Jawless fishes (Agnatha)

SUBPHYLUM - CEPHALOCHORDATA

ORDER AMPHIOXIFORMES - Lancelets

The lancelet is a protochordate, lacking a cranium, vertebrae, cartilage, bone, and red blood cells.  But it shares with vertebrates: a notochord, dorsal tubular central nervous system, paired lateral gill slits, postanal tail, hepatic portal system, and endostyle. There is no fossil record. Lancelets are small (up to 8 cm long), slender, fishlike animals, with an ancestry probably close to the ancestral vertebrate lineage. They live buried in sand or coarse shell gravel.  Feeding occurs by straining water that is constantly drawn through the mouth. Lancelets are distributed mainly in shallow-water in tropical and subtropical seas extending south as far as New Zealand.

One endemic species known in New Zealand waters, Epigonichthys hectori. Occurs north of Cook Strait, localised at depths of 0-100 m. Distinguished by having 19-21 pre-oral tentacles, a body with 84-85 myotomes (muscle blocks), and 28-33 gonads confined to the right side of the body.

SUBPHYLUM CRANIATA

SUPERCLASS AGNATHA – Hagfishes and lampreys

Hagfishes and lampreys are primitive fishes lacking jaws (derived from gill arches), pelvic fins, and vertebral centra; the gills are covered with endoderm, are directed internally, and are open to the surface through pores not slits; the skeleton is cartilaginous. The fossil record extends to the early Paleozoic, with the greatest radiation in the Silurian and Lower Devonian. Two extant groups occur in New Zealand: the hagfishes (order Myxiniformes, family Myxinidae) with at least three marine species, two endemic (one is very rare in collections); and the lampreys (order Petromyzontiformes, family Petromyzontidae) with one anadromous species, relatively common in freshwater.

Growing morphological evidence suggests that hagfishes are probably the most primitive agnathan and lampreys are more closely related to gnathostomes. Most similarities between hagfishes and lampreys involve superficial or primitive characters and cannot be convincingly used to support hypotheses of monophyly. Nelson (1994) and others considered the Agnatha to be paraphyletic. In contrast, molecular evidence suggests that all the jawless fishes have a common ancestor and thus form a monophyletic group, the Agnatha.