they need for themselves. The extra food (in the form of carbon and energy-packed glucose) is then given to the clam host, which is the same thing that occurs within most reef-dwelling corals.
Under optimal conditions, these zooxanthellae are constantly multiplying within a tridacnid, and some of these live algal cells can be digested by specialized amoeboid cells within the host, too. So, a host clam can rely on its zooxanthellae for more than just sugar, and is considered to be a "farmer" to some degree since it can consume these surplus zooxanthellae grown inside its body.
In addition, all tridacnids can also absorb a variety of nutrients directly from seawater. Their fleshy mantle is covered by a specialized tissue that can very effectively take in dissolved nutrients like ammonia, nitrate, and phosphates. So, here they have a third means of nutrient acquisition, with one more to go.
The last way they cover their nutritional needs is through filter-feeding. All tridacnids can eat fine particulate matter strained from surrounding waters by their specialized gills, which not only work to exchange carbon dioxide and oxygen, but can also act as sieves that can collect such particles. A tridacnid, like most other clams, pumps water into its body chamber, where it flows over the finely-branched gills and then flows out the other end of the body chamber, minus some particulates. These collected bits are can include phytoplankton, zooplankton, and detritus, meaning they can make use of a broad range of things.
When it comes to identification, once you know what to look for maxima is usually pretty easy to distinguish from all other tridacnids with the exception of T. crocea. So, I'll go over the basic features used to ID them, and then give you some tips on how to differentiate them from croceas, too.
When it comes to shells, they're almost always grayish-white when clean. However, one of the interesting things about the shells of this species is that sometimes they may be tinted with light yellow or pinkish-orange. Rarely, the shell may also be completely yellow. It's almost always strongly elongated in form, being much longer than it is tall, and some maximas are very thin from side to side while others are quite fat. Deformed shells are not particularly uncommon either, as maximas sometimes live in very crowded groups and/or partially burrowed into coral rock preventing them from producing a normally-shaped shell. Regardless, at its top each half of the shell typically has four or five smoothly-curved and inter-digitating projections that are symmetrical to those on the other, allowing the them to close together tightly. However, there are occasional individuals that have more elongated and even pointed tooth-like projections that don't inter-digitate as smoothly with those on the opposite side.
Some species of tridacnids have petal or shelf-like structures on their shells, which are called scutes, and maxima is one of them. In fact, their shells are typically covered by numerous tightly-spaced but thin scutes, which run in rows from the bottom to the top of the shell. However, when maximas partially burrow into the substrate, many of these scutes are either not formed in the first place, or are broken/eroded away in the process. So, maxima shells oftentimes have no scutes on the bottom portion, while numerous scutes are still present on the rest. Still, there are occasional individuals that have none at all for some reason, while aquacultured specimens that are not permitted to burrow typically retain most or all of their scutes.
Also note that it's possible for a maxima's shell to reach almost 16 inches in length, but that's the largest ever reported (Kinch 2002). Thus, you shouldn't expect any given specimen you purchase to get so big. In fact, McMichael (1974) did a survey of several hundred maximas in the wild and reported that only 3% were larger than 9 inches and the largest specimen found in the whole survey was only 9.8 inches. So, that record holding 16-inch specimen was quite an anomaly.
When it comes to the soft parts, maximas typically extend their zooxanthellae-packed mantle tissue well beyond the upper edges of the shell. In fact, it's typically extended to the point that it completely obscures the shell from view when looking down on one. The mantle can also come in such a wide range of colors and patterns that there really is no standard color, although blue is the most common. As noted, the patterns covering it may also be striped, sprinkled, spotted, blotched, marbled, etc. and quite fancy, which is why various specimens are often called things like teardrop maximas, striped maximas, super maximas, or even ultra maximas, etc.
Still, the only patterns that are relatively consistent in how they look are that of the teardrop and striped varieties. Teardrop maximas may vary significantly in color, but they tend to have the same sort of pattern covering their mantle, being covered in teardrop-shaped splotches, while striped maximas tend to have a dark, solid background color with thin radiating stripes of blue, yellow, or white. Other than that, the mantle has rows of simple, closely-spaced, dark eyes near the outer edge and sometimes has numerous eye-tipped tubercles/protrusions on its upper surface, too. The large mouth-like opening in it (called the inhalent siphon) is also ringed with numerous simple, small tentacles that usually lack anything more than very fine branches.
Now, as I said above, maximas are indeed easily confused with croceas because both species have relatively large and often brightly-colored mantles with small tentacles around their inhalent siphons. The vast majority of maximas has elongated shells with lots of scutes, while almost all croceas have shorter, taller shells that lack scutes or only have a few small ones. But, there are exceptions, which lead to this confusion. A typical crocea's shell lacks scutes and is far less elongated than the shell of a typical maxima, but there are individuals of each species that are in between. Croceas can be rather elongated at times, and may actually have a lot of scutes, while some maximas may have rather short shells and lack scutes. So, I'll give you some additional pointers for trying to figure out which is which in case it isn't clear who is who.
First, a maxima's shell usually has larger, much more pronounced waves or folds than that of a crocea, as crocea's shells are typically relatively smooth. A maxima's shell sometimes has very sharply-pointed, almost triangular projections at the shell's upper edge, but crocea's are always more rounded and never sharp. Maximas can reach significantly larger sizes than croceas, as the record-holding crocea was only 6 inches long. So, anything larger than about 5 inches in length is almost certainly a maxima. There is no such thing as a teardrop crocea or a striped crocea. Croceas may have some stripes on them at times, but I've never seen one that had a solid background color with thin radiating stripes on top, or the characteristic droplets of a teardrop. And lastly, the tentacles around the inhalent siphon of a maxima are typically simple and un-branched, while those of a crocea are usually finely branched at their tips.
So, there is no straightforward single way to always ID both species correctly, but by looking at a combination of these features you can usually figure out just about any of them. I will admit though, over the years I've come across a handful of specimens that have been quite difficult to differentiate. At such times most folks just throw up their hands and declare that a hard-to-ID specimen in a hybrid between the two species, but after doing a lot of searching, reading, and asking clam farmers questions I'm still far from convinced that these two species can/do hybridize. That's a topic for another day, though.
When it comes to caring for maximas, water quality requirements are typical for reef aquariums in general. Basically, if you're successfully keeping corals alive and well, then your water quality is good enough for a maxima. On the other hand, if you're having problems maintaining excellent water quality - don't fool with any species of tridacnid.
When it comes to water motion, tridacnids live in reef and near-reef environments, and are regularly exposed to strong currents and wave activity. This is especially so for maximas, which often live right at the crest of a reef where waves break hardest. Thus, they are no strangers to strong, surging and turbulent water motion. However, in aquariums the flow tends to be quite linear and constant, as a pump outlet might blast water in one particular spot day and night at about the same volume per minute, and rarely creates any real surge or turbulence. So, you need to think about this when it comes to the placement of a maxima (or any other tridacnid) in an aquarium.
It's okay to expose maximas to a low velocity surge, or to turbulent flow, but putting them in a position where a pump constantly hits them with a strong, non-stop linear current is not recommended. Basically, any sort of current that causes the mantle to fold upwards too much, or over onto itself all the time is bad, as is any current that makes a specimen chronically retract its mantle. Thus, you can put one anywhere you like with respect to current, as long as it doesn't bring on either of these reactions. I'll also add that while they're almost always found on hard substrates and rubble in the wild, placing them on such is highly recommended, but is not required. Placing a specimen on sand/gravel won't kill them, but they often move around a lot, trying to find something to attach their byssus to. Next is lighting, which not surprisingly is of critical importance.
Maximas live at relatively shallow depths where they receive relatively intense light, so fluorescent lighting will only suffice in shallow tanks, or if a specimen is placed on the rockwork near the water's surface in a deeper tank. I would try fitting as many bulbs into the canopy/fixture as possible at that, and mount the bulbs close to the water, and then place any specimens within a foot of the surface, preferably less. Some specimens may be able to get by at times with less light, or further down in deeper tanks, but I implore you to not take chances. Metal halide or comparable L.E.D. lighting is your best option.
I know that some people have gotten by with less, but when it comes down to it insufficient lighting is certainly one of the most common causes of losses. The problem is that corals are very simple organisms that have no real "guts" to speak of, while tridacnids have all the organs you'd expect to find in a higher animal. They've got stomachs, kidneys, gonads, gills, and even a heart. Thus, they are far more complex than you might think, and they use a lot of calories to keep everything running. So, it's a mistake to think that just because your lighting is bright enough to keep corals healthy and growing that they're necessarily bright enough to keep a maxima alive long-term.
To make matters worse, it can take a tridacnid months to slowly starve to the point of no return. So, everything can look fine for weeks on end, then a specimen may seem to just up and die for no apparent reason when it was really starving the whole time. Every maxima is genetically different at that, and long-term experience has proven that some individuals can get by with less while others need much more, even though they may be the same species and even the same size and color. To add, you cannot give a tridacnid too much light as long as a specimen is given time to adapt to intense lighting, so it's better to err on the bright side than the dim side. For more on this, refer to my article On Lighting for Tridacnid Clams in the March 2011 issue, and for even more than that see my book Giant Clams in the Sea and the Aquarium.
Lastly, there's the question of whether or not you need to feed a maxima in an aquarium. As covered, all tridacnids are filter-feeders, yet their zooxanthellae can cover a great deal of their nutritional needs, and they're able to absorb pretty much everything else they need directly from seawater. In fact, if provided with enough light, maximas of any size have no need to filter feed and can thrive in particulate-free water as long as there are enough dissolved nutrients present. Controlled experiments by Fitt & Trench (1981) proved that tridacnids can do without, and I kept them for years before anyone was talking about adding phytoplankton to aquaria, much less sold any in a bottle. You can get all the details in my article Tridacnid Clams (Usually) Don't Need to Be Fed in Aquaria in the July 2010 issue, but I'll give you some basic info on the subject anyway.
When you feed your fishes some amount of the food won't get eaten and becomes detrital particles, which maximas can filter out. Any uneaten food also releases nutrients into the water as it decomposes. Likewise, the food that is eaten by the fishes ends up becoming solid wastes that can also become detritus. However, even more importantly, fishes excrete dissolved substances that can be absorbed by a clam, too. For example, fishes give off dissolved ammonia as a waste product, but tridacnids can absorb it and use it as a source of nitrogen. Thus, when you feed your fishes, you're feeding your tridacnid(s), too.
So, the real question is whether or not there are enough fishes in your aquarium to support one or more tridacnids. While it's unlikely to happen, I suppose it is possible to have too low a fish load (or too high a tridacnid load depending on how you look at it) in an aquarium, which would mean that the amount of fish waste being produced would not be enough to support the needs of the clam(s). So, my advice is to refrain from taking any chances and use a quality phytoplankton product if you have any doubts. I have to say though, I imagine that very, very few hobbyists have problems due to nutrient levels that are too low since for most of us the fight is to prevent them from getting to high.
Anyway, I need to wrap things up, and unfortunately I'll going to end on a bad note. As gorgeous as they may be, most all experienced aquarists agree that maxima is the least hardy of the tridacnids. I've seen and heard of more losses of this single species than all the rest by far, and I think it's usually due to insufficient lighting. They are especially dependent on excellent water quality and intense lighting. So, if you don't have both, don't buy one of these.
I'll also add that despite their attractiveness, availability, and relatively low price, really small specimens are even more likely to pass away. In fact, I experienced so many losses of small maximas back in my selling days that I outright refused to order/sell them after a while, and I've heard the same from many other vendors, too. They didn't tolerate shipping and acclimation to aquarium life very well at all, and it was common for more to die in the first couple of weeks than to live. Stick with larger specimens, as in at least few inches long, and you'll have much better odds of success.
- Fitt, W.K. and R.K. Trench. 1981. Spawning, development, and acquisition of zooxanthellae by Tridacna squamosa (Mollusca, Bivalvia). Biological Bulletin 161:213-235.
- Jaubert, J. 1977. Light, metabolism, and the distribution of Tridacna maxima in a South Pacific atoll: Takapoto (French Polynesia). Proceedings of the 3rd International Coral Reef Symposium 1:489-494.
- Kinch, J. 2002. Giant clams: their status and trade in Milne Bay Province, Papau New Guinea. TRAFFIC Bulletin 19(2):1-9.
- McMichael, D.F. 1974. Growth rate, population size and mantle coloration of the small giant clam Tridacna maxima (Roding), at One Tree Island, Capricorn Group, Queensland. Proceedings of the Second International Coral Reef Symposium 1:241-254.
- Pasaribu, B.P. 1988. Status of giant clams in Indonesia. In: Copeland, J.W. and J.S. Lucas (eds.) Giant Clams in Asia and the Pacific. ACIAR Monograph Number 9, Canberra. 274pp.
- Rosewater, J. 1965. The family Tridacnidae in the Indo-Pacific. Indo-Pacific Mollusca 1:347-396.