The Mola mola, commonly known as the ocean sunfish, is one of the ocean’s most peculiar inhabitants. Its distinctive appearance resembles a massive, flattened disk with prominent fins. Named “mola” from the Latin word for millstone due to its rounded shape, this creature presents a unique silhouette in marine environments.
Unveiling the Ocean Sunfish: Physical Marvels
The ocean sunfish has a flattened, disk-like body. Instead of a typical tail fin, it features a unique, rudder-like structure called a clavus, formed from extensions of its dorsal and anal fin rays. Prominent dorsal and anal fins extend vertically from its body, which it flaps synchronously for propulsion.
This species is notable for its immense size and weight, ranking among the heaviest bony fish. Adult Mola mola typically weigh between 247 and 1,000 kilograms (545 and 2,205 pounds), with some reaching over 2,700 kilograms (6,000 pounds) and measuring up to 3.3 meters (10 feet 10 inches) in length. Its rough, leathery skin, with a sandpaper-like texture, covers a largely cartilaginous skeleton. The sunfish also has a relatively small, beak-like mouth with fused teeth, which it cannot fully close.
Life Beneath the Waves: Habitat, Diet, and Behavior
Ocean sunfish inhabit pelagic, or open ocean, environments across temperate and tropical waters worldwide, including the Atlantic, Pacific, Indian Oceans, and the Mediterranean Sea. While often observed near the surface, they are known for deep dives, reaching depths of over 600 meters (2,000 feet) in search of food. This vertical migration allows them to explore different thermal layers.
Their diet primarily consists of gelatinous zooplankton, such as jellyfish and salps, consumed in large quantities due to their low nutritional value. They also supplement their diet with small fish, squid, crustaceans, and larvae. Sunfish use their beak-like mouths to suck in and out prey, shredding it before swallowing.
A characteristic behavior of the Mola mola is “sunbathing” at the surface, where they often lie on their sides. This behavior is thought to serve multiple purposes, including thermoregulation after deep, cold dives and attracting cleaning fish or seabirds to remove parasites. Despite their large size, they are considered slow and somewhat awkward swimmers, propelling themselves mainly by flapping their dorsal and anal fins. Reproduction involves laying an extraordinary number of eggs; a single female can produce up to 300 million eggs in one spawning season, making them the most prolific vertebrate.
Mola Mola in Peril: Conservation Challenges
The Mola mola faces various threats in the marine environment, leading to its classification as “Vulnerable” by the International Union for Conservation of Nature (IUCN). A significant threat is bycatch in commercial fisheries, particularly in tuna and swordfish longline operations, where they are unintentionally caught. This can result in injuries or fatalities.
Plastic pollution also poses a substantial risk, as Mola mola can mistake plastic bags for their primary food source, jellyfish. Ingesting plastic can block their digestive systems, leading to starvation. Habitat degradation and boat strikes further contribute to the challenges facing these fish. Their unique biology, including slow growth rate and specialized diet, makes them susceptible to these anthropogenic pressures.