breathe by passing air through the gut. mudskippers breathe by absorbing oxygen across the skin (similar to frogs). a number of fish have evolved so-called accessory breathing organs that extract oxygen from the air. labyrinth fish (such as gouramis and bettas) have a labyrinth organ above the gills that performs this function. a few other fish have structures resembling labyrinth organs in form and function, most notably snakeheads, pikeheads, and the clariidae catfish family. breathing air is primarily of use to fish that inhabit shallow, seasonally variable waters where the water's oxygen concentration may seasonally decline. fish dependent solely on dissolved oxygen, such as perch and cichlids, quickly suffocate, while air-breathers survive for much longer, in some cases in water that is little more than wet mud. at the most extreme, some air-breathing fish are able to survive in damp burrows for weeks without water, entering a state of aestivation (summertime hibernation) until water returns. parasites on gills monogenean parasite on the gill of a grouper fish gills are the preferred habitat of many ectoparasites (parasites attached to the gill but living out of it); the most commons are monogeneans and certain groups of parasitic copepods, which can be extremely numerous. other ectoparasites found on gills are leeches and, in seawater, larvae of gnathiid isopods. endoparasites (parasites living inside the gills) include encysted adult didymozoid trematodes, a few trichosomoidid nematodes of the genus huffmanela, including huffmanela ossicola which lives within the gill bone, and the encysted parasitic turbellarian paravortex. various protists and myxosporea are also parasitic on gills, where they form cysts.