The Reefer: Giant Clam Placement in the Aquarium

Author: James W. Fatherree, MSc

The giant clams (species of the genera Tridacna and Hippopus) should be provided with intense lighting, bathed in low to moderate currents, and maintained in water of excellent quality in order to thrive in an aquarium. And then there’s the task of placing them in a suitable spot, which is also a very important part of successfully keeping them long-term.

Some clams are best placed on hard surfaces, while some will be fine on sand, and they all need to be properly oriented with respect to the lighting. However, it seems that proper placement oftentimes isn’t given as much thought as it should be, so this month I’ll go over some information on the topic and offer a few quick tips to help out.


All of the Tridacna species can be found living on hard substrates at times (such as living corals, the skeletons of dead corals, limestone, or other kinds of rock), and other times they can also be found on coral rubble and even on sandy bottoms—with the exception of Tridacna crocea. T. crocea is never found on sand, and I don’t think you’ll ever find one living on rubble either, as they bore their way into hard substrates in their natural setting.

I’ll also add that while T. maxima reportedly lives on sand on occasion, I have yet to see one that wasn’t attached to a hard substrate after many dives in many places. So I’m under the impression that it isn’t common for maximas to live on sand, unless maybe that’s all that’s available in a particular area. In contrast, the Hippopus species are most often found on soft bottoms and sometimes on rubble/gravel, and they apparently are very unlikely to be found on hard substrates.

Byssal Threads

All of these usually attach to something when they’re small by secreting a number of tough strands that are called byssal threads. These threads are produced by a specialized organ (the byssal organ), and they’re sort of like spider’s silk, as they’re produced in a liquid form that hardens quickly and can stick to bits of gravel, rubble, or solid substrates. The threads are produced on the underside of a clam, and can thus be used by them to stay put. Later in life some species will let go of their byssal threads, though, once they’re big enough for their own weight to keep them in place.

Choosing a Substrate

With all of this in mind, I think it’s best to place any given species on the same sort of substrate that you’d find it living on in its natural habitat. T. crocea and T. maxima are best placed on solid substrates, where they will stay attached for life, while Hippopus hippopus and H. porcellanus should be placed on sand or gravel. Conversely, T. squamosa, T. derasa, and T. gigas can be placed on either hard or soft substrates. These are the species that typically live attached to the bottom when they’re small, and then let go later.

So I say put small ones on a hard substrate and let them attach, then you can move them onto sand/gravel if they choose to release their hold at a later time. I’m not suggesting you absolutely must to do this by any means, but I do recommend it, as I always try my best to keep things as close to “normal” as possible in an aquarium, whether you’re dealing with clams, or corals, or anything else.


Regardless of the species, there are several ways to place a clam on a hard substrate. The preferred method is to put them on an appropriately sized flat piece of live rock, base rock, or coral skeleton, or one that is slightly bowl-shaped, and let them make their attachment. The piece of substrate should be considerably bigger in diameter than the clam itself, and it should allow for some growth, too, of course. If you can’t come up with something like this, a shell from another clam might work well, and so will a clean piece of tile, or even a piece of slate if you don’t mind the dark color.

It may take anywhere from a few hours to a couple of weeks, but a healthy clam will usually attach itself with at least a few byssal threads, or maybe a lot, unless they’re large enough in size that they don’t need to. However, for some unknown reason(s) even smaller ones may not do so at times, as I’ve seen apparently healthy specimens of squamosa, derasa, gigas, and hippopus refrain from attaching to substrates on a handful of occasions, even when they were small in size. I can’t say the same for crocea and maxima, though, as they’ve always formed a strong byssal attachment when given the opportunity.

Also note that if you don’t like the way this looks, having a clam sitting on a rock, etc., you can always wait for it to attach to whatever you put it on, then bury the piece. It’s not a problem to bury a rock or whatever else they may attach to just under the surface by covering it over with a bit with sand or gravel, as this can easily occur in the wild, too. However, you shouldn’t overdo it in such a way that the bottom part of a clam is down in sand, as it can become irritated if any of the grains manage to get inside when it opens its shell.

Tumbles and Migration

Unfortunately, sometimes it isn’t as easy as I’ve made it sound, as tridacnids are occasionally knocked over by various tankmates, may move around too much and fall over or tumble down from the rockwork, or they may even take a spill on purpose. Yep, sometimes they’ll open and close their shells, causing them to shift in position, and they just might end up falling to a different spot. This can be a very bad thing, as a clam may drop into a gap between the rocks, never to be seen again unless you tear the tank down, or it may even fall into the tentacles of an anemone or coral. Likewise, there are times when a clam may flop around on the bottom and end up on its side, or facing the wrong direction, etc.

Moving Around

They can actually move around a good bit when they want to, albeit quite haphazardly, which is not uncommon by any means. While there is no known specific reason for this type of behavior, I think it’s most likely a reaction to not being happy with the lighting and/or current where they have been placed. If conditions are unsuitable, a clam may simply do its best to move somewhere else where it might be better off.

Turning on Its Side

Likewise, if they’re already sitting on the bottom and are being over-illuminated, but can’t go anywhere, then it would make sense for a clam to turn onto its side, as this would allow it to sit with the shell open while keeping its mantle tissue from receiving so much light. A tridacnid’s shell is built to stay open when they relax their muscles, and they have to contract their muscles and use energy to keep the shell closed. So, sitting upright and trying to keep the shell partially or completely closed for long periods would wear it out. I imagine the same could apply if currents are hitting the mantle too hard, as well.

Position Adjustment

If my assumptions are correct, then it would seem obvious that you shouldn’t put a moving clam right back where it was, at least not under the same conditions anyway. Instead, you should try out a new spot, lower in the tank (unless they’re already on the bottom). Or, you can try putting them back where they were, but check to be sure that the currents aren’t too strong near them, and use some shade cloth to dim the lighting a bit. Then, if that still doesn’t work, it’s relocation time, and it’ll be up to you to make a decision as to what will be best for your situation. I’ll warn you though, they can be quite contrary sometimes, and it can take a while to get some clams to settle down.

Conversely, if you are confident that the location you have chosen is okay and that a fall was purely accidental, there are things you can do to prevent this from happening, or to at least reduce the chances of it. You can place some other pieces of rock or coral around the bottom of a clam’s shell to help keep it in place, or you can use a ring made of PVC pipe if the clam is relatively small in size.

All you have to do is find some pipe that is a little larger in diameter than the clam is long, and cut off a piece that is tall enough so that it can be placed around the clam like a little corral. The clam can’t go anywhere then, and will usually attach itself in time. Do be careful that the pipe isn’t so tight or so tall that the clam can’t fully open its shell though, as you may kill it in the process of trying to save it. Once the clam attaches, or at least seems to calm down if it’s overactive upon introduction, the ring can be removed. With all that covered, now let’s run through some things that youdefinitelyshouldn’t do.

Clam Placement Don’ts

Don’t Put a Clam with Corals or Anemones

Tridacnids can be especially resistant to the stings of many corals and anemones, but there’s no need for any possible aggravations. So avoid placing a clam where the tentacles of these stinging critters can come into contact with them.

Don’t Put a Clams on a Steep Angle

Don’t put a clam on top of a rock at some sort of precarious angle, which might lead to it falling down into the rockwork if it opens and closes its shell/tries to move around. The surface of a clam’s mantle should be close to perpendicular to the lighting anyway, and this is their typical orientation in the wild.

Don’t Put q Clams in a Tight Spots

Don’t put a clam in a tight crevice between rocks and such either, as it may restrict their ability to open fully and may also lead to it falling down into the rockwork should it decide to move into a better position.

Don’t Put a Clam in a Restrictive Space

Likewise, don’t put a clam down in a hole in a rock that might restrict its ability to open fully. And if you do put one in a large enough hole, don’t let a lot of detritus settle into the hole around the clam. You should blow out any such buildup of crud by using a turkey baster, powerhead, etc.

Don’t Put Croceas or Maximas on Sand or Gravel

Again, I also wouldn’t put a crocea or maxima directly on sand or gravel, unless it’s only temporary. No, they won’t drop dead if you put them on sand/gravel, but it’s totally unnatural for a crocea, and as I said above, it’s not what you’ll find too many maximas on either.

Don’t Grab an Attached Clam

If a clam has attached to a piece of rock, shell, etc., don’t try to move them by grabbing the clam. Doing so could injure it, so you should always pick up the clam and the piece together and be very careful when handling them. Likewise, you must never try to pull a clam off of anything to which it’s attached, as you can rip the byssal organ out in the process, which will kill it. If for some reason a clam must be separated from something it is attached to, you must very, very carefully cut its byssal threads with a knife or razor as close to the point of attachment as possible, without cutting any of the clam’s exposed tissue.

Don’t Move a Clam Repeatedly

And, last but not least, don’t move a clam repeatedly over a short period of time unless you absolutely have to. Clams have enough trouble adapting to changes in lighting and current when they’re placed in an aquarium, and quickly moving them from one spot to another, to another, to another may be too stressful for them. Moving them around too much can lead to reduced growth rates, a greater susceptibility to disease, and it may even outright kill them. So, if you must move a clam around or up closer to the lights in steps, give it plenty of time between each move, as in a week or two at the least. Patience can pay off.