Notostomum cyclostoma is an ectoparasite common in the northern Pacific Ocean south of the Bering Straight.  It lives in the coastal Alaskan subtidal zone to 200 fathoms (366 meter) in depth.  They are most often found adhering to and feeding off of marine fish such as sharks, Pacific halibut, and rays. Their egg cocoons and offspring are found attached to the carapaces of various species of crab, though they do not use the crab as a means of nutrition. 

It is thought that leeches can detect low levels of stimuli including light and vibrations.  This often occurs in leeches living on marine fishes, and is an adaptation for finding their host.  When feeding, the order Rhynchobdellae, which is the order to which Notostomum belongs, extends their proboscis out of the mouth and into the flesh of the host.