Argyropelecus lychnus, known as the Tropical Hatchetfish or Silver Hatchetfish, is a marine fish found mainly in tropical waters. Tropical Hatchetfish are deep bodied (i.e., they are deep back to belly relative to their length) and highly compressed laterally (i.e., from side to side). The main source of food for this species is apparently copepods (Harold 1993). Tropical Hatchetfish females are oviparous, laying planktonic eggs that hatch into planktonic larvae (Harold 1993).

Tropical Hatchetfish are absent from the Atlantic Ocean and only one specimen has been found in the Indian Ocean. They are found mainly in tropical Pacific waters (Harold 1993). They are usually found between 300 m and 400 m during the day. At night, however, the fish move up in the water column to between 200 m and 350 m. Although this is the primary depth range for this species, Tropical Hatchetfish are known to occur between 200 m and 700 m (Watson 1996).