ON THE HALF SHELL		BY DANIEL KNOP

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Reef Tank Design for Giant Clam Lovers

DANIEL KNOP

A cube-shaped clam tank with slanted front glass. This picture was taken before completing the rock decoration and set up.

Some years ago I experimented with different aquarium designs to make the beautiful colors of the tridacnid clams a bit more visible. One of the tanks that came out of these experiments was a cube-shaped tank with a slanted front glass (see accompanying picture). The advantage of this tank design is clear. When you look through the slanted glass, the effect is essentially the same as when viewing the clam from above — the colors of the siphonal mantle are as vivid as when viewing it from the top. After publishing this design in my book Giant Clams (1994), several aquarium designers in Germany started toying with it in order to develop a unique aquarium design. What came of all this is a wide variety of designs that make viewing even corals more interesting because they can be viewed from above. Some of these tanks even have slanted side panels.

There is no advantage without accompanying disadvantage, and the same holds true for tanks with a slanted front glass. These aquaria have a small surface area compared to water volume. This reduces the passive gas exchange. However, as long as a protein skimmer is used, this is probably negligible because the skimmer will take over the gas exchange. But, this type of aquarium seems to be more dependent on protein skimming than a conventional aquarium design, especially in the warmer months with the associated higher water temperatures and lower oxygen concentration of the water.

In order to ensure a high biodiversity of microfauna and to keep a good balance between oxydative and reductive processes, we decided to include a lot of live rock in this setup. Flat-shaped rocks were preferred because the clams can attach to their surfaces much better than to the rounded surfaces of a ball-shaped rock. Flat rocks that have indentations are even be better. The the rocks that were most suitable for the giant clams were placed in a location that received good lighting and water current.

Lighting is another aspect that is important for a giant clam tank. Under natural conditions, most of the smaller giant clam species inhabit the shallow, strongly illuminated portions of the reef. The small T. crocea, in particular, lives in extremely shallow water and is rarely found at greater depths, while T. maxima is also found in shallow water between 10 and 15 meters (30 to 50 feet) in depth — usually not deeper. The same holds true for T. squamosa.

At 15 meters (50 feet) in depth, Kelvin (K) values average 10,000 K. If we keep giant clams in aquariums with lamps that produce light in the 20,000 K range, we are keeping them under conditions that, in nature, would be found at depths of 20 to 50 meters (65 to over 160 feet) — depths at which giant clams cannot survive (except the extremely rare species T. tevoroa, which can still be found as deep as 30 meters). For this reason, I strongly suggest that lamps that produce light with values over 10,000 K not be used for giant clams.

If the clams come from a clam farm, this is even more important. We know that all clams that we buy from aquarium stores are adapted to the conditions of their former habitats, especially when it comes to illumination. On clam farms, clams are normally kept at locations between 2 to 4 meters (6 to 12 feet) in depth — seldom deeper. The reason for this is very simple — juvenile clams have to be easy to reach for cleaning and conducting research. This means that the farm-raised clams we buy in aquarium stores are most likely adapted to light in the 6000 to 7000 K range, even lower than that for wild-caught clams. If we now subject them to light conditions that in natural habitats can only be found at depths between 20 to 50 meters…well, it can only be regarded as an experiment with uncertain results.

Clams do have a variety of mechanisms for adapting to different lighting conditions, like intensity or spectral composition (Knop 1996), but some of these mechanisms are believed to be flexible only during early developmental stages, and it is not known how far they can be altered later on. Also, remember that in nature changes occur slowly, whereas the change in lighting conditions is very rapid once we put the clam into our tank. So, try to select lighting for the clam tank that comes close to the illumination of their natural habitats — tank illumination should serve primarily a physiological, not an aesthetic, purpose.

Of course, this is not to say that 20,000 K-rated lamps are unsuitable for reef aquaria in general. For many coral species that naturally occur at 15 meters or below, this lamp type surely makes sense — as is exhibited by the success of many reeferkeepers. But, for giant clams, K values should be kept between 5000 and 10,000.

The rule of thumb for light intensity should be close to 1 watt per liter. Of course this will not give exact values for illuminating every tank because it does not include the tank measurements, which also have a great influence on the light distribution and intensity in the tank. But at least it can give an average value as a starting point.

Following this rule of thumb, a 500 liter (130-gallon) tank should be provided with two 250-watt lamps. If the tank height exeeds 60 centimeters, it should be illuminated with 400-watt metal halides instead of 250-watt units. The tank with slanted glass shown in the picture did not follow the rule given above because of the square shape of the bottom. This allowed a very efficient use of a single lamp (250-watt halide, Osram TS 250 W/D, 5200 K) because most of the light reaches the rock formation, whereas the lamps hanging over an aquarium with normal design sometimes illuminate the carpet as well as the aquarium.

DANIEL KNOP

Normally, the beautiful colors of giant clams can only be seen when viewing them from the top of the tank. However, all these colors can easily be viewed through the slanted glass of the author’s innovative tank design.

Another important issue is filtration using activated carbon, a controversial topic among reefkeepers. Even a moderate concentration of yellow coloration to the water can have a significant affect on the penetration of light. Under natural conditions, the wavelengths visible as blue light penetrate deep into the water and the yellow wavelengths are quickly filtered out. In an aquarium with a yellowish tint to the water the opposite happens. The yellow tint filters out blue light and the yellow light can penetrate well down to the gravel of the tank. By using activated carbon, these conditions change very rapidly over hours or days, causing the conditions in the aquarium water to come closer to natural conditins, and leading to increased coloration of the syphonal mantle.

Even though we cannot say with certainty that using activated carbon is harmful to clams, it is surely unnatural and is best avoided. In addition, the strong adsorptive power of activated carbon can prove to have a disadvantage. It will change water conditions by quickly adsorbing a wide variety of dissolved compounds, among which are some that are essential for giant clams. If this happens suddenly after a long period of time with no carbon filtration, it can be disastrous to several groups of animals. Fossa and Nilsen report serious problems in maintaining xeniids when employing a lot of activated carbon for filtration (Fossa and Nilsen 1994). This is, of course, not an argument against the use of activated carbon in the clam tank. But we change the carbon once a month in clam tanks and use it in moderate amounts, so no sudden changes of water chemistry or the spectral light composition will occur.

Tridacnid clams like moderate to medium water current. Generally, the smallest species, T. crocea, can bear the strongest water movement, but a jet-like current that causes the syophonal mantle to fold up continuously should also be avoided in this species. Occasionally, strong water movement is tolerated, but continuous turbulence should not be directed at the clam’s location.

Because the distribution of the water current in the tank is greatly influenced by the way the rocks are placed in the aquarium, it is difficult to give exact values for a suitable pumping rate per hour. One pump size may be best for a specific tank, whereas it might be unsuitable for an aquarium of the same size if the rock formation and pump positioning differ significantly. But for giant clam husbandry the water current does not have to be as strong as it needs to be for an aquarium with many Acropora sp. and other small-polyped scleractinians, especially if those corals have grown to dense populations. For giant clams it should be enough to pump the water content of the aquarium at a rate of five times per hour, so a 500 liter tank would get a pump with a capacity of 2500 liters per hour. But, as mentioned above, the rock formation and other factors greatly influence this, so the best suited pump size for a specific tank might differ from this value.

Because all giant clam species take up the same substances from the water, they strongly compete with each other. In the shallow water clam-rearing cages on clam farms this situation is much different, because all necessary organic substances and minerals are available in quantity. But in the aquarium, we have to replace whatever gets lost. This is far more than just calcium ions and carbonates, and the clams tend to develop deficiencies when stocked in large numbers in a closed tank system. Also predators spread much more rapidly with this high stocking density. Even aquarium supply stores sometimes experience this when stocking many clams too densely for a prolonged period of time. In clam farms those problems in closed systems are being solved with the help of sodium nitrate additions, but for the aquarist it seems much better to try avoiding competition. Following our experience with dense clam populations versus a mixed community of reef invertebrates with a reasonable number of clams among them, we feel that mixing clams with a wide variety of inverts (not only corals) and avoiding overly dense clam populations is probably the best thing one can do, and much more important than a specific tank setup or a specific skimmer.

Ten hints for setting up a successful giant clam tank 1) The aquarium design should allow the clams to be seen from above 2) Include a lot of live rock 3) Ensure good foam fractionation 4) Change the granular activated carbon regularly (use small amounts, change once per month) 5) Illuminate the tank with metal halide lamps between 5000 K and 10,000 K, with a photoperiod of 10 to 12 hours 6) Water current should be gentle to medium — avoid jet-like currents and strong turbulence 7) Include a wide variety of invertebrates, not only strongly calcifying inverts 8) Don’t hesitate to add some fish, and be sure to feed them. Wrasses will help to control pyramidellid snails or bristleworm pests 9) Install a lime reactor for calcium and carbonate additions 10) Perform a monthly water exchange of 10 percent and add a good trace element supplement once a week. Ensure a sufficient supply of iodine.

Also, we see to it that a giant clam tank is not short of fish. The fish metabolism helps to ensure that the clams can satisfy their requirements for organic nutrients. One of those fish even has an extra job in controlling mollusc and worm populations. This can help reduce the risk of pyramidellid snails spreading among the clams, and also keep bristle worms from multiplying too much. Sometimes it is almost impossible to avoid introducing pyramidellid snails into the tank with a newly added clam (the snails are sometimes present only as a small egg mass on the underside of the shells). Manual control is very difficult and also disturbs the clams, so we prefer to include a wrasse in each clam tank. The species depends a little on the tank size and also on the sizes and ages of the clams. The genus Pseudocheilinus is relatively small and more compatible with smaller clams, while the bigger members of the genus Halichoeres are more effective and better suited for bigger clams, as are species in the genus Coris.

Last but not least, we must supply the clam tank with a calcium reactor. This is sometimes not necessary if the evaporated water can permanently be replaced with tap water high in calcium and carbonates, but most of the time this is not the case, so one must consider the effective addition of calcium and carbonate to the system. The last thing we do to keep the clams healthy is the weekly addition of a good trace element solution and the monthly water exchange of 10 percent.

Reefer’s Questions

Q. Daniel,
I do not have any clams yet, but I saw one at the local fish store with a huge hole in it. The local fish store guy was trying to talk me into getting it. I questioned him about the huge hole and he said that it was just the siphon and they had to have this. I have seen other clams with two little “pointy places” with just pin-hole size holes in the tops of these. I thought those were the siphons.

Could you please clarify whether clams are supposed to have huge holes in them or not? If not, and they do have one or more holes, will these heal or will the clam probably die? What might cause huge holes in the mantle? Thanks for having patience with newbie questions, and thanks, in advance, for your reply.
Sue C., LeRoy, Illinois (USA)

DANIEL KNOP

A T. crocea in an aquarium that is deficient in the nitrogen compounds it needs to thrive. It was one of almost 70 T. crocea in the aquarium, most of which exhibited signs of bleaching.

A. Sue, clams are not supposed to have huge holes in them. They have three natural openings — two on top (incurrent syphon and excurrent syphon) and one at the bottom (byssal orifice).

The condition of the incurrent syphon tells you pretty much about the clam’s condition. If it has been stressed (e.g., during transportation), the incurrent syphon (which is actually just a slit-shaped opening near the end of the clam) will gap. Normally the clam should keep it pretty much closed. If it is opened wide, the clam is probably in bad condition and you should not buy it.

The same thing goes for the excurrent syphon, which is the one you call the “pointy place.” It should also have a relatively small opening on top. I have seen stressed clams with a bigger opening. I have also seen a big T. gigas with a degenerating excurrent syphon (heavy metal poisoning in the tank). In that clam there was no “pointy place,” just a hole. You also should not buy a clam with this condition.

Another thing that I saw once was an additional hole between the two normal openings. That happened due to transport stress and occured immediately after importation from Southeast Asia to Europe. The hole got bigger during the first two days after import, and even though I did not do any specific examination, I am pretty sure that it was caused by a bacterial infection. The clam (T. crocea) was put into my big tank under moderate light and it recovered within about 10 days. The lost tissue regrew and the hole closed. There was a scar visible, and the coloration was missing in the new tissue, but the clam survived. Had that clam been put in a dealer’s tank and stocked under conditions that were less than ideal, I would not expect it to have survived. I would strongly advise you not to buy a clam in that condition.

REFERENCES Fosså, S. and A. Nilsen. 1995. Korallenriff Aquarium, Volume 4. Birgit Schmettkamp Verlag, Bornheim, Germany. Pp. 447. Knop, D. 1996. Giant Clams. A Comprehensive Guide To The Identification And Care of Tridacnid Clams. Dähne Verlag, Germany. Pp. 255.