Not all fish are cold blooded – science tells the story of the warm-blooded opah fish

The East London Museum oceanic gallery has a model of a fish that has captured the world’s attention this past week. Research on the opah fish (Lampris guttatus) has revealed that it is a warm-blooded fish that uses its fins to maintain endothermy (the production and retention of heat to warm the body temperature to above the ambient through metabolic processes).

The 80 centimeter East London Museum specimen washed ashore at Cebe on the Wild Coast in 2013. It was retrieved by the East London Aquarium and later handed to the museum. This was a first record of the opah from the Border region and most occurrences of this rare fish in South Africa are from the Western Cape. A model of the fish was made by museum technician Greg Brett and mounted in the oceanic gallery.

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Opah live in all parts of the world in the open ocean from just below the surface 50 m to depths of 400 m. Research by the NOAA Fisheries discovered that the fish circulates heated blood through its body similar to mammals and birds. Fish that live at great depths are normally sluggish and slow in their movements and as predators usually ambush prey rather than chasing them down. The opah with a heated body can speed its metabolism up resulting in faster movements – this helps in chasing down prey such as squid and lantern fish.

Fisheries biologist Nicholas Wegner of NOAA Fisheries’ Southwest Fisheries Science Center in La Jolla, California (lead author of the scientific paper describing the discovery) also found that the opah sees more sharply when acting as a high-performance predator in the oceans. In the study Wegner also noted that the gill design of the opah was different to other fish – a ‘counter-current’ system of warm blood vessels from the core of the fish moves into the fish’s gills winding around cold blood vessels and circulating this blood back to the core after oxygen has been absorbed. The discovery of the heat exchange within the gills contributes to the opah’s entire body being maintained at a higher temperature (4 – 5 degrees Celsius above the ambient).

At deeper, colder depths the opah’s ability to move faster and react quicker with better vision gives them a competitive edge over other fish predating in the vertical water column in which they feed. They swim rapidly by using large red pectoral fins much like wings through the water and this constant ‘flapping’ of the fins heats the body. This roughly round fish can attain a weight of up to 90 kg. Along with the coelacanth this unusual fish found along our shores has astonished fish scientists and defies long held beliefs that all fish are cold-blooded.

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Museum scientist Kevin Cole with a model of the opah fish from Cebe (Wild Coast 2013)

Journal reference:

Nicholas C. Wegner, Owyn E. Snodgrass, Heidi Dewar, John R. Hyde. Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus. Science, 2015

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About East London Museum Science

Conservation Biologist East London Museum South Africa
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