Mola Mola: The Weirdest Fish in the Ocean?

The mola mola. Photo: Per-Ola Norman, released into public domain.

The Mola mola. Photo: Per-Ola Norman, released into public domain.

By Craig Leisher

When it hatches, a Mola mola is the size of a pinhead but will grow to be the heaviest bony fish in the ocean—and the weirdest.

The weirdness begins with the eggs. A female Mola mola or ocean sunfish produces more eggs than any other vertebrate on earth.

One modest-sized female had an estimate 300 million eggs inside her.

At birth, the baby fish are protected by a star-shaped transparent covering that looks like someone put an alien head inside of a Christmas ornament—albeit a very small only a tenth of an inch across.

Even as a baby, the Mola mola has its parents’ surprised look with the wide eye and open mouth.

Mola mola larva. Photo: G. David Johnson under a Creative Commons license.

Mola mola larva. Photo: G. David Johnson under a Creative Commons license.

The baby will grow fast. Very fast. One individual in the Monterey Bay Aquarium gained 822 pounds in just 15 months (almost 2 pounds a day).

By the time it is an adolescent, the fish will have not tail fin, no ribs, a fused spine, and will swim by flapping its dorsal fin on the top and its anal fin on the bottom.

It will look like a giant swimming head.

Photo: U.S. National Oceanic and Atmospheric Administration

Photo: U.S. National Oceanic and Atmospheric Administration

Mola molas spend much of their lives in the open ocean chasing the sea jellie (a.k.a. jellyfish) they often eat. They have unusual teeth that are fused together inside a mouth they never close.

They are called the ocean sunfish because they are frequently seen catching rays on the ocean surface. One reason they float on the surface is so birds can peck out the parasites off their skin.

And they have a lot of parasites. More than 50 species of parasites have been recorded on and inside Mola molas.

Like sharks and rays, the female are far bigger than the males. The heaviest Mola mola on record is a female caught in 1996 that weighed 5,071 pounds (2,300 kg).

Here a picture from 1910 of a Mola mola that weighed an estimate 3,500 pounds. (1,600 kg).

Photo: Library of Congress, Prints and Photographs Division

Photo: Library of Congress, Prints and Photographs Division

The huge decline in shark populations and far greater numbers of sea jellies in the ocean mean Mola molas now have fewer predators and more food. The 21th century looks like a good one if you’re a Mola mola.

But who knows for how long. Given that they are one of the few large fish in the ocean that are doing well, don’t be surprised if someone gives the Mola Mola a catchy new name and starts selling them globally, just as marketers did for the Slimehead (Orange Roughy) and the Patagonia toothfish (Chilean sea bass).

You can see a Mola molas at a Nature Conservancy-supported marine protected area near Bali, Indonesia. The Mola mola congregate near Nusa Penida Island, and during the peak of mola season in October, there is a great chance of seeing the weirdness (and the parasites) of the Mola mola firsthand.

Opinions expressed on Cool Green Science and in any corresponding comments are the personal opinions of the original authors and do not necessarily reflect the views of The Nature Conservancy.

Posted In: Fish

Craig Leisher is a Senior Social Scientist who focuses on amplifying and measuring the benefits to people from conservation initiatives.

Comments: Mola Mola: The Weirdest Fish in the Ocean?

  •  Comment from TimBoucher

    Any idea how long they live?

    •  Comment from Craig Leisher

      Mola molas have a lifespan similar to ours. Females live longer than males, and 80 to 100 years is the upper end of the range.

  •  Comment from jon s

    Whoa! That’s some fish. But I vote for the blobfish. My kids love that thing.

    •  Comment from Craig Leisher

      Jon, great suggestion. The Blobfish is not only weird but is also the ugliest animal on earth according to the UK’s Ugly Animal Preservation Society.

  •  Comment from Stuart W

    How is this different from a Sun Fish? Or is “Sun Fish” just another name for them here in Australia?

    •  Comment from Craig Leisher

      Stuart, it’s the same as an ocean sunfish (but not the freshwater sunfish). Mola mola is its Latin name and its common name in many countries.

  •  Comment from josee roy

    300 million babys for one mom,,,, and eat jellyfish??? how come they have so many giant jellyfish problem in the ocean ??

  •  Comment from Frenchie

    Is this fat fish good to eat by humans?

    •  Comment from Craig Leisher

      Only if you like eating parasites.

  •  Comment from Janet

    Why do they have so many parasites and how does the number of their parasites compare to other fish?

    •  Comment from Craig Leisher

      One reason for so many parasites could be their jellyfish diet. Jellyfish provide shelter for lots of small creatures and their parasites.

  •  Comment from David Ehrlich

    I know that they have predators, which surprised me because I thought they were nearly all bone. But apparently the predators just chomp off their fins and leave the helpless molas to languish and die.

  •  Comment from robert

    Craig, great piece on mola molas.

    Any updates on the thinking that there are at least two different species here. I believe the M. mola in the first photo with the scalloped tail is the one commonly seen in US waters, while the M. mola in the later image from NOAA looks like ones we get here in Indonesia.

    •  Comment from Craig Leisher

      Good to hear from you Robert! There’s still only one Mola mola species, but there are different branches of the species (or clades). A meta review of the evidence states: “All individual and group analyses showed high genetic divergence among M. mola specimens indicating two clades; one consisting of animals exclusively from the southern hemisphere and the other containing individuals from both hemispheres.”

  •  Comment from Christel Platt

    There is a thing on TED about the Mola Mola & how they live on jellyfish, and that jellyfish are declining, but I saw pics the other day where jellyfish were all together in some odd event by the millions, miles long, all side by side…I hope this amazing fish stays around, its fasinating! (also called a sun fish)

  •  Comment from Liz Rauer

    I didn’t know TNC was supporting conservation efforts in Nusa Penida, that place is amazing! Went there last year, it was too early in the year for Mola Mola, but we did get to swim with manta rays by Nusa Lembongan. Happy to see you involved in marine conservation in that part of the world – keep up the good work!

  •  Comment from Jose

    See also this experiment on tracking these stragnge fish with robots:

  •  Comment from DanM


    >By the time it is an adolescent, the fish will have not tail fin…

    Should be “no tail fin”?

  •  Comment from Robert Black

    I read in a book that these fish tasted vile, so that might explain why they are still plentiful. Sea turtles, on the other hand, are very tasty, so there is incentive to kill them & tell no one.

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