Although toxic, physically destructive, and produced solely by cnidarians, cnidocysts are acquired, stored, and used by some predators of cnidarians. Despite knowledge of this phenomenon for well over a century, little empirical evidence details the mechanisms of how (and even why) these organisms use organelles of cnidarians. However, in the past twenty years a number of published experimental investigations address two of the fundamental questions of nematocyst acquisition and use by cnidarian predators: 1) how are cnidarian predators protected from cnidocyst discharge during feeding, and 2) how are the nematocysts used by the predator?
Nudibranchs that feed on cnidarians must defend themselves from the prey's nematocysts or risk their own injury or death. While a nudibranch's mucus has been thought to protect the animal from nematocyst discharge, an inhibition of discharge by nudibranch mucus has never been shown. The current study investigated whether mucus from the aeolid nudibranch Aeolidia papillosa would inhibit nematocyst discharge from four species of sea anemone prey. Sea anemone tentacles were contacted with mucus-coated gelatin probes, and nematocyst discharge was quantified and compared with control probes of gelatin only. Mucus from A. papillosa inhibited the discharge of nematocysts from sea anemone tentacles. This inhibition was specifically limited to the anemone species on which the nudibranch had been feeding. When the prey species was changed, the mucus changed within 2 weeks to inhibit the nematocyst discharge of the new prey species. The nudibranchs apparently produce the inhibitory mucus rather than simply becoming coated in anemone mucus during feeding. Because of the intimate association between most aeolid nudibranchs and their prey, an adaptable mucus protection could have a significant impact on the behavior, distribution, and life history of the nudibranchs.
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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