One example of commensalism is the relationship between Patiria miniata, known as the Bat star, and a segmented worm called Ophiodromus pugettensis. The star has several grooves protruding from its mouth. Some Ophiodromus pugettensis worms live inside these grooves, but they do not harm the starfish.
Commensalism is defined as a type of symbiosis that gives one organism an advantage without helping or hurting the other organism. In the case of the Bat star and the worms, the grooves in the starfish provide a safe place for the worms to reside. Because the starfish does not derive any benefit from the worms living there, the relationship is commensalistic rather than mutualistic.
Humans also participate in commensalistic relationships. For example, the Demodex folliculorum mite makes its home in human hair follicles, while Demodex brevis lives in oil-secreting glands on the skin. Another example of commensalism in humans is the presence of Streptococcus pyogenes bacteria in the digestive tract. The bacteria do not harm the digestive tract but they do not have any helpful functions. This commensalistic relationship can quickly turn parasitic if the bacteria migrate from the digestive tract to other parts of the body. If Streptococcus pyogenes moves into the throat, it causes strep throat.