Alepocephalidae

Slickheads, also known as nakedheads or Smoothheads, are deep water fishes that belong to the family Alepocephalidae. They are most commonly found in the bathypelagic layer which is approximately 3000 m below the surface. [1] They get their name from the lack of scales on the head. Similarly, the scientific name is from the Greek ᾰ̓- (a-, "not"); λέπος (lepos, "scale"); and κεφαλή (kephalē, "head"). It has about 22 genera with ca. 96 species.[2] In Japanese they are known as Sekitori Iwashi (関取鰯, "Massive Sardine").[3]

Slickheads
California slickhead, Alepocephalus tenebrosus
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Superorder:	Alepocephali
Order:	Alepocephaliformes
Family:	Alepocephalidae Bonaparte, 1846
Genera
Alepocephalus Asquamiceps Aulastomatomorpha Bajacalifornia Bathylaco Bathyprion Bathytroctes Bellocia Conocara Ericara Herwigia Leptochilichthys Leptoderma Microphotolepis Mirognathus Narcetes Photostylus Rinoctes Rouleina Talismania Xenodermichthys
Synonyms
Alepocephalini Bonaparte 1846 Leptochilichthyidae Marshall 1966 Bathyprionidae Marshall 1966 Bathypriidae Marshall 1966 Bathylaconidae Parr 1948 Aulastomatomorphinae Fowler 1934

The following characteristics are generally shared by the Alepocephalidae family:

Their mouths consist of 80 to 100 razor sharp teeth with their razor sharp teeth shape being small and feeble. They are shaped in an eel-like elongation, large eyes, gill rakers that range from moderate to long and numerous, fins without spines, the single dorsal fin would be located posterior to midpoint of body, no adipose dorsal fin present, pectoral fin ranging from small to rudimentary located below mid-flank, 4-18 rays, if pelvic fins present are abdominal and origin is before origin of dorsal fin, caudal fin forked, no tube above base of pectoral fin that connects to a luminous gland on shoulder girdle. They have no swim bladder.[2] Some species bear photophores.[4]

Their early life development is from large eggs directly into yolk sac juveniles that travel in deep waters.[5] There have been sighting of slickhead juveniles several meters above the sea floor which could suggest that some slickhead species may travel between different depths throughout their life stages, a vertical shift[6]

It’s been thought that as the Alepocephalidae age they move down layers in the ocean (mesopelagic waters and bathypelagic waters) due to mostly adolescent Alepocephalidae being reported near the surface.

Vertical shift could be due to predation being higher close to the sea floor and warmer water having a larger abundance of planktonic food. Juvenile slickheads may have a preference to travel upwards to take advantage of the lack of predation and abundance of food to increase survival rates.[7]

These fish can be found in many parts of the world, although they are difficult to find as they do typically live around the bathypelagic layer of the ocean.

The species of Alepocephalidae are widely distributed throughout the world and can be found in the Atlantic, Indian, and western Pacific.[8]