Smooth Giant Clam
Tridacna derasa
The smooth giant clam is one of ten known subspecies of the Tridacna family of bivalves (two part hinged shells). Species within the family are all similar with slight biological differences primarily based on their geographic location. The second largest subspecies within the family, Tridacna derasa, is found in the tropical reefs of the western Pacific. They can be recognized by their large size and vibrant color patterns. Heavily fished in many areas for both their meat and shells, they are listed as Vulnerable by the International Union for Conservation of Nature (IUCN). The bright colorations result from algae, with whom they have a symbiotic relationship, that provides cover for the algae while also gaining nutrition from the sugar and proteins produced by the algae. Fossil remains date the species to the late Pleistocene era (126,000—11,700 years ago).
SPECIES IN DETAIL
Smooth Giant Clam
Tridacna derasa
CONSERVATION STATUS: Vulnerable
CLIMATE CHANGE: Vulnerable
Geographic Distribution
They are located in the Western Pacific ocean and areas surrounding Australia, Fiji, Philippines, Micronesia, and surrounding island chains.
Habitat
These clams can inhabit a variety of locations within their ecosystem. They are normally found attached to coral and rocky locations. They have also been observed on sandy bottoms. They reside in depths up to 20 meters (68 feet).
Physical Characteristics
As the name implies, this species of giant sea clam is distinguished by the smooth outer shell known as the mantle. Their shell is also asymmetrical (lacking symmetry) in shape and has bright colorations that are produced by algae growth. The pale spots on the outside of the mantle are known as windows. The mantle has four to five folds which can never be completely closed to allow the algae to receive sunlight. They contain several hundred small pin size eye spots around the border of the mantle that can detect changes in light which can alert the clam to potential predators through detection of shadows on the mantle. A strong muscular foot is used to burrow into the ocean floor and hold the clam in place.
Size
Adult smooth giant sea clams typically have a shell width of 60 centimeters (24 inches).
Diet
These clams are filter feeders and pull in zooplankton and phytoplankton for nourishment. In addition, they have a symbiotic relationship with algae that lives within the shell mantle. The clam provides protection for the algae; and in turn, the clam feeds on the sugar and protein that the algae produce through photosynthesis. The giant sea clams never fully close their mantle in order to allow the algae access to sunlight.
Reproduction
Reproduction occurs through the broadcasting of eggs and sperm into the water column. Giant clams are functional hermaphrodites, being able to produce both eggs and sperm. Eggs are very small at 1 millimeter (.004 inches) and several million are released at one time. Within a few hours after egg fertilization, free-swimming larvae develop. After approximately nine days, the larvae settle and attach on the seafloor and begin to metamorphose into juvenile clams in 8—10 days. They reach the adult stage in approximately 2—3 years.
Behavior
During the pediveligers (pre-juvenile) stage the clam moves along the seafloor looking for a suitable location to settle. Once settled and having reached adulthood the movement of the clams is limited. The pinhole eyes on the edge of the mantle can alert them to possible predators triggering the response to close their shells to protect their soft bodies. Studies indicate that movement is nocturnal with speculation this is also as a predator defense mechanism.
Adaptation
The clam has a large band of light sensitive eyes around the edge of the mantle which aids in recognizing potential predators. Their large size eliminates several predators that often prey on smaller clam species.
Longevity
Their lifespan is up to 100 years. The age of clams can be determined by counting the rings on the shell.
Conservation
They are listed as Vulnerable by the International Union for Conserving Nature (IUCN). The seven subspecies of the genera are listed as candidates for addition to the Endangered Species Act of 1973 and are currently covered by a number of regional fishing regulations by the islands they inhabit. They are protected in some countries by the Convention of International Trade in Endangered Species of Wild Fauna and Flora (CITES) of 1975. This prevents international trade in specified protective species, but this is not a universally agreed to program and many of the giant sea clams’ habitat areas are not covered. They are currently protected in several island locations by Regional Fishery Management Organizations (RFMOs) and they are on the list of candidates to be included within the Endangered Species Protection Act of 1973. Threats to smooth giant clams include overfishing for food and their ornamental shells, and warming water temperatures due to climate change. They are also affected by ocean acidification which can impact their ability to produce shells for protection. This clam species was one of the first giant sea clam species to be commercially bred. The expansion of this aquaculture could have a positive impact on future wild population growth.
Special Notes
Giant sea clams, like some other bivalves, have a series of small pin like eyespots that act as photoreceptors around the edge of the mantle. Studies indicate they respond to light changes in a way to detect possible predators approaching and will close their shell or change the orientation of the mantle in response.
SPECIES IN DETAIL | Print full entry
Smooth Giant Clam
Tridacna derasa
CONSERVATION STATUS: Vulnerable
CLIMATE CHANGE: Vulnerable
They are located in the Western Pacific ocean and areas surrounding Australia, Fiji, Philippines, Micronesia, and surrounding island chains.
These clams can inhabit a variety of locations within their ecosystem. They are normally found attached to coral and rocky locations. They have also been observed on sandy bottoms. They reside in depths up to 20 meters (68 feet).
As the name implies, this species of giant sea clam is distinguished by the smooth outer shell known as the mantle. Their shell is also asymmetrical (lacking symmetry) in shape and has bright colorations that are produced by algae growth. The pale spots on the outside of the mantle are known as windows. The mantle has four to five folds which can never be completely closed to allow the algae to receive sunlight. They contain several hundred small pin size eye spots around the border of the mantle that can detect changes in light which can alert the clam to potential predators through detection of shadows on the mantle. A strong muscular foot is used to burrow into the ocean floor and hold the clam in place.
Adult smooth giant sea clams typically have a shell width of 60 centimeters (24 inches).
These clams are filter feeders and pull in zooplankton and phytoplankton for nourishment. In addition, they have a symbiotic relationship with algae that lives within the shell mantle. The clam provides protection for the algae; and in turn, the clam feeds on the sugar and protein that the algae produce through photosynthesis. The giant sea clams never fully close their mantle in order to allow the algae access to sunlight.
Reproduction occurs through the broadcasting of eggs and sperm into the water column. Giant clams are functional hermaphrodites, being able to produce both eggs and sperm. Eggs are very small at 1 millimeter (.004 inches) and several million are released at one time. Within a few hours after egg fertilization, free-swimming larvae develop. After approximately nine days, the larvae settle and attach on the seafloor and begin to metamorphose into juvenile clams in 8—10 days. They reach the adult stage in approximately 2—3 years.
During the pediveligers (pre-juvenile) stage the clam moves along the seafloor looking for a suitable location to settle. Once settled and having reached adulthood the movement of the clams is limited. The pinhole eyes on the edge of the mantle can alert them to possible predators triggering the response to close their shells to protect their soft bodies. Studies indicate that movement is nocturnal with speculation this is also as a predator defense mechanism.
The clam has a large band of light sensitive eyes around the edge of the mantle which aids in recognizing potential predators. Their large size eliminates several predators that often prey on smaller clam species.
Their lifespan is up to 100 years. The age of clams can be determined by counting the rings on the shell.
They are listed as Vulnerable by the International Union for Conserving Nature (IUCN). The seven subspecies of the genera are listed as candidates for addition to the Endangered Species Act of 1973 and are currently covered by a number of regional fishing regulations by the islands they inhabit. They are protected in some countries by the Convention of International Trade in Endangered Species of Wild Fauna and Flora (CITES) of 1975. This prevents international trade in specified protective species, but this is not a universally agreed to program and many of the giant sea clams’ habitat areas are not covered. They are currently protected in several island locations by Regional Fishery Management Organizations (RFMOs) and they are on the list of candidates to be included within the Endangered Species Protection Act of 1973. Threats to smooth giant clams include overfishing for food and their ornamental shells, and warming water temperatures due to climate change. They are also affected by ocean acidification which can impact their ability to produce shells for protection. This clam species was one of the first giant sea clam species to be commercially bred. The expansion of this aquaculture could have a positive impact on future wild population growth.
Giant sea clams, like some other bivalves, have a series of small pin like eyespots that act as photoreceptors around the edge of the mantle. Studies indicate they respond to light changes in a way to detect possible predators approaching and will close their shell or change the orientation of the mantle in response.