Genrebild på hav med brygga och båtar
Photo: Leon Green

Salty, fresh or both - what happens in the fish?

Ever since childhood, we have learned that there are different fish in our different waters, some thrive in salt water and others in fresh water. But what really happens inside the species that live and thrive in both? Daniel Morgenroth has with his dissertation "Cardiorespiratory function of euryhaline teleosts: Regulatory mechanisms, effects of warming and costs of osmoregulation" tried to sort it out.

Daniel Morgenroth
Daniel Morgenroth has spent many hours in the operating room at the Zoologen.

Fish are generally adapted to live in either freshwater or seawater and do not tolerate large changes in salinity. On the other hand, a small proportion of fish species can survive large changes in salinity and even perform migrations between freshwater and seawater, like for example salmon or eels. The latter is what we call euryhaline fishes.

To be able to tolerate such wide salinity changes, euryhaline fishes must undergo multiple behavioral and physiological adjustments. This includes changes in blood supply to the organs involved in maintaining the body’s salt and water balance, like the gills and gastrointestinal tract.
– Throughout my thesis, I have investigated some of the mechanisms involved in driving said changes in blood supply. Additionally, I explored the different effects warming has on fish depending on the salinity they experience it at, as well as the effects of salinity on energy turnover and growth, says Daniel Morgenroth, doctoral student at the Department of Biology and Environmental Science at the University of Gothenburg. 

What have been the biggest challenges during the process? 
– I would say that often the most challenging part is starting up new projects. We always have many hypotheses, but finding the right experimental approach to test them takes some work and thus, there is usually a bit of trial and error, polishing and fine-tuning of protocols preceding the actual experiments.

Illustration by Elin Norén
Photo: Illustration by Elin Norén

A dehydrating effect that increases with higher temperature

Seawater has a dehydrating effect on fish and therefore, they drink and increase blood flow to the gastrointestinal tract to help with the absorption of water.
– In one of my studies, I show that gastrointestinal blood flow seems to be partly regulated by some kind of osmo- or mechanoreceptors along the gastrointestinal tract. Additionally, at higher temperatures the dehydrating effects of seawater increase. In another study, I show that marine fish compensate for this by drinking more and by further elevating the amount of blood directed to the gut, which also requires larger elevations in the amount of blood pumped by the heart, says Daniel Morgenroth. 

Does anything surprise you? 
– Definitely! For example, as I mentioned, fish in seawater respond to warming with larger increases in blood flow compared to fish in freshwater, yet, the amount of blood the heart can pump is limited. This highlights that fish in seawater might run out of capacity to increase blood flow quicker with warming than their freshwater conspecifics. This could have important consequences for fish in seawater as we venture into a warmer future with climate change.