ON THE HALF SHELL
BY DANIEL KNOP

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Tridacna maxima — Widespread and Hardy

Tridacna maxima is one of the most common giant clam species and has the most widespread distribution of all the tridacnids. Not all clams of this species exhibit beautiful coloration, but some have incredibly beautiful blue syphonal mantles, especially specimens from the Red Sea.

Natural Habitat

T. maxima occurs from the Red Sea and East Africa to Australia to as far as Polynesia. This species has adapted to the upper, strongly illuminated portion of the reef. It occurs mainly in shallow habitats where it forms huge colonies, sometimes reaching a density of up to 60 specimens per square meter (Lucas and Copeland 1988). Within these dense colonies, the beautifully patterned syphonal mantles of the clams sometimes cover every square centimeter of substrate. When the shadow of a swimming fish falls on them, the numerous syphonal mantles retract simultaneously and the jaw-like shell margins become visible.

DANIEL KNOP
T maxima can develop a wide variety of shell formations. Pictured here are two examples. The longer shell formation is more typical for the species.

T. maxima is able to deal with intense changes in environmental conditions, much like the smaller T. crocea, and this seems to be the main reason for its wide distribution. T. maxima lives mainly in shallow habitats with strong water motion and lots of sunlight. In these areas we normally find clear waters, unlike habitats that are typical for the big giant clam species T. gigas (e.g., a muddy seagrass bed). However, T. maxima is not just found in shallow waters. It is also found at depths of about 15 meters, although at these greater depths it seldom forms colonies, occurring mostly as solitary individuals (e.g., among corals on a reef wall).

T. maxima is usually found on solid limestone substrate or on coral rubble. They create a shallow indentation when they settle on limestone, into which they bury the lower portion of their shells and adhere themselves tightly with their strong byssal apparatus. In contrast to the smaller T. crocea, which normally bores a hole so deep into the stone that it can become imprisoned, T. maxima always leaves a large part of its shell exposed. On coral rubble, they bury the hinge of their shells deeply within the rubble and attach their byssal apparatus to a large piece of the rubble.

Description

Before Dr. Joseph Rosewater revised the family Tridacnidae, there was much confusion as to the valid species. Numerous clams had been described under various names, including Hippopodes scutra Meuschen 1787, Tridachna elongata Humphrey 1797, Tridacnigenus Renier, Tridacnodites Krüger and Tridacna lanceolata Sowerby 1884. Many of these species names continued to be used until Rosewater published his revision in 1965. This revision turned out to be one of the most important publications about this family. Many of the formerly used names had been based on the color patterns of the syphonal mantles of the clams, which assumed that a specific pattern was always present within a certain species (T. punctulosa Lamarck 1819). It wasn’t until much later that it was determined that clams that exhibited totally different color patterns could, in fact, belong to the same species.

Biological Classification of Tridacna maxima (Roeding 1798)
Class:Bivalvia
Subclass:Heterodonta
Order:Veneroidea
Family:Cardiacea
Subfamily:Tridacnidae
Genus:Tridacna
Species:Tridacna maxima
Interestingly, T. maximahas had some close relatives in Europe during its evolutionary history. During the middle Tertiary period, arms of warmer seas extended to northern Europe. At that time, fossil species, such as Tridacna media Pusch 1837 and Tridacna wolfarti Chenu, could be found in this geographic region.

T. maxima is also known under the synonym T. elongata, a name based on one of the variations in shell shape — extremely asymmetrical. However, shell shape in this species is extremely variable, which may account for the numerous species names that have been used for it in the past. Even among specimens in one batch of juvenile T. maxima on a clam farm many different shell shapes develop. Sometimes the shells are even symmetrical, much like in T. squamosa.

The reason for this variability in shell shapes is quite simple. Under natural conditions the lower part of the shell is sometimes buried in a shallow indentation in the limestone and the clam lives firmly attached to the rock substrate. Given sufficient space the shells may grow and become more or less elongate. However, if their growth is impeded by some type of coralline obstacle, the valves may be malformed.

GIANT CLAM ANATOMY: THE EXCURRENT SYPHON
AND EXCURRENT CHAMBER

Tridacnid clams have a circulation system, similar to conventional mussels, that comes in contact with the environment via two syphons. The gills act as a filter between the incurrent syphon and the excurrent syphon. The water entering the incurrent syphon has to pass through the gills before entering the excurrent chamber. This excurrent chamber is located right under the syphonal mantle and extends from the gills over the whole clam. It forms a narrow cavity over the heart, the kidneys and the adductor muscle. The water leaves the chamber via the excurrent syphon, a pimple-shaped tube that is located almost in the center of the clam.

The wastes from the kidneys are excreted into the excurrent chamber. The anal papilla on the upper side of the adductor muscle that forms the end of the digestive tract is located in this chamber as well. The continuous water flow through the gills and the two chambers carries waste products from the digestive tract, urine from the kidneys and expelled zooxanthellae out of the clam. The latter is sometimes visible as a black or dark brownish thread-like band coming out of the excurrent syphon that is blown straight up into the water by the water flow leaving the syphon.

Sometimes, the muscle tissue in the excurrent syphon contracts and closes the opening just before the clam closes its shells. The continued closing of the shells results in increasing pressure in the excurrent chamber, and as soon as the excurrent syphon is opened again an extraordinarily strong burst of water leaves the chamber, sometimes blowing out floating particles. Bigger clams, such as adult T. gigas, T. derasa or T. squamosa, sometimes use this sudden burst of water as a defense mechanism when they are shaded by a fish swimming over the mantle.

Although T. maxima can attain a shell length of 40 centimeters, for the most part it remains smaller, usually around 30 centimeters. The shell has rows of scutes, as in T. squamosa, and may be confused with it. However, T. maxima is usually easy to differentiate from T. squamosa. The distance between the scute rows in T. squamosa almost equals the width of an individual scute. In T. maxima, the rows of scutes are much narrower and the lower part of the shell near the hinge normally has no scutes at all, because that is the part that is in direct contact with the solid limestone substrate.

T. maxima may also be confused with the closely related T. crocea. In addition to the difference in size, most T. maxima have a row of hyalin organs near the edge of the syphonal mantle that look like dark, pimple-shaped, pointed elevations. While these hyalin organs are also present in T. crocea, they are spread over the entire syphonal mantle.

T. maxima frequently exhibits a typical pattern that consists of spots and sprinkles (brilliant green, blue, purple and brown) that starts in the center of the mantle and extends straight to the periphery. Not all T. maxima exhibit this type of pattern, but it is seldom seen in other species. Some specimens exhibit a strong blue coloration consisting of large spots and, in some, the entire background color is blue. These extraordinarily beautiful clams normally come from the Red Sea and are highly sought after by reef aquarists.

Interestingly, the brilliance in color of the syphonal mantle of T. maxima depends on the geographic origin of the clam. At Eniwetok, Marshall Islands, as well as the Great Barrier Reef, the colors have been observed to be extremely bright, while in Malaysia, Thailand or off southern Sumatra the colors are much more subdued (Rosewater 1965). This may be a function of either genetic differences between clams of different locations or of environmental factors.

Aquarium husbandry

IlluminationT. maxima needs a lot of light and should only be maintained under strong metal halide lamps. If a newly purchased specimen is predominantly dark brown in color, with no pronounced color or pattern, it probably came from a deeper habitat in the sea. These specimens should be adapted to strong illumination gradually. As the brilliance of the color pattern on the mantle increases, the illumination can be increased as well. On the other hand, if a newly purchased specimen is extremely colorful, it can immediately be placed under a bright halide lamp.

Water motionT. maxima will tolerate water motion occasionally, just like T. crocea. However, water current should be gentle to medium so as not to cause the edge of the syphonal mantle to be inverted. Jet-like water flow is not welcomed by these clams.

Water quality — In nature T. maxima occurs mainly in habitats with extremely clear water. Therefore, we should also provide them with clear water in the aquarium, even though they are among the hardier species of giant clam.

Feeding — Just as with all the other giant clam species, it is not necessary to feed T. maxima. With some species feeding might even have a negative effect because they have adapted to relatively clear waters (e.g., T. maxima, T. crocea, T. derasa). Their gills have adapted to waters that are free of particulate matter. These species react negatively to extremely turbid waters, as well as to direct feeding with particulate or floating matter, because this can clog the gills (Adams et al. 1988). This is dangerous to the clams because they get their oxygen via the gills.

In nature, conditions of extreme turbidity (e.g., during a typhoon) develop very slowly, giving the clams sufficient time to react appropriately — they reduce the flow of water through their gills. But, in an aquarium, if the aquarist feeds the clam by applying particulate matter directly to the incurrent syphon of the clam with a pipette, the clam is unable to react the way it would in nature and the gills are overwhelmed and unable to rid themselves of the particulate matter. In my opinion, the highly sensitive tentacles that surround the incurrent siphon may actually detect floating particles and signal the clam to contract its muscles and close the incurrent syphon as soon as any particles are detected. This theory is supported by the fact that the two giant clam species that are not as sensitive to turbidity (T. gigas and H. hippopus) do not possess these tentacles, while all other species do. To my knowledge, however, this theory has not yet been verified scientifically.

DANIEL KNOP
The typical T. maxima shell is extremely asymmetrical and elongate on one side.

Difficulties in aquarium husbandry — Like all giant clam species, T. maxima is sometimes affected by pyramidellid snails. For the most part, however problems in keeping T. maxima are quite rare if conditions in the aquarium are suitable. In particular, if the intensity and spectrum of the lighting fail to supply the symbiotic dinoflagellates (zoothanxellae) with all their needs, the clam may stop growing and develop symptoms of bleaching. The exact cause of bleaching in clams is unknown, but it appears to be related to the clam’s ability to adapt to a specific habitat and the lighting conditions it finds there. Clams develop mechanisms that allow them to adapt to lighting conditions of a specific habitat at a very early stage in their development, and it is not known to what extent they can be altered later on. At any rate, in order to avoid problems it is best to keep light parameters, especially light spectrum, within the range found in the clam’s natural habitat.

Reefer’s Questions

Q. Daniel:
Do you have any additional information about white spots (pinhead size) on the mantle of a Tridacna clam as described in your book? I just noticed them on one of my clams and the prognosis in your book is not very promising — 100 percent fatal. I don’t know whether to destroy the animal so as to prevent infection in my other eight tridacnids. I have not seen this topic addressed on any of the newsgroups.

DANIEL KNOP
This beautifullly colored T. maxima from the Red Sea shows the brown row of hyalin organs near the edge of the syphonal mantle. The specimen here is infected with white spot disease, which is described in detail in my book Giant Clams.

My tank is a 120-gallon Berlin-style reef with metal halide lights, a skimmer and a calcium reactor. This affliction seems to only be affecting my newest (six weeks old) T. maxima. The other eight clams are doing well and growing and do not show signs of this problem yet. Any help or advice would be very much appreciated.
Thanks,
Mike Hughes

A. Hi Mike:
I’m sorry for your clam. It seems to have the white spot disease infection. In many cases we have found that chloramphenicol helps. My proposal would be to treat the clam in a separate tank. You can try 1 gram of chloramphenicol per 100 liters of water (with no granular activated carbon). After three days give another 0.5 gram per 100 liters.

That’s a dosage that could even be given in the reef tank without harming the inverts or bacteria, but it’s better to treat the clam separately just to be on the safe side. If you’re successful, please let me know. But, be sure to bear in mind that chloramphenicol is hazardous to humans, so make sure to wear protective clothing (i.e., gloves, apron) when using.

REFERENCES

Delbeek, J. C. and J. Sprung. 1994. The Reef Aquarium, Volume 1. Ricordea Pub., Florida. Pp. 545.

Knop, D. 1996. Giant Clams. A Comprehensive Guide To The Identification And Care of Tridacnid Clams. Dähne Verlag, Germany. Pp. 255.

Lucas, J. and J. Copeland. 1988. Giant Clams In Asia And The Pacific. ACIAR Monograph 9, Canberra:51-53

Rosewater, J. 1965. The family Tridacnidae in the Indo-Pacific. Indo-Pacific Mollusca 1:347-396.

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