The ocean sunfish, scientifically known as Mola mola, is a remarkable creature. It is the heaviest bony fish in the world. Often misunderstood as a passive jellyfish eater, the sunfish has a more diverse and dynamic diet than previously thought. This article delves into the fascinating world of the ocean sunfish diet, exploring what these gentle giants eat and how they obtain their food.
Physical Characteristics and Distribution
Before diving into dietary habits, it's essential to understand the sunfish's unique physical attributes. The ocean sunfish boasts a distinctive appearance. Its body is laterally flattened and round, resembling a millstone, which is reflected in its Latin name mola. They can grow to impressive sizes, reaching up to 3.3 meters in length and weighing as much as 2,300 kg. The most striking features are the elongated dorsal and anal fins, which it uses for propulsion. Lacking a true tail, they have a short, rounded structure called a clavus.
Sunfish are found in all oceans across the globe, but they are generally restricted to waters warmer than 10°C. Despite the common sight of them basking at the surface, they spend the majority of their time submerged at depths of 200 to 600 meters.
Challenging the "Jellyfish-Only" Myth
For a long time, the ocean sunfish was believed to be an inactive drifter, feeding solely on gelatinous plankton. This perception is now being challenged by recent studies. While jellyfish do constitute a part of their diet, it's not the whole story.
Scout Burns, whose fishy facts about sunfish went viral, stated that sunfish "mostly only eat jellyfish because [it has] a possibility of drifting into their mouths I guess. Everything they do eat has almost zero nutritional value and because it’s so stupidly fucking big, it has to eat a ton of the almost no nutritional value stuff to stay alive. Dumb."
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While it is true that jellyfish are a significant component of their diet, this statement requires closer examination. The idea that sunfish only passively consume jellyfish that drift into their mouths is inaccurate. Sunfish are active hunters, constantly patrolling the oceans in search of food, traveling long distances horizontally and vertically. When they find jellyfish, they selectively consume the most nutritious parts, such as the gonads and oral arms, leaving the rest behind.
A Varied Diet: More Than Just Jellyfish
Recent research indicates that ocean sunfish are generalist predators with a diverse diet. Genetic analysis has revealed that their diet consists mainly of small fish (such as flounder), fish (eel) larvae, squid, other molluscs, crustaceans, and other soft-bodied invertebrates, with jellyfish and salps making up only around 15% of the diet. They may also occasionally ingest eelgrass.
Smaller sunfish (less than 1 meter in total length) have been found to consume a mixed diet of benthic (bottom) and pelagic (open ocean) prey. These juvenile sunfish often school together and consume nearshore prey taxa, including fishes, squids, crustaceans, and jellies.
As they grow larger, sunfish may become more solitary and shift towards consuming pelagic gelatinous zooplankton. This shift may be due to increased energy efficiency and a potentially underestimated nutritional value of gelatinous zooplankton.
Dietary Adaptations and Feeding Strategies
The sunfish has several adaptations that aid in its feeding habits. They lack teeth, instead possessing beak-like plates in their mouths. This beak is used to break up harder-bodied prey, while teeth in the throat further grind the food before it is swallowed. They suck water in and push it out rapidly to break up soft-bodied prey.
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Sunfish are also known to dive to great depths in search of food, with evidence showing dives up to 844m. They may use their large, sensitive eyes to hunt prey in the deep sea.
How Sunfish Diet is Studied
Understanding the diet of ocean sunfish requires a variety of methods. Traditionally, stomach content analysis of recovered specimens was used. However, due to the rapid digestion of gelatinous prey, researchers now use complementary methods, including:
Stomach content analysis: Examining the contents of a sunfish's stomach to identify recently consumed prey.
Stable isotope analysis: Analyzing the tissues of sunfish to determine their trophic level and dietary history.
Genetic identification of prey items: Using DNA barcoding to identify prey species from sunfish tissue samples.
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Sunfish-borne cameras: Attaching cameras to sunfish to directly observe their feeding behavior in the wild.
Stable isotope analysis is a particularly useful technique. Isotopes are different forms of the same chemical element with varying numbers of neutrons. Nitrogen isotopes can help assign animals to a trophic level, as they become enriched as an animal moves up the food chain. Carbon isotopes, on the other hand, remain relatively unchanged by trophic level but vary due to biogeochemical factors in the local environment.
The Sunfish as Prey
While adult sunfish are vulnerable to few natural predators, young sunfish are preyed upon by various marine animals. Larger sunfish have been found inside large sharks, sea lions, and orcas. As sunfish start their lives as tiny eggs in the plankton, they are on the menu for almost every creature in the sea.
Humans also play a significant role in sunfish mortality. There are large markets for ocean sunfish meat across the Far East, particularly in Taiwan and Japan. They are also captured as unwanted bycatch by fisheries across the world.
Conservation Concerns
The ocean sunfish is listed as vulnerable by the IUCN, with its population decreasing. This decline may be caused by bycatch in fisheries, the increasing popularity of sunfish in human diets, and the ingestion of plastic debris mistaken for jellyfish.