The Atlantic Ocean is a vast and diverse marine environment, home to countless species. Its waters stretch across vast distances and plunge to extreme depths. Identifying a single “most common” fish within this colossal ecosystem is challenging, as many inhabitants remain largely unseen.
The Atlantic’s Most Abundant Fish
Beneath the vast surface of the Atlantic Ocean, a tiny fish known as Cyclothone microdon, often called the veiled anglemouth or bristlemouth, holds the title of the most abundant fish species. This small, slender creature typically measures between 6 to 12 centimeters. It possesses a dark brown or black coloration, and Cyclothone microdon has light-producing organs called photophores, contributing to a bioluminescent glow over its head and fins. These bristlemouths are considered the most numerous vertebrates on Earth by biomass, with estimates suggesting their numbers could reach into the quadrillions. They primarily inhabit the deep ocean’s mesopelagic and bathypelagic zones, found at depths ranging from 200 to 2,700 meters where sunlight is absent.
Why This Fish Thrives
The abundance of Cyclothone microdon stems from its biological adaptations and the unique characteristics of its deep-sea environment. These fish are adapted to the harsh conditions of the deep ocean, including immense pressure, perpetual darkness, and cold temperatures. They lack gaseous swim bladders, an adaptation that allows them to endure high pressure without changes in body volume.
Their reproductive strategy significantly contributes to their success. Cyclothone microdon breeds non-seasonally, with females capable of carrying between 2,000 and 10,000 eggs, ensuring a continuous supply of numerous small offspring. Many Cyclothone species are also protandrous hermaphrodites, beginning as males and transitioning to females, which offers reproductive flexibility in the challenging deep-sea environment.
The diet of Cyclothone microdon is broad and opportunistic, enabling efficient utilization of available food sources. They primarily consume small crustaceans like copepods and ostracods, common in the deep sea, along with other zooplankton and detritus. Their small size aids survival by requiring less energy, which supports higher population densities even with limited food.
The deep ocean’s immense habitat provides extensive space for these populations to flourish. Cyclothone microdon faces reduced predatory pressures compared to fish in shallower, more competitive waters. Their dark coloration and bioluminescent counter-illumination, utilizing ventral light organs to match faint overhead light, provide effective camouflage against predators viewing from below.
Its Role in the Ocean Ecosystem
Despite their hidden existence, Cyclothone microdon plays a significant ecological role within the deep-sea food web. As primary consumers, they convert detritus and small organisms into biomass, forming a crucial link in the food chain. This makes them an important food source for larger deep-sea predators, including various fish like dragonfish and fangtooths, and even some whales.
These abundant fish also contribute to the global carbon cycle through the biological pump. This process involves the transfer of organic carbon from surface waters to the deep ocean, effectively sequestering carbon away from the atmosphere. Cyclothone species, being non-migratory, are important in remineralizing organic carbon in deep ecosystems. Their sheer biomass means they play an outsized role in the oceanic cycling of carbon. Although Cyclothone microdon has no direct commercial value to humans, its widespread presence and ecological functions are fundamental to the health and functioning of the Atlantic deep-sea ecosystem.