Some myths die hard, especially when they get spread all over the internet these days. The idea that giant clams (tridacnids) have to be fed plankton to prevent starvation is one such myth, and I’ve been telling hobbyists this for about six years now. In fact, I devoted an entire chapter to tridacnid nutrition in my book Giant Clams in the Sea and the Aquarium, have made countless online posts, and have spoken on the subject at many conferences and club meetings, etc. But, the myth still lingers. I’m quite tired of talking about it though, so I’m going to address tridacnid nutrition one last time. Please read on, and help spread the word to those that haven’t heard.

Nutrient Acquisition in the Sea

Tridacnids filter feed by drawing water into their bodies through their inhalant siphon, which is the larger slit-like opening in their mantle.

Tridacnids are no different than any other life forms in that they need a wide variety of nutrients to stay alive, grow, and reproduce. Of these, carbon, nitrogen, and phosphorus are needed in relatively large quantities though, and are thus known as macronutrients. These are the three that I’ll focus on, and it can be assumed that if an animal is getting enough of these in its diet from some source, then it’s likely getting the micronutrients it needs, too. After all, beef, lettuce, peanuts, etc. aren’t made of just carbon, nitrogen, and phosphorus and neither is plankton. In addition to these, tridacnids need a source of energy, which comes primarily in the form of the simple sugar glucose.

We also need carbon, nitrogen, and phosphorus, etc. and a source of energy, but we really have only one way of acquiring them. We eat food or drink lots of protein shakes, smoothies, and sodas. On the other hand, tridacnids actually have four ways of acquiring nutrients, one of which is the hosting of zooxanthellae.

All species of tridacnids house large populations of these single-celled algae, which happen to be the same sorts that reef-building corals contain.

And as is the case with many corals, when provided with sufficiently intense lighting the zooxanthellae can provide their clam host with glucose (C₆H₁₂O₆), which is an excellent source of both carbon and energy. In fact, under optimal conditions, the zooxanthellae can make far, far more sugar than they need for themselves and give away the rest.

Of course, the question here is whether or not the zooxanthellae can cover all of a host’s carbon/energy (C/E) needs. Well, several scientific studies (ex. Klumpp & Griffiths 1994 and Klumpp & Lucas 1994) have looked at how much C/E several species and specimens of tridacnids need to survive, grow, and reproduce vs. how much of that need can be covered by its zooxanthellae, and what they found was that with sufficient lighting zooxanthellae can indeed provide all that a tridacnid needs. In fact, under good conditions, the zooxanthellae can typically supply a host clam of any size with an excess of C/E.

The water passes through the body chamber and over the gills, and is then expelled through the exhalent siphon, which typically looks like a little chimney at the other end of the mantle.

Still, it is important to note that this does not mean that tridacnids only need bright light in order to live. As stated above, they require nitrogen, phosphorus, etc., as well, which are not provided by their population of zooxanthellae. This is why tridacnids either have to eat some type of nutritious planktonic/particulate food by filtering it from the surrounding waters and/or absorb nutrients directly from the surrounding waters and/or digest some of their own population of zooxanthellae, which are their other three means of nutrient acquisition.

In order to collect planktonic/particulate food from surrounding waters, tridacnids (like almost all other types of clams) draw water into their body and use their gills as sieves. Planktonic/particulate matter of various sorts is stripped from the water and passed on to the stomach, and researchers have found that tridacnid stomachs and feces can contain zooxanthellae and various other types of phytoplankton, various types of small zooplankton, fine filamentous algae, detritus, very fine sand, and sponge spicules (Yonge 1936, Mansour 1946, Ricard & Salvat 1977, Trench et al. 1981, Reid et al. 1984, Klumpp et al. 1992, and Marayuma & Heslinga 1997). This, by the way, is quite contrary to the notion that these animals only eat phytoplankton if you’ve ever heard that before.

Some of these can be very nutritious and good sources of nitrogen and phosphorus, partially covering a tridacnid’s needs. However, it is important to note that no matter how much a tridacnid eats, it still needs bright light and apparently cannot make up for a lack of light with more feeding. Tridacnids are never found in dim/dark waters, regardless of the availability of planktonic/particulate foods, and I tried feeding a couple of them copious amounts of phytoplankton in a low-light aquarium with no luck, either. They eventually had to be removed to save them.

Here you can see the gills of a tridacnid, which can sieve plankton and other particles from water passing through the body chamber.

When it comes to absorbing nutrients, tridacnids can take them directly from seawater via the use of a specialized tissue that covers their surfaces (Fankboner 1971, Goreau et al. 1973, Wilkerson & Trench 1986, Fitt et al. 1993, Belda & Yellowlees 1995, Hawkins & Klumpp 1995, and Ambariyanto & Hoegh-Guldberg 1999).

Thus, any notion that their sole means of acquiring nitrogen, phosphorus, etc. is through filter-feeding is incorrect, as nitrogen and phosphorus are primarily taken directly from the surrounding seawater in forms other than plankton or detrital particles.

This tissue covering a tridacnid’s colorful mantle is laden with specialized microscopic structures called pinocytosing microvillous epidermal cells, which can take in “phenomenal” quantities of both fluid and miniscule particulate substances according to Fankboner (1971). And the later papers listed above reported that nitrogen can be taken up in the form of ammonia, ammonium, and/or nitrate, all of which are found in low concentrations in the environment, and phosphorus in the form of phosphates, too. Small quantities of amino acids can also be absorbed, as can various trace elements and other such things that naturally occur in seawater, albeit in low concentrations.

Lastly, under optimal conditions, the zooxanthellae living within a tridacnid are constantly multiplying, and some are actually ejected from the host alive when numbers get too high. But, some of these live zooxanthellae can be digested by specialized cells within the host, too (ex. Yonge 1936, Fankboner 1971, and Marayuma & Heslinga 1997). The zooxanthellae are able to take various simple substances provided by the host and combine them to make complex and useful compounds for themselves when living inside tridacnids. Thus, they can provide some nutrients to a host by becoming food themselves.

This healthy tridacnid is getting rid of excess zooxanthellae, which typically looks like a rope of reddish-brown goop coming out of their body. This rope is actually made of live zooxanthellae and mucus, and is not an indication of trouble.

Digestion isn’t carried out in the stomach though, as any old, dying cells are “harvested” by these specialized amoeboid cells that move around throughout a host, which then break them down intracellularly. Acquired nutrients can then be transported throughout the host by these same cells.

Nutrient Acquisition in the Aquarium

As explained above, well-illuminated tridacnids can acquire all the C/E they need from their zooxanthellae, can absorb nutrients from seawater, and can feed on their own zooxanthellae in the sea. They can filter out and make use of fine detritus particles, too. So, the question here is whether or not these methods can provide them with sufficient nutrients to forgo the need for any significant planktonic foods in an aquarium. Oddly enough, this will essentially depend on the fish load in an aquarium relative to the size/number of tridacnids in it.

The fish load matters due to the fact that fishes give off nitrogen-based ammonia as a waste product, which can be used by tridacnids as a source of nitrogen since they can absorb it directly from the tank water. Fishes also produce fecal material, which can provide tridacnids with a source of phosphorus and other nutrients, as well. Some is dissolved in the water and much becomes detritus, which can be filtered from the water. Additionally, any uneaten fish foods will also break down and become yet another source of dissolved nutrients and detrital particles. Thus, if there are enough fishes in an aquarium, providing them with food will also cover the needs of any tridacnids present.

This is why this article’s title has (Usually) in it, as there may be cases when an aquarium has a very low fish load (or none) and/or a very high tridacnid load. If there are few/no fishes in an aquarium, then there’s little/no fish food going into the tank, which means there is little/no ammonia being produced, and little/no phosphorus, etc. going in either. This is why some hobbyists have tried and failed to successfully maintain large numbers of tridacnids in aquaria housing few or no fishes, and is also the cause of some of the confusion about feeding.

Here’s an example of things going wrong: Many years ago my friend Barry Neigut (owner of Clams Direct) tried to test the effectiveness of a number of planktonic and particulate foods with respect to tridacnid survival and growth. He set up several small tanks, put clams in them, and used a different food product in each tank. He also had a control tank that contained the same number of clams, but didn’t feed them anything. All were given the same lighting.

Well, most of the clams that were fed survived (albeit some foods fared much better than others), but all of those in the unfed tank died. So, one of Barry’s conclusions was that apparently you had to feed them something or they’d starve. Unfortunately, at that time, he didn’t know about tridacnids’ ability to absorb nutrients, and his results would have been very different had he put a fish or two in the unfed tank and provided them with even a small amount of fish food. Hopefully at this point you can understand what went wrong, and how important fishes are.

Unfortunately, there is no sound guideline for how many fishes are required in order to keep a given tridacnid alive, though. There are too many variables to consider, such as tank volume, clam size, fish size, frequency and amount of feeding, competition from corals, etc. However, you probably shouldn’t worry, because the fish load and nutrient availability aren’t an issue in most aquariums anyway. In well-stocked reef aquariums the input of nutrients per gallon is usually much, much higher than it is in reef environments, and the vast majority of aquariums have a nutrient surplus rather than a deficit.

Still, if you have a low fish load and aren’t confident that any tridacnids present will be okay, I advise you to not take any chances. It is possible, although unlikely, to have too few fishes in your system to support the tridacnid load in addition to corals, algae, and anything else that can also absorb nutrients, and using quality planktonic foods certainly won’t do any harm as long as it’s not overdone. So, in such a case, you should go ahead and feed the tridacnids regularly.

Feeding and Growth

If water quality and lighting are up to par and a tridacnid is getting all the nutrients it needs, you should start seeing at least a little new shell material being added on within a few weeks, if not sooner. However, you need to keep in mind that some tridacnids grow very slowly, even under optimal conditions. For example, Tridacna crocea may grow less than 1mm per month, even in their natural habitat, and some individuals of other species may not grow much faster than that, either. Especially large clams may have significantly slower growth rates than smaller specimens, too.

Healthy tridacnids of any size should constantly add on new shell material, which can be seen as a bright, clean rim right under the mantle.

On the other hand, if you don’t see growth despite having excellent water quality and lighting, try some quality food for a while and see what happens. If growth starts after feeding starts, then you can be pretty sure that your fish load is too low to support a tridacnid. I say pretty sure because it’s possible that a specimen may be suffering from a disease/parasite or be stressed in some other way. So, you’ll need to do your best to identify and remedy any such problems before you decide that a tridacnid requires the addition of planktonic food.

Also note that just because one hobbyist might say they feed their tridacnid and that it’s growing quickly, that doesn’t mean your tridacnid will too, even if it’s the same species, and even if you are doing everything right and feeding it. Each individual tridacnid is different and I’ve seen some specimens grow more than twice as fast as others, even when they’re the same species, are the same size, and are being kept under identical conditions (in the same tank, under the same lights, etc.). Tridacnids are genetically diverse, just like everything else.

The “Three Inch” Myth

I have read and been told at some time or another that all tridacnids must be fed phytoplankton for long-term survival. Obviously, this is utter nonsense. However, much more frequently, hobbyists bring up the “three inch” rule, which sounds much more reasonable, but is also nonsense. So, let me kill this one off, too.

For several years I’ve heard that any tridacnids under about 3″ in length must be fed regularly or they’ll starve to death. The supposed twist here is that mature tridacnids can get all the C/E they need from their zooxanthellae, but immature clams cannot do so because they don’t yet harbor enough zooxanthellae in their mantles. The story goes that when young it takes them several years to build up a full complement of zooxanthellae, and that they must get the rest of what they need by filter-feeding until this happens.

I could go into great detail as to why this false, but I’ll keep it short since I already did that in my book. The fact is, small tridacnids, just a few weeks old, have all the zooxanthellae they need in order to thrive, and this population keeps up just fine as a clam grows, too. One study above all others bore this out. In a controlled experiment,

Fitt & Trench (1981) produced several larval specimens of Tridacna squamosa from sperm and egg, and then reared them for 10 months. These juvenile clams were kept in a flow-through system and provided with filtered seawater for the entire time, having no access to planktonic/particulate matter with the exception of being given some zooxanthellae early on in order to get their population started. For the duration of the experiment the clams not only stayed alive, but grew. Of course, the filtered water did contain dissolved nutrients, which the specimens could readily absorb, and that should be plenty enough to quell any arguments about juveniles requiring additions of plankton for survival.

Other Opinions

In order to beat a dead horse just a little longer, here are the words of a few well-respected authors in the hobby: Calfo (2001) says “Many clams have been maintained for years in aquaria without any deliberate feeding, but rather dependent upon light and dissolved compounds.” Knop (1996) states that “Altogether the food requirements of clams is so small that a special feeding scheme for them is superfluous if the tank is populated with a fair number of fish, especially if corals are regularly fed. In such a case a special food destined for the clams could even unbalance the whole system.” And, 16 years ago, Delbeek & Sprung (1994) wrote “…the effort required to feed these items is not worth it in our opinion. Tridacnid clams have been grown successfully in both culture systems and home aquaria for many years without any supplemental feedings.”

Okay, I’m done.

References

1. Ambariyanto and O. Hoegh-Guldberg. 1999. Net Uptake of dissolved free amino acids by the giant clam Tridacna maxima: alternative sources of energy and nitrogen. Coral Reefs 18:91-96.
2. Belda, C.A. and D. Yellowlees. 1995. Phosphate acquisition in the giant clam-zooxanthellae symbiosis. Marine Biology 124:261-266.
3. Calfo, A. 2001. Book of Coral Propagation, Volume One: Reef Gardening for Aquarists. Reading Trees, Monroeville, PA. 450pp.
4. Delbeek, J.C. and J. Sprung. 1994. The Reef Aquarium: Volume One. Ricordea Publishing, Coconut Grove, FL. 544pp.
5. Fankboner, P.V. 1971. Intracellular digestion of symbiotic zooxanthellae by host amoebocytes in giant clams (Bivalvia: Tridacnidae), with a note on the nutritional role of the hypertrophied siphonal epidermis. Biological Bulletin 141:222-234.
6. Fatherree, J.W. 2006. Giant Clams in the Sea and the Aquarium. Liquid Medium. Tampa, FL. 227pp.
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13. Klumpp, D.W., B.L. Bayne, and A.J.S. Hawkins. 1992. Nutrition of the giant clam Tridacna gigas (L.). I. Contribution of filter feeding and photosynthates to respiration and growth. Journal of Experimental Marine Biology and Ecology 155:105-122.
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16. Maruyama, T. and G. Heslinga. 1997. Fecal discharge of zooxanthellae in the giant clam Tridacna derasa with reference to their in situ growth rate. Marine Biology 127:473-477.
17. Reid R.G.B., P.V. Fankboner, and D.G. Brand. 1984. Studies on the physiology of the giant clam Tridacna gigas Linne – I. Feeding and digestion. Comparative Biochemical Physiology78A(1):95-101.
18. Ricard, M. and B. Salvat. 1977. Faeces of Tridacna maxima (Mollusca-Bivalvia), composition and coral reef importance. In: Proceedings of the Third International Coral Reef Symposium. Miami. 495-501.
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