Did you recognise this animal?
This species of fish is the incredible ocean sunfish, belonging to the family of Molidae. The scientific name of the ocean sunfish, Mola mola translates from Latin to “millstone”, which refers to their round shape. The ocean sunfish can be found worldwide, notably in the waters from New South Wales and Victoria, down to Tasmania, and along the southern coast from South Australia to Western Australia.
Unlike a lot of other fish, the ocean sunfish doesn’t have a tail. Instead, ocean sunfish have a clavus, a flattened, blunt structure that begins where their dorsal fin connects to their body and ends the start of their anal fin. The clavus is the result of the ocean sunfish being born with a back fin that never fully develops. Instead, it folds and grows into itself. In order to move, ocean sunfish move their fins from side to side continuously.
Ocean sunfish are omnivores with a diet mainly consisting of jellyfish and salps; however, they also feed on small fish, crustaceans, zooplankton and algae. These fish have pharyngeal teeth at the back of their throats forming a beak-like structure that prevents them from being able to fully close their mouths. As such they must push their food in and out of their mouth until it is broken up into small pieces.
Ocean sunfish are the heaviest bony fish in our oceans, weighing about 1-2 metric tonnes, making them as heavy as a camel! These gentle ocean giants have an average height of about 1.8 meters. Due to their large size, ocean sunfish must consume 1-3% of their body weight, often eating hundreds of jellyfish per day!
The ocean sunfish can dive up to 200 meters below the surface in their constant search for food. These deep dives last about 10 minutes, but because the water is so cold at such depths, the sunfish must return to the surface to warm up. This sunbathing behaviour is what gives the ocean sunfish its name!
The reproductive behaviours of the ocean sunfish are not very well known; however, what we do know is ocean sunfish reproduce through broadcast spawning, where females release eggs into the water and males release sperm simultaneously. This behaviour increases the chance that the eggs will be fertilized and not get eaten by predators. Female sunfish may even produce up to 300 million eggs at one time!
Ocean sunfish are known for having one of the most extraordinary alterations in size in the entire animal kingdom. When ocean sunfish hatch, they weigh in at less than a single gram. In the wild it is estimated that the ocean sunfish can live for about 100 years.
Written by Mili Janjic
If you would like to learn more about the huge diversity of life that can be found along our coasts and shorelines, please contact our Education Director at education@dolphinresearch.org.au for a public event, school holiday program or school incursion/excursion.
