Repugnatorial glands located in the marginal papillae of the intertidal ochidiid pulmonate Onchidella borealis secrete a viscous fluid in response to mechanical or chemical stimulation. In laboratory encounters, this fluid repels intertidal predatory asteroids, particularly Leptasterias hexactis, but not predatory gastropods, polyclad turbellarians, nemerteans, or fishes. Intertidal crabs consume dead O. borealis readily, but seldom consume living individuals capable of firing their glands. The vertical range of O. borealis overlaps that of L. hexactis, whereas limpets that are vulnerable to predation by the sea stars generally live higher on the shore. On a small scale, O. borealis and L. hexactis occupy similar microhabitats (e.g., crevices, algal holdfasts), but are seldom found together. Field and laboratory experiments suggest that this negative spatial correlation may result from expulsion of the sea stars by onchidiids.
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