Astronesthes richardsoni chin is ribbon-like and lacks a bulb or swelling at its tip. The dorsal fin has 12 to 14 soft rays and the anal fin has 13 to 18. The origin of the dorsal fin is behind the insertion of the pelvic fins. There is a dorsal adipose fin anterior to the caudal peduncle, and a small ventral adipose fin just anterior to the anus. There are two rows of photophores (luminous spots) on each side of the underside of the fish, numerous other photophores on the head and body and a luminous patch on the edge of the preoperculum;
Largetooth cookiecutter shark and sparsely scattered light-emitting photophores on the belly. Most specimens have lacked the dark "collar" found on the throat of "I. brasiliensis". However, a specimen caught in 2004 off the Azores did possess the collar. The maximum recorded length is . Based on its smaller dorsal and caudal fins, the largetooth cookiecutter shark is believed to be less active than "I. brasiliensis" and an overall weak swimmer. Much of its body cavity is occupied by an enormous oil-filled liver, which allows it to maintain neutral buoyancy in the water column with little effort. Unlike "I. brasiliensis", this shark possesses binocular