Basking sharks can maintain a body temperature that is higher than their environment, putting them among a small group of fish species that are warm-blooded.
Out of around 35,000 species of fish, only 35 are known to have the ability to generate or maintain the temperature in certain parts of their body, perhaps because it is so difficult to retain body heat in the water. They include great white and mako sharks, for whom body heat .
Basking sharks (Cetorhinus maximus), which feed on plankton, have always been thought to be cold-blooded. But their fast cruising speeds and ability to move great distances over a short amount of time are more characteristic of warm-bodied sharks.
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These behavioural clues raised questions for at Trinity College Dublin in Ireland, who wanted to see what she could learn by examining the anatomy of basking sharks.
Strandings of this species are rare, but, incredibly, four washed up in the UK and Ireland during Dolton鈥檚 PhD project. She worked with the UK Cetacean Strandings Investigation Programme to dissect the specimens.
What they found was very different to what had been previously described and resembled what would be expected from a warm-blooded fish, she says. There was red muscle close to the vertebrae at the trunk of the shark; a parallel artery and vein towards the outer edge of the red muscle, which may help to conserve heat; and a high percentage of compact myocardium 鈥� the middle layer of muscle in the heart 鈥� which may facilitate oxygen offload and uptake at the gills.
To investigate further, the researchers designed a new technique to record body temperature in a basking shark. They deployed a tag with two temperature probes: one to record the ambient water temperature and another in muscles at the base of the dorsal fin, where the tag was anchored to the sharks, to take internal readings. After around 12 hours, a dissolvable link caused the tag to pop off so the team could recover the data.
The results showed the body temperature of the sharks was consistently about 1 to 1.5掳C above ambient water temperature, suggesting they are regionally endothermic 鈥� meaning they can warm up parts of their body.
Regionally endothermic fish can migrate large distances in shorter amounts of time. Dolton and her colleagues believe such swimming ability may be what helps basking sharks overcome the drag caused by opening their large mouths while feeding.
鈥淸This] completely changes what we knew about them and what they could be capable of,鈥� says Dolton. 鈥淪hould we stop thinking about basking sharks as these slow moving, non-predatory sharks?鈥�
We still know very little about basking sharks鈥� life history or biology, she says, which is problematic for conservation efforts. Discovering they can raise their body temperature could help scientists better predict their distribution, possibly improving conservation policies, such as the creation and enforcement of marine protected areas, she says.
Endangered Species Research
