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The first time I saw one of these fish I was maybe seven or eight years old. What looked to be a huge shark dorsal fin sticking up in the air actually belonged to one of the weirdest, and largest, fish you’ll likely encounter. Instead of cutting through the water like a shark, this giant dorsal simply flopped side to side as the fish slowly made its way through the ocean.

Found worldwide, the mola mola (Mola mola) is a fish that basically looks like it is nothing but a giant head. It’s also one of the few animals who’s common name is the same as its scientific name, although it is also known as the Ocean Sunfish.

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The most distinguishing features of this fish are the fins. The dorsal and ventral fins are enormous and are not only used for balance, but for locomotion as well. In a process known as sculling these two fins wave back and forth propelling the fish forward. While most fish use their tails for propulsion, the mola lacks this caudal feature entirely. In its place is a useless lobed structure known as a clavus, formed not from tail fins but from extensions of the dorsal and ventral fin rays.

The teeth of the mola are fused into solid bony plates and are used to bite chunks out of large jellyfish and salp colonies. In addition to the fused teeth in the mouth, this fish also has a set of pharyngeal teeth in its throat, used to grind up its prey before it reaches the stomach. The mola is immune to the sting of Coelenterates and, like sea turtles, gets most of its nourishment from feeding on jellyfish. Considering jellyfish are 96% water, they need a lot of these animals to make a decent meal. Again like sea turtles, this feeding behavior is now a leading cause of death for these animals when they mistake plastic sandwich bags for food. After enough bags are consumed they clog the intestinal tract, leading to a slow and painful demise. One reason they are so susceptible to ingesting this flotsam is that the mouth is unable to be closed. Another very strange feature of this very strange fish. Anything that it encounters in its travels is sucked into the belly, whether edible or not.

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Molas gets their common alias "sunfish" because of their habit of lying on their sides at the waters surface, appearing to sunbathe. What they are actually doing is waiting for seagulls to land on them and pick off parasites. Sunfish are especially prone to parasites, mainly because of their slow, pelagic habits. Another way of ridding themselves of these pests is by breaching. Like whales, molas sometimes launch themselves out of the water, crashing back in with a tremendous splash. This is believed to be an effort to dislodge ectoparasites from their skin.

Despite their size, and they are the largest bony fish on earth, capable of reaching weights approaching 5,000 pounds (and may live for over 100 years), these gentle giants are susceptible to predation by several types of marine mammals. Orcas (killer whales) feed on them regularly. Sea lions, too, will kill them, and in a particularly cruel way. Having no tail, molas are helpless without their two large median fins. Sea lions will gang up on one and nibble away at the fins, removing any of the fish’s ability to swim. They then carry the mola to the bottom and wedge it under a rock. Essentially providing themselves with fresh meat whenever they feel like snacking.

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This 1200 pound mola was caught in 1926.

Molas aren’t normally eaten outside of the odd Japanese market (the eyeballs are considered a delicacy for some reason). Most of those killed by marine fisheries are bycatch trapped in oceanic gill nets. Although the flesh is edible, the internal organs contain neurotoxins similar to that of their relatives the pufferfish.

The relationship to pufferfish can be seen in the planktonic larval stages of the mola. The tiny young do resemble miniature puffers. Although molas are normally solitary creatures, during the breeding season they can be found swimming in pairs. In addition to holding the world record for size of a bony fish, they are also the champions in egg production. Each female will lay as many as 300 million eggs, far more than any other known fish. And the larvae are tiny, less than one tenth of an inch in length. Here’s something to blow your mind: A mola will increase it’s weight by 50 million times over the course of its life.

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Fun Fact: "Mola" is Latin for "millstone", named because of its round shape, grey color and rough skin.

Other diaries in this series can be found here.

Originally posted to Mark H on Fri May 18, 2007 at 12:16 PM PDT.

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