Rise of the Mola mola, by Coly Cancino

The Mola Mola, more commonly called Ocean Sunfish, has been a fairly quiet fish when it comes to news. It was recently thrust into the spotlight when a person on a forum called Reddit Rants posted about how worthless it is due to its many deformities. The three deformities called out were that it doesn’t have a caudal fin, it lacks a swim bladder, and its nutrient intake. He went on to say “EVERY POUND OF THAT IS A WASTED POUND AND EVERY FOOT OF IT IS WASTED SPACE.” The redditor could not be more wrong; the Mola Mola is one of the more fascinating fish due to its abnormalities, and it does not let them hold it back.

Common Mola.  Mola mola.  By Mike Johnson, Earthwindow

            The Ocean Sunfish may be considered “half a fish” to some people, due to its obscurities, but it does just fine. The mola are the heaviest of all the bony fish, with large specimens reaching 14 feet vertically and 10 feet horizontally, and weighing nearly 5,000 pounds (Thys 2015). They generally grow and mature at a fast rate due to their short lifespan of 10 years. They are commonly found in temperate and tropical oceans around the world, but that seems to be changing. Mola Mola can seen basking in the sun near the surface and are often mistaken for sharks when their huge dorsal fins emerge from the water. Their teeth are fused into a beak-like structure, and they are unable to fully close their relatively small mouths. Though the fish has many abnormalities, it’s important to consider the organism as whole: more than the sum of its parts.

One of the deformities the redditor was ranting about was the fact that the Ocean Sunfish does not have a caudal fin, which most fish use to swim and navigate through the seas. Instead of the caudal fin, the Ocean Sunfish has a clavus, which is essentially where the end of the fish folds in on itself forming a rounded rudder type of appendage. The Ocean Sunfish overcomes its lack of caudal fin by swimming with its anal and dorsal fins moving at the same time laterally. This way, it generates a lift type thrust that moves it forward and up at the same time. This type of movement has not been seen in many other fish. It may not move fast at .04-.07 m/s, but the Ocean Sunfish has recently been found to have expanded its migratory pattern (Pope 2010). The Mola mola was recorded sporadically in artic waters throughout the 20th century, but never more than one fish in a given year. Since 2000, however, there has been a considerable increase in both the annual frequency and the number of fish observed in Icelandic waters far off from its usual tropic habitat (Palsson 2017). They have been tracked as early as October in these waters and have been recorded staying in the cooler waters for approximately a month until continuing their migration back toward tropic water. The recent expanded range of the sunfish debunk the myth that the sunfish is not a swimmer and cannot migrate.

About the Mola mola.  Source. 

             The redditor also called out that the Mola Mola does not have a swim bladder, which most fish do have. Instead of a swim bladder, the Ocean Sunfish has subcutaneous gelatinous tissue, which is low in density causing the Mola Mola to be neutrally buoyant (Watanabe 2008). It also uses its abnormal swim style, stated above, to stay afloat. Mola Mola also use this gelatinous tissue to help float at the surface of the water. The fish needs to float at the surface so that birds can land on them to eat off parasites and algae. They tend to collect these organisms on their skin due to the fact that they move slow throughout the water. Even without the swim bladder, the Mola Mola is able to swim throughout the ocean like a regular fish.

            The third obscurity of the Ocean Sunfish that the blogger called out was the fact that it has to eat so much because its diet consists of non-nutritional organisms. Ocean Sunfish are often referred to as obligate, or primary feeders, on gelatinous zooplankton. Large Mola Mola appear adapted for capturing and ingesting large scyphozoan jellyfish. Many also say that the Mola Mola eat a wider variety of organisms including algae, crustaceans, mollusks, and fish (Nakamura 2014). The Ocean Sunfish can get away with this type of diet due to its passive lifestyle not requiring excessive amounts of nutritional benefits.

Close up into the mouth of the Mola mola with fused beak teeth and throat teeth.     Source. 

Overall the Ocean Sunfish is not as helpless as perceived by the redditor and the general public, and are quite the fascinating fish. They can function just like any other fish, but just tend to do it in their own unique way. From their unusual shape to their unusual behavior, there is not a lot known about the Ocean Sunfish. They are still a giant mystery swimming about our oceans.

References 

Thys, T. M., et al. "Ecology of the Ocean Sunfish, Mola Mola, in the Southern California Current System." Journal of Experimental Marine Biology and Ecology, vol. 471, 2015, pp. 64-76doi:10.1016/j.jembe.2015.05.005.

Palsson, J. and Astthorsson, O. S. (2017), New and historical records of the ocean sunfish Mola mola in Icelandic waters. J Fish Biol, 90: 1126–1132. doi:10.1111/jfb.13237

Nakamura, I. & Sato, K. Mar Biol (2014) 161: 1263. doi:10.1007/s00227-014-2416-8

Pope, E.C., Hays, G.C., Thys, T.M. et al. Rev Fish Biol Fisheries (2010) 20: 471. doi:10.1007/s11160-009-9155-9

Watanabe Y, Sato K (2008) Functional Dorsoventral Symmetry in Relation to Lift-Based Swimming in the Ocean Sunfish Mola mola. PLoS ONE 3(10): e3446. https://doi.org/10.1371/journal.pone.0003446