Fringeback NudibranchFringeback nudibranchs (Dendronotus frondosus) Known to feed on hydroids (Hydrozoa). The hydroids typically belong to genera such as Obelia and Laomedea. Nudibranchs will eat hydroids by using their specialized radula (toothed organ) to scrape the polyps from the colony. The nudibranchs also have the ability to incorporate the hydroid's stinging cells (nematocysts) into their own tissues, which may serve as a defense mechanism.