The bump-head sunfish, Mola alexandrini, holds the certified record as the heaviest bony fish in the world. As a member of the Mola genus, it is one of three recognized species of ocean sunfish. Its remarkable size and unusual appearance make it a subject of fascination, representing one of the ocean’s most unique inhabitants. This species navigates the open seas as a giant, dwarfing most other fish.
Physical Characteristics and Size
The most striking feature of Mola alexandrini is its unconventional body plan. The fish is laterally compressed and tall, with a truncated posterior that gives it the appearance of being just the front half of a fish. Instead of a traditional caudal fin, it possesses a unique structure called a clavus. This rounded, rudder-like lobe is formed from extensions of the dorsal and anal fin rays and helps steer the fish through the water. Large dorsal and anal fins propel it, swishing back and forth to move its massive body.
What truly sets Mola alexandrini apart are its distinct head features. It develops a prominent, fleshy bump on its head that becomes more pronounced with age and size. This, along with a protruding, rounded chin, gives the species its common name, the “bump-head sunfish.” Its skin is leathery and covered in rough denticles, typically colored in shades of brown or gray with lighter patches.
In late 2021, a specimen discovered off Faial Island in the Azores set a new world record. This individual weighed an astounding 2,744 kilograms (6,049 pounds) and measured 3.25 meters (10.66 feet) in length. To put its weight in perspective, this single fish weighed more than a white rhinoceros. This discovery surpassed the previous record-holder, another Mola alexandrini from Japan that weighed 2,300 kilograms (5,070 pounds).
Habitat and Global Distribution
Mola alexandrini is a pelagic species, meaning it inhabits the open ocean rather than coastal or bottom environments. Its distribution is vast, with confirmed sightings in temperate and tropical waters across the globe. The species has been documented in the Pacific, Atlantic, and Indian Oceans, with specimens found off the coasts of Japan, Taiwan, New Zealand, Spain, and South Africa, among others. Despite this wide range, encounters with these giants are relatively infrequent, and they are known to undertake deep dives into colder waters to forage for food.
Diet and Unique Behaviors
The diet of Mola alexandrini consists primarily of gelatinous zooplankton. They feed on creatures like jellyfish, salps, and siphonophores. Their teeth are fused into a sharp, beak-like structure that allows them to tear apart their soft-bodied prey. They must consume large quantities to sustain their body mass.
A well-known behavior is surface basking, where Mola alexandrini is frequently observed lying on its side at the ocean’s surface. This is thought to be a method of thermal recharging after deep, cold-water dives for food. Additionally, this surface time allows seabirds and certain species of fish to clean parasites from the sunfish’s rough skin.
Distinguishing From Other Sunfish Species
For many years, Mola alexandrini was mistaken for its more famous relative, the common ocean sunfish (Mola mola). The most definitive feature of Mola alexandrini is the prominent bump on its head and its distinct, protruding chin, both of which are absent in Mola mola. These characteristics become more obvious as the fish matures.
Subtle differences also exist in the shape of the clavus, the rudder-like tail structure. The specific contours and ossicles (small bony plates) can differ between the species. The combination of these morphological distinctions and modern genetic analysis finally confirmed Mola alexandrini as a separate species, which resolved decades of confusion.