Wednesday, May 24, 2023

Temperature Regulation in a Shark Species

Environmental & Science Education, STEM, Biodiversity, Biological Evolution, Nature of Science, Models

Ed Hessler

Leslie Orgel's second rule, well known among evolutionary biologists, is that "Evolution is cleverer than you are."  If you are unfamiliar with the rules or it has been a while since you read about them, I urge you to read the Wiki entry for a short explanation of  their origin as well as their meaning.

This quote from Nature Briefing, May 12 provides an example and is linked to a short news item in Nature News by Bianca Nogrody (May 11) . "When diving for delicious squid in the cold ocean depths, scalloped hammerhead sharks (Sphyrna lewini) apparently ‘hold their breath’. Being fish, they don’t breathe as such, but they seem to shut off water flowing over their gills, to keep their bodies warm. It’s the first time such behaviour has been spotted. Heat loss from gills is a key weakness for diving fish, because gills are 'essentially just giant radiators strapped to your head', says shark researcher Mark Royer."

These sharks are deep fishers for squid, making up to six dives per evening of some 800 m (2624 feet) where the water is about 20 degrees Celsius colder (5 to 11 degrees Celsius) than surface waters.

Here is a description of the techniques used. "To understand how sharks were coping with the temperature changes, postdoctoral shark physiologist (Mark) Royer and his colleagues developed a device consisting of instruments that measured depth, water temperature, location and movement, as well as a probe embedded into muscles near the dorsal fin that recorded the shark’s core temperature. The device was designed to break off after several weeks, float to the surface and send out a signal to enable its recovery."

The essay includes  a discussion of the issue of heat loss, examples of solutions used by tuna and whales and other fishes - tuna, marlin and two shark species, and keeping warm. This is the first report of the use of this particular mechanism but one that researchers would not be surprised to find in other fishes although the mechanism is yet to be settled (not the use of the modifier "apparently" above). 

The original report is in the journal Science to which there is a link and where the full article may be read. I include this link again for insurance. It Includes an editor's summary, a very useful, diagrammatic figure of deep diving behavior. i.e., what happens during the dives, including times, and a detailed description of the descent, time at the bottom of the dive, and the ascent. You will notice the reference to the use and power of models, mathematical models, which indicated the "suspension of convective heat transfer."

The study has led to two hypotheses regarding the mechanism: shunting blood away from the gills OR active gill closing. These are considered in the discussion section of the paper where it is pointed out that there are distinct differences between high performance fishes (fast swimmers) and scalloped hammerhead sharks.
This is another example of what technology has made possible for scientists to investigate.

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