Aquarium Invertebrates: A Look at Giant Clam Spawning in Aquaria

by | Aug 15, 2009 | 0 comments

From time to time giant (tridacnid) clams will spawn in aquaria, ejecting clouds of sperm, and sometimes eggs, too. Usually this is a sign of excellent health, but it can also be a reaction to heavy stress. Unfortunately, such events won’t lead to a tank full of baby clams, and can lead to real trouble in some cases, too. So, I thought I’d explain a bit about how tridacnids reproduce, why they spawn, and why it can be a problem. I’ll also give you some advice on what to do if it happens in your tank.


Tridacna crocea

Reproductive Biology

Tridacnids, like other clams, are broadcast spawners. This means that when it’s time to reproduce, sexually mature clams will eject clouds of sperm and eggs into the water where they can mix with those of other nearby clams. I say sperm and eggs because fully mature tridacnids are both male and female at the same time, which means they are simultaneous hermaphrodites. Thus, they can make both and spawn both in the same event. They have to be fully mature to spawn both though, as tridacnids develop testes first with quite some time passing before they develop ovaries. So, sub-mature clams may spawn, but release only sperm.

Typically it’s a relatively energetic act, as the shell and body contract vigorously making the sex cells spew quite far out of the smaller opening in a clam’s fleshy mantle called the exhalent siphon. But, at other times these cells may just slowly sort of ooze out of the siphon and allow currents to do the rest (Alcazar 1988). Even a relatively small species, like Tridacna crocea, may release several million gametes (sex cells) at one time, but a big Tridacna gigas can cough out about half a billion! However, because their sperm have the ability to fertilize their own eggs, successful spawning requires a bit of timing, as self-fertilization is not a good thing.

Usually all of the sperm are released first, over a period of a several minutes, and requires numerous contractions of the body. There are typically only a few sperm coming out at first, but as a spawning event progresses, more and more will be ejected with each contraction, and then the amount will decrease with each contraction as the clam becomes spent. Then, as the sperm runs out, or maybe even quite a few minutes later, the eggs are released in the same manner. Just a few at first, with the numbers then increasing and later decreasing until the clam is spent again (although, again, many times no eggs are released at all). This works a lot better than releasing all of the gametes at the same time, as the sperm can be dispersed and moved away from the parent by currents before the eggs are released.


Here you can see a cloud of sperm being ejected from the exhalent siphon of my own Tridacna derasa. I just happened to be at home and watching the tank when spawning initiated.

When spawning occurs, communicative substances are ejected with the gametes (spawn-inducing pheromones), and these can elicit spawning in other nearby clams, too. So, any tridacnids that are mature enough to produce gametes will start the same routine, releasing their millions of sperm and eggs into the mix with everyone else’s. Thus, there’s a much higher probability that there will be some genetic mixing within a population of neighboring clams. This may occur year-round near the equator, but it happens more often in the warmest times of the year in higher latitudes. There are numerous recorded exceptions though, as changes in temperature, salinity, lighting, and the tides may all play some part in the timing (Fitt & Trench 1981 and Trinidad-Roa 1988).

Regardless, when fertilization occurs, a new clam is only around 100µm in diameter, and within 12 hours or so the rapidly dividing mass of cells develops into what’s called a trochophore larva (Ellis 1998). This is a free-swimming stage, when the miniscule larva can be carried along by currents to new localities. At this time there are rows of tiny hair-like cilia on the outside of the larva, which allows a larva to swim under its own power to some degree, as well.

After another 12 to 36 hours or so, a trochophore larva will pass into the veliger larval stage (Ellis 1998). A velum is a specialized organ that develops at this time, which is also covered by cilia and is used for locomotion, but also for feeding. So, it’s during this phase that a larva can start to filter particles from the water. In fact, within the first 2 or 3 days, a clam typically has a well-developed velum, stomach, and intestines, and the shell is already forming, as well. It’s fascinating to me that they can go from an egg to all that in a matter of a couple of days.

Timing can start to vary a great deal at this phase and through the rest of development though, as the developmental schedule varies from species to species, and on individual genetics, the condition of the gametes at spawning, and environmental conditions, too. After somewhere between 3 and 10 days, a veliger also develops a foot and is called a pediveliger (“ped” means foot) for some time (Ellis 1998). This foot is a muscular structure that can reach out and help the larva to move around on the bottom, and may also be used to collect food particles for a short time, as well (Reid & King 1988). It looks and acts almost like a sticky little tongue, and once formed, the larva will start to alternate between swimming around and crawling around on the bottom. The digestive system will continue to develop, as well, and so will the shell, while the larva moves about trying to find a suitable place to live. Then, once a spot in chosen, it’ll settle down for the next step.

When the timing is right, which can be anywhere from 8 to 29 days after fertilization, the pediveliger will stop moving around and will begin to change into a juvenile clam (Ellis 1998). This change is appropriately called metamorphosis, and involves the atrophy and sloughing away of the velum and the loss of the cilia that were used for locomotion, too. After this, there is no more swimming about, and a post-metamorphic clam, for the most part, looks just like a tiny version of an adult, still being in the neighborhood of only 200µm (1/5 of 1mm) in size.

The gametes don’t carry any zooxanthellae from their parents, so a clam must capture its own by filtering them from the water. This can start at the veliger stage at times, even before the specialized system of tubes that hold them in the fleshy mantle has started to develop, and when zooxanthellae are collected so early on, they’re simply held in the stomach and stay there until after metamorphosis.

As a post-metamorphic juvenile develops, and the tubular holding system has begun to form, the zooxanthellae can typically be seen moving into the growing mantle within 2 to 7 days (Fitt & Trench 1981). Then they start to reproduce rapidly, while even more are being collected via filter-feeding, and they spread throughout the mantle in just 1 to 3 weeks. From this point on, a juvenile functions essentially the same as a small adult.

The clams that make it through metamorphosis to become juveniles typically grow at relatively slow rates at first. In fact, it may take them several months to make it to just a few millimeters in length. However, at some point their growth rate usually increases and continues at a faster pace until they approach sexual maturity and become adults themselves. Growth during this period, between the early parts of the juvenile phase and the mature phase of life, is also usually fairly linear, although things like changing environmental conditions, predation, disease, etc. can slow things down, of course.

As mentioned above, they don’t become completely mature all at once, though. Depending on the species, an individual’s genes, and the environmental conditions, a clam may become a fully-functioning male that can eject millions of sperm in as little as a couple of years, but may not become a functioning female for many more years after that (Lucas 1988). For example, Braley (1998) reported that it can take Tridacna gigas 4 to 5 years to become male-mature, and then another 5 to 6 years to become female-mature.

Many clams can become fully mature faster than this though, as Nash et al. (1988) reported that Tridacna gigas can become male-mature at 25 to 35cm, which is likely a year or so faster than Braley’s observation of 4 to 5 years. And, Tridacna derasa can become female-mature by that size according to Adams et al. (1988), which is also in the 5-year range. Likewise, Heslinga (1993) said that Hippopus porcellanus could become a female-mature in four years, and Benzie (1993) said Hippopus hippopus could do it in only three. That’s quite a difference in schedules.


My artist’s rendering of a tridacnid pediveliger larvae. Not much to see other than the valves, the velum (blue), and the foot (green) sticking out. (pediveliger)

Spawning in Aquaria

So, they do spawn in aquaria from time to time, but this can lead to serious trouble in the closed confines on a tank if you don’t take immediate action. In fact, if the gametes aren’t cleaned up quickly, there’s a chance that a spawning event can lead to the death of everything in the tank! So, let’s look at some of the things that might cause a clam to spawn (so that you can avoid them), and then what to do if it happens anyway.

First of all, spawning is a natural occurrence, thus healthy and mature tridacnids may spawn in aquariums as a normal part of life. However, they may also spawn if they’re mature and are subjected to too much stress. When something goes wrong a clam may respond by ejecting any gametes it’s holding, as this may ensure the survival of some of its potential offspring if it should be killed. Remember that tridacnids also give off spawn-inducing pheromones, too. So, if something entices a clam to spawn early, the rest of the clams in the area will likely do the same. In the wild, the gametes and fertilized eggs may be carried away by currents to other areas that have more favorable conditions, and the population’s genes would stand a much better chance of survival if things went really bad for the parents.

Natural spawning will occur when a clam becomes ripe with large stores of gametes, but “survival spawning” as I call it, can occur at any time, whether a clam has only a few gametes or a full complement. It also doesn’t matter if a clam is only at the male-phase of sexual maturity and can release only sperm, rather than both sperm and eggs, either. It’s simply a last-ditch effort to save their genes, so whatever is there will be ejected, whether a clam is really ready to spawn or not.


I managed to make a short movie using a digital camera. Here’s part of it, so you can see the strong contractions that typically occur during spawning. Note: everything else looks so still because I momentarily turned off the pumps for better viewing. There were only two clams in this 125 gallon tank, and only the derasa spawned, so didn’t bother to remove it until after it released several spurts of sperm. Click the image to watch the movie.

The cues for something like this to occur can be wide-ranging, as essentially anything that over-stresses a clam can bring it on. A rapid change in temperature, up or down, can certainly do it. So can generally poor water quality, rapid changes in salinity, rapid over-illumination, predation or pestering, disease, physical damage, or anything else that’s potentially life threatening.

In aquariums in particular, temperature change is likely the most common reason for survival spawning, but it can also happen after doing a really big water change rather than regular smaller changes. This can shock a clam due to some rapid fluctuation in water quality and possibly salinity, and adding too much freshwater at once to replace evaporated water can have the same effect, particularly in relatively small tanks (because it rapidly decreases salinity). Delbeek and Sprung (1994) also reported that adding a lot of new carbon can do it, as can waiting too long to replace old bulbs, both of which can lead to a rapid increase in light.

Of course, if something affects several clams in one tank simultaneously, then they may all spawn as a reaction to the environmental stress. However, if a single clam is suffering from an injury, disease, pestering, etc. it may undergo survival spawning, which can chemically trigger spawning in other clams, even if they aren’t stressed. So, if you have more that one clam, having all of them spawn simultaneously doesn’t necessarily mean that they are all stressed.

Now you might be thinking that survival spawning is a bad thing, but that a natural spawning in your aquarium by a healthy clam, or several, would be really neat. I remember bragging (to myself) about how great my own reef aquarium was and how healthy my clams were the first time this happened in one of my tanks, as it can certainly make you feel successful. However, I can assure you neither type of spawning is a good thing when it comes to the effects that they can have on a tank’s water quality. As I said above, spawning can end up killing everything.

No, the gametes aren’t toxic. In fact, they can be a very nutritious food source for many other things in an aquarium. Instead, the problem is that they’ll all die in an aquarium and basically rot. They’ll do it quickly, at that.

The gametes will start to die in about an hour, if not sooner, and because they all die and begin to decay relatively simultaneously, they can potentially cause oxygen levels in a tank to drop so low that all the fishes and other tank inhabitants may drop dead within a couple of hours. If eggs are released along with sperm and some of them are fertilized, they’ll die too, due to the over abundance of sperm. In the wild, the gametes are immediately diluted into millions of gallons of seawater, but this obviously isn’t going to happen in an aquarium. The problem is that when too many sperm try to fertilize a single egg, it kills that egg (it’s called polyspermy). The precipitous drop in oxygen levels, when the gametes decay can also cause a spike in ammonia and nutrient concentrations, as well. That’s bad too, as too much ammonia can also kill everything in an aquarium, and increased nutrient levels invariably lead to outbreaks of unwanted algae. Obviously, there’s nothing good about
any of that.

Of course, if something like a single 8cm male-mature Tridacna crocea spawns in a relatively large tank, it may not be a big deal. But, if larger clams spawn, or if a tank is small in size, you must take action immediately. The first thing to do is remove a spawning clam if at all possible. The clam can be temporarily placed into another container with some tank water, where it can release all of its gametes without fouling up the aquarium. If you have more than one clam, this may also prevent any others from joining in if you catch the first one in time.

Keep in mind that a clam may need to stay in the container for quite some time before it is completely spent, so you may need to pour out the water in the container and refill it a couple of times, and you should also add a powerhead pump or an air pump/airstone to keep oxygen levels up if possible. While this is going on, you should also make up a new batch of seawater to replace whatever you remove from the aquarium, too.

If the spawning clam(s) can’t be removed, if possible, you can use a length of hose to create a siphon and suck up the gametes as they are being discharged. You can hold one end of the hose near the exhalent siphon of the clam, where the gametes are ejected, and run the other end to a bucket. However, instead of draining the whole tank while waiting for a clam to become spent, you’ll need to hold your finger over one end of the hose to stop the flow of water, and only remove it and let the water flow when a pulse of gametes is released.

Likewise, you may be able to use a hose attached to a mechanical filter to suck the gametes up without removing water form the tank. I don’t usually use cartridge-type filters on reef aquariums, but if you have one handy you can affix a hose to it and let the filter element collect the gametes. Of course, if you don’t have a hose, you can just let such a filter run as it normally would, on the side of the tank or underneath it, too. But, it will take much longer to clear the water if the gametes aren’t slurped up as quickly as they are released. Also note that the filter media should be cleaned out thoroughly afterwards, as the gametes will break down in it just as fast as they will in the tank.

Another method, described by Knop (1998), would be to use a pump and hose of some sort to collect the gametes and then direct the water through some form of perforated container filled with cotton or nylon wool. According to Knop, the gametes will stick to these materials due to a static charge, but I’ve never tried this myself.

Aside from these means of filtering out gametes, you should also do a substantial water change. I would say 25% for sure, and then see how things are looking. This will help to remove some of the remaining gametes, and can also reduce the concentration of ammonia if it has spiked. You should also test the water for ammonia afterwards if you are worried, and change even more water if you feel it is necessary.

Aeration in the tank should be increased in any way possible to keep oxygen levels up, from adding some airstones to positioning any powerheads that you have in such a way that they really make the water’s surface roil. Many powerheads have little hose attachments that come with them, which can be used to blow large numbers of bubbles into the water, too.

Once everything is under control, it’s also important to stop and think about what might have brought on the spawning event in the first place. If you feel that is was a natural, healthy event, then don’t worry. However, if you feel that a clam(s) spawned due to stress, you need to try to determine the source of the stress and take action to correct the situation. Fortunately, if you can get things cleaned up, can figure out the reason a survival spawning event occurred, and then correct the situation, the clams that spawned will still have a good chance of recovering and everything else will be fine.

No Baby Clams

Lastly, I want to point out why you won’t end up with a tank full of baby clams after a spawning event. Even under the best conditions, only a small percent of the eggs ejected in a spawning event will actually get fertilized, and of those that do, maybe 5% will make it through metamorphosis, or even far fewer than that. For example, Fitt & Trench (1981) reported that more than 95% of all the veligers in their study that had made it at least 3 days did not make it on through metamorphosis, regardless of the experimental conditions. This is due to microbial attacks for the most part, as the use of various antibiotics has been shown to greatly increase survival (Ellis 1998).

So, mortality can obviously be staggering when they’re young, as thousands upon thousands succumb to microbial attacks and other problems. All those sperm, eggs (fertilized or not), veligers, etc. are also subject to being eaten by various filter-feeders and tiny predatory organisms, and then as they grow they may be eaten by crabs, worms, fishes, and myriad other critters, as well. Any sort of mechanical filtration would also strip them out of the water, as can a skimmer, and it’s questionable that they could survive a trip through a powerhead or other pump. Polyspermy will ensure that few, if any, eggs would survive long in an aquarium, anyway. So, don’t get too excited…


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  2. Alcazar, S.N. 1988. Spawning and early larval rearing of tridacnid clams in the Philippines. In: Copeland, J.W. and J.S. Lucas (eds.) Giant Clams in Asia and the Pacific. ACIAR Monograph Number 9, Canberra. 274pp.
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