Despite the Lion's Mane jellyfish or Cyanea capillata, having a fearsome sting, juvenile whiting - Merlangius merlangus (a member of the cod family) often make their home amongst the tentacles. It is not known for sure how they avoid being stung and eaten but it is assumed that there is a protective mucous on their scales. This seems to be a "one way street" of commensalism with the whiting being very well protected from predators but there being no obvious benefit to the jellyfish. These fish will also associate with other jellyfish and even flotsam at times.
North Sea. St. Abbs Marine Reserve