I think it’s time to introduce you to my favourite fish ever. I know that’s a big call because there are plenty of fish in the sea, let alone the rivers, but I’m sticking to my decision because holy mola this fish is cool.
Yeah I said mola, because I want you to meet the ocean sunfish, scientific name Mola mola. The much larger, stranger relation to the pufferfish, who’s shiny, mucus covered body looks like one giant floating fish head. Not only have sunfish evolved away their caudal fins, a.k.a tails, but they are the largest and heaviest bony fish in the sea and the most fertile.
So let’s dive in and learn about this big dish of a fish.
How does it move?
Well tails aren’t everything you know, you can get along fine without one so long as you evolve your dorsal and anal fins into two big flapping wings. And luckily for the sunfish, this is exactly what it’s done.
Surprisingly enough sunfish locomotion has been described as similar to that of penguins. Both flap their fins synchronously to thrust them through the water, the sunfish just looks that little bit stranger doing it because it’s done away with its tail, its pelvic fins and has shrunk down its vertebral column. All adding to the swimming head illusion. Looks aside, their underwater wing-beats are an effective way of getting around and the sunfish is a powerful swimmer, capable of deep-water dives and even breaching. What I would give to see that…
What does it get up to?
Something the sunfish is more commonly caught doing, is resting on the surface of the water, eyes facing up as if counting clouds. However its real purpose here is a little more productive. The sunfish uses this time to recharge thermally after foraging for jellyfish in deeper water, about 200 – 600m down. It swims down to feed, then swims up to get warm. A neat little cycle they can perform 20 times a day.
The sunfish will also commonly cosy up to gulls floating on the surface, as they’ll graciously feed on any parasites it’s picked up along the way.
Saving the best till last: From little things big things grow
Perhaps the most surprising thing about the sunfish however, is its incredible growth. Not only are sunfish the largest, and heaviest of the bony fish, reaching up to 3m in length and 2235 kg (4927 lbs) – they’re also the most fertile fish in the water too. Sunfish carry more eggs per individual then any other vertebrate and one medium sized female is estimated to contain 300 million eggs. Naturally for all these eggs to fit, they’re teeny tiny, so when a larval sunfish hatches it’s a meer 0.25cm big.
One scientist, Gudger calculated that “the larval sunfish is to its mother as a 150-pound (68kg) rowboat is to sixty Queen Marys” – and the Queen Mary weighed 80,773 tons (80,773,000kg)! Therefore its growth from this small fry into the biggest bony fish in the ocean is pretty staggering.
Who would have thought a big floating head could be this interesting…. Yeah nah, it was always going to be cool. These bad boys are the biggest and heaviest of the bony fishes yet start their lives as some of the smallest. They are the greatest egg producers in the vertebrate world and they even enjoy the odd sun bathe.
Without a doubt, my favourite fish.
Now then, what’re your favourite fishes? Do dish! Or feel free to leave any questions on the mighty mola in the comments below.
Houghton, J. D., Doyle, T. K., Davenport, J., & Hays, G. C. (2006). The ocean sunfish Mola mola: insights into distribution, abundance and behaviour in the Irish and Celtic Seas. Journal of the Marine Biological Association of the United Kingdom, 86(05), 1237-1243.
Nakamura, I., Goto, Y., & Sato, K. (2015). Ocean sunfish rewarm at the surface after deep excursions to forage for siphonophores. Journal of Animal Ecology.
Pope, E. C., Hays, G. C., Thys, T. M., Doyle, T. K., Sims, D. W., Queiroz, N., … & Houghton, J. D. (2010). The biology and ecology of the ocean sunfish Mola mola: a review of current knowledge and future research perspectives. Reviews in fish biology and fisheries, 20(4), 471-487.