Hello everyone, decided to drop by and start hitting this thread with minor updates as technology advances and new information have been made available.
I hope this will answer most of your questions in regards to providing your clam with the best lighting conditions possible. Most of the information here is based on personal experience and experiences shared by other fellow clam lovers. Supplemental information is credited to James Fatherree and his book, Giant Clams in the Sea and the Aquarium, and many thanks to Chris for his help. Please read the entire post as the information is designed to support each other. (especially the Tridacna Crocea section)
Please remember: Squamosas, Deresas and Gigas WILL outgrow your tank fairly quickly!
Tridacnid Lighting Requirements:
These clams require the most light of the Tridacna family. They occur in the wild mostly in very clear waters less than 15 feet in depth. (6 meters according to J.F.) The minimum recommended lighting requirement for a healthy Crocea is under a 150 watt metal halide of a reliable 14k bulb at a maximum depth of 20 inches, preferably less. To be on the safer side, a 10k bulb is recommended, and is closer to the "true" color of natural sunlight. I have seen some Croceas under 70 watts of halide, but these are normally placed high up in the tank. 70 watt bulbs are also considered by some as the least developed bulb, therefore being more inefficient in terms of their output compared to their 150 watt counter parts. 150 watts of (14k) metal halides would be the minimum “safe” amount of lighting. They will do even better under a 250 watt or 400 watt metal halide. If you are using 250 watts and above, you can place the Crocea even deeper in your tank, or even use 20k bulbs. If using T-5s, make sure the bulbs have individual parabolic reflectors on them, and the clam is placed at the upper half of your tank to be “safe”. Power Compacts will in a lot of cases NOT be adequate for Croceas.
In the case of T-5 lighting, remember only the 10k bulbs produce enough photo energy for PAR on clams, and the actinics produce little to no PAR depending on color temperature. It is also noted that the longer the bulbs are, the more penetrating power they carry. Thus a Crocea should be fine in a 20 inch depth under a 36 inch fixture. Having a Crocea in a 20 inch depth under a 24 inch fixture raises doubts, as most 24 inch fixtures only have 4 T-5 bulbs under them, with 2 of them usually carrying heavy actinics or some shade of high color temperature bulbs. In this case a retrofit is recommended to be able to cram in more lower temperature bulbs.
These clams require almost as much light as Croceas if not as much. In the wild they have a maximum depth of occurrence slightly deeper than Croceas, but are again mostly found in very clean and shallow waters. The best bet is to place them in the same lighting conditions as you would light a Crocea. The “safe” amount of lighting would be under a 100 watt metal halide if they existed, so I’m going to say 150 watt 14k metal halide under 20 inches or less as well.
Based on hobbyist experience, these clams are slightly more forgiving in terms of light than the upper two Tridacna species. In the wild however, they occur at the same depth T. Maximas do. This essentially means their lighting requirements are similar to Maximas, and should be treated as such. (with intense lighting) Assuming they are not placed in a nano due to the potential size they can reach, they will be happy under metal halides. There is no exact number for the wattage of the bulbs, but the “safe” amount of lighting would be anything more than 150 watts. I can no longer recommend Squamosas to power compact lighting, although I do know several clams that have been fine under them. (every clam is unique) T-5s are recommended over your regular power compact lighting, and a Squamosa will soak up Metal Halide lighting very happily.
Can suffice with even less amount of light compared to the Squamosa. They can thrive under most pc /vho combination, but again will be happier with lighting closer to that of its natural environment. Gigas can be found at depths up to 20 meters (J.F.) but some of the most successful aquarists have them under some type of metal halide lighting system and have recorded tremendous growth.
These clams are probably the most forgiving in terms of light than the rest of the mentioned clams. Their maximum depth of occurrence is about 25 meters (J.F.), but like the Gigas, they will be much happier with the lighting requirement given to Squamosas.
Exceptions to the Rule
Many people claim that Croceas and Maximas can survive under power compact lighting. These cases are far and few between, but there are certain individuals that are able to tolerate lower lightings, at which most of their counterparts would not be able to survive. Nevertheless, you should not take the risk and hope to get lucky, it is better to be prepared to give them what they will surely thrive in. It is always good to provide at least enough light of the given species to thrive in, and NOT the bare minimum you think you can get by with. Plus, it is the least we can do for our animal, as well as our responsibility as hobbyist.
LEDs have successfully been implicated to SPS and clams. The information advertised by one specific manufacturer suggests a total power output of 70 watts and above for clams. There still remain concerns on whether LED bulbs are able to maintain their PAR outputs for over a year, as well as the wavelength of light it covers.
There is no artificial lighting stronger than the great ball of fire in our sky, so all clams can adjust to your lighting provided they are given proper acclimation. This is especially important to prevent light shock, and gives time for the natural population of the clam’s zooxanthellae to adapt to their new habitat and photo conditions. Acclimation can easily be done using egg crate screening, or other materials that cut/diffuse lighting intensity. Remember, switching from fluorescents to metal halides is a big change, and so is switching from 150 watts to 250 watts. Light switch from any system to LEDs must be done slowly and cautiously as some have reported bleaching of coral.
Fluorescent light bulbs need to be changed about every 10 months as they lose their intensity from old age. For metal halides, people have reported that 20ks lose their intensity at 6 months, 14ks around 8-10 months, and 10ks almost one year. I have not seen solid evidence in regards to this matter, but personally would change my 14ks every 10 months. The best way is to see if your clam has any loss in zooxanthellae. Fading or less vibrancy in the mantle is usually an indicator of this.
I do not know the origin of the pictures as they were saved in my computer so I cannot credit them appropriately. The ORA photo of the Deresa is obviously credited to Ocean Reef and Aquariums. The sole purpose of this thread is for the benefit of everyone, so please understand I hope this thread has been helpful to all of you.