The gas bladder is absent in most species, and the lateral line is uninterrupted. The branchiostegal rays (bony rays supporting the gill membranes behind the lower jaw) number two to four. The javelin spookfish ("Bathylychnops exilis") is by far the largest species at standard length; most other species are under . Barreleyes inhabit moderate depths, from the mesopelagic to bathypelagic zone, "circa" 400–2,500 m deep. They are presumably solitary and do not undergo diel vertical migrations; instead, barreleyes remain just below the limit of light penetration and use their sensitive, upward-pointing tubular eyes—adapted for enhanced binocular vision at the expense of lateral vision—to survey the waters above. The high number of rods in their eyes' retinae allows barreleyes to resolve the silhouettes of objects overhead in the faintest of ambient light (and to accurately distinguish bioluminescent light from ambient light), and their binocular vision allows the fish to accurately track and home in on small zooplankton such as hydroids, copepods, and other pelagic crustaceans. The distribution of some species coincides with the isohaline and isotherm layers of the ocean; for example, in "Opisthoproctus soleatus", upper distribution limits coincide with the 400-m isotherm for . What little is known of barreleye reproduction indicates they are pelagic spawners; that is, eggs and sperm are released "en masse" directly into the water. The fertilized eggs are buoyant and planktonic; the larvae and juveniles drift with the currents—likely at much shallower depths than the adults—and upon metamorphosis into adult form, they descend to deeper waters. "Dolichopteryx" species are noted for their paedomorphic features, the result of neoteny (the retention of larval characteristics). The bioluminescent organs of "Dolichopteryx" and "Opisthoproctus", together with the reflective soles of the latter, may serve as camouflage in the form of "counterillumination". This predator avoidance strategy involves the use of ventral light to break up the fishes' silhouettes, so that (when viewed from below) they blend in with the ambient light from above. Counterillumination is also seen in several other unrelated deep-sea families, which include the marine hatchetfish (Sternoptychidae).