I am interested in getting a giant clam for my reef tank. I wanted to get a small one because they are cheaper, but I was told that small clams 1 to 2 inches in size are more dependent on filter feeding than on lights. Is this true? Do I have to feed my clam if I get a small one?
Well, the answer to whether or not juvenile tridacnid clams are more dependent on suspension feeding than older clams is both yes and no. It is not quite true that small clams in the 1-2 inch size range are more dependent on filtering particles from the water column than from nutrients provided by photosynthetic symbionts, because it depends to a great deal on the species of giant clam in question. It is true, however, that as tridacnid clams grow, their dependence on nutritional input from suspension feeding tends to decrease, but again, the relative amount of nutrition derived from each photosynthesis and suspension feeding, and the degree to which the ratio changes with age and/or size of the clam, depends on the species in question and, to a large extent, the conditions under which they find themselves.
The information that you've been told comes from the research of Klumpp and colleagues which showed that ~75% of the phytoplankton (2-50 Φm) passing over the Great Barrier Reef was captured and eaten by giant clams (Tridacna gigas). These same researchers found that juvenile T. gigas obtained ~65% of their carbon needs from filtering phytoplankton rather than from photosynthetic inputs by their zooxanthellae (and under some conditions, filter-feeding could provide up to 100% of their needs). However, this proportion changed as the clams aged, and in adult clams the proportion of their nutritional requirements met by photosynthesis as compared to suspension feeding was essentially reversed – depending on the conditions under which the clam was found, output from photosynthetic symbionts provided roughly 60-100% of the carbon budget of the clam. The pattern of reliance on suspension feeding in juveniles switching to autotrophy (living off the photosynthetic output of symbionts) in adults was strongest in the clams T. gigas and Hippopus hippopus, and for these species it is likely true to make the generalization that you have been told. For other species, such as T. crocea and T. derasa, however, the claim that juveniles are more dependent on filter-feeding than on light for their endosymbionts is almost certainly false.
Although all clams studied to date do show decreased reliance on suspension feeding and increased ability to provide for their energetic needs via photosynthesis of their symbionts with increased size, not all clam species show nearly such a strong effect as those described above. In T. derasa, for example, filter-feeding under natural lighting (at depths of less than 5m) accounts for only about 14% of the total carbon budget of juvenile clams (although that percentage increases rapidly with decreased irradiance in cloudy or deep water). Because the average aquarium is unlikely to have lighting intensity equal to that of full-strength sunlight in the habitats from which these clams are naturally found, it is unlikely that T. derasa would obtain that much of their energy budget from photosynthetic outputs in captivity, but the reliance on light as opposed to particulate feeding is likely to be much lower than described for T. gigas or H. hippopus above. In fact, for clams adapted to clear water conditions (especially T. crocea and T. derasa, but to a lesser degree T. maxima as well), light is always more important than filter-feeding in terms of nutritional requirements of the animal, because photosynthesis is the primary source of carbon, and suspension feeding of the clam accounts for something on the average of only 10-30% of the total carbon requirements (again, this depends on the specific species in question and the specific conditions under which they find themselves). Thus, for species such as T. crocea,T. derasa and T. maxima, it is not true to make the generalization that juvenile clams are more dependent on filter-feeding than on light for meeting thier energetic requirements. It is still true that these species are more reliant on filter-feeding as juveniles than as adults, and that clams deprived of sufficient light intensity may make up for some amount of deficit by filter-feeding, but in these species, the majority of their nutritional requirements appear to always be met by the photosynthetic output of their symbionts rather than by filter-feeding.
Given that, it is going to be much more important to provide some supplementary feeding to juvenile clams in an aquarium than to adults, and that some small degrees of insufficient light intensity in the aquarium can be overcome with supplemental feeding. So, if you’re going to feed something to your giant clam, what should it be? Well, again, the answer depends somewhat on the species in question and how capable these animals are of handling high concentrations of particulates in the water column. In general, however, giant clams feed primarily on large phytoplankton and very small zooplankton in the range of roughly 2 to perhaps 100 Φm. This means that traditional zooplankton surrogates such as baby brine shrimp and even rotifers are too large for the animals to filter, and it is only some tiny invertebrate larvae, ciliates and the like that are likely to be captured by the clams. There are a variety of suppliers now providing live, frozen or spray-dried phytoplankton cultures for feeding home aquaria, and any of these should be a good source of particles in the correct size range for the majority of clam species. However, it is important to realize that clams use their gills for both respiration (breathing) and filtering food from the water, and if a clam is overloaded with food such that the gills become clogged, the animal suffers from an inability to both feed and breath properly. That means that species particularly adapted to clear water (especially T. crocea and T. derasa) should not be fed heavily, and in fact, frequent heavy feedings may actually decrease growth rates of these animals rather than increase them, because the animals are forced to reduce pumping rates after concentrated phytoplankton is added to the aquarium, and then also expend energy to clear their gills to allow for proper gas exchange. Other species (such as T. gigas and H. hippopus) will likely only benefit from frequent heavy feedings. Supplemental feedings of phytoplankton for species poorly-adapted to turbid water (such as T. crocea and T. derasa) should be fed infrequently or with low cell concentrations such that the filtering capacity of the animal are not overloaded. Because these species are much less dependent on filtered particles than the others, the risk of clogging their gills may outweigh the potential benefits of specifically providing particulate food if you are not careful in moderating your feeding of phytoplankton.
Having said that, however, I still think that it is important to provide some extra nutrition in the form of phytoplankton, especially to juvenile clams but to adults as well, because suspension feeding provides additional nutrients besides carbon. I think this is a bit of a dirty trick that authority figures play by quoting percentages of the carbon budget provided by photosynthesis (whether talking about corals or giant clams). While carbon is indeed the currency of energy needs, nitrogen and other trace nutrients are also important and necessary for the health and growth of all animals, and I think it is important to realize that meeting energetic requirements (as measured by carbon) does not necessarily equal total nutrition. A number of studies have now shown that the photosynthetic rate of zooxanthellae is linked to feeding by the host animal, and host starvation leads to decreased output by the symbiotic algae as well. The general pattern that emerges from these studies is that well-fed animals (corals, anemones or giant clams) transfer nitrogen from the digestion of plankton prey to their symbiotic zooxanthellae, and this leads to a significant increase in the photosynthetic rate and energy transfer to the host. This result makes feeding all the more critical to the health and survival of even “photosynthetic animals,” because by starving the animals they are struck with a ‘double-whammy’: namely they are deprived of both the energy provided by the capture and digestion of prey, as well as losing the ability to properly fertilize their endosymbionts and therefore having decreased ability to gain energy from photosynthesis as well! In fact, in all cases where researchers have experimented with nutrient supplementation for giant clams, they have found that ammonium, nitrate or particulate enrichment (as nitrogen sources) has led to increased growth rates (sometimes as much as ~20% or more, although this again depends on the species and the conditions under which it is kept).
It is also worth noting that aside from the issue of feeding, natural survivorship in juvenile clams of these different species varies greatly, and studies following the growth and survival of protected baby clams in the field found that over a 2 year trial in the Solomon Islands, T. deresa had the best growth (15 cm) and survival rates (>90%), while T. maxima averaged about 8 cm and T. crocea about 5 cm of shell growth in the same time period. Survival rates of juvenile T. maxima and T. crocea, even in the wild, were much lower than that of T. deresa, with less than 40% of the T. maxima and T. crocea clams surviving beyond 18 months. Given the poor survival of caged juveniles of these species in the wild with natural lighting and particulate food availability, it is not surprising that survival of juvenile clams in our aquaria is less than stellar, and if you're buying one of these more sensitive clam species as a juvenile you should take into consideration that the chance of survival to adult size is less than 50% as well....