*A ninth species. Tridacna rosewateri Sirenko and Scarlato, has recently been described from Saya de Malha Bank, an isolated region in the Indian Ocean. Its validity is not generally accepted with great confidence because it was described from shells only. It has large, widely spaced scutes on the primary radial folds, like T, squamosa, a thin shell, and dorsal valve margins having 4 to 5 strongly elongated triangular, medially projecting interdigitating processes, a trait in common with large T. gigas. The byssal orifice and most other morphological features are like T. maxima. The shells appear to have a nearly centrally located umbo, more like T. squamosa, though we have seen T. maxima with this characteristic. We have seen apparent hybrids of T. maxima and T. squamosa that share traits, and specimens ot both species that may not be hybrids but share traits. For example, the clam on the top of page 364 in this book has widely spaced scutes and a central umbo, but still is readily identifiable as T. maxima. The clam in the center photograph on page 381 is another example. Since both T. maxima and T. squamosa occur near the region where T. rosewateri occurs, if it is a valid species, T. rosewateri my hybridize with them, See: Sirenko, B.I. and O.A. Scarlato. Tridacna rosewaterisp. n. A new species of giant clam from Indian Ocean. La Conchiglia 261:4-9 (1991).
** See note about name change for T. tevoroa on page 383.
In popular fiction, no picture of a coral reef environment is complete without a giant clam maliciously capturing an unwary diver in its cavernous maw. This image is based more on romanticism than fact. In truth the giant clam, Tridacna gigas, is not dangerous, but it is the largest bivalve in the world, reaching lengths greater than 1 m (3 ft.) and weights up to 400 kg (800 lbs.). It represents only one species and there are other, smaller species, that are much more suitable for the home aquarium. Tridacnid clams belong to the order Bivalvia which includes the various families we commonly refer to as clams. The family Tridacnidae currently contains eight species* in two genera, Hippopus and Tridacna: Hippopus hippopus, H. porcellanus, Tridacna crocea, T. derasa, T. gigas, T. maxima, T. squamosa and T, tevoroa**.
The exhalent siphon of a Tridacna squamosa. Sonja VanBuuren.
Tridacnid clams are found throughout the Indo-Pacific and Red Sea, and are usually associated with coral reefs, either amongst live corals, or on sand and rubble areas adjacent to reefs. Tridacna squamosa and T. maxima have the widest distributions, being found throughout the Indo-Pacific, from the Red Sea in the west to Tonga and Pitcairn Island in the east, respectively. Tridacna crocea, T. derasa, and T. gigas are found from the Nicobar Islands in the west to Fiji in the east (see sidebar note, p.374), while 11. hippopus is found from the Nicobar islands to Tonga (Yonge, 1975). Hippopus porcellanus has a limited distribution being found only between eastern Indonesia and western Papua-Newr Guinea (Lucas, 1988), while T. tevoroa has so far only been found in eastern Fiji and islands within the Ha'apai and Vava'u groups, Tonga (Lucas et al., 1991).
At first glance tridacnids resemble normal clams by having two valves (shells). However, it soon becomes apparent that they are different in a number of important facets. The major factor that has resulted in these differences is the presence of symbiotic zooxanthellae in the mantle tissue. It is thought that it is the presence of these algae that has allowed these clams to do as well as they have in nutrient poor areas. Due to the presence of these zooxanthellae, tridacnids have undergone a number of behavioural and physical changes.
The inhalent siphon of T. squamosa has numerous, large fringing tentacles. J. Sprung.
Tridacnids are generally limited to shallow waters where they can receive the maximum amount of light. In fact, some specimens are found in water so shallow that they are exposed to the air during periods of low tide. Tridacna gigas can be found as deep as 20 metres (66 ft. ), however, and T. tevoroa is only found in deep water (Crawford and Nash, 1986; Lewis and Ledua, 1988). Physical adaptations include a large, fleshy mantle that increases the surface area available for exposure to light. Actually, the mantle of a tridacnid is simply an extension of the inhalant and exhalent siphons. The inhalant siphon consists of an elongated opening, often surrounded by fringing tentacles, which act to strain out large particles. The exhalent siphon is located further along the mantle and forms a raised cone, through which water leaves the body cavity after being filtered by the gills. In order for the siphons and mantle to be in an upper position, the internal organs have been twisted 180 degrees such that the heart, inhalant and exhalent siphons, and the stomach lie near the top of the body, just below the mantle. This allows the siphons to be on top, further increasing the available surface area. As a result of this rotation, the muscular foot, so prominent in other clams, has become greatly reduced and is found next to the hinge of the valves. To compensate for the small, functionless foot, tridacnids have a much more prominent byssus gland. The byssus gland produces filaments (byssal threads) that extend through an opening between the two valves and fasten the clam to the substrate. The larger species, T. gigas, T. derasa, T. tevoroa, and Hippopus spp. lose these glands as they grow larger, relying instead on their size and weight to hold them in place (Lucas, 1988).
Anatomy of a cockle
After Yonge, 1975
anterior pedal retractor muscle anterior adductor muscle posterior pedal retractor muscle posterior adductor muscle anus exhalent
Tridacnid Clam Anatomy
After Yonge, 1975
Close-up of the blue UV-absorbing pigment pattern on a Tridacna maxima. J.C. Delbeek.
Tridacna maxima. Note the dark eye spots along the edge of the mantle. J.C. Delbeek.
The siphonal tissue (mantle) contains the majority of the zooxanthellae as well as fixed cells called iridophores that contain numerous pigments. Mainly in the colour range of blue to brown, or green to yellow, combinations of these pigments give rise to the wide range of colours and patterns that make these clams so desirable to the marine aquarist. However, the main function of these pigments is to protect the clam against excessive light and UV radiation (Yonge, 1975). If the clams do not receive the proper light intensity and quality, they will quickly lose their bright colours. Loss of bright colour results in the underlying brown colouring of the zooxanthellae becoming visible. Unless conditions are improved, the zooxanthellae may begin to disappear too, and the clam will take on a whitish-brown colour (see topic "bleaching" chapter 10). Once this stage is reached, death shortly
follows (Achterkamp, 1987a). We have seen bleached clams that were highly illuminated by intense metal halicle lighting. Though this lighting is normally ideal, bleaching may occur when the trace element iodine is depleted. See topic iodine in chapter 8 under trace element additions, and in chapter 6 with respect to lighting.
Tridacnids have hundreds of eyes along the edges of the siphonal tissue and some specimens of T. crocea and T. maxima can also have eyes on top of raised tubercles scattered over the mantle surface. These eyes are used primarily to detect shadows, warning the clam of the passing of potential predators (Wilkens, 1986). The eyes are also sensitive to green, blue and ultraviolet light (Wilkens, 1984). It is felt that these sensitivities help the clam to orient toward the light, in order to maximally expose the zooxanthellae. Even clams that are lying on their sides will stretch their mantle toward the light (Wilkens, 1986). Given the transparency of reef waters to UV light, the eyes could also function to detect excessive amounts of these potentially harmful wavelengths. Tridacnid clams also have light concentrating organs in their mantles called hyaline organs. These are translucent "windows"1 that allow more light onto pockets of zooxanthellae, thereby further enhancing their metabol ism (Rose water, 1965).
'Recent research has shown that glucose is the primary carbohydrate released to clams by zooxanthellae, followed by a group of glucose-based oligosaccharides, then glutamate, aspartate, succinate, alanine, and glycerol (Griffiths and Streamer, 1988),
Most clams obtain nutrition from a variety of sources such as filter j feeding and absorption of dissolved organic compounds from the water. Tridacnid clams have gone a step further by harbouring symbiotic algae (zooxanthellae), that manufacture food for them just as in hermatypic corals. The zooxanthellae of tridacnid clams are located within zooxanthellal tubules that extend from the stomach into the mantle tissue, not within individual cells as in corals (Norton, et al., 1992). Through photosynthesis the zooxanthellae provide clams with the same products corals receive: carbon, in the form of glucose*, and amino acids such as alanine. Under sufficient light, zooxanthellae can provide 100% of a clam's respiratory carbon requirements (Fisher et al., 1985). In return, the zooxanthell ae use the nitrogenous wastes produced by the clam, primarily ammonia, as a nitrogen source. This method of nutrient recycling benefits the tridacnids a great deal, allowing them to utilize a highly efficient internal food source that minimizes energy loss between trophic levels (Heslinga and Fitt, 1987). It has also been shown that tridacnid kidneys contain large amounts of calcium phosphate (Trench et al., 1981). In low phosphate areas such as reefs, it is tempting to speculate on the
Tridacnid Clam Anatomy showing zooxanthellal tubules extending from the stomach into the mantle tissue.
After Norton eta!., 1992.
Secondary Zooxanthellal tubes
Tertiary Zooxanthellal tubes
Primary Zooxanthellal tube
The gills are evident through the inhalent siphon of this T. maxima. J.C. Delbeek.
possible role this phosphate could play in zooxanthellae nutrition. However, clams without zooxanthellae also have these deposits.
The large, convoluted mantle is not only efficient in capturing light but also in absorbing dissolved nutrients from sea water. In light, zooxanthellae in the mantle take up ammonia, nitrate, phosphate and sulfate from the surrounding water and use them to make amino acids. This accounts for the ability of tridacnid clams to
lower levels of these substances in closed systems (SeaScope, 1991). The clams can also expand and contract the mantle as light intensity changes, depending on their need to eliminate excess ammonia (Benson, 1984). Finally, it has recently been
demonstrated that additions of ammonia, nitrate and ammonium, primarily in the form of ammonium nitrate, to culture systems has improved the growth rate of juvenile tridacnid clams (Heslinga, 1989; Hastie, et al., 1992). The presence of ammonium, however, interfered with the uptake of nitrate. Ammonium is the preferred nitrogen source since it does not require energy to be absorbed or reduced, as nitrate does (Fitt et al., 1993). It should be pointed out that these were not recirculating systems, but open ones that received constant exchange of nutrient poor ocean water. In a closed system like an aquarium, where these nutrients are generally 10 to 100 times higher than in natural sea water, no such additions are necessary .
Studies by Klumpp et al., (1992) have shown that filter feeding of particulate organic matter alone in J. gigas can meet 64% of the carbon requirements of 4,2 cm specimens. This percentage declines to 34% in 16.7 cm specimens. A similar study conducted on T. derasa and T. tevoroa showed that these species could easily gain their carbon requirements from zooxanthellae alone (Klumpp and Lucas, 1994). It may be that the rapid growth rate and large size attained by T. gigas is a function of its ability to utilise both autotrophic and heterotrophic feeding strategies, thus allowing more carbon to be allocated to growth than to respiration.
The role of phytoplankton in tridacnid nutrition is not clearly understood. It is believed that phytoplankton provides the clam with some of its protein, but it is more likely a source of carbohydrates. However, as Yonge (1975) points out, the amount of phytoplankton available on tropical reefs is probably not sufficient to meet the needs of the clam. It has been argued that since clams possess feeding appendages such as gills, palps, and an efficient digestive system, they must be actively feeding. However, the gills are still required for respiration, ammonia expulsion, and possibly nitrate uptake (Fitt et al., 1993), the palps are greatly reduced and the digestive system is used to expel excess zooxanthellae (Norton, et al., 1992). Furthermore, studies of T. maxima have shown that zooxanthellae can produce excess oxygen, far above what is required by the clam (Trench et al., 1981). High levels of oxygen are potentially lethal and need to be eliminated, either through the mantle or perhaps via gills. It has been speculated that tridacnids actually digest their zooxanthellae, in effect "harvesting' excess senescent zooxanthellae as a source of protein (Yonge, 1975). Yet, several studies have shown that many
Measuring the growth in captive-bred Tridacna derasa in the nursery beds off of the Micronesian
Mariculture Demonstration Centre, Palau. G. Heslinga.
of the zooxanthellae in the stomachs, rectum and feces of tridacnids are still viable and fully functional (Trench et al, 1981).
Zooxanthellae are introduced into juvenile clams via their feeding organs and move from the stomach of the clam into the mantle via the tubule system. Zooxanthellae are generally resistant to digestion so it should not be surprising that viable cells can be
isolated from the feces (Heslinga and Fitt, 1987). If the zooxanthellae were not resistant, they could not survive long enough to make it into the mantle. This is easily shown in closed systems as clams are often observed to release thin, brown strands from their exhalent siphons, especially after periods of stress. When examined under a microscope these strands can be seen to be composed mainly of viable zooxanthellae (Achterkamp, 1987a; J.C. Delbeek pers. obs.X It is even possible to cultivate these expelled zooxandiellae (Trench et al, 1981); a potentially useful technique for those interested in breeding tridacnids.
In summary, zooxandiellae photosynthetic products appear to be the major source of nutrition in tridacnid clams. However, other sources such as dissolved nutrients in the surrounding w ater, phytoplankton and senescent zooxanthellae may contribute to a leaser extent.
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