The ocean sunfish, \(Mola\) \(mola\), is an unmistakable resident of the world’s temperate and tropical oceans, recognized by its flattened, disc-like body. As the heaviest known bony fish species globally, this immense creature appears to be all head and no tail. Attaining its massive scale suggests a requirement for significant longevity. Although the \(Mola\) \(mola\) is one of the most widely distributed oceanic fishes, much of its life history remains a mystery due to its preference for the open sea.
Understanding Ocean Sunfish Lifespan
Determining the precise lifespan of the ocean sunfish in its natural environment is a challenging task for researchers. Unlike many other fish species, the \(Mola\) \(mola\) possesses anatomically primitive otoliths, or “ear stones.” These bony structures, commonly used to estimate age by counting annual growth rings, are ineffective for this species.
Scientists rely on alternative methods, such as analyzing growth rings found in the fish’s vertebrae or using growth curves developed from captive individuals. One study on a related species, the sharptail mola (\(Masturus\) \(lanceolatus\)), estimated life expectancies based on vertebral growth rings. These estimates suggested a wild lifespan ranging from 2 to 23 years for females and 1 to 16 years for males.
A separate analysis extrapolated growth data from captive fish to larger, wild individuals, suggesting a sunfish reaching three meters could be approximately 20 years old. While these estimates offer a glimpse into their potential longevity, the maximum age for a wild \(Mola\) \(mola\) is not definitively established. Observations of sunfish in controlled environments show they can live for over a decade.
Achieving Maximum Size
The size achieved by the ocean sunfish represents one of the most astonishing growth trajectories in the animal kingdom. The \(Mola\) \(mola\) starts life as a minuscule larva, measuring only about 0.25 centimeters in length. The transition from this tiny larval stage to a full-grown adult represents an increase in body mass of over 60 million times.
Adult sunfish typically weigh up to one metric ton (2,200 pounds) and measure around 1.8 meters (6 feet) from snout to the edge of the clavus, or pseudo-tail. Their height, measured from the tip of the dorsal fin to the tip of the anal fin, can reach up to 4.26 meters (14 feet). The heaviest bony fish specimen ever recorded was a closely related species, \(Mola\) \(alexandrini\), which weighed 2.3 metric tons (over 5,000 pounds).
This massive scale is attained through a rapid growth rate, observed in aquariums. For example, one young sunfish at the Monterey Bay Aquarium gained 364 kilograms (800 pounds) in just 14 months. This tremendous size is partly due to a diet consisting largely of gelatinous zooplankton, such as jellyfish, requiring them to consume large quantities to meet energy needs.
Factors Limiting Longevity
Many ocean sunfish do not survive to their maximum lifespan due to various ecological pressures. Natural predation plays a role, with large marine predators like orcas and sea lions known to target sunfish. Sea lions, in particular, have been documented attacking sunfish by tearing off their fins, often leaving the maimed fish to sink to the seafloor.
Human interaction represents a major source of mortality, primarily through fisheries bycatch and entanglement. Sunfish are frequently caught accidentally in fishing gear like drift gillnets, longlines, and trawls, leading to injury or death. Entanglement in marine debris and discarded “ghost nets” also poses a significant threat to the sunfish population worldwide.
The \(Mola\) \(mola\) carries a heavy parasite load, with up to 40 different genera recorded on a single individual. This burden includes both internal and external parasites, and managing these infestations affects the fish’s overall health and survival. The characteristic behavior of sunfish basking at the surface or visiting cleaning stations is thought to be an attempt to enlist the help of small fish or seabirds to remove these parasites.