Management of nematocysts in the alimentary tract and in cnidosacs of the aeolid nudibranch gastropod<Emphasis Type="Italic"> Cratena peregrina</Emphasis>

The nudibranch gastropod Cratena peregrina (Opisthobranchia: Aeolidaceae), when feeding on polyps of the hydrozoan Eudendrium racemosum (Cnidaria: Anthoathecata), devours masses of small microbasic eurytele and holotrichous isorhiza nematocysts. Large proportions of these nematocysts were found undischarged in the alimentary tracts of the snails. Feeding experiments in this study tracked the fate of nematocysts as they passed through the alimentary canal to the digestive gland in the dorsal appendages, the cerata, to the cnidosacs, and finally in the faeces. In digestive cells, many structurally intact nematocysts were present in large phagosomes that remained unaffected even after 2 days fasting, and phagosomes containing nematocysts were found in the faeces. Thus, it is inferred that fusion of nematocyst-containing phagosomes with lysosomes and subsequent digestion of nematocysts is blocked. Masses of exposed, undigested and structurally intact nematocysts were discarded in the faeces. In the tips of the cerata, other nematocysts were phagocytosed by cnidophages and stored in the cnidosacs. After release, in contact with seawater, cnidosac nematocysts were able to discharge. When cnidophore tentacles of E. racemosum with only holotrichous isorhizas were fed to the snails, the isorhizas arrived in the cnidosacs about 2 h after feeding and mixed with existing small euryteles. Some cnidosacs also contained very large microbasic eurytele or large macrobasic eurytele nematocysts, possibly from Eudendrium ramosum and Eudendrium glomeratum, respectively. This indicates that the various types of nematocysts from food were all incorporated into the cnidosacs. Evidence for a selection process or digestion of a certain nematocyst type in the cnidophages was not obtained. It is concluded that a large proportion of the nematocysts ingested with the food are not digested, but are eliminated, structurally and functionally intact, via the alimentary canal and the tips of the cerata.