Hatchet Fish: An Evolutionary Race Against Time and Predators

As long as there have been predators, there have been prey developing mechanisms to avoid them. Whether through the development of offensive strategies or through concealment, prey have adapted countermeasures to avoid predation. This has resulted in an evolutionary arms race, each competing against the other for survival.

Take for example the Hatchetfish, Sternoptyx pseudobscura [2]. As shown in Season 1 Episode 2 of Blue Planet: A Natural History of the Oceans, hatchetfish have evolved to have photophores [3] on their underbellies to conceal themselves from predators [4]. These photophores are actually crystalline structures [2] that emit dim lights to help them blend in with the water surface miles above them, a defense tactic called counterillumination [4]. Their sides are also covered in mirror like scales that further protect the hatchetfish from detection [2]. Both defenses make the hatchetfish nearly imperceptible to their predators. However, not all hatchetfish species exhibit these traits, indicating that Sternoptyx pseudobscura have evolved specifically to increase their survival in deep sea conditions [1].

Although the hatchetfish employs these mechanisms to manipulate the light in their surroundings, predators are coevolving offensive mechanisms to better discover their prey. An inability to locate food sources infringes on the predators’ ability to survive, and as a result, predators with better detection mechanisms have higher chances of surviving. Selection favors predators with eyes that can filter out the reflected light of the photophores [4], allowing them to locate their prey and more successfully hunt. And thus, the arms race between predators and prey continues.

by Samantha Noda, Ava Papetti and Akshar Patel

Blue Planet: A Natural History of the Oceans, Season 1, Episode 2, A Natural History of the Oceans, starting at approximately 6:24


  1. Carnevale, Giorgio. 2008. Miniature deep-sea hatchetfish (Teleostei: Stomiiformes) from the Miocene of Italy. Geological Magazine 145: 73-84.
  2. Iwasaka, M. & Ohtsuka, S. 2017. Modulation of light localization in the iridophores of the deep-sea highlight hatchetfish Sternoptyx pseudobscura under magnetic field. AIP Advances Vol7: Issue5: 1-6.
  3. WJ Clarke. 1963. Function of bioluminescence in mesopelagic organisms. Nature 198: 1244-1246.
  4. Johnsen S, Widder EA & CD Mobley. 2004. Propagation and perception of bioluminescence: factors affecting counterillumination as a cryptic strategy. The Biological Bulletin 207, no. 1: 1-16

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