Last week I was digging through my old marine biology papers, and found one I wrote many years ago titled…
“Osmoregulation in euryhaline teleost fishes with emphasis on the role of chloride cells.”
Now that sounds really boring…
But I think it’s actually pretty interesting and here’s a bit of fish trivia for you.
Did you know???
- Saltwater fish drink heaps of salt water, yet freshwater fish don’t drink at all.
- It ain’t pretty when you place a freshwater silver perch into saltwater… (WARNING GRAPHIC CONTENT)
“For the first 2 hours, individuals were generally calm, swimming slowly near the bottom of the tanks, after which they moved closer to the water surface where many attempted to gulp air. Later, individuals turned black in colour and some developed skin haemorrhages. Finally, individuals lost the ability to swim and lay on the bottom until breathing activity ceased completely.” (Guo et. al. 1995)
- Some fish like barramundi and mangrove jack CAN tolerate extreme changes in salinity and they do this via the use of chloride cells in their gills.
So how come some fish can swim between fresh and salt with impunity, while others drop dead in a matter of hours if the mediums get switched up?
The answer lies in how a fish transfers concentrations of NaCl (salt) and H2O (water) between their blood and the surrounding water.
But first let me just explain the difference between a true “euryhaline” fish such as a barra or jack, that regularly swim between fresh and salt during the course of a tidal cycle, and other fish that migrate between the two as distinct parts of their lifecycle.
1. True Euryhaline: Live their entire life with wide salinity tolerance. Often found in estuaries e.g. barramundi, mangrove jack.
2. Migratory Euryhaline: Migration requires physical transformation. Abrupt transfers between environments at inappropriate times can be lethal;
a. Anadromous: Bulk of life at sea and migrate into rivers to reproduce (salmon).
b. Catadromous: Adults migrate from fresh to salt to spawn (Long-finned eel)
So how can salt water, or freshwater as the case may be, be lethal to a fish?
It’s because nature loves to balance things out.
In saltwater, the concentration of salt and other ions in the water way exceeds that in the blood plasma of the fish. And because a fishes gills contact both the water and blood to breathe, salt naturally tries to move from the ocean into the blood until it balances out. Water (H2O) in the blood also tries to balance by moving out into the surrounding seawater.
The opposite occurs in freshwater.
Fish counteract these natural balancing movements of H2O and salt with chloride cell in the gills.
- Saltwater adapted α chloride cells move salt against the gradient and back into the surrounding water.
- Freshwater adapted α chloride cells move tiny traces of salt from the surrounding water and into the blood.
- β Chloride cells are found only in freshwater fish and assist in this process.
As shown in the diagram, saltwater fish swallow heaps of seawater as a source of H20, and eliminate salt via chloride cells and small amounts of highly concentrated urine.
Freshwater fish on the other hand are at risk of their blood being diluted by the surrounding water. They don’t drink at all and pee lots. Freshwater adapted α and β chloride cells suck as much salt and other ions from the water and they also get some of these vital nutrients from food.
True euryhaline fish such as a barramundi and mangrove jack cope with increases in salinity in the short term (phase I) by increasing their drinking rate and decreasing urine dilution. e.g. they drink more and pee less!
In the longer term over 3-4 days (phase II), changes include a degeneration of β chloride cells in the gills while the α type get bigger and transform into a typical saltwater adapted chloride cell.
But what of the migratory fish. How do they cope?
Well they undergo sometimes permanent changes (some die after spawning as they cannot return).
The short-finned eel Anguilla australis has a remarkable life-cycle living in freshwater as a yellow form and migrating at depth as a silver one towards the spawning area deep in the Coral sea. The spawning migration is performed without feeding. During the transition the eyes and pectoral fins enlarge, the skin thickens, stomach degenerates and the anus constricts to reduce water loss. They also undergo permanent changes in the proportions of α and β chloride cells.
Well there you have it…
Did you know that some fish drink and some don’t?
And for more interesting fish biology facts, plus some tips on how to catch them, check out some of our previous posts by clicking the links below;
