What is the story with Flame Scallops? They are beautiful and cheap at the local shops, and there have recently been some really gorgeous red flame scallops with bioluminescent "U"s inside the shell. I really like these guys, and I Want one in my tank, but I’m sure that you’ll tell me some good reason not to get one . . .
Let me start off by saying that “flame” or “flashing scallops” (as the bioluminescent ones are known) are all species in the genus Lima. Although these animals are called “scallops” they are not related to scallops by anything other than appearance, and are really file clams (a simple distinction is that while scallops swim forwards, file clams swim backwards). I’m sorry to agree with you right off the bat, but there are plenty of good reasons not to get one for your tank. Flame and flashing scallops have a very poor survival record in aquaria, and the typical experience of people who buy them is for the scallop to try its best at hiding, and slowly wasting away (over a couple to a maximum of about six months).
By my estimation, the single most common cause for the demise of Flame Scallops in any aquarium is simply starvation. Although I should also point out that these animals are very short-lived (something on the order of about three years), there are precious few reports of these animals surviving in captivity for more than about six months, and that is also a best-guess estimate of how long it should take a well-fed animal to starve to death in a sterile aquarium. So if you’re really set on trying to add one of these guys to your tank, you need to make a serious effort to feed it.
Let me start off by explaining a little about the biology of these clams. These animals are specialist filter feeders that require a lot of planktonic food because unlike the more popular and expensive giant clams (Tridacna), Flame Scallops lack any photosynthetic symbionts to help out with feeding the clam. In fact, Flame Scallops gain all their energy through filtering tiny particles (consisting entirely phytoplankton and tiny zooplankton) from the water around them. These tiny plankton are much smaller than even newly hatched brine shrimp (which the Flame Scallops seem incapable of swallowing even if they manage to filter them), and without heavy plankton supplements, no reef tank produces enough plankton of this size to support a decent-sized filter feeder like a Flame Scallop. The majority of food by weight in the gut of Flame Scallops collected from the wild is usually phytoplankton in the 5-40 micrometer (Φm) range (1/25 to 1/200th of a millimeter), but invertebrate larvae (up to a maximum size of just less than 200 Φm or roughly the size of an unhatched brine shrimp egg) appear to be preferred if the animals are offered a choice in feeding experiments, and these tiny zooplankton are quite likely to be equally important or perhaps even the primary source of energy intake for the clams in the wild.
In most filter-feeding species, the addition of phytoplankton to the aquarium along with the presence of zooplankton actually increases the rate of feeding on both for bivalve molluscs such as clams and oysters. This does not seem to be the case with Lima, however, which feed on both at roughly the same rate whether fed together or apart, but it is still almost always a good idea to feed both phytoplankton and tiny zooplankton (the primary zooplankton in the appropriate size range for a flame scallop are rotifers, and small invertebrate larvae) to the tank at the same time if you plan to keep any filter-feeding bivalves.
Many bivalve molluscs (such as oysters, mussels, clams and scallops) appear to be capable of selecting particles directly by their surface flavor prior to ingesting them, and flame scallops seem to be among them, rejecting the majority of food typically offered in an aquarium. However, the importance of flavor depends very strongly on the size of particle being eaten by the animal as well. In laboratory feeding experiments, researchers found that tasteless plastic beads of the approximate size of phytoplankton (5 - 40 Φm) were ingested at the same rate as the phytoplankton themselves, suggesting that the animals are incapable of using taste to decide which particles to ingest in this size range. However, larger plastic beads of the approximate size of invertebrate larvae (100-200 Φm) were uniformly rejected by the clams, showing that taste plays an important role in feeding for larger particle sizes. Given that it has traditionally been difficult to obtain food as small as even 200 Φm, it is not surprising that success rates with suspension-feeding animals like Flame Scallops has been so low.
In studies with oysters, the addition of phytoplankton to the experiment led to ingestion of some the larger plastic beads despite their lack of taste, and if the beads were flavored with phytoplankton juice prior to being fed to the oysters, they were also eaten at a significantly higher rate than unflavored beads. This does not seem to be the case with Lima, however, and the addition of phytoplankton to the medium did not affect the ingestion rate of the larger (100-200 Φm) plastic beads. In the case of these clams, at least, the animals actually seem capable of selecting particles directly by their surface flavor regardless of what other tasty scents are in the aquarium. This result probably explains why pea-flour and yeast based liquid invertebrate foods available in the hobby so often fail at supporting these animals in captivity, and we have actually done demonstrations for students in our Invertebrate Zoology classes of the animals rejecting several commonly available aquarium invertebrate foods based on pea-flour and yeast.
If bottled invertebrate foods are not the answer, what can you do to feed your Flame Scallop? Well, the primary prey in the feeding experiments where these animals were offered a choice of phytoplankton and zooplankton prey, were invertebrate larvae. In fact, on average, ~75% of larvae offered were consumed by a clam in any given feeding trial (it is virtually impossible to get a higher ingestion rate because the larvae become so dilute in the aquarium after that point that the clams can’t pump enough water to catch the last few). Naturally it is hard to provide a lot of tiny invertebrate larvae to your aquarium, although people running well-established deep sand beds are finding that there are actually quite a few larvae produced by the polychaetes (aka bristleworms) and other invertebrates living in the sand bed. This will certainly help the health of the Flame Scallop, but I think it is a mistake to rely solely on the in-tank production of plankton to try to support an active suspension feeder like a Flame Scallop. That means that you have to be prepared to supplement the tank with both phytoplankton and zooplankton of the appropriate size range if you intend to keep a Flame Scallop alive for any length of time in your aquarium. A good diet of mixed phytoplankton and enriched rotifers ought to be appropriate for keeping these animals, and the number of phytoplankton and zooplankton products becoming available to the hobbyist at home is steadily increasing.
If you are not feeding phytoplankton and/or zooplankton of the size of rotifers on a regular (at least every second day) basis, your clam is simply going to starve to death, and no matter how cool you think the animal is, you should not add one to your aquarium. If you are willing to start feeding phytoplankton (which has recently become readily available through a number of vendors – just check the ads in any recent issue of this magazine) and could be convinced to maybe start culturing rotifers to feed as well (I discuss the biology and culture of rotifers in another article) then it may be possible for you to keep one of these clams in an aquarium. There have recently also become available several dried powder foods that produce tiny particles in the size range of very small zooplankton or large phytoplankton, and these prepared foods may offer an easy and inexpensive alternative to feeding real phytoplankton and zooplankton to your Flame Scallop, but to be honest I have never tried them, and simply do not know whether the nutritional profile is sufficient even if the animals will readily accept these foods as surrogate invertebrate larvae... Only time and some experimentation will tell.
Another important consideration is that Flame Scallops are pretty reclusive by nature, and part of the reason I think so many people fail with them (aside from the diet issue above) is that they force the animals to stay where they are visible (or keep moving them back to the front where they are “supposed to be”). The animals prefer a deep crevice in which they can hide and gain some measure of protection from predators, against which they have very few natural defenses. Most bivalves escape predation by having tightly closing valves (the halves of the shell are each called a valve) and a special type of muscle that allows them to lock the valves closed. Like their more colorful and popular cousins the giant clams, Flame Scallops lack those adaptations, and are easily preyed upon by a number of wimpy sea stars that would not be able to eat a mussel or oyster of equivalent size, so predation is more of an issue for these clams than for most bivalves. Most sea stars are predatory, and with few exceptions I think that keeping sea stars out of a reef tank (especially one with clams or Flame Scallops) is a good idea. Certainly there are many people that report a sea star in their tank has eaten their scallop, but I suspect that rather than the star being the problem, it is more likely that the stars simply move in to finish off an easy meal when the clams stop resisting due to other problems. My guess is that the most common problems are those I just outlined above: stress caused by placing these animals out in the open for easy viewing and improper or inadequate feeding. Because they are easy prey, and prefer deep crevices into which they can retreat, they will continue to move away from an easily accessible and viewable spot (much to the chagrin of the aquarist), and that takes a lot of energy from the animals (they are not all that motile to start with). Especially if they are getting limited or inappropriate food in the first place, when the animals are placed into a stressful situation in which they continually have to expend an enormous amount of energy (such as continually moving back into a crevice to hide), it is not at all surprising that the animals frequently die in captivity... Finally, you should know that these animals are protandrous hermaphrodites, switching sex from male to female as they grow. This means that with very few exceptions, small individuals (~2.5 - 5 cm shell) will be predominantly male, and large individuals (~5 - 7.5 cm shell) will be predominantly female. Obviously this little tidbit of information is important to keep in mind if you have any intent to try to breed them, because you’ll need a range of sizes from small to large in order to ensure you have both males and females. It is worth keeping in mind anyways, however, because getting a male (small scallop) will mean that you’ll have a chance of keeping the animal alive for a few years. If you select a large animal, it is almost certainly a mature female, and may already be 2 years or more old. When the lifespan of the animal is about 3 years on average, that means that you have little chance of keeping your Flame Scallop alive for very long, even if aquarium conditions are ideal.
All of these factors need to be taken into consideration if you plan to add a “flame or flashing scallop” to your tank, and if you are not prepared to feed them properly and allow them to crawl back into the rock-work to a place that probably makes them difficult to view (but where they will feel comfortable and remain for the long-term), then you should certainly not consider purchasing one of these animals. However, if you are willing to accept those limitations, and provide for the needs of the animal, they can make a beautiful and interesting addition to a reef tank. With the gaining popularity of feeding reef aquaria phytoplankton and the advances in technology allowing the preparation of some excellent dried foods that provide particles of the appropriate size range, I have now heard a couple of reports of the animals living for a year or more in captivity – so all hope is not lost. With a little more effort and information about these animals and their needs, perhaps success with them will become more commonplace. Good luck!