Cnidosac-Related Structures in Embletonia (Mollusca, Nudibranchia) Compared with Dendronotacean and Aeolidacean Species

In defense against attack by predators, cnidosacs in the tips of cerata (dorsal appendages) of aeolidacean nudibranchs discharge masses of mature nematocysts that are derived from cnidarian food. Cnidosac-related structures in various nudibranchs may provide a reconstruction of potential steps in the evolution of cnidosacs. Such structures in the cerata of the two valid species of Embletoniidae, an enigmatic nudibranch family, are described in this report, and compared to cnidosacs in dendronotaceans and aeolidaceans. The Embletonia spp. ceratal tips are characterized by cnidophages, which are largely undifferentiated cells that take up the nematocysts from the digestive tract lumen and transport them to different surface locations of the cerata, where they are released. Organized muscular bags that force out the nematocysts, as those found in cnidosacs, are absent. These cnidosac-related structures and other characteristics weaken the case of including Embletoniidae incertae sedis within Aeolidacea.