Reproduction and development in the
molluscan class to which clams belong) is highly
variable, but generally, the sexes are separate
and many are broadcast spawners. Fertilization
often takes place in the plankton and an early
larval stage, the trochophore, develops.
This stage is relatively short lived and soon
transforms into a veliger. It is at this
point in development where shell formation usually
begins, originating from a plate-like area on the
dorsal surface of the larva.
At the time of metamorphosis, the velum
(used for propulsion through the water column
during its early planktonic existence) is shed and
the juvenile settles, typically spending the
duration its life on the ocean floor.
There are however exceptions
to this traditional developmental scheme. Direct
development and brooding are characteristic of
several species of small marine clams, with the
young being released from the parent's mantle
cavity as fully developed little clams.
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