Becky Crew is a Sydney-based science communicator with a love for weird and wonderful animals. From strange behaviours and special adaptations to newly discovered species and the researchers who find them, her topics celebrate how alien yet relatable so many of the creatures that live amongst us can be.
IF YOU’VE EVER wanted to know what life in the deep sea is really like, take a moment to look into the tortured eyes of a marine hatchetfish.
Sure, I’m projecting emotions onto a fish that just happens to have the gaping mouth and vacant eyes of a shell-shocked wretch who’s seen all too much, but when you’re living more than a kilometre below the surface of the ocean where food is scarce and you make the perfect meal, you’re probably not having a very good time.
But that doesn’t stop the hatchetfish from trying its darndest to survive in one of the most inhospitable and competitive environments on Earth.
Marine hatchetfish are members of the family Sternoptychidae, which also includes the wonderfully named bottlelights, pearlsides and constellation fishes, and are found all over the world in the Atlantic, Indian, and Pacific Oceans.
Deep-sea fish evolved to live in the dark depths
Split into 40 known species, their sizes range from 2.8cm to 12cm long, and like many mysterious deep-sea creatures, they carry an array of evocative monikers, including half-naked (Argyropelecus hemigymnus), lovely (Argyropelecus aculeatus), and false oblique (Sternoptyx pseudodiaphana).
They also carry with them an ingenious strategy for avoiding detection by predators – a custom lightshow that keeps them hidden in plain sight, at whatever depths they choose to roam.
Even in the pitch-black darkness of the deep sea, the faintest silhouette could give up the location of a little fish to a bigger fish lurking nearby. So along with their slender shape and reflective scales, marine hatchetfish have equipped themselves with a row of light-emitting organs called photophores to conceal their shadows.
Located on their bellies, these little organs allow the hatchetfish to cast a glow from their undersides that perfectly matches the colour and intensity of the light all around them, making them invisible to any hungry eyes below.
It’s the perfect cover, because it allows the hatchetfish to conceal themselves wherever they go, from the darkness of the deep sea to the brighter environment further up, where they migrate at night to find food. Known as ‘bioluminescent counter-illumination’, this strategy is one of the most effective, and most popular, forms of camouflage found in marine organisms all over the world.