Actually, when they hatch some clams don't look
much like clams at all. Clams are a form of
bivalve (2 shell) mollusks. Bivalves usually have
larvae that swim freely in the water or become
parasites on fish gills, unlike their adult forms.
Why does it make sense for an animal that
moves around very little as an adult to have a
very mobile offspring? Northeast Fisheries
Science Center has some fun facts on bivalves.
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
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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