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Nature & Environment

Why are some types of fish not cold-blooded ?

July 6, 2021 · Comment icon 7 comments

The opah is an example of a warm-blooded fish. Image Credit: CC BY 2.0 NOAA / Nick Wegner
Fish physiologist Lucy Harding, and colleagues, have solved the mystery of why some fish are warm-blooded.
For over 50 years now, scientists have known that, despite their reputation, not all fish are cold-blooded. Some shark and tuna species, the white shark and the Atlantic bluefin tuna, have evolved the ability to warm parts of their bodies, such as their muscle, eyes and brain.

About 35 species of fishes - accounting for less than 0.1% of all described fishes - have this ability, which allows them to stay warmer than the water around them. Until recently, however, the reason this ability evolved was a mystery.

Some scientists believed being warm-blooded allowed the fish to swim faster, as warmer muscles tend to be more powerful. Others thought it allowed them to live in a broader range of temperatures, making them less susceptible to the effects of ocean warming caused by climate change. With this in mind, an international team of marine biologists and I set out to answer the puzzle of why some fish warm-blooded when most aren't.

Our study found fishes' ability to warm their bodies provides competitive advantages - they can swim faster than their cold-blooded relatives. However, this doesn't necessarily mean they'll be able to adapt to changing ocean temperatures under climate change better than cold-blooded fish, according to our results.

Catching fish

Our team - from Australia, USA, Tasmania, Hawaii and Japan - collected data from wild sharks and bony fish, as well as using data which had already been collected. We attached biologging devices - waterproof, electronic devices that can remotely record data - to the fins of the animals we caught. Animals were caught by hook and line and secured alongside a boat. This allowed us to the attach the devices and release the animals immediately after.

These devices gathered information such as water temperatures encountered by the fish in their habitats, the speeds at which the fish swam for most of the day and the depths of water the fish swam in.
By comparing the speed and temperature data of these warm-blooded and cold-blooded animals we could calculate the range of temperature these animals were swimming in and what speeds they were swimming at, accounting for their body weights. It turns out that warm-blooded fish can swim 1.6 times faster than cold-blooded fish. This is some of the first direct evidence of the evolutionary advantage of being warm-blooded.

This extra speed provides advantages when it comes to things like predation and migration. It's likely that this makes them better hunters or travellers. The faster swim speeds also aid the fish in identifying prey. The quicker they swim, the faster an image moves across their eye, allowing them to process and identify the image - perhaps of prey - faster than slower counterparts.

It has previously been suggested that these warm-blooded fishes may be better able to deal with changing ambient temperatures by stabilising their body temperatures. This would be useful under current climate change scenarios, such as global ocean warming.

That may be the case, but our results indicate the ability to warm their bodies doesn't allow them to occupy a broader temperature or depth ranges. This means we may have been overstating the resilience warm-blooded fish have for facing changing ocean temperatures.

Many of these animals are already facing threats from ocean warming, and human-induced risks. The Atlantic bluefin tuna is an endangered species while the white shark is classed as vulnerable. We hope that taking these findings into account could better inform future work on the conservation and protection of these unique but threatened animals.

Lucy Harding, PhD Candidate in Fish Physiology, Trinity College Dublin

This article is republished from The Conversation under a Creative Commons license.

Read the original article. The Conversation

Source: The Conversation | Comments (7)

Recent comments on this story
Comment icon #1 Posted by Biggz 2 years ago
And this is worth researching why?
Comment icon #2 Posted by Still Waters 2 years ago
The same reason anything is worth researching. In this case it tells you in the article:  
Comment icon #3 Posted by NCC1701 2 years ago
Because climate change theory has highjacked science.
Comment icon #4 Posted by Troublehalf 2 years ago
I mean, lets for a moment say that the temperatures of the oceans are not rising. How is finding out information about how fish work and the differences between cold-blooded and warm-blooded fish a bad thing? Why does expanding human knowledge have to have an 'agenda' beyond the desire to know something we did not already know? Finally, lets just say the temperatures are not rising, but could possibly rise in the future... knowing this information could assist organisations in preserving the species. So, better to know it now, eh? Also it is spelled hijacked, for future usage. See, you learned... [More]
Comment icon #5 Posted by Tatetopa 2 years ago
If you fish for tuna you know they are wicked fast and hot blooded. If you like to eat them somebody has to catch them.  If we understand their life cycle we have a better chance of keeping them around as a healthy food source.  That is a selfish reason.  There are plenty of others.
Comment icon #6 Posted by RAyMO 2 years ago
I saw what you did there - well done. 
Comment icon #7 Posted by Tatetopa 2 years ago
Scientists are not generic nerds in white coats.  They specialize.  Some study climate science a lot don't.  You see a lot of research because the military, NASA and the weather service are interested, not to mention farmers and others that depend on temperature and water to produce their products. The DOD ponies up some of the research grants.  

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