Hammerhead sharks are first fish found to ‘hold their breath’ | Hacker News Reader

Hammerhead sharks are first fish found to ‘hold their breath’

nature.com

121 pointsgmays38 comments

For anyone not reading the article, they close their gills to (we assume) not lose heat when they are diving in deep and cold water - very interesting - hadn't thought that gills are the fastest way to exchange heat for fish. “Diving to over 1,000 metres from tropical temperatures at the surface down to just a couple of degrees centigrade to feed is a fairly extreme movement to do on a regular basis,” Simpfendorfer says.

It makes a lot of sense when it's explained. Gills basically act like a heat exchanger for oxygen. Lungs work very differently. They don't have continuous flow and humans have mechanisms to heat up and moisturize air before it reaches their lungs.

Salmon sharks are warm blooded and can stand temps down to about 36 degrees. I wonder if they have any adaptations in their gills to prevent heat loss or if they don’t care because they warm their bodies.

Down to? That only a single degree below human body temperature.

I think they might be talking farenheits

What I like about having an input method editor (IME) is that I can easily type units like ℃ or ℉ to avoid ambiguity. For a Japanese IME, just type ど (do, for degree), press spacebar until it pops out a conversion candidate list, then just find the symbol you want. Of course, you need to have known that the symbol appears ℉ when you type ど. There isn't an obvious way to find out which word is mapped to which symbol. Usually, I type きごう (kigou, for symbol) to get a list of symbols but ℉ does not appear in that list.

Or, you can also use WinCompose (or Xcompose for X Windows users. For tty users, tough luck). At least, on my installation, it can be typed as "Compose Compose d e g f".

"Compose o o" "C" °C

They move heat via blood into their bones such that their bones sheild the heat and can be re-pumpted back into the muscles over time to allow for deep divings.

I have a crazy suspicion this mechanism is a thing in humans. After cold weather sets in the fall, it takes me a day or two before I start to be bothered by the chill. Mid-winter, I start to crave saunas.

My assumption is the skeleton, marrow and body fat (or some other body structure with high thermal mass) act as a heatsink that expels some built up heat.

Maybe this is where "I can feel it in my bones" comes in

Bones? I thought that sharks have no bones.

I wonder if cartilege vs bones are a better heat / oxygen moderating permeable filter/exchanger, thus no bones....

OK this is going to be a crazy post, just hold your horses

cartelige is a permeable material for various occupants, oxigen being one.

So we find that highly oxigenated creatures are larger and more carnivorous

we also know that high ingestion of oxygen results in larger creatures

so if a shark has a larger ability to permeate oxygen into the muscles and have the ability to store heat, plus the ability to transfer heat/oxy btwn bodies...

This is why we can see a body +500 years old..

buoyed by the buoyancy of less dense bones, such as cartilage.

BTW I love "crazy suspicions" they oft prove true///

Not quite warm blooded:

> It is known for its ability to maintain stomach temperature (homeothermy), which is unusual among fish. This shark has not been demonstrated to maintain a constant body temperature.

https://en.wikipedia.org/wiki/Salmon_shark

But there is indeed a vascular heat exchanger involved:

https://en.wikipedia.org/wiki/Rete_mirabile

I have wondered about bird ability to keep their feet in cold water, but not enough to look up how they've adapted. Thanks for that second link.

Yea I could link to two different sources who say endothermic, one of which is a university. The others are articles with spelling and grammar mistakes and that’s why I didn’t link a source.

The shark can raise its body temperature 14-20 degrees more than the environment. I think it fits the definition of warm blooded.

https://en.m.wikipedia.org/wiki/Warm-blooded

Wim Hof would like a word.

I used to be able to do homethermy by thought when I was younger... but I have lost the ability.

(SERIOUS); I used to be able to direct heat to various extremities by thought when I was laying in bed in really cold environs... but I seem to have lost that ability.

> Lungs work very differently. They don't have continuous flow

I was under the impression that bird lungs do have continuous flow.

Water is a much more efficient heat exchanger than air.

> The mammalian lung has reciprocating ventilation with large terminal air spaces (alveoli) while the avian lung has a flow-through system with small air capillaries. As a result the environment of the pulmonary capillaries is very different between the mammals and birds.[1]

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2681264/#:~:tex....

The way a paleontologist can tell if an animal was warm blooded is to look for the structures (basically an energy recovery ventilator, but made of bone) in the animal's airways. In mammals, it's in the nasal passages.

I knew that breathing through the nose heats up the air before it hits the lungs but the energy recovery aspect is news to me. How does that work? Do we have a heat pump in our noses?

Think of the nose tissues as of a simple heat reservoir.

When we inhale cold air, these structures warm it, and thus become colder.

When we exhale hot air, it heats these structures, making them warmer again, to allow heating the next breath in.

It's of course not 100% efficient, but it's better than nothing. I wonder uf there are more advanced strictures, like circulating liquid, to improve efficiency.

> When we inhale cold air, these structures warm it, and thus become colder. When we exhale hot air, it heats these structures, making them warmer again

How so? When you breathe in, your nose tissues will cool down to a temperature T_N and the air you inhale will heat up to a temperature T_A. Physics (the second law of thermodynamics) dictates that T_N >= T_A.

Moreover, the nose tissues are connected to a large heat reservoir (the body), so we might as well assume they go back up to body temperature very quickly, so T_N = 37°C after a second or two.

Either way, when you breathe out you still have T_N >= T_A. In this situation, there is no way the air could transfer back heat to the nose without a violation of the second law or, well, a heat pump. Hence my question.

The key point is that the temperature varies across the nose tissues. Close to the outside, it's low; farther in, it's high.

I realized I made a mistake: A quick Google search reveals that air gets heated up to full body temperature in the lungs while the nasal turbinates, after inhalation, remain below body temperature for a bit longer. So upon breathing out T_N < T_A and heat indeed gets transferred back from the air to the nose.

It's an example of countercurrent heat exchange, basically exchanging heat between outgoing and incoming air with very high efficiency.

When breathing in, the air is gradually heated by the nasal passages. The temperature in the nasal passages increases along the channel, so at all points there's a small temperature drop, minimizing production of entropy.

When breathing out, the air is cooled in the same way. Again, there's a small temperature drop (now of the opposite sign) along the channel, again minimizing entropy production.

Moisture is also conserved here in a similar way.

Countercurrent heat exchange is also used in biology to minimize heat loss in extremities. The arteries of your arm, and the deep vein returning blood there, are wrapped around each other to transfer heat. In some vertebrates elaborate artery/vein structures have evolved to make this more effective.

https://en.wikipedia.org/wiki/Rete_mirabile

Countercurrent exchange is a fundamental engineering concept I feel more people should know about.

Thanks, I hadn't heard of countercurrent heat exchange!

I do find it remarkable, though, that the nasal turbinates don't seem to thermalize with the rest of the body in the time between inhale and exhale (which is of the order of several seconds). Or maybe it does thermalize quite a bit (even though not fully): A quick search for "nasal turbinates countercurrent heat exchange" reveals that human nasal turbinates are not particularly great heat exchangers, with air exiting at temperatures of up to 33°C during exhales.

Many examples of the principle in action:

https://en.wikipedia.org/wiki/Countercurrent_exchange

> To understand how sharks were coping with the temperature changes, 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.

> In a paper published in Science1, the team reported that the sharks would dive several times — six in an evening, for one shark — into deep water at temperatures of 5–11 °C, around 20 °C colder than at the surface, and remain there for 5–7 minutes at a time before surfacing.

> Body temperature remained constant for most of the dive until the final stage of their ascent back to warmer waters, when it would decline rapidly.

> Royer suggests that the sharks are keeping their core temperature stable by simply not opening their gills or mouth during the dive; effectively ‘holding their breath’. “If you don’t have water going over your gills, then you won’t be dumping your body heat into the environment,” he says.

So they only measured the shark's temperature, the water temperature, and location (in 3 dimensions). Hammerhead sharks holding their breath is a hypothesis that aligns with the rapid heat loss as they ascend and reenter medium-temperature waters (and start breathing again presumably). It seems a bit early to say this is the exact mechanism, but I am no marine biologist.

edit: ah, they have more evidence, including videos of sharks closing their gills https://www.science.org/doi/10.1126/science.add4445#sec-4

“There is a big advantage in being able to hunt when you’re warm and everything else is cold.”

Pardon my ignorance, does anyone know why? Is it because the warmer creature is likely to have faster metabolism?

What about fish that jump out of the water?

They were either not first, or they simply keep breathing regardless of availability of oxygen.

IIRC, they keep blood pumping (inefficiently) through their gills while above water. This is less an analogue for "holding your breath" and more for "being on top of Mt. Everest." You're physically trying to get oxygen, it's just not working.

"You're physically trying to get oxygen, it's just not working"

Nitpick, but not working as good, because some humans do manage to get enough on Everest, to not need extra supplied oxygen.

I understood it as thing that could be tolerated for a short time by some humans. They will die as their saturations are falling, but they get up and then down within a narrow window.

https://en.m.wikipedia.org/wiki/Death_zone

You surely could not live there, but some people managed for astonishing amount of time.

(and before they proofed it, it was thought to be literally impossible without extra oxygen)

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