I assume you are asking about living bivalves?  There are a number of
anesthetizing or "narcotizing" agents that work well with bivalves, with a
reasonable margin of safety (I mean the safety of the bivalves. The safety
of the investigator of course depends on proper chemical-handling
technique).  Among the best are magnesium chloride, magnesium sulfate,
chloral hydrate, phenoxyethanol, and phenoxypropanol.

If you are asking about opening dried specimens, just soaking overnight in
water will usually do it.

Paul M.

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