I was searching online to see if I could find that aquaculture study on baby
T. maxima clams that showed that the growth rate and survival rate was substantially higher in a hatchery fed with live phytoplankton than it was in a control hatchery with no feeding of any kind but I didn't find it.
However, I did come across something interesting. Baby gigas clams get as much as 65% of their nutrition from filter feeding, falling off to 34% when they reach 10g dry weight. I guess that's still in line with the 40% figure one of the leading clam hobby experts uses because the 65% figure is for really, really tiny clams.
"Giant clams of the family Tridacnidae are familiar and conspicuous residents of shallow coral reefs throughout much of the tropical Indo-Pacific. Their most characteristic feature is the enlarged, upwardly directed and usually brightly coloured mantle, which is packed with symbiotic dinoflagellate zooxanthellae. An estimated 95% of the carbon fixed by these algal symbionts is translocated to the host (Fitt 1993, Klumpp & Griffiths 1994), where it normally provides sufficient energy to cover at least the immediate metabolic needs of the hosts (Tench et al. 1981, Fisher et al. 1985, Mingna 1988, Klumpp & Griffiths 1999).
"Giant clams are also able to filter food particles from the water column (Yonge 1936), although the nutritional significance of this has only recently been quantified. Fitt et al. (1986) first quantified ingestion and digestion of C14-labelled phytoplankton cells by Tridacna gigas, while Klump et al. (1992) showed that particulate food constituted 65% of carbon needs in small (0.1g dry tissue) T. gigas, declining to 34% in individuals of 10g weight. This capacity to exploit both heterotrophic and autotrophic sources of nutrition, plus an ability to divert an unusually high proportion of energy to growth (Klumpp et.al. 1992), are no doubt factors that have allowed
T. gigas to become the largest bivalve ever to have existed."
Reference
