The Mola mola, commonly known as the Ocean Sunfish, is a marine animal of exceptional size and unique form. It holds the record as the world’s heaviest known bony fish, with large individuals weighing over 2,200 kilograms and reaching a fin-to-fin height of up to 3.3 meters. Its appearance is unmistakable, characterized by a massive, disc-shaped body that is flattened laterally. The fish lacks a true caudal fin, instead possessing a rudder-like structure called a clavus at the rear of its body.
Global Distribution Across Earth’s Oceans
The habitat of the Ocean Sunfish spans the globe, making it a circumglobal species found in the open waters of nearly all major oceans. These pelagic creatures prefer the warmer regions, inhabiting the temperate and tropical zones of the Atlantic, Pacific, and Indian Oceans. Their range also extends into enclosed seas, such as the Mediterranean, where surface sightings are common in summer months. They are notably absent from polar waters, as their physiology requires water temperatures generally above 10°C for extended periods. This wide but thinly spread distribution reflects their migratory, open-ocean lifestyle, where they often travel long distances across ocean basins.
Vertical Habitat and Depth Range
Despite being frequently observed near the surface, the Mola mola spends a significant portion of its life in the deeper parts of the water column, exhibiting diel vertical migration. This species is an accomplished deep-sea diver, routinely descending to depths greater than 200 meters. Tagging studies have recorded dives reaching up to 644 meters, into the cold, dark mesopelagic zone. The purpose of these extensive vertical excursions is primarily to forage for food, which includes gelatinous zooplankton like jellyfish and salps. These prey items are often concentrated at depth, frequently below the thermal layer known as the thermocline, where temperatures are significantly colder.
The Purpose of Surface Basking
The characteristic behavior of the Ocean Sunfish floating on its side at the surface is a deliberate action with multiple functions. The primary reason for this “sunbathing” is thermoregulation, often referred to as “thermal recharging.” After spending time hunting in the cold, deep waters, the fish returns to the warmer surface layer to raise its body temperature, as prolonged exposure to cold can lead to disorientation and death. A secondary function of surface presence is the removal of parasites, which the Mola mola is prone to carrying in high numbers. The fish will visit shallow water cleaning stations where smaller reef fish, such as bannerfish, pick off external parasites, or allow seabirds, like the Laysan albatross, to pick ecto-parasites from its thick skin while it basks.