Smooth Giant Clam/Southern Giant Clam/Derasa Clam

Smooth Giant Clam/Southern Giant Clam/Derasa Clam Tridacna derasa

Type of Animal:

Outer reefs/reef edges, intertidal coral areas, lagoons, coral outcrops, within Acropora corals, sandy flats, rubble piles, rock faces, found at depths of 1-65 ft

Indo-West Pacific, C Pacific, & S Pacific

Very large clam, smooth shell, lack of scutes (scale-like shell protrusions), colorful mantle has wavy stripe or spot pattern, large plain shell, 2nd largest of the giant clams

Make food for themselves in 4 ways-firstly, they have large amounts of symbiotic single-celled dinoflagellate algae (zooxanthellae) in tissues-due to these clams needing moderately lit to well-lit habitats, algae may make too much food for themselves giving extra carbon/glucose to clam. Secondly, zooxanthellae can be eaten by amoebid cells within host clam. Thirdly, they can absorb nutrients from water. Fourthly, they can act as filter-feeders, straining phytoplankton/zooplankton/detritus/algae w/ gills.

Status in Wild:

Use in aquaculture reducing demand for wild-caught clams

Mostly in colonies of 15-30 clams, though sometimes solitary as well

Additional Info:

Young: Trocophore
Group: Colony/Aggregation/Bed

90-400 lbs

12 hours

Life Span:
30-70 years in captivity, 150-250 years in wild

Adult: 18-23 in
Young: 4 in

Main predators are parasitic/predatory snails, fish, sea snakes, sea stars, crabs, lobsters, shrimp, bristleworms, fireworms, octopus, burrowing sponges, clam worms, & anemones.

Threatened due to aquarium trade, hunting for food, hunting for shells, medicinal use, habitat degradation/destruction, water pollution, & coastal development.

They’re very slow growers after reaching maturity.

Reproduction stimulated by lunar cycle, time of day, & presence of other eggs/sperm.

They detach byssus (tuft of long tough filaments) as they mature.

Most readily available giant clams in aquarium trade.

Mantle colors result of crystalline pigment cells.

Fun Fact(s):
Start life as free-swimming larvae, then developing into new larval stage capable of filter-feeding. Develop feet at 3rd larval stage, allowing it to alternately swim/rest. Become juveniles at around 10 days old, when they acquire zooxanthellae, starting symbiotic relationship. Juveniles become males at 2-3 years old, eventually becoming hermaphroditic once they reach 12 in.

Due to being hermaphrodites much of their life, sperm released 1st followed by eggs, avoiding self-fertilization.

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