 Some teleosts are parasites . <unk> have their front dorsal fins modified into large suckers with which they cling onto a host animal such as a whale , sea turtle , shark or ray , but this is probably a commensal rather than parasitic arrangement because both remora and host benefit from the removal of ectoparasites and loose flakes of skin . More harmful are the catfish that enter the gill chambers of fish and feed on their blood and tissues . The snubnosed eel , though usually a scavenger , sometimes bores into the flesh of a fish , and has been found inside the heart of a shortfin mako shark . 
