I don't know of any chemical method of defense (toxins, repellents,
disagreeable tastes, etc.) utilized by bivalves.  Most bivalve
methods of avoiding becoming someone else's lunch seem to fall into
one of 3 categories - active escape, modification of the shell, and
concealment.  In the first category, Pectinidae are the obvious
champions.  When threatened, they just clap their shell valves
together and swim away (with the exception of some forms that attach
by a byssus).  Some related groups, like Limidae, can do the same
thing.  Other bivalves may escape capture by rapidly burrowing deeper
into sediment.  But most bivalves are not very agile, and if they have
any defense at all,  it is usually of a more passive nature.  Shells
too heavy to penetrate, and projecting spines that prevent their
being swallowed whole (Acanthocardia aculeata, Spondylus imperialis,
Pitar lupinaria) are the commonest modifications that provide
protection against at least some potential predators.  The last
category includes burrowers in hard substrates (Pholas, Zirfaea,
Martesia, Teredo, Lithophaga, etc.).  It also includes commensal
species which live in the bodies of, or in the burrows of other
organisms (Lepton, in the burrows of shrimp and worms;
Phylyctaenachlamys, in shrimp burrows; Vulsella, in sponges;
Entovalva, in sea cucumbers; Modiolaria, in sea squirts; etc.).  And
of course a number of bivalves share a method of defense against
parasites and other small invasive organisms, namely, covering the
intruder with secreted shell material.  A parasite looking for an
oyster dinner may end up instead as the nucleus of a pearl.  Many of
the irregular lumps and bumps on the inner surfaces of pearl oysters
and other bivalves represent such entombed organisms.  This is the
only method I can think of by which a bivalve can actually take
direct action against another organism that is attacking it.  It
seems that the more vulnerable bivalve forms, which have little or no
means of defense, compensate by reproducing in large numbers.  If
there are enough of them, then a large percentage of each generation
can fall prey to predators, while still leaving sufficient numbers of
individuals to produce the next large generation.
Paul M.
Rhode Island