Acnidosac is ananatomical feature that is found in the group ofsea slugs known as aeolidnudibranchs, a clade ofmarineopisthobranchgastropodmolluscs.[1] A cnidosac containscnidocytes, stinging cells that are also known as cnidoblasts or nematocysts. These stinging cells are not made by the nudibranch, but by the species that it feeds upon. They are obtained while digesting prey in a process calledkleptocnidy. Once the nudibranch is armed with these stinging cells, they are used in its own defense and to capture prey.[2]
The sea slugs within the nudibranch cladeAeolidida have protrudingcerata (singular "ceras") on their dorsal surface. At the tip of each ceras is a small sac in whichnematocysts (stinging cells) are stored. These nematocysts originate in thecnidarians (such assea anemones,hydroids,jellyfish,corals,siphonophores, etc.) that are the food source for aeolid nudibranchs.[3]
Glaucus atlanticus is a bluepelagic aeolid nudibranch. Individuals in this species can be dangerous for humans to handle; the cnidosacs ofG. atlanticus often contain particularly powerful venomous stinging cells from one of its food species: the pelagicsiphonophore known as thePortuguese Man o' War,Physalia physalis.[4]