How flippin’ clever! Why do sunfish look like pancakes?

Taiyaki

A fishy looking pancake if ever I saw one…

So it’s Pancake Day once again and to get in the flipping spirit of things, I thought this blog post should focus on the biggest pancake in the sea, our favourite fish; the ocean sunfish!

But why are ocean sunfish so round and yet so slim? There have been many suggestions over the years, however a recent study lead by Itsumi Nakamura from the University of Tokyo in Japan, has provided an exciting new theory! The team captured several sunfish around Funakoshi Bay, and attached data recorders to measure their temperature and depth use. The data revealed that the sunfish regularly dived down into deep water where attached camera systems recorded them feeding on siphonophores (gelatinous animal colonies often mistaken for jellyfish, including the Portuguese Man of War). After feeding, the sunfish would then return to the surface waters and resume the classic “basking” behaviour. But why would this great silver pancake of a fish need to be in surface waters at all?! Especially if their ideal dinner lives at depth?

basking sunfish

The BBC termed the sunfish an “Ugly Sunbather” (well beauty is in the eye of the beholder!)

Well, as I mentioned in a previous blog post, sunfish have an ideal temperature range which limits their time in cold water and which might prevent them diving very deep (as water temperature quickly decreases with depth). It appears that the sunfish may spend up to half of each day basking at the sea surface where the water is warmest, to increase their core body temperature and this enables them to maximise their time foraging in deeper, colder waters. Each deep dive causes the sunfish’s body temperature to decrease, but by having such a slimline body with a large surface area (like a pancake), the sunfish can warm up much faster when it returns to the surface!

The team suggested that the warming process was much faster than they had expected and they believe that sunfish have some unknown physiological mechanism to increase heat gain. The enormous body size of adult sunfish means that they can retain heat much longer, losing it slowly as they hunt, which explains why larger sunfish can forage for longer.

So in the sunfish world, there are warm rewards to be had if you resemble a big pancake!!

If you would like to read more about this fascinating new research, check out this BBC article on Nakamura’s work (http://www.bbc.com/earth/story/20150209-giant-sunfish-reveals-secrets) or if you have access to the Journal of Animal Ecology read the whole paper for yourself (http://onlinelibrary.wiley.com/doi/10.1111/1365-2656.12346/abstract).

As always, if you have any comments or have seen a sunfish dead or alive, please get in touch!

Twitter: @SunfishResearch

Email: nphillips01@qub.ac.uk

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