Thou shalt covet thy neighbor’s cnidocytes

A small clarification, dear reader: in a recent post about the fantastic stinging cells of the Cnidaria (jellyfish and their relatives), it was stated that only cnidarians possess these cnidocytes. It is surely true that only cnidarians can make these barb cells. However, the animal kingdom has found these diverse structures useful enough that thievery of a sort has evolved, in lineages as distinct as comb jellies and sea slugs.

Consider the case of Microstomum lineare, a common resident of organic slimes in slow segments of flowing waters worldwide. These tiny flatworms spend most of the year eating detritus and dividing asexually into new clones. When in need of defense, however, the worms seek out and consume bits of the freshwater cnidarian Hydra, a favorite study organism of biologists.  The parts of Hydra that are consumed are digested by enzymes in the gut which leave intact only the stinging part of the cnidocyte. Cells of the gut then enclose these stinging cysts, pass them off to cells of the connective tissue, and ultimately, to the skin, where they are used as a means of defense and prey capture, much as the Hydras themselves use them. Remarkably, Microstomum has found a way to prevent these otherwise hypersensitive cysts from firing until the very end stage of this process of manipulation. The cysts persist in the skin until used, and can be passed onto clonal offspring, grand-offspring, and beyond. Even clonal lines that have not been exposed to Hydra for tens of generations will exhibit this behavior, but a Microstomum with a full stock of cnidocytes will ignore Hydra completely. Photos by Christopher Laumer.

--Christopher Laumer