Splashy Sunfishes: Secrets of the World’s Heaviest Bony Fish Surface

The esoteric ocean sunfishes have been splashing across the media with the latest big discovery, led by Japanese researcher, Etsuro Sawai, being announced this week: World’s heaviest bony fish identified and correctly named.

The Sawai team assigned an old species name to a group of sunfishes who sport a large bulbous bump on their heads when adults. This renaming has replaced the beloved Mola mola with Mola alexandrini as the world’s heaviest bony fish. It’s rare for non-commercial oddballs to garner such public attention, but in this case it underscores an important discovery regarding how many sunfish species exist, their distribution, and how much we still have to learn about these magnificent, albeit morphologically abridged, behemoths.

As one of the most fecund vertebrates in the world, ocean sunfishes occupy a significant yet poorly understood role in the marine food web. They consume a variety of prey species, including fish and squid when juveniles, but appear to shift their diet to eating more jellies when adults.

One big worry is that molas are common victims of bycatch in fisheries worldwide, including the California drift gillnet fishery where thousands are caught each year. To better manage these fishes and reduce bycatch, we need all the taxonomic and behavioral information we can muster.

The latest taxonomic discoveries also highlight the long process of naming a species. Shockingly, a 2012 survey found that the time it takes from species discovery to species naming is on average 21 years! (“New” species gather dust on museum shelves for 21 years). Taxonomists and others in this line of work are certainly champions of delayed gratification!

Mola alexandrini was named 178 years ago in 1839 by the Italian naturalist Camillo Ranzania–two years before he died. Linking it to today’s living sunfishes took more than a decade of diligent digging into the old literature, detailed dissections, accessing museum collections, and conducting genetic analyses like Sawai’s team did.

Another sunfish species, Mola tecta, named recently by Nyegaard et al. 2017, was also “discovered” more than 10 years ago by Japanese researchers and assigned the name Mola sp C. It took many  years of collecting enough samples, performing genetic analyses and coupling those with careful dissections of whole specimens from the Southern Hemisphere to formally describe it as a new species.

National Geographic has held a long-standing interest in these colossal fishes, from supporting satellite-tagging research in California (1), Galapagos (2,3), South Africa (4), Japan (5) and Bali, Indonesia (6), to helping host international underwater biotelemetry training workshops (7). Nat Geo has also featured sunfish discoveries in its magazines, educational publications, and school textbooks (8).

When I first started studying sunfishes in 2000, there were very few of us. Now, battalions of talented sunfish researchers collaborate and span the globe from Australasia to Japan, South America to the UK. We now know sunfish can dive to 1000m, and that they don’t just eat jellies but change their diets as they get larger and older. We know that they migrate latitudinally and target frontal systems — and are eaten by orcas, sea lions and sharks.

Many more wondrous discoveries are to come. As with nearly every ocean animal, there are plenty of questions to be answered. I hope this latest sunfish media splash inspires a whole new generation of fish biologists to dive in and join the effort of researching and protecting not only the magnificent Molidae but all the life in the sea.

Here are links to sunfish papers, workshops and pubs that have been supported by Nat Geo grants.


1. http://www.sciencedirect.com/science/article/pii/S0022098115001239

2. http://biotelemetry.ucdavis.edu/publications/37_MolaRamsayi.pdf

3. https://www.hindawi.com/journals/jmb/2017/7097965/

4. http://citeseerx.ist.psu.edu/viewdoc/download?doi=



7. http://www.oceansunfish.org/expeditions8.php

8. http://ngl.cengage.com/search/productOverview.do?N=201+4294918604&Ntk=NGL&Ntt=PRO0000000004&Ntx=mode%2Bmatchallpartial and http://ngl.cengage.com/assets/html/life/

9. https://www.jstage.jst.go.jp/article/specdiv/22/1/22_220106/_article

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