Fish diversity in the one of earth’s least explored environments: the mesopelagic zone
The mesopelagic zone comprises the entire water column intermediate from the epipelagic zone (up to 200 m depth) to the deep pelagic layers (bathypelagic zone), located ca. 1000 m depth extending down to 4000 m. Also defined as twilight zone, the mesopelagic zone is the transition from the upwards-epipelagic photic zone to the deep aphotic zone, where the sunlight is completely absent.
- Pelagic zonation
Environmental conditions at mesopelagic depths are very particular showing dim light, cold waters and low oxygen levels, reduced turbulence, increased hydrostatic pressure, high inorganic nutrient concentrations and irregular food supply.
Mesopelagic fishes are well adapted to these conditions: they show bigger and more sensible eyes, dark or silver bodies, and luminous organs called photophores. Bioluminescence is the most common communication mode in the deep pelagic zone, being used in predation, defense (camouflage) and communication (mating, warning the presence of predators, migrations). Many mesopelagic fish species have also the capacity to adjust the intensity or color of the light emitted by their photophores. Bioluminescence may occur by symbiosis with bioluminescent organisms, such as bacteria (less frequently in fishes), or due to the oxidation of light emitting molecules (luciferin).
Most mesopelagic species perform extensive diel vertical migration (DVM) between the surface and 1000 m depth, moving at night into the epipelagic zone, following similar migrations of zooplankton, and returning to greater depths during the day to avoid predators. They also play an important role in biogeochemical cycles, transporting organic matter from the epipelagic zone, to greater depths and fuelling the benthic fish communities.
Families Myctophidae, Gonostomatidae, Stomiidae and Gempylidae are the most diverse and exhibit a global distribution, the first two being the most abundant.
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Examples of mesopelagic fishes collected in Cape Verde using a midwater trawl between the surface and 500 m.
Some features:
– Mesopelagic fishes are the most abundant vertebrates on earth.
– Inhabit the mesopelagic zone of all oceans.
– Bristlemouth fishes (belongs to family Gonostomatidae represented by 8 genera and 32 species) and lanternfishes (Family Myctophidae divided in 38 genera and approx. 250 species.) are the most abundant.
– Gonostomatidae and Myctophidae accounts for over 90% of the trawl catches at pelagic depths
– They exhibit reduced size (2.5-10 cm)
– Perform daily vertical migration between the surface and 1000 m
– Feed on zooplankton
– Some mesopelagic fish species use bioluminescent appendages to attract prey
– Others have large and extensible jaws to capture larger prey
References:
Catul, V., Gauns, M., & Karuppasamy, P. K. (2011). A Review on Mesopelagic Fishes belonging to family Myctophidae. Reviews in Fish Biology and Fisheries, 21, 339–354
Haddock, S. H. D., Moline, M. a., & Case, J. F. (2010). Bioluminescence in the Sea. Annual Review of Marine Science, 2(1), 443–493. doi:10.1146/annurev-marine-120308-081028
Sutton, T. T. (2013). Vertical ecology of the pelagic ocean: classical patterns and new perspectives. Journal of Fish Biology, 83, 1508–1527. doi: 10.1111/jfb.12263
Whitehead, P. J. P., Bauchot, M-L., Hureau, J-C., Nielsen, J., Tortonese, E. (eds) (1986). Fishes of the North-eastern Atlantic and the Mediterranean, vol. 1-3. UNESCO Press, Paris