This is one myth that we have all heard and most likely passed on.
Clams under 2" must be fed to stay alive, because they are not able to live photo-synthetically yet.
This is one of the most commonly passed on pieces of clam wisdom that you can find on the web, and as an avid forum poster, I am no exception. When I heard that, it seemed to make sense to me and I was happy to pass it on to others, but has anyone actually looked into this word of mouth rumor to find the fact behind it?
Watching my tiny clams thrive, I started to wonder if this was truly based on fact, so I decided to look into the subject more deeply. I was more than a little surprised when I found that this is in fact a complete myth. I must say right now before getting into this further that smaller clams ARE in fact less hardy, but not for reasons of nutrition.
First and foremost we must look at why a clam feeds in the first place. They feed as a source of nitrates which they require to live, but this is not the only source of nitrates that exist in a reef by any means. Clams are fully capable of assimilating nitrates directly from the water at rates capable of replacing feeding all together.
Clams begin ingesting their symbiotic algae (zooxanthellae) before they are even one millimeter long! At this point there growth rate is extremely slow and the zooxanthellae reproduce at an amazing rate making them capable of photosynthesis at a very tiny size. This was my first surprise, but there is still the argument that their tiny mantle has less surface area and thus cannot gather as much light as a larger clam.
This is where the real science started to come out. While the fact that they possess less surface area for zooxanthellae to live on while they are small is obvious, you must consider density of the zooxanthellae. It has been found in several prominent studies that for unknown reasons density actually goes down as a clam matures, which means that a small clam actually has more ability to produce nutrients from light proportionally to mantel size. You must also consider how much energy a clam must produce to live and grow. As a clam's mantel grows it's shell grows rapidly out of proportion and consumes more and more energy the bigger the clam gets. Clams internal soft parts also gain mass out of proportion and need much more energy the bigger the clam gets. So a larger clam not only has a less efficient mantel, but it also requires more energy.
With all that said, I must again point out that in no way am I saying small clams are easy. They are more susceptible to parasites, illness and damage, but it should be known that they in no way require more food than large ones.
As a side note, hippopus clams are in fact exactly as the rumor states. When smaller they begin thickening up the bottom of their valves right away and consume much more energy doing so. Hippopus clams should be fed regularly when young.
If you are interested in further information on the topic, James Fatherree's book, Giant Clams in the Sea and the Aquarium, and all cited sources are a great place to start.
