The short-headed lamprey, a member of the Australian lampreys, family Mordaciidae. At least, that's the taxonomic scheme used by Nelson (2006). Wikipedia apparently uses a slightly different taxonomy, such that the family is reduced to sub-family status: Mordaciinae. Oh well. This species spends most of its time at sea as a parasite of marine fish, but breeds in the fresh water rivers of southern Australia, particularly the Murray.