Many might picture vast schools of sardines or tuna. The most abundant fish on Earth, however, is a creature few people have ever seen, residing in the ocean’s twilight depths. This challenges common assumptions about marine life.
Unveiling the Most Common Fish
The title of the most common fish, and indeed the most abundant vertebrate on Earth, belongs to the bristlemouths, specifically species within the Cyclothone genus. While not a single species, this genus comprises numerous closely related fish that collectively dominate marine ecosystems by sheer numbers. Estimates suggest their global population ranges from hundreds of trillions to potentially a quadrillion individuals.
These tiny deep-sea dwellers far outnumber well-known fish like herring or anchovies. The Cyclothone genus is believed to have more individuals and biomass than any other fish genus globally. Despite their immense numbers, bristlemouths remain largely unknown to the general public due to their inaccessible habitat.
Life in the Deep
Bristlemouths primarily inhabit the mesopelagic zone, often called the “ocean twilight zone,” which extends from about 200 to 1,000 meters (650 to 3,300 feet) below the surface. This environment presents extreme conditions, including high pressure, perpetual darkness, and cold temperatures, with scarce food resources. Their small size, typically 2 to 7.5 centimeters (1 to 3 inches) long, is an adaptation to this resource-limited environment, requiring less energy for growth and maintenance.
Many bristlemouth species exhibit bioluminescence, producing light from specialized organs called photophores along their bodies. This light is used for various purposes, including camouflage through counter-illumination, signaling to attract mates, and possibly luring prey. Their dark coloration also provides camouflage against the murky backdrop of the deep ocean. Some species are protandrous hermaphrodites, meaning they begin life as males and later transition to females, a reproductive strategy that may contribute to their success.
Ecological Significance
Bristlemouths play an important role within the marine food web, particularly in the deep-sea ecosystem. They serve as a primary link, consuming zooplankton and small crustaceans, with crustaceans making up 92 to 98% of their diet. In turn, these abundant fish become a food source for a variety of larger deep-sea predators, including squid, larger fish like dragonfish and fangtooths, and even marine mammals.
Their vast numbers mean they contribute to the biological pump, a process that transfers carbon from surface waters to the deep ocean. As bristlemouths feed on sinking organic matter and undergo daily vertical migrations, they help transport carbon to deeper layers. When they are consumed or their waste and bodies sink, carbon is sequestered, influencing global carbon cycling.
Beyond the Surface
The immense abundance of bristlemouths contrasts with their obscurity to most humans. Their preferred habitat in the deep mesopelagic zone makes them largely inaccessible to conventional fishing practices. Consequently, they hold no commercial significance and are not a target for human consumption.
Their existence highlights the biodiversity of the deep ocean and the specialized life forms that thrive there. Unlike familiar fish species that interact directly with human activities, bristlemouths remain in their hidden domain. This lack of interaction means their populations are not directly impacted by fishing pressures, contributing to their continued dominance.