Ocean Sunfish Size: How Big Do They Get?

Inspired by their round disc-like shape, ocean sunfish were given the nickname mola, which in Latin means “millstone”. In addition to their odd appearance and elusive behavior, ocean sunfish are known for being the heaviest bony fish in the world. They are a marine giant that has been studied by many curious marine biologists and ocean enthusiasts.

What Do Ocean Sunfish Look Like?

Unlike other fish, which have long elongated bodies, ocean sunfish are flat and round. From the side, they do not appear to have a tail. Instead, they use a small fin called a “clavus” to swim and move through the water. The clavus is short and does not grow over a sunfish’s lifetime. As the sunfish grows, the fin actually folds into itself, making the sunfish appear even more circular. The “clavus” is very aptly named as it translates into “rudder” in Latin.

Currently, there are five species of sunfish that belong to the same Molidae family. All species are silver in color and possess a head that is disproportionately large relative to its body. On the contrary, their mouths are quite small. Their beak-like structure makes it perfect for consuming aquatic organisms, such as jellyfish, smaller fish, and zooplankton. Ocean sunfish are omnivores, so they also eat algae and other vegetation.

Where Are Ocean Sunfish Found?

Ocean sunfish are found in temperate waters around the entire globe. They can be found in every ocean in the world and prefer warmer tropical regions, staying close to the surface. More specifically, the waters in which they swim are usually warmer than 10 °C. Temperatures lower than this can cause confusion and even death.

With their huge dorsal fins, ocean sunfish are often mistaken for sharks. Luckily, they are typically not aggressive towards humans. They usually travel alone but have at times been seen in pairs. Using a gas-filled bladder, ocean sunfish glide gracefully in search of their next meal. They can get to speeds of up to 3.2 kilometers per hour, which is quite sluggish compared to the 40 kilometers per hour that sea lions, one of their main predators, can reach.

Just How Large Can Ocean Sunfish Grow?

Ocean sunfish ordinarily grow up to a whopping 5,000 pounds. For some context, that is more than some SUVs. Amazing, right? The fish can reach 14 feet horizontally from head to tail and 10 feet vertically. They are bony fish, so their skeletons are almost entirely made of bone. Salmon, tuna, and goldfish are some other common examples of bony fish. On the other hand, there are cartilaginous fish, such as sharks and rays.

In December 2021, José Nuno Gomes-Pereira, a marine biologist working with the Atlantic Naturalist Association, made an incredible find. He came across an ocean sunfish that shattered records. Weighing in at 6,049 pounds, the sunfish was a sight to behold. Prior to this discovery, the sunfish that held the world record was caught near Japan in 1996. Unfortunately, this sunfish was dead when it was found, and evidence suggested it was likely hit by a large ship. Gomes-Periera expressed this was a positive sign as “it means the marine ecosystem is still healthy enough to sustain these large animals”. However,  the fact that the monster fish had been killed raised concerns because it was a reminder that humans still posed lethal threats to the ocean’s giant creatures.

The Life of An Ocean Sunfish

Due to their mysterious nature, the reproductive behaviors of ocean sunfish are not well studied. It is known that they reproduce via broadcast spawning, where males release sperm that fertilize eggs simultaneously released by females. Some have been recorded releasing as many as 300 million eggs at a time, providing ample opportunities for new offspring.

After hatching, ocean sunfish measure only a tenth of an inch long. In contrast to how large it will be as an adult; this is 60 million times smaller than its final size. In adulthood, humans are only 30 times larger in size. Ocean sunfish don’t stay small for long. In just 15 months, they can reach almost 900 pounds. They grow remarkably fast compared to other bony fish. In captivity, ocean sunfish can live for up to 10 years under ideal conditions.

How Does the Ocean Sunfish Get So Big?

Because of their astonishing size, ocean sunfish have few natural predators. Their sheer size makes hem difficult to handle and predators, in turn, have to be large and ferocious enough to take down such a massive creature. Killer whales, some larger species of shark, and sea lions have been documented feeding on ocean sunfish. These predators are powerful and skilled hunters that are capable of overcoming not only the sunfish’s size but other natural defenses.

Ocean sunfish carry a large amount of parasites on their skin. These parasites can be harmful to the animals that consume them, making them less appealing as prey. Furthermore, the bony structure of ocean sunfish don’t make them particularly nutritious food sources.  Ultimately, ocean sunfish are able to grow to such immense sizes because of various factors, including their efficient feeding habits, slow metabolism, and special adaptations that allow them to competently evade predators.