Jawless Fish: Hagfish, Lampreys and Extinct Agnathans

Jawless fish are vertebrates that never evolved jaws: they feed by suction through a round, muscular mouth lined with rows of teeth (National Park Service). The scientific name, Agnatha, comes from Greek for 'without jaws' (Wikipedia). Today the group means two kinds of animal, hagfish and lampreys, but for most of the early history of vertebrates jawless fish were the main fish in the sea.

Scientists treat Agnatha as a paraphyletic group: jawed vertebrates, including every other fish and ultimately people, evolved from jawless ancestors, so 'jawless fish' is a description rather than one branch of the family tree (Wikipedia).

• No jaws: a round sucking mouth with rows of teeth instead (NPS)
• No paired fins and, in living species, no scales (NPS; Wikipedia)
• Long, cylindrical, eel-like body (NPS)
• Notochord kept in both larvae and adults, rather than replaced by a full backbone (Wikipedia)
• Seven or more gill pouches; lampreys show seven round gill openings on each side (Wikipedia)
• A single nostril on top of the head in lampreys (Wikipedia 'Lamprey')
• No true stomach, and cold-blooded like other fish (Wikipedia)

Yes, two groups:

• Hagfish are marine scavengers that live in muddy burrows in cold, moderately deep water and eat dead or dying fish (NPS). Most are about 50 cm long; the largest known, Eptatretus goliath, reached 127 cm. Their slime expands to 10,000 times its volume in 0.4 seconds, and Wikipedia calls them the only known living animals with a skull but no vertebral column, although they have rudimentary vertebrae
• Lampreys are mostly freshwater fish of temperate zones that breed in rivers and lakes (NPS). Wikipedia counts about 38 species, of which 18 feed on other fish; the rest are non-parasitic. Larvae (ammocoetes) filter-feed for up to eight years before they transform

The counts vary by source. Wikipedia gives about 120 living jawless species in total; the National Park Service page, last updated in 2017, gives about 20 hagfish and about 30 lampreys. Numbers change as species are split and described, so treat any single figure as approximate.

Two living and three extinct, to show the range:

• Sea lamprey: the largest lamprey, up to about 1.2 m; it became invasive in the North American Great Lakes in the early 20th century and is controlled with lampricides and barriers (Wikipedia 'Lamprey')
• Goliath hagfish (Eptatretus goliath): the largest known hagfish at 127 cm (Wikipedia 'Hagfish')
• Conodonts: eel-like animals that lasted from the Cambrian to the early Jurassic (Wikipedia)
• Osteostracans: heavily armoured ostracoderms (Wikipedia)
• Thelodonts and anaspids: smaller jawless fish (Wikipedia)

Other extinct groups include the pteraspidomorphs and galeaspids, also armoured ostracoderms (Wikipedia).

The fossil record shows the timing more clearly than the cause. Jawless fish dominated early Paleozoic seas, peaked in diversity in the late Silurian, then 'declined in the Devonian and never recovered' (Wikipedia). Over the same period jawed vertebrates, which had evolved from jawless ancestors in the early Silurian, spread and diversified. The sources we read do not settle on one cause for the decline, so be wary of pages that state a single reason as fact.

All three are long and slimy, so anglers mix them up. Check the head:

• Lamprey: a round, toothed sucking disc for a mouth, seven gill openings in a row on each side, one nostril on top, no paired fins (Wikipedia). Often found attached to other fish
• Hagfish: simple eyespots without lenses that cannot form detailed images, and a flood of slime when handled (Wikipedia). A sea-floor scavenger living in muddy burrows (NPS)
• True eel, such as the American eel: real jaws and paired fins (Wikipedia)

A photo of the head from the side is usually enough to decide; see key features to identify fish.

Both groups have been eaten. Lampreys were prized in medieval Europe, especially during Lent; King Henry I of England is said to have died after eating them, though the cause is disputed, and a lamprey pie was made for Elizabeth II's coronation in 1953 (Wikipedia 'Lamprey'). In Korea hagfish is skinned, coated in spicy sauce and grilled or stir-fried, and hagfish skin is sold as 'eel skin' leather (Wikipedia 'Hagfish'). If you catch a lamprey, check local rules: sea lampreys are controlled as an invasive species in the Great Lakes (Wikipedia).