When it comes to caring for tridac-nid clams in aquariums, one of

the most commonly asked questions is whether or not they need to be fed by their keeper. Different opinions come from different people, but for the most part the answer is actually quite straight-forward. In the vast majority of reef aquariums there is no need to provide a tridacnid clam with any sort of particu-late food. To some hobbyists this may be a surprise, but I assure you that once you understand how tridacnids “work”, you’ll see why this is the case. So, I’ll tell you how they cover their nutritional needs and why they typically don’t need to be fed.

Nutrients and their acquisition in the wild:Like all other living things, tridacnids

need a long list of nutrients to stay alive, but the big ones are carbon, nitrogen, and phosphorus, and they need a source of energy too. While it may sound simplistic, if they are getting all of these that they need from some source, you

can safely assume that they’re getting all the trace elements and such that they require along with these as well. The neat thing is that unlike humans, which get all their nutrition in one way (eating food), tridacnids actually have four means of doing so.

First of all, tridacnids house large populations of single-celled algae called zooxanthellae, which are the same algae that reef-building corals contain. When provided with sufficiently intense light-ing, the zooxanthellae can provide their clam host with a great deal of carbon-rich food, primarily in the form of the simple sugar glucose (C6H12O6). Basically, under good conditions, the zooxanthel-lae can make far, far more sugar than they need for themselves and the excess is donated to the clam host that they live in. This supplies a tridacnid with both carbon and energy, as sugars like this are energy-providing molecules that can be broken down to supply it through cellular respiration.

But, the big question is whether or not the zooxanthellae can cover all of a tridacnid’s carbon/energy (C/E) needs. And the answer is – yes, they can. Sci-entific studies (ex. Klumpp & Griffiths

1994 and Klumpp & Lucas 1994) have looked at how much C/E a tridacnid needs 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 the zooxanthellae can indeed provide all that a tridacnid needs. In fact, under opti-mum conditions, the zooxanthellae can typically supply a host clam of any size with much more C/E than it requires. That may sound hard to believe, but it’s a fact.

So, does this mean that tridacnids only need bright light in order to thrive? This time the answer is – no. Tridacnids can-not live off light/C/E alone, as they still require the nitrogen and phosphorus, etc. They have to get these things in other ways, with three options to choose from. To get these, tridacnids either have to eat some type of nutritious particulate food by filtering it from the surrounding waters, and/or they can absorb nutrients directly from the surrounding waters, and/or they can digest some of their own population of zooxanthellae.

When it comes to filter-feeding, tridac-nids (like almost all other types of clams) use their gills to filter particles from the

By James W. Fatherree, M.Sc.All images © James W. Fatherree, M.Sc.

To Feed ... or not to Feed?

Tridacnids, like most other clams, have the ability to strain par-ticulate matter from seawater with their dual-purpose gills.

eating any particulate foods. However, there’s still plenty to go over before we’re done.

Nutrients and their acquisition in aquariums:Tridacnids can get all the C/E they

need from their zooxanthellae and can also absorb nutrients and feed on their own zooxanthellae in the wild, but can they also cover all of their needs in an aquarium without being provided any sort of particulate foods by you? This will essentially depend on the fish load in an aquarium relative to the size/number of tridacnids in it. The fish/tridacnid load is the key because the fishes get fed and tridacnids can use their waste products as nutrient sources.

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 faecal 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 tridacnids can filter from the water, too. On top of this, any uneaten fish food will also break down and become yet another source of dissolved nutrients and particulates. Thus, if there are enough fishes in an aquarium, providing them with food will also cover the needs of any tridacnids present.

However, there may be cases when an aquarium has a very low fish load and/or a very high tridacnid load. Likewise, corals, bacteria, and other

organisms can also compete with tridacnids, so we’re still not done quite yet.

If there are no fishes, then there’s no fish food going into the tank, which means there is no ammonia being produced, and no phosphorus, etc. going in either. So, adding a fish and keeping it well fed would solve the problem of nutrient availability for a tridacnid, as long as the fish is big enough and enough fish food is going in. I say it must be big enough because you obviously can’t keep a single damselfish in a tank with a 12 inch tridacnid and expect the little fish to give off enough stuff to keep the big clam going, no matter how much food it eats.

Yes, you’d need more fishes than that. Get the idea?

Unfortunately, there is no “formula” for how many fishes/how much fish food has to go into an aquarium in order to keep a tridacnid, or several, alive, as there are entirely too many variables to consider. You shouldn’t worry though, as the fish load and nutrient availability won’t be an issue in most tanks anyway. In the typical well-stocked aquarium the input of nutrients per gallon is usually much, much higher than it is in reef environments, and there’s much more likely to be a nutrient surplus rather than a deficit.

Still, with that said, in the event that you feel like you have a relatively small fish population compared to your clam population, and/or have a lot of competition in your tank and aren’t feeling confident that they’ll be okay, I wouldn’t take any chances. It’s certainly possible, although unlikely, to have too few fishes in your system to support the tridacnid load, and feeding clams using quality foods definitely won’t do any harm to anything as long as it’s not overdone. So, in such a case, you should go ahead and feed the tridacnids regularly.

If you do choose to supply a tridacnid with food, you can always try to slowly wean it off the food if possible, if you want to. Assuming you have sufficient lighting, all you have to do is watch it carefully,

monitor your water quality, and look for white shell growth along the edges of its shell.

If everything is all right 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, after it has become acclimated to its new home. You can keep feeding it for as long as you like, of course, but if a tridacnid is growing, you can try to cut back on the amount/fre-quency of feeding and see what happens. If you cut back and the growth stops, then you should return to feeding the same amount/frequency that you were feeding beforehand and see what happens.

On the other hand, if the clam keeps growing after you cut back, then you can assume that you’ve been adding more food than it needs. So, you can cut back

water that is drawn into their body. Some of the particulates found in tridacnid stomachs and faeces include: zooxanthel-lae and various 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). So, you can see that they’ll take in a wide variety of things, some of which can be very nutritious while others have no nutritional value at all (the sand and spicules). The particu-lates that do have some nutritional value are a source of nitrogen and phosphorus and other things, so to some degree they can cover a tridacnid’s needs. However, before moving on, I do need to point out that no matter how much a tridacnid eats, it still needs bright light and cannot make up for a lack of light by feeding more. So, don’t try any such thing in an aquarium.

Next, they can also absorb large amounts of nutrients from sea water through 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 sea water in forms other than plankton or detrital particles.

Fankboner (1971) was the first to write about this when he reported that the outer surface of the tridacnid mantle is covered by specialized microscopic struc-tures called pinocytosing microvillous epidermal cells, and that they can take in “phenomenal” quantities of both fluid and minuscule particulate substances. And the later papers shown above covered the uptake of various substances directly from sea water, as well. Nitrogen can be taken up in the form of am-monia/ammonium (NH

3/NH

4) and/or

nitrate (NO3), all of which are found in

low concentrations in the environment, and phosphorus can come in the form of phosphates, which are present in low concentrations in reef waters, too. Even small amounts of dissolved amino acids can be taken in, as are trace elements and other such things that naturally occur in sea water, 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 the numbers get too high. But, some of them can be digested by specialized cells within the host, as well (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 their own respira-tion, growth, and reproduction when living inside tridacnids (and outside of them, too). Thus, they can provide some nutrients to a host by becoming food themselves. Some of them, particularly any old,

dying zooxanthellae are “harvested” by specialized amoeboid cells that move around throughout a clam and its blood, collecting them from the tubes they’re held in within the host. These zooxanthellae are then digested inside these cells rather than in the stomach, and the nutrients acquired can be transported to other clam cells.

So, now you can see just how amazing tridacnids can be. When it comes to getting everything they need, they’re fortunate enough to have an array of means at their disposal. As you may have guessed at this point, these alternative means of acquiring the nutrients are the reason tridacnids can survive without

A good indicator of tridacnid health is the appearance of new shell material. The older shell material slowly becomes discoloured, but the

new shell is typically bright white.

One last thing:There something of a story that has

been around for a while in the hobby proclaiming that any tridacnids under about 3” (or sometimes 4” depending on the source) in length must be fed regularly or they’ll starve to death. This is false, but the story seems to be a persistent one for whatever reason, so I’ll try to convince you that it isn’t true.

The basic idea of this story is that mature tridacnids can get all the C/E they need from their zooxanthellae, while immature clams cannot do so and must be fed until they reach a size of 3 or 4 inches. Supposedly, small tridacnids don’t harbour enough zooxanthellae in their mantles when young to provide them with enough C/E and they must get the rest of what they need by filter-feeding. Wrong.

I could add a couple more pages here to fully explain why this false (I cover it in great detail in my book), but I’ll make it easy. Small tridacnids, just a few weeks old, have all the zooxanthellae they need in order to thrive, and I’ll give you a good example. In a controlled experi-ment, Fitt & Trench (1981) maintained several specimens of Tridacna squamosa for 10 months, which they reared from sperm and egg. The specimens were kept in filtered sea water for the duration, hav-ing no access to plankton, detritus, etc. for nearly a year – and they still grew. Yes, they didn’t just survive, but grew in size, too. Do note that the filtered water did contain nitrogen, phosphorus, etc. though, which the clams could absorb.

To finish up, I’ll also tell you that not too many years ago I was still running my reef aquariums with a bare bottom and never added any sort of foods for my tridacnids (or corals either), and had no food-related troubles with them. To the contrary, I was constantly siphoning up detritus that settled on the bottom in areas of weak flow, and had to do large regular water changes to keep dissolved nutrients low and the growth of unwanted algae in check. “Plankton-in-a-bottle” products hadn’t even been “invented” yet, or at least weren’t available in those days, but my tridacnids lived and grew anyway. And to show you what a few well-respected experts have said on the same topic, 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.”

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 Calfo (2001) says “Many clams have been maintained for years in aquaria without any deliberate feeding, but rather dependent upon light and dissolved compounds.”

Convinced now?

ReFeReNces:

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.

Belda, c.A. and D. Yellowlees. 1995. Phosphate

acquisition in the giant clam-zooxanthellae symbio-

sis. Marine Biology 124:261-266.

calfo, A. 2001. Book of coral Propagation, Volume

One: Reef Gardening for Aquarists. Reading Trees,

Monroeville, PA. 450pp.

Delbeek, J.c. and J. sprung. 1994. The Reef

Aquarium: Volume One. Ricordea Publishing,

coconut Grove, FL. 544pp.

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.

Fatherree, J.W. 2006. Giant clams in the sea and

the Aquarium. Liquid Medium. Tampa, FL. 227pp.

Fitt, W.K. and R.K. Trench. 1981. spawning,

development, and acquisition of zooxanthellae by

Tridacna squamosa (Mollusca, Bivalvia). Biological

Bulletin 161:213-235.

Fitt, W.K., G.A. Heslinga, and T.c. Watson.

1993. Utilization of dissolved inorganic nutrients

in growth and mariculture of the tridacnid clam

Tridacna derasa. Aquaculture 109:27-38.

Goreau, T.F., N.I. Goreau, and c.M. Yonge. 1973.

On the utilization of photosynthetic products from

zooxanthellae and dissolved amino acids in Tridacna

maxima cf. elongata (Mollusca: Bivalvia). Journal of

Zoology (London) 169:417-454.

Hawkins, A.J.s. and D.W. Klumpp. 1995. Nutri-

tion of the giant clam Tridacna gigas (L.). II. Relative

contributions of filter-feeding and the ammonium-

nitrogen acquired and recycled by symbiotic alga

towards total nitrogen requirements for tissue growth

and metabolism. Journal of experimental Marine

Biology and ecology 190:263-290.

Klumpp, D.W. and c.L. Griffiths. 1994. contribu-

tions of phototrophic and heterotrophic nutrition to

the metabolic and growth requirements of four species

of giant clam (Tridacnidae). Marine ecology Progress

series 115:103-115.

Klumpp, D.W. and J.s. Lucas. 1994. Nutritional

ecology of the giant clams Tridacna tevoroa and T.

derasa from Tonga: influence of light on filter-feeding

and photosynthesis. Marine ecology Progress series

107:147-156.

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.

Knop, D. 1996. Giant clams: A comprehensive

Guide to the Identification and care of Tridacnid

clams. Dahne Verlag, ettlingen, Germany. 255pp.

Mansour, K. 1946b. source and fate of the zoox-

anthellae of the visceral mass of Tridacna elongata.

Nature (London) 158:130.

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.

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 Physiology 78A(1):95-101.

Ricard, M. and B. salvat. 1977. Faeces of

Tridacna maxima (Mollusca-Bivalvia), composi-

tion and coral reef importance. In: Proceedings of

the Third International coral Reef symposium.

Miami. 495-501.

Trench, R. K., D. s. Wethey, and J. W. Porter.

1981. Observations on the symbiosis with zooxan-

thellae among the Tridacnidae (Mollusca, Bivalvia).

Biological Bulletin. 161:180-198.

Wilkerson, F.P. and R.K. Trench. 1986. Uptake of

dissolved inorganic nitrogen by the symbiotic clam

Tridacna gigas and the coral Acropra sp. Marine

Biology 93:237-246.

Yonge, c.M. 1936. Mode of life, feeding, digestion

and symbiosis with zooxanthellae in the Tridac-

nidae. scientific Reports of the Great Barrier Reef

expedition 1:283-321.

a little more and continue to look for growth, and so on. Eventually, you’re likely to find that you can cut back the feeding more and more until you aren’t feeding at all, with the tridacnid still showing new growth anyway.

There are a few other things to keep in mind, though. It’s possible that a tridacnid may not grow at all in the first place, even if you start feeding it from the beginning. In that case, the lighting may be insufficient, the clam may be suffering from a disease/parasite or be stressed in some other way, or maybe there’s a problem with the calcium/pH/alkalinity

that prevents it from producing shell material, etc. If any or all of these are an issue, then all the food in the world won’t make a tridacnid grow. Thus, you’ll need to do your best to identify and fix any such problems before you decide that a tridacnid needs more or less food.

You also have to keep in mind that some tridacnids grow very slowly, even when things are going great. 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. Thus, you may have to

look very closely for new shell material, and have some patience. 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, too. Each individual tridacnid is different and I’ve seen one specimen grow more than twice as fast as another, even when they’re the same species and being kept under identical conditions (in the same tank, under the same lights, etc.). Tridacnids are genetically diverse, just like everything else.

A collection of T. croceas