As a rule, fish are as cold as the water around them - but there are exceptions. However, partial warm-bloodedness was previously only known from nimble predators such as the great white shark and tuna. But now researchers have also found the rare trait in a more “comfortable” filter feeder: the basking shark. The discovery raises ecological and evolutionary questions and could have implications for protecting the endangered plankton feeder, the scientists say.
As far as body temperature is concerned, there are two basic concepts in the animal world: the cold-blooded, so-called ectothermic creatures adapt to the ambient temperature in an energy-saving manner. The warm-blooded, endothermic animals, on the other hand, afford self-generated body heat in order to be able to be agile even in the cold. Fish generally belong to the group of cold-blooded creatures - but there are interesting exceptions: At least partial endothermy is known from a few representatives of the bony fish and a few shark species such as the great white shark: These predatory fish can bring certain areas of their body to temperatures higher than those of the water. What is believed to be the largest shark of all time, the extinct megalodon, was probably also partially endothermic, as certain substances with a "thermometer function" in its fossil teeth suggest.
Targeting the second largest fish on earth
The previously known fish with endothermic characteristics were therefore predators at the top of the food chain. Therefore, it was also assumed that the ability to generate heat primarily served their particularly pronounced "sportiness". But as the current discovery now suggests, the concept can apparently also be worthwhile for a comparatively easy-going plankton eater: the basking shark (Cetorhinus maximus) is the second largest fish on earth with a length of up to ten meters. In contrast to the slightly larger whale shark, it does not suck in the water by moving its mouth, but swims with its mouth wide open to filter small organisms out of the current. The basking sharks are in this way in both hemispheres of the earth in the temperate to cold sea areas.
An international team led by Trinity College Dublin is investigating the still mysterious sea giants. Studies of dead basking sharks that washed up in Ireland and Great Britain, the scientists report, provided the first evidence of partially endothermic characteristics: They found that they have special swimming muscles deep in their bodies, similar to great white sharks and tuna. In other fish, however, these reddish muscles are usually found in more external areas of the body. The team also discovered that basking sharks have a strikingly muscular heart, which is also typical of partially endothermic species. Fish, on the other hand, normally have “spongy” hearts, the scientists say.
Confirmed by temperature measurement
In order to back up these anatomical indications of partial endothermy with measurements, the team then equipped basking sharks off the coast of Ireland with special data loggers. These devices recorded the animals' muscle temperatures just under the skin for up to 12 hours before they automatically released and could be collected by the researchers. They were finally able to confirm: The temperatures of the muscles are permanently above the water values. The extent corresponds largely to the level known from partially endothermic fish species, say the scientists.
“Previously, it was assumed that partial endothermy only occurs in apex predators. However, we've now discovered the concept in a species that feeds on tiny plankton, so the way we view the trait may now need to be adjusted," says senior author Nicholas Payne of Trinity College Dublin. But what use could the ability to heat up to the basking shark? So far, there are only conjectures: "In cooler water, it could enable the sustained mechanical power required to overcome the sustained water resistance that occurs when filtering at speeds averaging 0.9 meters per second," the researchers write.
Finally, first author Haley Dolton from Trinity College Dublin emphasizes the possible importance of the discovery for the protection of the sea giants, whose populations have declined sharply in recent times: "It can be assumed that partially endothermic fish consume a comparatively large amount of energy and therefore possibly react differently to the warming of the oceans than other fish." Climate change may therefore be affecting the basking shark in a special way. "We hope that this type of research can help protect these fascinating animals," said the scientist.
Source: Trinity College Dublin, professional article, Endangered Species Research, doi: 10.3354/esr01257
Video exploring partial endothermy in basking sharks. © Trinity College Dublin