The oarfish is a mysterious deep-ocean creature, rarely seen by humans and often only encountered when it washes ashore. This serpentine fish has fueled legends of sea serpents for centuries due to its massive length and ribbon-like appearance. Its unique adaptations allow it to thrive in the dark, cold depths far below the surface. Because of its elusive nature, much of its life cycle and behavior remains poorly understood by scientists.
Defining Features and Deep-Sea Habitat
The giant oarfish (Regalecus glesne) is the world’s longest bony fish, capable of reaching lengths of up to 8 meters. Its body is laterally compressed, giving it a distinctive ribbon shape. Despite its length, the oarfish lacks true scales, instead having a coat of silvery, guanine-covered skin that is easily damaged outside of its high-pressure environment.
A bright red dorsal fin runs the entire length of its back. The first few rays of this fin form an ornate, crown-like crest above its head, which is thought to be the origin of one of its common names, the “King of Herrings.” The oarfish also possesses long, paddle-like pelvic fins.
This species is pelagic, meaning it lives in the open water column rather than near the ocean floor. Its preferred home is the mesopelagic and bathypelagic zones, often called the twilight and midnight zones. These depths range from about 200 meters down to over 1,000 meters below the surface. It is a cosmopolitan species, found in tropical and temperate oceans worldwide, but not in polar regions.
Understanding Oarfish Longevity
Determining the exact lifespan of the oarfish is challenging because its deep-sea habitat makes long-term observation impossible. It cannot be studied effectively in captivity due to its specific physiological needs related to pressure and temperature. Therefore, scientists must rely on indirect methods to estimate how long these fish live.
The primary method for estimating a fish’s age is by analyzing its otoliths, small calcium carbonate structures found in the inner ear. These structures lay down growth rings, known as annuli, similar to tree rings. Scientists examine thin sections of these otoliths under a microscope and count the rings to determine the fish’s age in years.
Because the oarfish is rarely captured alive or intact, data on its otoliths is scarce, leading to a wide range of estimates. Analysis suggests that some of the largest giant oarfish may live for up to 30 years. However, the lack of a robust sample size and difficulty validating the yearly nature of the annuli mean this figure remains an upper-end estimate.
Diet, Behavior, and Why They Surface
The oarfish is a carnivore, but its diet consists mainly of small marine organisms, which is surprising given its massive size. It primarily feeds on zooplankton, tiny crustaceans such as krill, and sometimes small fish and squid. It captures prey using a suction-feeding mechanism, drawing water rapidly into its relatively small, toothless mouth.
Instead of actively hunting, the oarfish is thought to swim in a near-vertical orientation, with its head pointed upward. This posture may be a strategy to silhouette planktonic prey against the faint, downwelling light from the surface, making them easier to spot. Its primary mode of locomotion is an undulating, wave-like motion of its long dorsal fin, allowing it to glide subtly through the water column.
A common question is why this deep-sea inhabitant occasionally appears at the surface or washes up on beaches. Such sightings are unusual because the fish is adapted to its deep-water environment and typically dies from depressurization if brought up. Most surface appearances are attributed to the fish being sick, injured, or disoriented. Strong underwater currents or severe weather can also force the weakened fish out of its normal depth range and toward the shore.