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:

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

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, (B) the ribs, and (C) 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 (B) 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 (B), and the beautifully-colored and extended mantle (C) 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

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

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

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

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

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

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.

Bivalves for the Reef Aquarium by Rick Martin
Reef Aquarium Guide to Bivalves
Clam Care - Guide Picks
Clam Picture Database by James Fatherree