Though rarely caught, opah are prized trophies for deep-water anglers as their large size and attractive form lend themselves well to taxidermy. Occasional bycatch from the longline tuna industry is also marketed and prepared as sashimi, as well as being broiled and smoked; opah flesh (despite being stringy and hard to fillet) has a moderate flavor and is well appreciated in this regard, especially in Hawaii. Opah are garnering increasing interest from restaurateurs as other staple species become unavailable. An average of 35 percent of an opah's weight is utilized for consumption.
If coloration is ignored, opah closely resemble the unrelated butterfish (family Stromateidae). Both have falcate pectoral fins and forked, emarginate caudal fins. Aside from being significantly larger than butterfish, opah have enlarged, falcate pelvic fins—with ca. 14–17 rays, which distinguish opah from superficially similar carangids—positioned thoracically; adult butterfish lack pelvic fins. The pectorals of opah are also inserted (more or less) horizontally rather than vertically. The anterior portion of an opah's single dorsal fin (with ca. 50–55 rays) is greatly elongated, also in a falcate profile similar to the pelvic fins. The anal fin (ca. 34–41 rays) is about as high and as long as the shorter portion of the dorsal fin, and both fins have corresponding grooves into which they can be depressed.
The snout is pointed and the mouth small, toothless and terminal. The lateral line forms a high arch over the pectoral fins before sweeping down to the caudal peduncle. The largest species, Lampris guttatus, may reach a total length of 2 metres and a weight of 270 kilograms. The lesser-known Lampris immaculatus reaches a recorded total length of just 1.1 metres.
Squid and euphausiids (krill) make up the bulk of the opah diet; small fish are also taken. Pop-up archival transmitting (PAT) tagging operations have indicated that (aside from humans) large pelagic sharks, such as Great White Sharks and Mako Sharks, are primary predators of opah. The tetraphyllidean tapeworm Pelichnibothrium speciosum has been found in L. guttatus, which may be an intermediate or paratenic host (Scholz et al. 1998).
The planktonic larvae of opah initially resemble those of certain ribbonfishes (Trachipteridae), but are distinguished by the former's lack of dorsal and pelvic fin ornamentation. The slender hatchlings later undergo a marked and rapid transformation from a slender to deep-bodied form; this transformation is complete by 10.6 millimetres standard length in Lampris guttatus. Opah are believed to have a low population resilience.