The sunbathing habits of the Oceanic Sunfish

The elusive Oceanic Sunfish (Mola mola) is the heaviest bony fish in the world. With an average weight of 1,000 kg, average length of 1.8 m and a fin-to-fin length of 2.5 m, this peculiar looking species is native to tropical and temperate waters around the globe. The species belongs to the order Tetraodontiformes, which also includes pufferfish, porcupinefish, and filefish. These large and docile animals are pelagic in nature and are known to dive to significant depths where they feed on various species of jellyfish1, but are often observed floating at the ocean surface, seemingly basking in the sun.

A recent study led by Itsumi Nakamura of the University of Tokyo, published in the Journal of Animal Ecology, investigated this basking behaviour of the Oceanic Sunfish in an attempt to understand its physiological purpose. The team of scientists caught several sunfish off the coast of Funakoshi Bay, and through the attachment of a combination of an animal-borne accelerometer and camera with a light source, evidence of ocean sunfish feeding in deep water was obtained. After four to six days these instruments naturally separated from the sunfish, allowing the researchers to collect and analyse the data. Siphonophores, an order of Hydrozoan Cnidarians, were the most abundant prey items captured by ocean sunfish and were typically located at a depth of 50–200 m where the water temperature was <12°C. Ocean sunfish were diurnally active, made frequently deep excursions, and foraged mainly at 100–200-m depths during the day.

Additionally, the team of researchers attached thermometers to measure changes in the fish’s body temperatures during foraging periods and basking. Body temperatures were measured under natural conditions, with body temperatures decreasing during deep excursions and recovering during subsequent surface ‘basking’ periods. Heat-budget models indicated that the whole-body heat-transfer coefficient between sunfish and the surrounding water during warming was 3–7 times greater than that during cooling. Through these results, the team concluded that the main function of this surfacing behaviour of the Oceanic Sunfish is the recovery of body temperature, potentially being able to increase heat gain from the warm surface water by physiological regulation.

Nakamura was surprised to find just how quick the warming process was.

“Beyond our assumption, their body temperature increased rapidly during surface warming, suggesting they have some physiological mechanisms to increase heat gain from the surrounding water.” 2

Nakamura also goes on to suggest that the results explain why larger sunfish can forage for longer and that they provide a possible answer to why Mola mola have such large bodies – as it serves to help them adapt to their hunting environment, losing heat very slowly.

Article Source: Nakamura. I, Goto Y, Sato K (2015) Ocean sunfish rewarm at the surface after deep excursions to forage for siphonophores – J Anim Ecol. 2015 Feb 2. doi: 10.1111/1365-2656.12346

Header photo credit: Alan Friedlander

Additional References:

1. Thys, Tierney. “Molidae Descriptions and Life History”. Retrieved 2007-05-08.


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