Genera: 60; 228 species; 6 to 7 families.
English: Marine smelts and allies.
Fish: they have a physoclistous gas bladder or lack it entirely; teeth are absent in almost all; hypaxial muscle is unusually extended to forward at its upper end, attaches to the neurocranium below the spine, perhaps to snap the upper part of the skull down when catching prey; primordial ligament attaches posteriorly on the upper surface of the coronoid process; the autopalatine is peculiarly expanded to above and below at its caudal end; the caudal part of the mesethmoid appears compressed when seen from above; autopterotic and dermopterotic bones are not fused together; crumenal organ, also called epibranchial organ, consists of the additional cartilage and gill rakers on the fifth ceratobranchial, well-developed.
Color: silvery or dark.
Habitat: bathypelagic ocean.
Fins: adipose fin, unusually for Protacanthopterygii; dorsal fin is located in the second half of the body.
Argentiniformes formerly were included in the Osmeriformes. Studies show they may actually be the most basal lineage of the living Protacanthopterygii. It would probably require either inclusion of the supposed superorders Cyclosquamata and Stenopterygii in the Protacanthopterygii. Or Euteleostei is used for this entire group, restricting the Protacanthopterygii to the Osmeriformes and either Esociformes or Salmoniformes.
• Alepocephalidae: Slickheads), includes Bathylaconidae; Leptochilichthyidae.
• Platytroctidae: including Searsiidae.
• Argentinidae: Herring smelts.
• Bathylagidae: Deep-sea smelts.
• Microstomatidae: Pencil smelts.
• Opisthoproctidae: Barreleyes.