The ocean sunfish is a very large, oddly shaped fish that gets its name from his habit of floating on its side, at the sea surface, warming itself in the sun. This species has no tail and swims with its very large dorsal and anal fins. It is the heaviest bony fish in the world, reaching weights of up to 5100 pounds (2300 kg), and the distance from the top of the dorsal to the bottom of the anal fin can be as much as 14 feet (4+ m).
Ocean sunfish are foraging predators that will eat a variety of food, but their preferred prey are jellyfishes. Jellyfishes are almost exclusively made up of water and are low in calories/nutrients, so a fish with a body as large as the ocean sunfish’s has to eat a whole lot of jellyfishes to support its weight. They have a surprisingly high growth rate and can gain hundreds of pounds in a year, so these jellyfish specialists are always on the hunt. Adults are too large to be threatened by any but the absolute largest potential predators, but medium-sized individuals are eaten by sea lions, killer whales, and large sharks. California sea lions are known to bite the fins off of small ocean sunfish and then play with them like frisbees.
The reproductive behaviors of ocean sunfish are not well known, but they reproduce via broadcast spawning, where females release eggs and males release sperm into the water column at the same time. This behavior increases the likelihood that eggs will become fertilized and that fertilized eggs will not be eaten by egg predators. However, scientists are not sure if ocean sunfish reproduce in groups or in pairs. Female ocean sunfish produce more eggs than any other vertebrate. They can release as many as 300 million eggs at a time and spawn several times throughout their lifetimes. Newly hatched ocean sunfish weigh less than a gram, and this species is noted for one of the most impressive transformations in size in the animal kingdom. The biggest adult ocean sunfish are 60 million times larger than when they hatched.
Ocean sunfish are occasionally captured in net fisheries targeting other species. This species has not been assessed by conservationists, but it is likely naturally rare and may be depleted in some areas. Further monitoring of ocean sunfish populations and research into their life history are important to understand this interesting species.
Oceana joined forces with Sailors for the Sea, an ocean conservation organization dedicated to educating and engaging the world’s boating community. Sailors for the Sea developed the KELP (Kids Environmental Lesson Plans) program to create the next generation of ocean stewards. Click here or below to download hands-on marine science activities for kids.