Particularly as aquarium animals, the giant clams (Tridacna and Hippopus spp.) attain
impressive sizes. They are, in fact, among the largest of all invertebrate animals. The group
claims the most massive living marine bivalve mollusk species. Some giant clams really are
gigantic; THE giant clam, T. gigas, can weigh in at well over 400 pounds and live for a century.
These strange and beautiful creatures do not become hulks overnight. Still, their growth rates
are pretty high, especially considering where they live. They undoubtedly owe their rapid growth
to their broad, and somewhat opportunistic, feeding strategy.
Giant Clams have a diverse palate
Coral reefs are typically very nutrient-poor environments. A good meal just doesn’t come easy
on a reef. Many animals there adapt by specializing on a particular food source whereas a few
others such as giant clams become highly inclusive.
Tridacnids are somewhat unusual among their bivalve brethren in that they form mutualistic
relationships with endosymbiotic dinoflagellates (zooxanthellae). These zooxanthellate algae
(the same species that live in “photosynthetic” corals, actually) reside mainly in the clam’s outer
mantle tissue where they have best access to sunlight. During the daylight hours, the clam
opens its shell widely and exposes its mantle to the sun. The algae (in return for getting a great
place to live) provide a substantial amount of the carbon that is assimilated or respired by the
clam–something like 34% for mature individuals and 65% for very young individuals.
Some experts insist that (along with nutrients taken up directly from the surrounding waters via
special epithelial tissues) it is possible for tridacnids to survive indefinitely off of the fuel
produced by their zooxanthellae. To be clear, the clam’s association with the photosymbiont is
obligate. Even so, it appears that giant clams prefer a rather varied diet. One might even
consider them to be omnivorous. Aside from that produced by their symbionts or absorbed from
the waters, all of a giant clam’s food is obtained by regular ol’ bivalve filter feeding. Though
(even heavily fed) tridacnid clams cannot survive solely by filter feeding, the foods they capture
seem to contribute significantly to their growth and development.
A whole lot of things can be found in a giant clam gut. These items are all miniscule in size
(mostly within 2-50 microns); they are, however, highly varied in composition. Gut contents
range from detritus to tiny zooplankton. Tridacnids even consume unneeded zooxanthellae. But
a substantial portion of the suspended material that giant clams capture and eat consists of
assorted phytoplankton. This shouldn’t be all that surprising as many of the giant clam’s closest
relatives (e.g. mussels) live almost entirely on phytoplankton. And let’s not forget that
phytoplankton is incredibly nutritious!
Microveggies and macronutrients
Phytoplankton is indeed some wholesome stuff. While the specific content of each species
varies a bit, phyto is generally complete and well-balanced nutritionally. It also includes a host of
essential vitamins (e.g. vitamin C), many of which cannot be synthesized by animals.
For tridacnid clams and all planktivores, phytoplankton is especially valuable for its high long-
chain fatty acid content. While these unsaturated fatty acids can be utilized as an energy
source, they are required by the feeding animal (which cannot produce them itself) to maintain
proper cell structure. Marine phytoplankton are a huge source of omega-3 fatty acids. Omega-3s
such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are essential
components in the diets of pretty much all marine animals.
Thus, while phyto is not necessarily required to satisfy a giant clam’s energy needs, it does act
as a rich vitamin/dietary supplement. And it even provides a little roughage (i.e. cellulose) for the
animal’s digestive system.
Feeding your captive bivalves
If you keep giant clams in captivity, you will definitely want to try dosing phyto. Lots of other
inverts, including a wide range of corals, will appreciate the feedings as well. Where feedings
are frequent, it may be possible to keep other (yet obligately phytoplanktivorous) bivalves such
as certain oysters and scallops. Tridacnid clams certainly do not mind generous feedings so
long as the aquarium filter system is adequate and water is replaced often.
For maximal nutrition, for minimal waste and for best palatability, a live phytoplankton product is
ideal. Better yet is a live product such as OceanMagik phytoplankton that includes a nutritionally
complementary mix of phyto species.
Put simply, you will not succeed in keeping giant clams unless you provide them with an
intense, full-spectrum lighting system such as the Kessil h380. Tridacnids depend upon their
zooxanthellae, and the zooxanthellae depend upon bright illumination.
That being said, tridacnid clams do eat phyto. And they love it. Younger, smaller, fast-growing
specimens in particular appreciate the additional source of energy, protein, vitamins, fatty acids,
etc. Considering how beautiful and delicate they are, and how long they can potentially live, it is
worth the small effort to provide your captive giant clams with a complete diet. And regularly
using live phyto is certainly the first step in that direction!