Giant Clams are hermaphrodites (having both male &
female organs). However, they reproduce sexually,
not by self-fertilization.

A 'Spawning Induced Substance' is released into the water
from the outlet siphon. Detection of SIS through the
inlet siphon stimulates other giant clams to swell their mantle,
contract adductor muscles & vigorously release eggs & sperm.

Since Tridacnas cannot move, this broadcast spawning
('synchronized release' of sperm & eggs) reduces the burden
of finding a compatible mate, simultaneously doubles
the number of offspring produced, and ensures that new
gene combinations are passed to further generations.