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Warmer water means that the amount of gases- especially oxygen- dissolved in it reduces, since solubility of gases in a solution in most cases decrease with an increase in temperature.

I'm interested in knowing if tropical fish have any adaptations to account for this reduced oxygen compared to fish in colder waters. Any modification of the gills maybe?

Referencing the below articles, I have come to the conclusion that tropical fish end up migrating to shallower waters or cooler habitats due to the oxygen depletion, showing that this decrease in oxygen is serious enough to affect them. However, this may be due to excessive heating of oceans due to global warming. I'm curious about whether fish in tropical regions have any adaptations for living in ocean water that is heated purely by natural means, not anthropogenic ones.


References:

https://www.usgs.gov/media/images/temperature-affects-dissolved-oxygen-concentrations https://www.pca.state.mn.us/sites/default/files/wq-iw3-24.pdf https://www.sciencedaily.com/releases/2015/06/150604162455.htm https://www.nationalgeographic.com/science/article/150313-oceans-marine-life-climate-change-acidification-oxygen-fish

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  • $\begingroup$ How having access to less oxygen would be accounted for by swimming faster? $\endgroup$ – Winston Mar 12 at 11:56
  • $\begingroup$ I'm not sure, that was just me speculating that a higher rate of respiration would mean more oxygen could be inspired in less time. $\endgroup$ – Meta xylene Mar 12 at 14:37
  • $\begingroup$ Welcome to Biology.SE! While dissolved gasses do tend to decrease with increasing water temperature there are many other factors that contribute to the actual concentrations (e.g.s: rate of use, rate of mixing with the atmosphere). Please edit your post to include evidence that dissolved O₂ is actually lower in tropical waters, by how much, and whether this difference is biologically significant. Without that background research (which is expected) your question is based on speculation and is thus off-topic for this site. Thanks! $\endgroup$ – tyersome Mar 13 at 20:10
  • $\begingroup$ Thank you; I've edited my question for the same. I wasn't able to find how much in magnitude this oxygen depletion was, and most articles I checked were about how global warming is decreasing the (especially tropical water) oxygen levels. I understood that a faster metabolism ('swimming faster' @Exocytosis) does not allow for adaptations to less oxygen. $\endgroup$ – Meta xylene Mar 14 at 5:11
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Fish that live in cold water for most of the year actually have particularly strong hypoxia-tolerance adaptations, i.e. from summer heatwaves and algal blooms, compared to fish that are better oxygenated by waves and flowing rivers.

Goldfish and other fish are able to rapidly change the surface area of their gills in less than a day. enter image description here

Carp, killifish, goldfish and oscars have 2/3 times as much glycogen in their bodies than river fish i.e. rainbow trout.

There's a lot of research into the biochemistry of fish's oxygen response, i.e. gene expression changes, things like decreased expression of genes involved in the citric acid cycle.

Probably you will find most of the information on the wiki page, there are also summaries of current advances in the field.

On coral reefs, oxygen is a limiting currency for all the fish and zooplankton. The reading suggests to me that hypoxia adaptations depend mostly on heatwave events and low current waters vs rivers and coastal waves than by tropical vs pelagic and temperate regions.

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  • $\begingroup$ Thanks for the answer, this is what I was looking for. These adaptations are only viable to a certain extent and not seen in all fish, correct? – $\endgroup$ – Meta xylene Mar 14 at 16:10
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Many can breath air directly ; examples - bettas, gouramis, paradise , some catfish . No doubt others that do not come to mind .

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  • $\begingroup$ Thanks.This is an adaptation seen only in tropical fish (Labyrinth fish are endemic to fresh waters of Asia and Africa), while @aliential 's excellent answer was more to do with fish that live in cold water. $\endgroup$ – Meta xylene Mar 15 at 1:20

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