Spawning of the lugworm Arenicola marina (L.) (Annelida: Polychaeta) occurs in both sexes following the release of a sex-specific maturation hormone from the prostomium. In males this has been termed sperm maturation factor (SMF). Chromatographic purification of SMF, using an in vitro bioassay, has led to it being putatively identified as the 20- carbon fatty acid 8,11,14-eicosatrienoic acid and this paper describes the induction of spawning in vivo by injection of 8,11,14-eicosatrienoic acid into sexually mature individuals. It is shown that spawning in male A. marina occurs following the injection of prostomial homogenate or 8,11,14-eicosatrienoic acid into the coelomic cavity of sexually mature specimens. The injection of 8,11,14-eicosatrienoic acid into the coelomic cavity of females does not cause spawning although it can be induced by a hormone present in the prostomium. There are, therefore, clear sex-specific differences in the chemical signal that causes spawning in this species. Sex-specific differences in the behaviour of spawning animals are also described. The study identifies a role for the fatty acid 8,11,14-eicosatrienoic acid in the spawning of male A. marina. This is the first description of a hormone in the annelids, for which both the chemical structure and the endocrine role are known.

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