I tried talking about this here a while ago and people jumped all over me instead or reading the info posted, hopefully people will read the research this time. Saying that clams under 3" are total reliant on filter feeding is incorrect. They can sustain themselves fully through the energy provided by their symbiotic algae. Clams do not need to filter plankton to gain nutrients for themselves or their symbiotic algae, they can absorb raw nutrients (nitrogen&phosphorus)right from the water. I did a write up on this with links to research that proves this here Incase you dont like the link ill copy and paste it here. Please give it a look over and follow the links. So what is the truth about feeding tridacnids? How do tridacnid clams acquire nutrients? In this thread I will do my best to lay everything out for you. I will post quotes and links from, and to research in the hopes you will have a better understanding of how these animals work. Its an old myth that clams under X inches need to filter feed. Tridacnid clams are not dependent on filter feeding phytoplankton no matter what size they are. They are primarily photosynthetic and can sustain themselves on the food provided by their zooxanthellae, through light alone. Clams will extract dissolved nitrogen and phosphorus from the water and pass this onto there zooxanthellae and then the zoox gives the clam sugars as food. They do not need to filter particulate or extract nutrients through the gills. They can absorb nutrients directly through their mantles (Hypertrophied Siphonal Epidermis). From INTRACELLULAR DIGESTION OF SYMBIONTIC ZOOXANTHELLAE BY HOST AMOEBOCYTES IN GIANT CLAMS (BIVALVIA:TRIDACNIDAE), WITH A NOTE ON THE NUTRITIONAL ROLE OF THE HYPERTROPHIED SIPHONAL EPIDERMIS Clams will filter phyto (and bacteria and zooplankton) but when they do this all they are doing is extracting the same N & P and passing it to the zoox. One of the arguments for feeding clams is that clams mantles are not fully developed untill they are 3" or 4" in length. This is completely false. From the paper. The Zooxanthellae Tubular System in the Giant Clam Clams mantles are fully developed and full of zoox within weeks of metamorphosis. The above link and quote is from a study on the ZTS and in it it states “During a study of the anatomy and histology of giant clams, numerous Tridacna gigas, from a few millimeters to 35 cm in shell length, were dissected.” And not once does it mention a different stage of development of the ZTS between the clams of “a few millimeters to 35 cm”. From this paper Establishment of the photosymbiosis in the early ontogeny of three giant clams So this paper states that with in 20 days the ZTS is developed and the symbiosis is established. Another one is that clams mantles are not large enough to house enough zoox to support the clam untill its 4", false again. The size of a clams mantle is proportionate to the size of the clam through out its life. Another argument some people have for feeding phyto is that clams have a fully functioning digestive system and that if they didn't need to filter feed they wouldn't have this. So lets look at this. Clams gills are multifunctional, they are use for respiration and capturing particulate matter. They can't get rid of the gills or they wouldn't be able to breath , clams also constantly replenish there zoox, they use their gills to do this. From this paper The Zooxanthellal Tubular System in the Giant Clam A clams stomach is connected to it's zooxanthellae tubular system (where the zoox live) the stomach passes new zoox from the gills to the ZTS , processes the sugars the zoox make (to feed the clam) and pass old, dead and unviable zoox to the anus. Even though the digestive system isn't needed for filtering phyto, it is still used as a basic function of the clam. If you want to feed your clams phyto, go ahead but don't think that they will die if you don't. As long as you have strong lighting and N & P (fish pee and poo) in the water the clam will do just fine ! Here's a few more snippets from research papers to back up what i say. From Contributions of phototrophic and heterotrophic nutrition to the metabolic and growth requirements of four species of giant clam (Tridacnidae) This study actually indicates that clams may need to acquire additional nutrients through filter feeding as they grow larger. However their zoox through photosynthesis can still provide them with at least 2x there Carbon Energy needs. This go against the notion that very small clams need to be feed phyto. This next study was done to determine how clams acquired there zoox and what they did with them. Two sets of clams were used, one was given zoox the other was not. They were both kept in micro filtered water and not allowed to receive any particulate matter. The only particulate that one set received was its initial dose of symbiotic zoox. These are very tiny clams, the kind everyone says cant live through photosynthesis alone. They did just fine. JSTOR: An Error Occurred Setting Your User Cookie Fatherree 2006 So according to that they are getting C/E from photosynthesis just fine. People often bring up an article that was written by Dr. Shimek commissioned by DT's. How much money do you think DT's would have paid him if the conclusion was that clams are not dependent on phyto? No offense to Dr. Shimek, but i dont think this was his best work. The references used are old and out dated. Some of the claims made in it are completely false and show either sloppy research by the author, or selective research to come to a desired conclusion. Whats the magic # in that article? 4" I think. A crocea at its fastest growth rate, grows about 3/4" per year. So it would take a crocea at least 5.33 years to get to 4". Gigas has an average growth rate of about 3" per year, and at its fastest growth almost 4.5" per year. So it can achieve the magic 4" in one year. Why more then 5 years for crocea and barely 1 year for gigas to fully develop there mantles or house enough zoox to support the clam? It doesn't make any sense. If its going to take clams so long to be able to use zoox for photosynthesis why do the start collecting them between 2 and 4 weeks after fertilization while there still pediveligers? Only to wait 1 to 5 years to see the benefit. Lets look at just crocea for a minute. I think everyone would agree that crocea is considered to be the most light demanding of all the clams. They are most commonly found in very shallow water of just a few feet. They can be sporadically found down to about 20', no more. And we all know that clams are broadcast spawners. There eggs and sperm are at the mercy of the currents for up to a month then settle out. They have no control where they settle and I'm sure that many more larva settle deeper then 20' then that do. If they are so dependent on filtering phyto how come there aren't a bunch of small croceas under 4" at 30', 40', 50' deep? There's plenty of phyto down there for them. They should be able to do just fine down there filtering away untill the magic 4" comes along and then they would just die. I want to give special credit to James Fatherree for this write up. His book was a real eye opener for me and some of the comments I made above were sparked from personal communications with James.