Giant clams (Tridacna spp.) inhabit the shallow coral reefs of the Indo-Pacific Ocean, and their colorful mantles make them perennial favorites of aquarists. Healthy Tridacna specimens will contain millions of tiny symbiotic algae (technically, dinoflagellates of the genus Symbiodinium), thus proper lighting is a prerequisite for success. This article will discuss the lighting requirements of commonly available Tridacna species, including T. crocea, T. derasa, T. gigas, T. maxima, T. squamosa and Hippopus hippopus.
Proper lighting of a reef aquarium has always been controversial, and it seems it remains so as new products enter the market. Recommendations for effective lighting for any photosynthetic organism may be based on observed depths at which organisms are collected (see Figure 1). While these observations can lend valuable information, they are not without caveats. For instance, an organism may be found in only a few inches of water, and this might lead an aquarist to believe it requires intense lighting when it in fact it lives in a crevice or in shadows for most of the day. On the other hand, a photosynthetic organism living in deeper water might possess the ability to adapt to very intense lighting. Sometimes lighting decisions are based on casual observations of what works and what does not. A Tridacna clam may seem to be doing well under weak lighting for a few months only to succumb to problems associated with “light starvation.”
Fortunately, researchers have developed tools to estimate the lighting needs of plants, algae and those animals maintaining a symbiotic relationship with zooxanthellae, including corals and Tridacna clams. In addition, sophisticated instruments can identify the different types, or clades, of zooxanthellae living within the tissues of various Tridacna species. With this information, the hobbyist can make informed decisions on just how much light these clams need in order to survive in captivity.
This article will examine several ways to estimate the lighting requirements (and pitfalls) of Tridacna clams, including:
1. Estimating lighting requirements by depth of distribution
2. Types of zooxanthellae known to inhabit Tridacna species
3. Photosynthetic capacity
4. Other considerations
Before getting started, perhaps we should define some of the terms used in this article.
Autotrophic. Possessing the ability to be self-sufficient without feeding. Plants are autotrophs, but animals having a symbiotic relationship with algae (such as many corals and Tridacna clams) may receive enough nutriment from zooxanthellae to be considered autotrophic. If an animal must feed in order to survive, it is said to be heterotrophic. If the animal receives a portion of its food through symbiosis but must feed to make up the remainder, it is mixotrophic.
Clade. A group of organisms considered to have evolved from a common ancestor.
Compensation point. Every plant and algal species (including zooxanthellae) has a minimum requirement of light called the compensation point. This is the point at which oxygen production by zooxanthellae meets their requirements and that of the Tridacna clam (or any other photosynthetic organism).
At the other end of the scale is the saturation point. This is when the zooxanthellae have absorbed as much light as possible and are photosynthesizing at maximum capacity. Increasing the light intensity will not increase the rate of photosynthesis but will in fact possibly damage the zooxanthellae in a process known as chronic photoinhibition. See the “Xanthophylls” entry of this glossary for the definition of dynamic photoinhibition.
Mycosporinelike amino acids (MAAs). A group of proteins capable of absorbing ultraviolet radiation and thus protecting the photosynthetic apparatus from damage. Originally isolated from a mycosporine fungus.
Photosynthesis. The link between the inorganic and organic worlds. In corals and clams, chlorophyll A and chlorophyll C2 (as well as accessory pigments including peridinin) harvest sunlight (in the form of photons) and convert it to electrons. This flow of energy converts water and carbon dioxide into simple sugars and other organic compounds, including amino acids (some of which are leaked to the animal host, thus providing nutriment).
Photosynthetically active radiation (PAR). Radiation falling between that which promotes photosynthesis, generally considered to be between 400 nanometers (violet) and 700 nm (red). Maximum PAR in the tropics at noon on a clear day is about 2,200 microMol-photons per square meter per second (µmol/m2/sec). A good estimate for converting lux to PAR is to multiply lux by 50.
Ultraviolet radiation. Radiation just below that visible to the human eye, including UV-A, UV-B and UV-C. UV-A is the least harmful of these rays and is considered to be those wavelengths between 315 nm and 400 nm. UV-B is that bandwidth of radiation that causes sunburn (280 nm to 315 nm). UV-C is filtered out of solar radiation by the Earth’s atmosphere, but it can be generated in small amounts by some metal halide and mercury vapor lamps and other means (such as arc welding). UV-C’s bandwidth is between 100 nm and 280 nm.
Xanthophylls. Two xanthophylls (diadinoxanthin and diatoxanthin) act as “pressure relief valves” for photosynthesis in zooxanthellae. When photosynthetic rates are very high, xanthophylls shunt blue light away from photosynthesis. This process is called dynamic photoinhibition.
Our questions when addressing lighting issues of Tridacna clams should include:
What is the minimum light requirement (compensation point) for each of the Tridacna species?
Is it possible to provide too much light? What about ultraviolet radiation?
What other factors are involved?
It is a pipe dream of many hobbyists to have a listing of depths at which Tridacna clams and corals are collected. It would then be such an easy matter to estimate lighting requirements. Or so it would seem. Examine the maximum depth distributions of Tridacna species listed in Figure 1 (Fatherree, 2006).
Now, look at the information supplied in Figure 1 for T. squamosa and compare it to Figure 2 (Jantzen et al., 2008).
Only a glance at Figure 2 will reveal major differences in depth distribution for T. squamosa at different locations. Why the difference? It could be due to water clarity. Or perhaps another factor. Let’s dig a little deeper for further clues, starting with the types of zooxanthellae that inhabit clams.
Types of Zooxanthellae
Many types (clades) of zooxanthellae have been identified through genetic fingerprinting. Of hundreds of clades, seven are known to infest the tissues of Tridacna clams. These are clades A, A2, A3, A3a, A5, A6 and C1. Table 1 lists them by the host Tridacna species. With a bit of detective work, we can determine the environmental preferences of the zooxanthella clade and hence its host clam.
As a footnote, Tridacna clams are not “born” with Symbiodinium species infesting their tissues. Instead they acquire zooxanthellae in a process called “horizontal transmission,” where symbionts must be acquired from the ambient water column. There is good reason to believe that Tridacna species exhibit a high fidelity to certain zooxanthellae clades.
Clade A zooxanthellae are generally considered relatively hardy (resistant to a number of environmental shifts) and are found in scleractinian corals, octocorals, hydrocorals, clams, anemones and zoanthids. Most hosts of clade A zooxanthellae are found in the Caribbean, with sporadic reports of occurrences in Australia’s Great Barrier Reef, Hawaii, the Red Sea and the western Pacific (Korea). Clade A is considered ancestral to all other Symbiodinium lineages.
What follows is a breakdown of each of the various A clades. Refinement of genetic fingerprinting has revealed quite a number of variations of the clade originally described as A.
Reported depth range: less than 33 feet
Reported geographical range: Caribbean and Pacific
Host species: Zoanthus sociatus, stony coral Meandrina meandrites, the photosynthetic clam Corculum cardissa, Gorgonia ventalina, Bartholomea annulata, a Pacific hydrocoral Heliopora and the “giant clam” T. gigas.
Symbiodinium pilosum. Found in the Caribbean zoanthid Zoanthus sociatus. These are high-light adapted (they respond poorly to low-light levels), tolerate high temperature swings and are able to produce and incorporate protective xanthophylls (diadinoxanthin and diatoxanthin) into chlorophyll protein complexes. Iglesias-Prieto and Trench (1997) found this zooxanthella to be the least adaptive in respect to light intensity of six zooxanthellae examined (high light is tolerated while low light is not).
Symbiodinium meandrinae. This zooxanthella was discovered within the tissues of the Atlantic stony coral Meandrina meandrites. It is now considered clade A2 (LaJeunesse, 2001). Banaszak et al. (2000) found two zooxanthellae clades (A and C) within M. meandrites, while Baker and Rowan (1997) report clade B. This leads to confusion over the actual identity of S. meandrinae. Trench (1997) clarifies the situation by listing S. meandrinae as clade A.
Symbiodinium corculorum. This species was originally isolated from the photosynthetic Pacific clam Corculum cardissa. Iglesias-Prieto and Trench (1997) suggest this zooxanthella species has limited photoacclimation capability, and the symbionts (S. corculorum and C. cardissa) perform best under high-light intensity. This clam is limited to a depth of 33 feet (Gosliner et al., 1996) and is thus considered tolerant of high light. Symbiodinium corculorum is now considered clade A2 (LaJeunesse, 2001).
Besides those animals previously listed, clade A2 is also reported to be found in Gorgonia ventalina (Puerto Rico and Jamaica), the pesky anemone Bartholomea annulata, the Pacific hydrocoral Heliopora and T. gigas.
Clade A3 (Symbiodinium fitti)
Reported depth range: 7 to 49 feet
Reported geographical range: Atlantic and Pacific
Host species: Tridacna clam (species unreported, Baille et al., 2000), T. crocea, T. derasa, T. gigas, T. maxima and another “giant clam” (Hippopus hippopus; LaJeunesse, 2001).
Comments: Tolerant of higher light levels (Hennige et al., 2006). A3 zooxanthellae are known to produce one ultraviolet-absorbing compound — the mycosporinelike amino acid (MAA) mycosporine-glycine (absorption maximum in the UV-A range of approximately 331 nm; Banaszak et al., 2006). Thornhill et al. (2008) found A3 zooxanthellae lowered their chlorophyll content when exposed to a very low temperature (50.9 degrees Fahrenheit) and did not recover within three weeks of exposure (the end of the experimental period).
Reported depth range: Not indicated in study
Reported geographical range: Philippines
Host species: Tridacna species (LaJeunesse studied clams in several genera but found A3a only in an unidentified Tridacna species)
Reference: LaJeunesse, 2005
Reported depth range: Not indicated in study
Reported geographical range: Palau (Caroline Islands)
Host species: Tridacna squamosa
Reference: LaJeunesse, 2001
Reported depth range: 3 to 33 feet
Reported geographical range: Okinawa, Philippines
Host species: Tridacna species
Reference: LaJeunesse et al., 2004
Clade C1 (S. goreaui). Originally found within the sea anemone Ragactis lucida (Trench, 1996) but expanded by LaJeunesse et al. (2003) to become the pandemic generalist zooxanthellae clade C1. Considered to be adaptable to a wide variety of environmental conditions.
Reported depth range: up to 65 feet
Geographical range: Bahamas, Japan, Korea, Great Barrier Reef, Atlantic and Mexican Caribbean, Taiwan, Hawaii and Indonesia
Host species: Tridacna derasa, T. gigas, T. maxima and others
Photosynthetic capacity describes how, in this case, zooxanthellae perform in given amounts of light: What is the minimum required, how much is enough, and how much is too much? Recall that the compensation point is defined as the minimum; the saturation point indicates maximum photosynthesis and photoinhibition (whether chronic or dynamic) is too much.
Sophisticated scientific instruments can measure compensation and saturation points as well as photoinhibition, and fortunately, researchers have provided some pertinent information to us.
Figure 3 shows the average photosynthetic capacity of five T. maxima specimens (experiments I have performed generated similar results).
As we have seen, no single method of determining the lighting requirements of Tridacna species is foolproof, and each has a degree of uncertainty. However, when the information we have is combined, trends begin to emerge, and we can make informed decisions.
Tridacna crocea. Known to be infested by only clade A or A3. Known depth distribution of A3 zooxanthellae is 7 to 49 feet, although T. crocea inhabits waters to depths of about 20 feet. The zooxanthellae is tolerant of high light and can produce at least one natural sunscreen, allowing exposure to ultraviolet radiation. The clam also benefits from these sunscreens, as during low tide clams can be exposed to the atmosphere and UV-A and UV-B in the same amounts as any beachgoer. Low temperatures (such as those encountered during shipping) can harm this clam’s zooxanthellae.
Metal halide lamps will provide maximum flexibility in placement of the clam. If other sources are used, situate the Tridacna closer to, instead of further from, the lamps. Based on results of scientific research, it is practically impossible to provide too much light to T. crocea.
Tridacna derasa. This Tridacna species is known to harbor at least two zooxanthellae clades (though likely not simultaneously): clades A3 (7 to 49 feet) and C1 (up to 65 feet). Tridacna derasa is reportedly found at depths up to 80 feet. Clade A3 zooxanthellae are intolerant of short-term exposure to low temperatures.
Recommendations for lighting are difficult to make because we cannot tell which zooxanthellae clade inhabits the clam. Provide as much light as possible, acclimate the clam slowly to the new lighting for a period of about two weeks and watch for signs of zooxanthellae loss (bleaching).
Tridacna gigas. As far as we currently know, T. gigas can contain the most varied number of zooxanthellae and is reported to contain the most varied types, including clades A, A2 (less than 33 feet), A3 (7 to 49 feet) and C1 (up to 65 feet). Tridacna gigas is found at depths of up to 65 feet. (See lighting recommendations for T. derasa.)
Tridacna maxima. Tridacna maxima harbors zooxanthellae clades A3 (7 to 49 feet) or C1 (up to 65 feet), and this compares well to T. maxima inhabiting depths of up to about 50 feet. Research has shown that T. maxima is a strict autotroph (meaning its zooxanthellae supply almost all of the clam’s nutritional requirements). Obviously, light intensity would be extremely important to this clam species. Researchers have reported that a minimum of 274 µmol/m2/sec or about 13,700 lux is required to meet minimal requirements. This light intensity corresponds to the maximum amount of light seen at a depth of 50 feet. Laboratory tests found the best survival rates (100 percent) when T. maxima specimens were maintained under metal halide lamps; while 90 percent was observed under mercury vapor lamps and only 70 percent under 54-watt fluorescent lamps. (See Figure 3 for a photosynthesis/irradiance curve for T. maxima.)
Tridacna squamosa. Data indicate that T. squamosa inhabits depths of up to about 135 feet. Water clarity probably plays a role, but there is more to the story. Genetic fingerprinting of zooxanthellae found within this clam species has revealed clades A and A5. Interestingly, clade A is known to require a great deal of light and, in fact, research has shown that T. squamosa requires a minimum amount of light (170 µmol/m2/sec, or about 8,500 lux). If this minimum is not meet, this species must feed in order to survive.
Hippopus hippopus. Hippopus hippopus is known to contain only zooxanthellae clade A3, which has been reported from depths of 7 to 49 feet. Hippopus hippopus is found in shallow water up to about 20 feet deep. Previous comments about the lighting requirements of T. crocea are applicable to H. hippopus, too.
Feeding Tridacna Clams
In suboptimal lighting conditions, feeding might take on an added importance for at least some Tridacna species. Researchers have used green algal species such as Tetraselmis subcordiformis to feed some tridacnids. In the old days, we had to culture algae for use as a food source for filter-feeders, but today is a different story. With the number of prepared foods for filter-feeders nowadays, finding a suitable off-the-shelf product should not be difficult.
Tridacna clams are known to absorb nutrients such as nitrate and phosphorus from the water column, but research has shown that this occurs only under optimal lighting conditions.
In conclusion, Tridacna clams will likely fare better under more intense lighting sources.