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