DIVE DRY WITH DR. BILL #818: LAND OF THE GIANTS
Back in my days in Boston while at Harvard, I developed a real taste for clam chowder. None of that red Manhattan nonsense... give me the real thing! I still eat it for lunch and occasionally dinner, thanks in part to our new Vons market with its great variety of choices. Of course I still stick to the tried and true... and buy Progresso New England clam chowder... but only when it is on sale since I'm a starving marine biologist.
On several of my international dive trips to the tropics of the western Pacific, I've filmed clams that would require a large cement mixer or huge vat to make chowder from. Yep, I'm referring to the monstrosities known as giant clams (Tridacna ssp.). I've observed and or filmed them in places like Australia, the Philippines and Palau. Their original geographic distribution was throughout much of the Indo-Pacific.
There are several species in the genus, but the one that most often is called the giant clam is Tridacna gigas. It is often difficult to tell smaller giant clams from their relatives, but as they age the distinguishing features emerge. Adult T. gigas clams are unable to fuilly close their shells, thus portions of the mantle can always be seen. Their shells also have fewer vertical folds (4 to 5) than other species.
The giant clam has been documented as large as four feet six inches in length for an individual found in Indonesia back in 1817. Its weight including the shell was estimated at 550 pounds. Today they may still reach four feet across and about 450 pounds if allowed to grow to a ripe old age. Their lifespan is believed to be about 100 years. Not so many years ago it was referred to as the "man-eating clam" and was thought to be able to trap humans by closing its shell. Recently a false "news" article was spread about a diver in Hawaii whose male organ was trapped inside one.
The giant clam is generally a shallow water species, living above 66 feet depth. In part this is due to the fact that larger specimens incorporate dinoflagellates (zooxanthellae) in their mantle tissues and they require light to photosynthesize. These tiny organisms are responsible for the varied colors of the exposed mantle. Tridacnids are filter feeders. The young receive much of their nutrition from plant plankton filtered out from the surrounding waters. Older ones get more of their sustenance from the zooxanthellae. In turn the zooxanthellae receive nutrients and protection from the clams.
I don't want to disappoint my readers by not talking about their sex lives. Actually it is pretty boring from a human perspective. T. gigas is a hermaphrodite, containing both male and female organs. They cannot fertilize their own eggs, but can with any other giant clam in the neighborhood. There is no "courtship" involved... however there is a chemical substance that triggers mass spawning. They simply cast their gametes into the surrounding water through contractions of the shells... and then hope and pray. The planktonic larvae are rather short-lived and settle to the bottom after about a week.
Their marine predators include various fish, moray eels, starfish and even smaller molluscs! Sadly, it and some of its relatives are also endangered due to human exploitation. Some are harvested for food while others enter into the aquarium trade. The giant clam is considered a delicacy in some countries. The Chinese would sell parts of the adductor muscle, which closes the two shells, as an aphrodisiac! I've tried oysters but never giant clams it so I can't verify its effects, but scientific researchers have discovered that amino acids in the muscle can increase the production of sex hormones.
I have even heard of T. gigas being sold for display in home aquaria. It makes me think of people who used to purchase baby alligators when I lived in Florida back in the 1950s. The little ones might be cute... but they get big and unmanageable with time. Imagine the size aquarium a mature giant clam would need! And where would an aquarist obtain the massive amount of plankton to nourish them?
Conservationists are concerned that these clams may be vulnerable to extinction. Fortunately there are "captive breeding" (hatchery) programs that raise various species of Tridacna to release them back in the ocean. Some operations also raise them for the aquarium trade which decreases the pressure on wild stocks. There are also aquaculture operations that grow them for human consumption, especially in the South Pacific and Asia.
© 2019 Dr. Bill Bushing. For the entire archived set of over 800 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Giant clam with shell open and filtering gills; two color variants of tridacnid clams due to photosynthetic zooxanthellae.
Back in my days in Boston while at Harvard, I developed a real taste for clam chowder. None of that red Manhattan nonsense... give me the real thing! I still eat it for lunch and occasionally dinner, thanks in part to our new Vons market with its great variety of choices. Of course I still stick to the tried and true... and buy Progresso New England clam chowder... but only when it is on sale since I'm a starving marine biologist.
On several of my international dive trips to the tropics of the western Pacific, I've filmed clams that would require a large cement mixer or huge vat to make chowder from. Yep, I'm referring to the monstrosities known as giant clams (Tridacna ssp.). I've observed and or filmed them in places like Australia, the Philippines and Palau. Their original geographic distribution was throughout much of the Indo-Pacific.
There are several species in the genus, but the one that most often is called the giant clam is Tridacna gigas. It is often difficult to tell smaller giant clams from their relatives, but as they age the distinguishing features emerge. Adult T. gigas clams are unable to fuilly close their shells, thus portions of the mantle can always be seen. Their shells also have fewer vertical folds (4 to 5) than other species.
The giant clam has been documented as large as four feet six inches in length for an individual found in Indonesia back in 1817. Its weight including the shell was estimated at 550 pounds. Today they may still reach four feet across and about 450 pounds if allowed to grow to a ripe old age. Their lifespan is believed to be about 100 years. Not so many years ago it was referred to as the "man-eating clam" and was thought to be able to trap humans by closing its shell. Recently a false "news" article was spread about a diver in Hawaii whose male organ was trapped inside one.
The giant clam is generally a shallow water species, living above 66 feet depth. In part this is due to the fact that larger specimens incorporate dinoflagellates (zooxanthellae) in their mantle tissues and they require light to photosynthesize. These tiny organisms are responsible for the varied colors of the exposed mantle. Tridacnids are filter feeders. The young receive much of their nutrition from plant plankton filtered out from the surrounding waters. Older ones get more of their sustenance from the zooxanthellae. In turn the zooxanthellae receive nutrients and protection from the clams.
I don't want to disappoint my readers by not talking about their sex lives. Actually it is pretty boring from a human perspective. T. gigas is a hermaphrodite, containing both male and female organs. They cannot fertilize their own eggs, but can with any other giant clam in the neighborhood. There is no "courtship" involved... however there is a chemical substance that triggers mass spawning. They simply cast their gametes into the surrounding water through contractions of the shells... and then hope and pray. The planktonic larvae are rather short-lived and settle to the bottom after about a week.
Their marine predators include various fish, moray eels, starfish and even smaller molluscs! Sadly, it and some of its relatives are also endangered due to human exploitation. Some are harvested for food while others enter into the aquarium trade. The giant clam is considered a delicacy in some countries. The Chinese would sell parts of the adductor muscle, which closes the two shells, as an aphrodisiac! I've tried oysters but never giant clams it so I can't verify its effects, but scientific researchers have discovered that amino acids in the muscle can increase the production of sex hormones.
I have even heard of T. gigas being sold for display in home aquaria. It makes me think of people who used to purchase baby alligators when I lived in Florida back in the 1950s. The little ones might be cute... but they get big and unmanageable with time. Imagine the size aquarium a mature giant clam would need! And where would an aquarist obtain the massive amount of plankton to nourish them?
Conservationists are concerned that these clams may be vulnerable to extinction. Fortunately there are "captive breeding" (hatchery) programs that raise various species of Tridacna to release them back in the ocean. Some operations also raise them for the aquarium trade which decreases the pressure on wild stocks. There are also aquaculture operations that grow them for human consumption, especially in the South Pacific and Asia.
© 2019 Dr. Bill Bushing. For the entire archived set of over 800 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Giant clam with shell open and filtering gills; two color variants of tridacnid clams due to photosynthetic zooxanthellae.
