The ocean sunfish, Mola mola, is the largest bony fish in the world, with adults commonly weighing between 545 and 2,205 pounds and sometimes reaching over a ton in weight. Its massive, laterally flattened body resembles a giant, swimming head, making it one of the ocean’s most unusual inhabitants. It thrives in temperate and tropical waters across the globe, sustaining its colossal size on a diet primarily composed of food sources that are mostly water. This unique feeding habit often leads to a misunderstanding of how the Mola mola manages to thrive.
The Primary Role of Gelatinous Zooplankton
The historical understanding of the Mola mola’s diet was that it consisted almost entirely of gelatinous zooplankton, which are low-calorie organisms. These items include true jellyfish, salps, and siphonophores, which are soft-bodied and energy-poor. The sunfish’s mouth structure is specifically adapted for this soft prey, featuring fused teeth in both the upper and lower jaws that form a strong, beak-like structure. This strong beak is not designed for biting, but rather for grasping and tearing soft tissue, with the fish consuming prey primarily through powerful suction.
Recent research suggests that gelatinous prey may account for only about 15% of the Mola mola’s total diet. While the sunfish is an abundant eater of these animals, it often consumes only the most energy-rich parts of the organism, such as the gonads and oral arms of jellyfish, leaving the water-filled bell behind. This selective feeding ensures the fish maximizes its caloric intake even when consuming low-density prey.
Opportunistic Consumption of Higher-Calorie Prey
Sunfish are generalist predators and incorporate a variety of higher-calorie items into their feeding regime. These items, though representing a smaller portion of the overall mass consumed, provide a necessary caloric boost. Non-gelatinous prey includes small fish, eel larvae, squid, and crustaceans like amphipods. Analyzing stomach contents confirms these energy-dense food sources are crucial, challenging the long-held belief that the Mola mola subsists almost exclusively on jellyfish.
The inclusion of these prey items suggests the sunfish is an opportunistic feeder that takes advantage of any available food source in its environment. Since it can forage throughout the water column, from the surface to depths of over 600 meters, it encounters a wide range of potential meals. This flexible diet supplements the low-nutrient intake from gelatinous zooplankton, supporting the sunfish’s rapid growth and massive adult size.
Sustaining Massive Size on Low-Calorie Food
The paradox of the ocean sunfish is how it can grow to such an immense size while primarily consuming food with low nutritional value. The solution lies in the sheer volume of food consumed daily, which is estimated to be between 1% and 3% of the fish’s massive body weight. For a one-ton sunfish, this means consuming between 20 and 60 pounds of food every day to meet its metabolic demands. This high-volume consumption is necessary because the gelatinous prey is composed of up to 95% water.
The sunfish employs a specific feeding strategy to maximize hunting efficiency, which revolves around deep-water foraging dives and surface basking. Sunfish perform daily deep dives, sometimes over 3,300 feet, often below the thermocline into cold water, to hunt for deep-dwelling gelatinous prey, like siphonophores, which undergo diel vertical migration. These dives cause the sunfish’s body temperature to drop significantly, sometimes from 68°F to 53.6°F.
To recover from this thermal drop and recharge its muscles for the next hunt, the fish will ascend and “bask” near the surface, lying on its side to absorb solar energy. The Mola mola’s unique anatomy supports this feeding and diving cycle. Unlike most bony fish, the Mola mola does not possess a swim bladder, which would make rapid changes in depth difficult due to pressure changes. Instead, its neutral buoyancy is maintained by a thick, positively buoyant, and incompressible hypodermis layer. This lack of a swim bladder allows the fish to travel up and down the water column many times a day, maximizing its chances of encountering the vertically migrating prey.
Dietary Changes from Larvae to Adult
The life cycle of the Mola mola involves a significant shift in diet and feeding strategy as the fish develops from a tiny larva into a massive adult. Newly hatched larvae are minuscule, measuring about 0.25 cm in length, and resemble miniature pufferfish with a tail and body spines. During this early stage, the fish primarily feeds on small zooplankton and micro-crustaceans.
Larvae possess a crushing surface behind the beak-like jaws, a feature common in their relatives, the pufferfish, which helps them process hard-shelled micro-prey. This diet supports an astonishingly rapid growth rate, with juveniles gaining up to 1.8 pounds per day. As the sunfish grows, its body structure transforms, losing the tail and spines. The diet shifts from hard-shelled micro-prey to the soft-bodied macro-prey of adult sunfish, correlating with the development of the adult body form and the loss of the hard-prey crushing plate.