If my wet suit sleeve rides up a couple of inches, my exposed skin contacts often-inconspicuous hydroid colonies that cover rock surfaces. These sessile coelenterates inflict very intense stings like a series of bee stings. The pain lingers for days, and the sting causes minor but permanent scarring. Anywhere in the tropics, a serious sting can appear seemingly from nothing as detached but still functioning jellyfish or siphonophore tentacles drift invisibly in the water. The offending entity was just such a drifting, but still very painful, bluebottle tentacle.
Many coelenterates are polytrophic feeders, able to derive nutrition in more than one way.
For example, most coral polyps on a reef act like miniature greenhouses during the day and become efficient micro-predators by night. The algae make food products by photosynthesis and pass on nutrients to the coral. At night, when photosynthesis ceases, coral tentacles armed with nematocysts capture food from the passing plankton. Conversely, the species with few zooxanthellae may have especially abundant nematocysts and well-developed digestive structures, indicating dependence on external sources of food. Species that derive little or no nutrition from algal symbionts rely on a battery of nematocysts, which provide their sole source of food, whereas corals that obtain plenty of nutrients from their symbiotic algae have less need for abundant and efficient nematocysts.