Mesopelagic fish are small, abundant fish living in the mesopelagic zone between the depth of 200-1000m. They exist and inhabit all of the worlds oceans. These are central to the SUMMER project, as they – together with other mesopelagic organisms, have recently been estimated to have a standing biomass of 10 000 million tons and are thought to be one of the last unexploited marine resources on our planet. In this blogpost, we explore 5 interesting facts about these fascinating mesopelagic fish that you might not have known before!

  1. Mesopelagic fish (namely bristlemouths) are the most abundant vertebrate on Earth.
    Photo Cred: Leif Grimsmo, SINTEF Ocean

    Photo Cred: Leif Grimsmo, SINTEF Ocean

There is one family of mesopelagic fish, the Gnostomatidae AKA “Bristlemouths” that are thought to be the most abundant of all mesopelagic fish,  making them the most abundant fish worldwide. As a matter of fact, this also makes bristlemouths the most abundant vertebrate on our planet, outnumbering humans, birds, and amphibians! The upper estimations of these species suggest a total of one quadrillion individuals – that’s one million billion, or 10^15 fish!

 

  1. Mesopelagic fish glow in the dark by producing their own light: bioluminescence!

    Lanternfish with blue bioluminescense. (From Adobe Stock, Sam)

You have probably seen fireflies – insects which produce their own light to glow in the dark when the sun is down during the night. This light is produced through a chemical reaction and is known as bioluminescense. In the deep ocean twilight zone light is scarce, so instead of solely employing bioluminescence during the night the fish have evolved to use it both during the day and night to light up the dark with brilliant flashes! The color of this light is most often blue. Mesopelagic fish have whole organs dedicated to this light production, called photophores! In the lanternfish family these light organs can be found along the entire length of the fish’s body. Mesopelagic fish use bioluminescence for a variety of reasons, such as communication, hunting, and scaring off predators.

 

  1. Mesopelagic fish help combat climate change by transporting CO2!

Mesopelagic fish perform diel vertical migration (migrating to surface waters during the night, and back to the depths during the day). Their diet consists mostly of small, free-floating organisms – zooplankton! These are more abundant in the upper sunlit zone compared to the twilight zone, as zooplankton feed on photosynthesizing phytoplankton which only exist in the upper zones. The photosynthesizers absorb CO2 turning it into sugar, which is transferred when they are predated by zooplankton. The carbon transfers again as mesopelagic fish eat them. After feeding, the fish migrate down to the depths where they excrete and thus deposit carbon even deeper into the ocean as it sinks. As such, carbon can be transported and stored away in the depths for centuries, sequestering it from the atmosphere, all thanks to the small migrating mesopelagic fishes.

  1. They are tasty!

Since SUMMER seeks to explore the possibility of sustainably harvesting mesopelagic fish in order to use it as food or feed, we have included a fact about eating mesopelagic fish! Several scientists have tasted cooked mesopelagic fish species along with mesopelagic krill – and disclosed that it had a great taste! In addition, mesopelagic fish oil and meal has been experimentally added to fish feed and has been found palatable by fish species such as Atlantic Salmon! Nutritional analyses also show that several mesopelagic fish species are nutrient dense, containing healthy omega-3 fatty acids. It seems both humans and fish could enjoy eating this nutritious marine resource if we find it harvestable.

  1. They live and interact with many species!

    Salp chain (from Adobe Stock, Brook Peterson)

Of course, Mesopelagic fish don’t live in the dimly lit mesopelagic zone all by themselves. The zone is teeming with other inhabitants such as jellies, squid, krill and salps. Haven’t heard of salps before? They are colonial, jelly-like animals, consisting of multiple individuals, often chained together to form long links – aptly named “sea grapes”. These fascinating animals float freely in the epipelagic and mesopelagic zone, filtering water through their bodies as they move while feeding on small particles and plankton. Marine mammals are also known to visit the twilight zone to feed on mesopelagic organisms! Both emperor penguins and sperm whales are examples of deep divers that can submerge themselves into extreme depths in search for food. Who would’ve thought that the seemingly inhospitable mesopelagic zone hosts such a large span of organisms?

 

And there you have it – 5 fascinating facts about the oceans twilight zone, hope you learned something new! Thanks for reading and happy holidays!

Written by Kristian Fjeld, SINTEF Ocean. Photo credit given under each picture.