The ocean’s depths hold countless inhabitants, leading many to wonder which fish species is the most numerous. Understanding the distribution and population of marine life provides insights into ocean ecosystems. This exploration reveals a surprising candidate for the title of the sea’s most common fish.
The Unsung King of the Ocean
The genus Cyclothone, commonly known as bristlemouths, is the most abundant fish, and indeed vertebrate, on Earth. These small fish typically measure one to three inches. They have elongated bodies, large mouths with bristly teeth, and light-producing organs called photophores along their undersides.
Bristlemouths inhabit the deep sea, primarily residing in the mesopelagic and bathypelagic zones, from approximately 200 to 1,500 meters below the surface. Despite their immense numbers, estimated to be in the hundreds of trillions (10^14 to 10^16 individuals), they remain largely unknown to the general public. Their deep-water habitat means they are rarely seen by humans.
Secrets to Its Success
Several adaptations contribute to the bristlemouth’s abundance in the marine environment. Their small size means they require fewer resources and less energy, a significant advantage in the nutrient-scarce deep ocean. Bristlemouths also exhibit a rapid reproductive cycle, producing many offspring through planktonic eggs and larvae, which increases the likelihood of survival. Some species are protandrous, beginning life as males and transitioning to females as they mature, providing reproductive flexibility.
Their diet is generalized and opportunistic, consisting primarily of small invertebrates such as copepods, chaetognaths, and other crustaceans, along with detritus. This broad diet allows them to utilize various food sources. Their deep-sea habitat offers a relatively stable environment with low light. Their bioluminescence provides counter-illumination, helping them blend with faint light from above to avoid predators. Many bristlemouth species possess a fat-filled swim bladder, which aids in buoyancy control without pressure issues at extreme depths.
Its Vital Role in Marine Ecosystems
Bristlemouths play a significant role in marine ecosystems. They serve as a link in the food web, consuming smaller invertebrates and zooplankton, and becoming a food source for larger deep-sea predators. Their predators include larger deep-sea fish like dragonfish and fangtooths, as well as squids and some whales.
The biomass of mesopelagic fish, including bristlemouths, is substantial, accounting for a large fraction of the total fish biomass in the world’s oceans. Their vast numbers contribute to the global carbon cycle through their metabolic processes at depth. While not all Cyclothone species undertake extensive vertical migrations, some do, ascending to feed and then descending. This helps transport organic carbon to deeper waters, contributing to the biological carbon pump by consuming organic matter in the upper layers and remineralizing it at greater depths.