Identifying The Tridacid Clams

[Steel Toe]

http://reefkeeping.com/issues/2002-07/jf/feature/index.htm

Quite a few years ago when I first began the transition from being a fish-only marine aquarist to a reef aquarist, I became particularly fascinated with the tridacnid clams. Aside from their obvious beauty, a large part of my interest was due to the fact that I found it simply amazing that such a wide array of color schemes and patterns could be exhibited by a mere handful of species. While making the clams aesthetically intriguing, this variety of appearances also led to the early realization that it was rather pointless to simply glance at the decorated, fleshy extensions of many of the clams' bodies in order to attempt to identify these animals to species. In fact, as I would learn, it is the shell of each specimen which is typically the key to identification, not just the flesh. However, with the help of a knowledgeable friend, Nancy Stone, and a handful of clam shells, it was easy work to learn the key features to look for, and to make accurate identifications.

Like all other clams, these fascinating animals are placed in the Phylum Mollusca, along with the snails and cephalopods, and a number of other related organisms. Within this phylum, the tridacnids are placed within the Class Bivalvia (two valves or halves), which also includes the oysters, scallops, and cockles, etc. The vast majority of these are filter-feeders, which use specialized dual-purpose gills to capture tiny food particles from seawater that is circulated through the interior of their bodies, and to carry out gas exchange. However, the tridacnids also acquire nutrition through the harboring of internal algal symbionts. Just as the reef-building corals do, the tridacnids maintain populations of single-celled zooxanthellae in parts of their bodies that can produce photosynthesis-derived "food" for a clam host when provided with sufficient sunlight.

All clams have a specialized tissue structure called the mantle which forms a thin, taco-shaped flap that envelops the whole of the clam's body, and is also responsible for the precipitation of calcium carbonate to form the clam's shell. It also typically forms or houses sensory apparatuses like tentacles and light-sensitive eye-spots, and the openings through which seawater enters and leaves the body chamber inside the shell. Yet, unlike almost all other clams, the tridacnids have greatly oversized mantles, and this is where the clams' complement of zooxanthellae are maintained. Thus, this enlarged mantle tissue is typically extended well outside the edges of the shell to act as something of a solar collector, increasing the mantle's surface area exposed to sunlight and therefore enhancing photosynthesis. It is these extensible mantle flaps which make the clams so attractive, often covered with exotic patterns of dots, circles, stripes, and waves in a broad spectrum of colors.

As mentioned, the problem in identification of clams is that these patterns/colors of the mantles may vary greatly from one individual to the next, even if they are the same species and are found in close proximity to one another. A further complication is the fact that any one species may have numerous combinations of colors and patterns; some of those may look very much like one or more of the similar combinations of a different species. Admittedly, with experience, some hobbyists can indeed identify some of the more common tridacnids simply by the appearance of the mantle, but such identification can actually be quite tenuous at times when not armed with the knowledge of other diagnostic features.

For these reasons, it is easily understandable that hobbyists typically have difficulty identifying the tridacnid clams at the species level. There are relatively few literary references available to hobbyists and, to make matters worse, I have personally seen countless wrongly-named specimens for sale in retail stores. While working in various capacities for collectors and trans-shippers in the past, it was not overly uncommon to find clams misidentified from their sources, as well. However, I should add that I have noticed a substantial decrease in misidentifications in the last few years, as tridacnids are being farmed and raised in captivity by knowledgeable operators.

With these complications in mind, I have provided as much information as possible about each of the species common to the hobby to use when a specimen is unidentified, or when a given identification is in question. The guidelines below should be of great help and, while many of these can be variable to some degree, even within a single species, those features that are the strongest indicators are noted with an asterisk (*). These noted features are relatively inflexible within a species and should be given the greatest weight during the identification process.

Observable features of the shell which may serve as identification aids:

— the nature of the shell's overall shape and symmetry — the nature of the shell's ribs or folds, which are the undulations that give the shell a corrugated appearance — the nature of or lack of scutes, which are shelf or scale-like structures on the shell — the nature of the shell's upper margin which gives the two valves an interlocking appearance — the nature of, or lack of, the shell's byssal opening, which is an opening at the bottom of the shell where numerous tough fibers (collectively called a byssus) can be secreted to attach the clam firmly to the substrate

Observable features of the mantle which may also serve as identification aids:

— the degree of, or lack of, extension of the mantle beyond the top margin of the shell — the type of, or lack of, tentacles/projections around the edge of the incurrent siphon/aperture, which is the centrally-located opening in the mantle where water is brought into the body cavity.

Figure 1: Shell symmetry is seen as how similar in dimensions a shell is in both directions away from its center. For example, symmetry can be determined by looking at the length of the two heavy lines drawn from the center of this Tridacna squamosa shell. Both lines are approximately the same length, thus this shell is very symmetrical. The more unequal in length, the less symmetrical a shell would be. Also labeled are: (A) the scutes, ( the ribs, and © the upper margin.

Figure 2: The byssal opening (A), when present, can be seen as an opening on the underside of the shell and can vary greatly in size. This Tridacna crocea shell has a very large byssal opening. Also labeled is the upper margin ( as seen from the underside of the shell, through the byssal opening. The margin of this shell forms a tightly interlocking closure.

Figure 3: The exhalent siphon, through which water is expelled (A), the incurrent siphon ( , and the beautifully-colored and extended mantle © of this Tridacna maxima specimen can all be seen clearly.

Figure 4: The incurrent siphon of this Tridacna maxima specimen is lined with relatively small, simple tentacles.

Figure 5: The incurrent siphon of this Tridacna derasa specimen is lined with larger, more elaborate tentacles.

Tridacna squamosa

— most commonly available as 4 to 6 inch specimens — maximum shell length is approximately 16 inches (typically 12 inches, or less) — shell is strongly symmetrical in form* — shell typically has 4 or 5 large, well-spaced distinct ribs — ribs have numerous relatively large, well-spaced, heavy scutes* — upper margin is strongly curved and each valve is symmetrical to the other* — byssal opening is variable in size, being moderate to almost non-existent; typically smaller in larger specimens, as they rely more on their own weight to hold them in place rather than a byssus — mantle extension can be well past the margin, completely hiding the shell and scutes — incurrent siphon is ringed with numerous large and often elaborate tentacles*

Figure 6: Two Tridacna squamosa shells showing strong symmetry, large ribs, and large scutes.

Figure 7: The underside of a Tridacna squamosa specimen with a very small byssal opening.

Tridacna maxima

— most commonly available as 2 to 4 inch specimens — maximum shell length is approximately 16 inches (typically 12 inches, or less) — shell is strongly asymmetrical in form, typically being much longer than tall* — shell typically has 5 distinct ribs — ribs have numerous very tightly-spaced, but light scutes; however, these are typically eroded away by the burrowing activities of this species when in their natural habitat. Thus, specimens that have been collected "in the wild", typically have numerous scutes present only on the upper portion of the shell. Those raised in captivity are not provided the opportunity to burrow into substrates and thus retain most, or all of the scutes. — upper margin is strongly curved and each valve is symmetrical to the other* — byssal opening is variable in size, being moderate to relatively large — mantle extension can be well past the margin, completely hiding the shell and scutes — incurrent siphon is ringed with numerous small, simple tentacles*

T. maxima is occasionally confused with T. squamosa. However, the overall elongation/asymmetry of the shell, the closely spaced nature of the smaller scutes, and the presence of small, simple siphonal tentacles of T. maxima help in differentiating the two.

Figure 8: Two Tridacna maxima shells showing strong asymmetry, distinct ribs, and closely-spaced scutes.

Figure 9: The underside of a Tridacna maxima specimen with a relatively large byssal opening.

Tridacna crocea

— most commonly available as 2 to 3 inch specimens — maximum shell length is approximately 6 inches — shell is moderately asymmetrical in form, typically being somewhat longer than tall* — shell typically has 5 to 6 low ribs — ribs have numerous tightly-spaced, but light scutes; however, these are typically eroded away by the natural burrowing activities of this species when in their natural habitat. Those specimens that have been collected "in the wild" typically have no scutes present, or have only a few scutes at the upper margin of the shell. Those raised in captivity are not provided the opportunity to burrow into substrates and thus retain most, or all, of the scutes. — upper margin is moderately curved and each side is symmetrical to the other* — byssal opening is very large in size* — mantle extension can be well past the margin, completely hiding the shell and scutes — incurrent siphon is ringed with numerous small, simple tentacles*

Tridacna crocea is often confused with T. maxima. In this case, the more exaggerated elongation/asymmetry of the T. maxima shell is again a strong identifier. Scutes, when present on T. crocea, tend to be further spaced and smaller, as well. Also, the byssal opening of T. maxima, while being large at times, still is not as large as that of the typical T. crocea, and is more likely to be considerably smaller.

Figure 10: Two Tridacna crocea shells showing moderate symmetry, low ribs, and a lack of scutes.

Figure 11: The underside of a Tridacna crocea specimen with a very large byssal opening.

Tridacna derasa

— most commonly available as 2 to 3 inch specimens — maximum shell length is approximately 24 inches (typically 20 inches, or less) — shell is strongly symmetrical in form* — shell typically has 5 to 7 moderate ribs — ribs typically lack scutes, although some very low ridge-like scutes may be present on some specimens* — upper margin is typically only slightly to moderately curved, and each valve is symmetrical to the other — byssal opening is narrow and relatively small in size* — mantle extension is highly variable and ranges from barely extending past the margin to extending well past the margin, completely hiding the shell — incurrent siphon is ringed with numerous relatively large and often elaborate tentacles*

Figure 12: Two Tridacna derasa shells showing strong symmetry, moderate ribs, and a lack of scutes.

Figure 13: The underside of a Tridacna derasa specimen with a small byssal opening.

Tridacna gigas

— most commonly available as 4 to 8 inch specimens — maximum shell length is approximately 54 inches (typically 48 inches, or less) — shell typically has 4 to 5 distinct ribs — shell is slightly asymmetrical in form* — ribs lack scutes* — upper margin is very strongly curved, and each valve is asymmetrical relative to the other, forming large finger or tooth-like projections which do not form a tightly closing shell. This characteristic becomes more prominent with age* — byssal opening is very small in size to non-existent * — mantle extension is highly variable and ranges from not extending past the margin at all, to extending well past the margin, completely hiding the shell — incurrent siphon lacks tentacles*

Tridacna gigas is often confused with T. derasa, especially when juveniles. When comparing larger specimens, the large tooth-like form of the margin of the T. gigas shell is a strong identifier. With other specimens the larger, more prominent ribs of the T. gigas shell, and the lack of siphonal tentacles help in differentiating the two.

Figure 14: A living Tridacna gigas specimen, easily differentiated from Tridacna derasa by a lack of siphonal tentacles.

Hippopus hippopus

— most commonly available as 4 to 6 inch specimens — maximum shell length is approximately 14 inches — shell typically has 7 to 8 distinct ribs, but may have many more less distinct, minor ribs — shell is asymmetrical in form and quite distinct* — ribs lack scutes* — upper margin is strongly curved and each valve is symmetrical to the other* — byssal opening is very small in juveniles and is lost completely in adults — mantle does not extend past the margin* — incurrent siphon lacks tentacles*

Figure 15: A living Hippopus hippopus specimen, easily identified by its unusual shell form and recessed mantle, which does not extend past the shell's upper margin.

A Final Note:

Lastly, I should also mention that there are a few lesser-known tridacnids, such as T. rosewateri, T. tevora, and H. porcellanus, but these are rare and are not often seen in the hobby, thus they are not discussed here. However, it is also important to note that there are a number of hybrid tridacnids occasionally offered. These are individuals that share characteristics of more than one species, which are the product of the mixing of genetic material between two species. Tridacnids, like so many other marine organisms, are broadcast spawners which can eject hundreds of thousands of freely-mixing sperm and eggs into the water in the process of sexual reproduction. So, it's easy to imagine the occasional intermingling of genes if the ability to cross-fertilize is present. Among these, some of the most prevalent hybrids seen in the hobby are crosses between T. derasa and T. gigas. These specimens typically have shells more like those of T. derasa and have relatively large, elaborate tentacles surrounding the incurrent siphon like T. derasa, but have coloration patterns that are clearly a mix of those commonly exhibited by the two species. Unfortunately, because the degree of outward, physical expression of the hybridized genetic material can be highly variable, as you might guess there are occasional clams which seem to defy most, or even all identification guidelines. Fortunately, for those easily frustrated by not having an answer for every question, these clams are typically few and far between.

[ASHWIN]

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