Other fauna, such as hydrozoa, bryozoa and brittle stars, often dwell among the branches of gorgonian and coral colonies. The pygmy seahorse not only makes certain species of gorgonians its home, but closely resembles its host and is thus well camouflaged. Some organisms have an obligate relationship with their host species. The mollusc "Simnialena marferula" is only found on the sea whip "Leptogorgia virgulata", is coloured like it and has sequestered its defensive chemicals, and the nudibranch "Tritonia wellsi" is another obligate symbiont, its feathery gills resembling the tentacles of the polyps.