#aulopiformes

Aulopiformes seem to have diversified around the Early Cretaceous, and their members had some of the earliest adaptations for deep sea life. By the Late Cretaceous, they were the dominant nektonic (swimming in the open water column) fish… before the extinction of most of their members around the K–Pg extinction event. Those that survived were those that had already adapted to the deep sea, where the effects of the meteor crash and rapidly changing climate were not as intense.

Most aulopiform species are hermaphroditic, many of which are able to self-fertilize.

Lancetfishes (family Alepisauridae) (image 2) are known to eat a wide variety of prey, “steal” fish from fishermen, and have even been documented with plastic refuse in their stomachs.

Most Barracudina (family Paralepididae) species have a subdermal layer of guanine in their skin, giving them a kaleidoscopic color that refracts the dim light of mesophotic waters around their bodies.

Barracudina of the genus Lestrolepis have a bioluminescent organ along their stomachs which likely helps in countershading against downwelling light, camouflaging their silhouettes to would be predators below. First-hand accounts reported behavioral observations of Lestrolepis intermedia rapidly swimming vertically through the water column like "silvery javelins" glowing "a brilliant yellow" from their bioluminescent organs. They were also observed hovering in the water and rapidly reorienting themselves from facing upwards to facing downwards as if scanning up and down for predators and/or prey.

The Pearleyes, family Scopelarchidae, are distinguished by their unique visual system, with two retinas in each eye. Unlike any other fishes, their eyes possess a "pearl organ": a white spot on the surface of the eye that may help to pick up light from the side of the fish, out of the normal visual field. The pearl organ is associated with the secondary retina, allowing the fish an unusually wide field of view.

Greeneyes (family Chlorophthalmidae) are named for their disproportionately large, iridescent (as well as fluorescent) eyes. Some species also have iridescent patches covering their heads.

The Tripod Fish (Bathypterois grallator) (image 1) is one of the most iconic deep-sea fish, known for its elongated pectoral and pelvic fins. In this particular species, these fins are three times as long as the body, up to a meter in length, and are used for standing on the “ooze”: fine-grained pelagic sediment made up of at least 30% microscopic, calcareous, or siliceous shells of phytoplankton or zooplankton, along with clay-size siliciclastic sediment and detritus (marine snow). The Tripod Fish will stand on its three fin extensions (the namesake tripod) on the ooze, facing upstream with the antennae-like pectoral fin rays extended out, waiting for food (shrimp, copepods, and tiny fish). Even though the fins are presumably quite stiff to be able to support its weight, observations of swimming fish shows that their fins seem flexible. It is suggested that fluids are pumped into these fins when the fish is “standing” to make them more rigid.

Some Aulopiformes have bizarre larvae, which look nothing like the adults. Often, these larvae were initially described as distinct species, sometimes even separated into their own genera or even families! Here is the larvae of a Tripod Fish:

Deepwater Lizardfish (genus Bathysaurus) (image 3) are one of the world's deepest living apex predators, and will not hesitate to eat anything they meet, including their own kind. Deepwater Lizardfish have an expansive liver constituting up to 20% of their total weight, which serves as an energy reserve to sustain growth between sporadic feeding episodes.