Hatchetfish

Marine hatchetfish are small, deep-sea fish in the family Sternoptychidae, named for their distinctive laterally compressed, blade-like body shape that resembles the head of a hatchet. Argyropelecus aculeatus, the silver hatchetfish, is a widely distributed representative species found in tropical and temperate oceans. Hatchetfish are notable for their tubular, upward-pointing eyes and extensive rows of photophores on the underside of the body, adaptations that support both prey detection and camouflage in the mesopelagic zone. Despite their small size, hatchetfish are extremely abundant in deep, open ocean habitats and form an important part of the diet of larger predatory fish and squid that hunt in the mesopelagic zone.

Hatchetfish have one of the most distinctive body shapes among deep-sea fish, easily separating them from other mesopelagic groups.

• Extremely laterally compressed, deep-bodied, blade- or hatchet-shaped profile
• Large, tubular eyes oriented to look upward
• Silvery body with a metallic sheen
• Rows of small photophores lining the lower body and belly
• Small mouth and slender tail region relative to the deep forward body

The dramatic hatchet-shaped body and upward-pointing eyes make this family unmistakable compared to the elongated, snake-like shape of dragonfish or the needle-toothed profile of viperfish. Adults are typically quite small, generally under 10 cm in length.

Hatchetfish are found in tropical and temperate oceans worldwide, inhabiting the mesopelagic zone at depths generally between 200 and 1,500 meters, depending on species. Like many mesopelagic fish, they perform daily vertical migrations, moving toward shallower, more food-rich water after dark and returning to deeper, darker water during daylight hours. Their upward-pointing tubular eyes are especially suited to detecting the silhouettes of prey against faint downwelling light from above, an adaptation well matched to their mid-water lifestyle. Hatchetfish live entirely within the open water column, showing no association with the seafloor, and their habitat overlaps broadly with lanternfish and viperfish in the same depth zone.

Hatchetfish are small, schooling predators of the mesopelagic zone that feed primarily on tiny crustaceans and other zooplankton detected using their large, upward-oriented eyes. Their belly photophores produce counter-illumination that matches faint light filtering down from the surface, helping to camouflage their silhouette from predators looking upward from below. Hatchetfish take part in nightly vertical migrations, moving toward the surface to feed under cover of darkness before retreating to deeper water during the day. As a highly abundant group within the mesopelagic community, hatchetfish serve as an important food source for larger deep-water predators, including squid and mid-water fish.