Serotonin‐induced parturition in the fingernail clam Sphaerium (Musculium) transversum (Say)

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Abstract
Parturition in the freshwater fingernail clam Sphaerium (=Musculium) transversum (Say) was induced by external application of both 10−4 and 10−3 M serotonin [5‐hy‐droxytryptamine (5‐HT)]. 5‐HT induced release of viable juveniles within 30–60 min of application. Parturitions in 5‐HT (10−3 M) occurred sooner and more often than in 5‐HT (10−5 or 10−4 M) or in artificial pond water. The numbers of juveniles per parturition in 5‐HT varied from one to nine. The parturition response to 5‐HT was completely blocked by 10−4 M methiothepin (a vertebrate 5‐HT1/5‐HT2 receptor antagonist) known to bind with high affinity to gastropod 5‐HT receptors and to block 5‐HT‐induced spawning in other freshwater bivalves. However, parturition was not induced by 8‐OH‐DPAT (8‐hydroxy‐dipropylaminotetralin HBr, a vertebrate 5‐HT1A receptor agonist), a potent inducer of spawning in freshwater bivalves. Thus, the pharmacological profile of the 5‐HT receptor(s) mediating parturition in S. transversum may be unlike that mediating spawning in other bivalves. This is the first report of 5‐HT‐induced parturition in any brooding bivalve.