INVERTEBRATE NON-COLUMN by ROB TOONEN

Flame scallops

Each month, I see a number of posts on the various reef message boards asking about what to do for a flame or flashing scallop that is in decline in someone’s reef aquarium.  Although these animals are beautiful, relatively cheap, and easily available in the pet trade, I find that there is precious little information about them available.  Therefore, I want to spend some time discussing the biology of these animals to help people understand why they typically do so poorly in captivity. 

In terms of their lifestyle, flame scallops are specialist filter feeders that quite simply require a lot of planktonic food of the correct particle size.  Unlike the more popular and expensive giant clams (members of the genus Tridacna), flame and flashing scallops lack any photosynthetic symbionts to help out with providing nutrition to the animal. The brilliant red color of the tissue and long tentacles of these animals is due instead to an unusually high concentration of caroteinods in their tissues (Lin and Pompa 1977) .  Without the aid of photosynthetic symbionts from which to draw some nutritional support, flame scallops must rely solely on their ability to filter tiny particles from the water passing over them in order to support themselves.  However, these animals do not simply filter any passing particles.  They must also be of the correct size and flavor for the flame scallops to collect and ingest the particles.

Numerous studies have now shown that growth and reproduction in tropical bivalves are linked to phytoplankton production (e.g., (Barber and Blake 1991; Giese and Kanatani 1987; Sastry 1979) , and flame scallops are no exception to that pattern – animals in the wild show the highest rate of both shell growth and reproductive output during the season of peak phytoplankton abundance (Lodeiros and Himmelman 1999) .  The finding that maximal growth and reproductive output for Lima clams in the wild are strongly linked to phytoplanktonic food availability suggests that the primary source of food for these animals is phytoplankton, and as we would predict from this finding, the majority of food by weight in the gut of flame scallops I collected from the wild appeared to be phytoplankton in the range of 5-40 micrometers (mm – roughly 1/25 to 1/200th of a millimeter).   

However, just because these animals eat primarily phytoplankton in the wild does not mean that they cannot ingest other prey if it is available.  For example, (Sprung 2001) reports that in captivity flame scallops capture a wide range of foods including dissolved and particulate organics, bacteria, and phytoplankton.  Likewise, although oysters collected from the wild have roughly the same sized particles in their guts as flame scallops, feeding trials with captive oysters showed that invertebrate larvae (from roughly 50 Fm up to a maximum size of just less than 200 mm – about 1/5th of a millimeter) were the food item most often preferentially ingested by the oysters if they are offered a choice in feeding experiments (Tamburri and Zimmer-Faust 1996) .  In fact, more that 75% of the larvae tested in these experiments were captured and digested by oysters (and the ones that were rejected are known to be chemically defended from ingestion by fishes as well).  Given these results, suspension feeding bivalves are obviously capable of capturing particles other than phytoplankton if it is available to them, and such alternate foods may be just as important as phytoplankton to the overall health of these animals.  Tamburri and Zimmer-Faust (1996) argue that even though invertebrate larvae are captured far less frequently than are phytoplankton, the nutritional value of these larvae are greater and these tiny zooplankton are likely to be a very important source of energy intake for bivalves in nature.  Unfortunately, without heavy plankton supplements, no reef tank produces enough plankton of the appropriate size to support a decent-sized filter feeder such as a tropical bivalve mollusc (e.g., (Lodeiros and Himmelman 1994; Vélez and Epifanio 1981) . 

Furthermore, the gills of bivalve molluscs serve a dual purpose: they are used for both respiration (gas-exchange, or breathing), and filtration (feeding).  Although they have some limited ability to actively reject particles that they do not want, the animals continuously filter particles within a specific size range regardless of whether or not they intend to consume them (e.g., (Beninger and Veniot 1999; Hawkins et al. 1998; Shimeta and Koehl 1997; Tamburri and Zimmer-Faust 1996; Ward et al. 1998) .  In a survey of a number of suspension feeding taxa, (Hawkins et al. 1998) found that even after filtering particles from the water column, on average more than 70% of the particles captured by suspension feeding marine invertebrates are never ingested!  The collected particles which are not considered “tasty” or have too little nutritional value (due to a high inorganic content, for example) are typically balled up in mucus and ejected as “pseudofeces” (a fancy term meaning quite simply “fake poop”) without the animal ever eating them.  That is why I make such a big deal here and elsewhere about the difference between animals capturing something added to the aquarium and feeding on it.  Just because you see a suspension-feeding animal capturing particles (which are so tiny that it’s often hard to be really sure), does not mean that they are gaining any nutritional value from that product.

I like to use this behavior as a demonstration for the invertebrate zoology class at UC Davis, where students are asked to observe the feeding response of the local blue mussel (Mytilus) to a pea-flour and yeast-based invertebrate food from the local pet shop.  This is a great demonstration because the students always think that the mussels are feeding, until we point out with the aid of a microscope that the bivalves are actually filtering the particles from the water, collecting them into balls of mucus and then spitting out pseudofeces pellets of the “food” they have collecting without ever ingesting any of it.  The fact that these mussels collect and spit out these old-style commercial invertebrate foods without eating any of it suggests that they are not a particularly good choice to feed these kinds of animals.  In fact, I have observed the same behaviors and ejection of pseudofeces from flame scallops in aquaria that have been fed with the same pea-flour and yeast-based invertebrate food.  I have previously discussed the relative pros and cons of various planktonic foods for the reef aquarium (Toonen et al. 2002) , and rather than covering the same subject again here, I will refer interested readers to the Marine Fish and Reefs USA 2002 Annual article.  However, from the data presented in that article, the size range of particles provided by pea-flour and yeast-based invertebrate foods was roughly 1/1000^th to nearly 2 millimeters in diameter, with the vast majority of the particles falling well above the 40 mm size range that comprise the vast majority of the gut-contents of wild-collected animals.  Given that particle size range, and the fact that the animals do not seem to like the taste of the pea-flour particles, it is not particularly surprising that these traditional types of prepared invertebrate foods have not proven particularly successful over the past few decades for maintaining obligate suspension-feeding animals in reef aquaria.  Fortunately, there is a flurry of new phytoplankton and zooplankton food products coming onto the market all the time now.  When we take into consideration that it has traditionally been difficult to obtain food as small as even 200 mm (let alone less than 40 mm), it is not at all surprising that success rates with suspension-feeding animals like flame scallops has been so low in the past.

The reason that I keep mentioning the particle size ranges is that, in the simplest terms, the smaller the particles, the more likely they are to be captured by these animals.  For example, (Tamburri and Zimmer-Faust 1996) have shown that ingestion of unflavored plastic beads of the same particle size as phytoplankton (< 40 mm) are eaten at the same rate as are real phytoplankton.  However, it is not simply the correct size range of these particles that is attractive to the animals: the scent of phytoplankton also appears to be a feeding stimulant for most bivalves tested to date.  In fact, even when offered the invertebrate larvae on which the oysters were shown to prefer, the addition of phytoplankton to the aquarium along with the presence of zooplankton actually increases the rate of feeding on both types of prey.  The addition of phytoplankton to the experimental feeding trials even led to some the larger plastic beads (100 - 200 mm, roughly the size of invertebrate larvae) being eaten despite their lack of taste.  If the larger unflavored plastic beads were soaked in phytoplankton juice prior to being used in the feeding trials with the oysters, they were also eaten at a significantly higher rate than unflavored beads not soaked in phytoplankton before feeding (Tamburri and Zimmer-Faust 1996) .  On the other hand, in the absence of phytoplankton, these larger unflavored beads were uniformly rejected.  This result is of interest because it suggests that regardless of whether or not the food being offered “tastes good” to oysters, they will ingest a wider array of particles (within a certain size range, at least) provided that there is the scent of phytoplankton in the water around them.  That is good news for aquarists, because it means that it is easier to feed the animals by providing phytoplankton when attempting to feed a bivalve.

Unfortunately, the same may not necessarily be the case with Lima, because I once saw a presentation at a meeting in which a student reported that the addition of phytoplankton to aquaria did not affect the ingestion rate of the larger (100-200 mm) plastic beads for flame scallops (however the study has never been published, and I am not sure how carefully the project was done).  If this result holds up to further study, then unlike the oyster example mentioned above, flame scallops are more picky about what they eat, and may be capable of selecting particles directly on the basis of their surface flavor, regardless of what other tasty scents are in the aquarium.  Even if flame scallops still reject tasteless or distasteful larger particles in the presence of phytoplankton, the fact that phytoplankton tends to increase the ingestion rate of both small (< 40 mm) and large particles (100 – 200 mm) is worth keeping in mind.  When feeding animals in a closed system in which the added food is expensive, and not in super-abundance, any chance that your animal will get more food from the same amount of product when both phytoplankton and zooplankton are added to an aquarium together is worth trying.  In addition to there being “more bang for your buck,” the other reason that I suggest that both food types be provided simultaneously is that doing so covers a wider particle size range than feeding either a phytoplankton (should be less than 40 mm particles) or zooplankton (greater than 50 mm particles) replacement product alone.

So having explained that, if the traditional types of bottled invertebrate foods from the local pet shop are not the answer, what can you do to feed your flame scallop?  Well, as I mentioned above, when given a choice of phytoplankton and zooplankton prey, the preferred item that bivalves liked to eat most were invertebrate larvae.  In fact, on average, ~75% of larvae offered were consumed by a clam or oyster 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 on a regular basis, although people running well-established deep sand beds (see (Toonen 1998-99; Toonen 2000a) or Ron Shimek’s articles, linked from his web page here, for more details on 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 (e.g., (Shimek 1997) .  This will certainly help the health of the flame scallop, but it is a fatal mistake to rely solely on the in-tank production of plankton to try to support an active suspension feeder like a flame scallop – even the most productive of reef tanks has insufficient concentrations of such planktonic food for a large-bodied, and efficient filter feeder such as 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 reasonable length of time in your aquarium.  A good diet of mixed phytoplankton and enriched rotifers (Joyce (Wilkerson 2001) has a great discussion of the biology and culture of rotifers in her book if you are not sure how to go about this) is likely the best option for keeping these animals well-fed in captivity.  However, the number of phytoplankton and zooplankton products becoming available to the hobbyist at home is steadily increasing, and many of these “new generation” planktonic foods are likely to provide a suitable diet for these animals in the aquarium.  Again, I will refer interested readers to my review of planktonic foods (Toonen et al. 2001) or my phytoplankton feeding talk for #reefs (Toonen 2000b) for more information on the relative benefits and drawbacks of each of the various types of planktonic food products currently available on the market.  Given the proliferation and availability of such plankton foods in the hobby today, it is easier than ever before to provide a reasonable concentration of tiny plankton particles of the appropriate size and flavor for obligate suspension feeders such as flame scallops.  If you are feeding phytoplankton and could be convinced to either start culturing rotifers to feed as well or adding a combination of planktonic food products that provide the appropriate size range for obligate suspension-feeders, then it may be possible for you to maintain a flame in your aquarium.  If, however, you are not feeding mixed phytoplankton and zooplankton of the appropriate size on a regular (and by this I really mean at least daily) basis, adding a flame scallop to your aquarium is quite simply a death sentence for the animal, because it is going to starve to death.  If that is the case, then no matter how cool you think the animal is, you should not add one to your aquarium. 

Assuming that you are willing to make the effort to feed a flame scallop properly, there is another important consideration that I feel is almost never considered when introducing one of these animals into your aquarium.  Flame scallops are pretty reclusive by nature, and 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 shells and a strong muscle that allows them to lock the valves closed.  Unfortunately, flame scallops and flashing scallops are not really able to close their shell tightly, and are therefore easily preyed upon by a number of relatively wimpy sea stars that would not be able to eat a mussel or oyster of equivalent size.  Because of that, predation is more of an issue for these clams than for most bivalves of a similar size.  Most sea stars are predatory, and with relatively few exceptions (such as the Linckia sea stars (Toonen 2002) I covered in the May issue of AAOM), I think that keeping sea stars out of a reef tank (especially one with clams or flame scallops) is always a good idea.  Instead of closing tightly to avoid being eaten, flame scallops rely on their ability to swim away from a sea star, and to find a deep tight crevice in which to minimize their exposure to potential predators.  Part of the reason that I think so many people fail to maintain these animals in captivity is that they force the animals to stay where they are visible (or keep moving them back to the front of the tank where they are “supposed to be” for easy viewing).  Because they 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 for an animal that is getting limited or inappropriate food in the first place, this additional cost of always trying to move back into the rockwork, coupled with the animals being placed into a stressful situation in being forced to remain in what the animal perceives as an unsafe environment, it is not at all surprising that the animals frequently die in captivity...

Finally, before I end here, I want to discuss a little bit about the reproductive biology of these animals.  Lima are protandrous hermaphrodites, switching sex from male to female as they grow to larger sizes (e.g., (Gómez et al. 1995; Gómez et al. 1990; Lodeiros and Himmelman 1999) .  This means that with very few exceptions, small individuals (roughly 2.5 - 5 cm across the shell) collected from the wild are predominantly male, and large individuals (those greater than 5 cm across the shell) are predominantly female (Lodeiros and Himmelman 1999) .  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.  Furthermore, these animals tend to occur in aggregations of mixed sizes in the wild, and these mixed size groups appear to increase reproductive success in the wild (Lodeiros and Himmelman 1999) .  Even if you have no intention of attempting to breed these animals, however, it is worth keeping this information in mind anyway, because getting a male (small scallop) will mean that you’ll have a much better chance of keeping the animal alive for a few years.  Remember that the natural lifespan of these animals appears to be on the order of three years or so, and that the growth rate even under ideal conditions is slow enough that a shell width of 5 cm or so is likely to be approximately two years old (Lodeiros and Himmelman 1999) .  Given an average lifespan of about 3 years, even with ideal aquarium conditions an animal that is something on the order of 2+ years of age at the time that you purchase it is unlikely to last very long!  Thus, if you select the largest flame scallop available at the local retailer, it is almost certainly a mature female, and may already be old enough that its days are numbered no matter what conditions you provide in your aquarium.