Petromyzonidae
by V. D. Vladykov
Body snake-like. Cartilaginous skeleton, absence of jaws, mouth in metamorphosed individuals is a circular sucking-disc with horny teeth, seven external gill openings close to the head, nasohypophysial sac not opening into pharynx, only vertical fins (caudal, one or two dorsal) supported by cartilaginous rays are present. Very extended larval stage known as ammocoete. Ammocoetes lack sucking-disc with teeth but have horseshoe-shaped mouth with a fleshy hood-like upper lip overhanging a short lower lip; their eyes are covered by skin.
Metamorphosed individuals can live in fresh- or salt-water areas, but for spawning they ascend to freshwater streams, where the ammocoetes remain from egg hatching to the completion of their metamorphosis. Although the diet of ammocoetes of all lampreys is similar, consisting of microscopic algae (principally diatoms), the feeding habits of metamorphosed individuals are one or the other biotype: parasitic or non-parasitic. Among parasitic species, individuals after completing metamorphosis from an ammocoete (without teeth) to an adult stage provided with well keratinized teeth retain a functional intestinal tract and feed upon the blood of fish and cetaceans. All species from Clofnam area are parasitic.
Genera 8 in the Holarctic region; in Clofnam area 3.
Recent revisions: Vladykov (1974), Hubbs (1975), Vladykov and Gruchy (1975), Vladykov and Kott (1979a).