The Ocean Sunfish, Mola mola, is the world’s heaviest bony fish. These massive animals can reach lengths of up to 11 feet and weigh over 2.5 tons in the open ocean. Its distinctive, flattened, and truncated appearance makes it resemble a giant swimming head. The Mola mola inhabits temperate and tropical pelagic zones across the globe, often basking near the surface. Despite their immense size, these slow-moving fish are preyed upon by certain marine predators.
The Primary Predator
The primary shark species documented to successfully predate upon adult Ocean Sunfish is the Great White Shark, Carcharodon carcharias. These apex predators possess the size and powerful bite force required to breach the sunfish’s defenses. Great White Sharks typically target large, high-calorie prey like marine mammals, but their habitat often overlaps with the Mola mola in temperate coastal regions.
The Great White Shark, which can reach lengths exceeding 20 feet, is able to overpower the sunfish, which is not an agile swimmer. Other large species, such as the Tiger Shark, Galeocerdo cuvier, have also been documented in rare instances of preying on sunfish, particularly in Australian waters. The Great White’s specialization in attacking large, tough-skinned prey makes it suited to overcome the sunfish’s physical resistance. Evidence of this predation has been confirmed through the discovery of Mola mola remains in the stomach contents of captured Great White Sharks, including a documented case in Italian waters.
The Challenge of the Meal
The Mola mola presents a challenging meal primarily due to its unique anatomical features, which make it a poor energetic investment for a predator. The sunfish is protected by an incredibly thick, rubbery, and highly elastic hide that can be up to three inches thick in some areas. This tough skin is covered in denticles and a layer of mucus, making it difficult for a shark’s teeth to gain a solid, deep purchase.
Beyond the physical barrier, the sunfish offers a remarkably low yield of energy for the effort exerted. The adult Mola mola feeds predominantly on gelatinous zooplankton, such as jellyfish and salps, which are largely composed of water. This diet results in a body composition that has a low caloric density, meaning a shark must consume a large amount of sunfish flesh to gain sufficient energy.
The sunfish’s laterally flattened, disc-like shape also creates an awkward target for a shark trying to execute a clean, incapacitating bite. Unlike a streamlined seal or tuna, the sunfish lacks the compact girth that would allow a shark to easily tear off a substantial, nutritionally rich piece. Its internal structure is also unusual; despite being a bony fish, its skeleton is largely cartilaginous, contributing to its low overall food value.
Observed Predatory Behavior
The predatory strategies employed by sharks against the Mola mola reflect the difficulties presented by the sunfish’s anatomy and low caloric reward. Sharks often employ an initial, non-committal “sampling” bite rather than a full, sustained attack. This exploratory behavior is a common strategy for sharks encountering difficult prey to assess nutritional viability.
Sharks and other predators, like Sea Lions, tend to target the fins of the sunfish. Sea Lions have been observed biting off the sunfish’s dorsal and anal fins—the primary means of propulsion—leaving the debilitated fish to drift. Sharks are more likely to inflict lethal wounds, but they often consume only the softer tissues, leaving large, tough pieces of the hide uneaten.
This selective feeding suggests the attack serves a dual purpose: providing a necessary meal when other prey is scarce, or removing vulnerable individuals. The ecological role of this predation is often to cull the sick or slow. The presence of larval shark tapeworms in Mola mola suggests that being eaten by a shark is an expected part of the parasite’s route to maturity, linking the two species in a parasitic relationship.