If a clam is blue, that means it's reflecting blue light. Most of the lights we use in the hobby are emitting the vast majority of their energy in the blue wavelength, which means that the vast majority of the light energy is quite possibly being reflected by blue clams, leaving only a small fraction of the measured PAR to penetrate the tissue to reach the clam's zooxanthellae. This fraction may be enough energy for the clam when it's smaller, but as it grows (mass increases to the cubic exponent and surface area to the square exponent - not quite accurate with clams, but more accurate than not) this energy could become insufficient to support the clam's energy requirements. This could also be exacerbated by the fact that often times clams are under the typical BRS recommended SPS PAR levels (~275-350), which are insufficient to keep clams healthy in the long run as they increase in size, IMO/E. I think it's safer to keep any/all clams under 400+ PAR, *especially* if it's a blue clam.
Remember, you can't over illuminate a clam, the PAR they're exposed to at noon on a reef measures in the several thousand (really). Also don't forget that all clams were under that level of lighting shortly before they ended up in our aquariums, because they were either wild collected or aquacultured nearby their natural habitat, in vats under natural sunlight. Even the supposed 'deeper water' clams (if you can consider up to ~20m 'deep') were aquaculture in 50cm of water or less, and thus were receiving, you guessed it, several thousand PAR worth of light every single day.
All of my clams over ~15cm receive minimum 500 PAR, most ~650. Even at ~350-400 PAR I see their growth slow dramatically. And, when using lighting with significantly more energy in the blue spectrum, my blue clams always grow significantly more slowly than the clams with less/no blue.
Final thoughts, clams are far more dissimilar to coral than they are similar. These are much more complex organisms with much higher energy requirements, and those energy requirements change exponentially with growth. Coral is a lot more similar to, say, a plant, and a clam is a lot more similar to, say, a dog. A plant will typically find a way to stay alive (even if it doesn't thrive) with limited resources, but if you fed your dog the same amount when it's 2 years old as you did when it was 2 months old, it's going to slowly(ish) starve to death. Coral is essentially thin layers of tissue without a centralized energy requirement, clams have a circulatory system, eyes, organs (including a heart, gills, stomach, etc) large muscles, a large shell, etc and the entire organism needs enough energy to live, or it dies. There's no outward evidence of this slow death due to lack of energy, because a clam doesn't 'get skinny' like a dog would that was slowly starving, but 'mysterious clam deaths' really aren't mysterious: if it wasn't a parasite or disease or water chemistry problem, the culprit was almost certainly a lack of energy, most typically a lack of light energy.
Remember, you can't over illuminate a clam, the PAR they're exposed to at noon on a reef measures in the several thousand (really). Also don't forget that all clams were under that level of lighting shortly before they ended up in our aquariums, because they were either wild collected or aquacultured nearby their natural habitat, in vats under natural sunlight. Even the supposed 'deeper water' clams (if you can consider up to ~20m 'deep') were aquaculture in 50cm of water or less, and thus were receiving, you guessed it, several thousand PAR worth of light every single day.
All of my clams over ~15cm receive minimum 500 PAR, most ~650. Even at ~350-400 PAR I see their growth slow dramatically. And, when using lighting with significantly more energy in the blue spectrum, my blue clams always grow significantly more slowly than the clams with less/no blue.
Final thoughts, clams are far more dissimilar to coral than they are similar. These are much more complex organisms with much higher energy requirements, and those energy requirements change exponentially with growth. Coral is a lot more similar to, say, a plant, and a clam is a lot more similar to, say, a dog. A plant will typically find a way to stay alive (even if it doesn't thrive) with limited resources, but if you fed your dog the same amount when it's 2 years old as you did when it was 2 months old, it's going to slowly(ish) starve to death. Coral is essentially thin layers of tissue without a centralized energy requirement, clams have a circulatory system, eyes, organs (including a heart, gills, stomach, etc) large muscles, a large shell, etc and the entire organism needs enough energy to live, or it dies. There's no outward evidence of this slow death due to lack of energy, because a clam doesn't 'get skinny' like a dog would that was slowly starving, but 'mysterious clam deaths' really aren't mysterious: if it wasn't a parasite or disease or water chemistry problem, the culprit was almost certainly a lack of energy, most typically a lack of light energy.
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