About the Hatchetfish
The hatchetfish was born ready. With pelvic bones tilted downwards and large eyes pointing upwards, this little fish is poised for whatever morsel—or threat—comes its way. Its blade-like body cuts through the water column with barely a trace, thanks to pale blue lights on its belly that mask its silhouette from below.
Hatchetfish aren’t the only species to evolve this sort of counter-illumination camouflage—lots of other twilight zone animals use a similar strategy to avoid predators. But the hatchetfish (especially Argyropelecus) has developed ways to gain a subtle edge. With large pupils and sensitive, protruding eyes, it can pick out camouflaged prey swimming in the murky light above it. Hatchetfish also have a unique ability to regulate the intensity of their bioluminescent lights and can focus on objects at a short or long distance. Even with these skills, however, it’s not invincible. Its gaping, downturned mouth and wide eyes make the hatchetfish appear permanently surprised, perhaps in preparation for an almost inevitable demise in the mouth of a lancetfish or tuna.
Some species of hatchetfish take part in the world’s most extensive migration, traveling from depths of 1,500 meters (about 5,000 feet) to shallower waters. Under the cover of darkness, they join their twilight zone neighbors at the zooplankton buffet, feeding on ostracods and copepods, floating fish larvae, and crustaceans. When the sun rises, however, it’s time to retreat to the twilight zone. A hatchetfish never knows when the axe will fall.
[Editor’s note: marine hatchetfish shouldn’t be confused with the freshwater species bearing the same name, which are popular in home aquarium settings.]
|Scientific Name||Family Sternoptychidae, comprising 74 species, including seven species of Argyropelecus (including A. aculeatus, the silver hatchetfish), four species of Sternoptyx and 33 species of Polyipnus|
|Other Names||Silver hatchetfish, lovely hatchetfish, half-naked hatchetfish, giant hatchetfish|
|Size||Ranges from 2.8 cm (1.1 inch) to 12 cm (4.7 inches), depending on the species.|
|Discovery||Sternoptyx diaphana first described by Johann Hermann, 1781|
|Eats what?||Ostracods, copepods, small crustaceans, fish larvae|
|Eats how?||Some less-abundant species migrate to the surface at night to feed|
|Is eaten by?||Larger fish such as tuna and lancetfish|
|Bioluminescence||Yes, light producing organs cover its body, pointing downward to counter-illuminate against the faint light from above|