Fish are aquatic vertebrate animals that breathe through gills These animals can be divided into three large groups: agnathus or fish without jaws, chondrichthyans or cartilaginous fish and osteichthyans or bony fish. All of them drink water from which they capture oxygen to be able to breathe, with the exception of lungfish, which breathe air and there are only six species.
If fish take oxygen from water, why do some live in fresh water and others in s alt water? and what would happen if a freshwater fish was placed in the sea?
In this article on our site we will talk about fish respiration, analyzing how oxygen behaves depending on the environment and why a freshwater fish cannot live in s alt water.
Fish breathing
Each group of fish has a different shape of gills and way of breathing.
Gills and respiration in lampreys and hagfish (agnate fish)
- Hagfish: they present bags or branchial sacs on the Upper part of the body. What is observed is that the water enters through the mouth, passes through the gill sacs, and exits through the gill opening or openings, which are located on the side of the animal.
- Lampreys: if they are not feeding they breathe like hagfish. In the case of feeding, being parasites they adhere to another fish, and in this case they have a concurrent respiration, the water enters and leaves through the same hole of the gill openings.
Gills and ventilation in teleost fish (osteichthyan fish)
The oral cavity communicates with the outside both through the mouth and through the opercular cavity, this is where the gills are located.
They have four gill arches and from each gill arch two groups of gill filaments will emerge, arranged in a V-shape. These filaments overlap with those of the neighboring gill arches and form a species sieve
Each of the filaments will have perpendicular projections called secondary lamellae, this is where theoccurs respiratory exchange , have a thin epithelium and are highly vascularized. The flow of water passes through the lamellae in one direction and the blood goes in another direction, this is where gas exchange occurs (oxygen enters and carbon dioxide leaves).
These fish have an oral pressure pump and an opercular suction pump, which means that, on the one hand, pressure will be generated in the oral cavity that will push the water towards the cavity opercular, and also in the opercular cavity, the pressure drops so much that it sucks the water out of the oral cavity.
Gills and ventilation in elasmobranchs (chondrichthyan fish)
Water enters through the mouth and through the spiracles (nostrils on the sides of the head). They are very active fish, they swim with their mouths open, which causes a lot of water to enter at great pressure due to their speed and this is what causes the entrance to the opercular cavity, where the gas exchange takes place. Here the ventilation system is slightly different, since they don't have both pumps. The cons of these is that they spend more energy than in the previous case and must always be in motion.
Why can't a freshwater fish live in s alt water?
The first thing we must bear in mind is that all living beings seek to maintain homeostasis, which helps them maintain balance internal chemical.
Each animal is adapted to its environment, so a s altwater fish needs the exact concentration of oxygen found in this water and the right concentration of s alts. What if we put a marine fish in fresh water? Fresh water has a higher concentration of oxygen and a lower concentration of s alts, which would modify its homeostasis causing blood acidosis due to the greater production of carbon dioxide and accumulation of s alts, causing the death of the animal. And if a freshwater fish is placed in the sea, the opposite would happen, the concentration of oxygen is lower and that of s alts is higher, so it would not be able to maintain its vital functions.
Living things that can live in fresh and s alt water
Despite all of the above, some fish, throughout their lives, change from a s alty medium to a sweet one, as is the case of salmon or eel. These animals have developed mechanisms to maintain their body's homeostasis despite changes.
The skin of these fish is very slightly permeable, to prevent water loss. When they go from the sea to the river they increase urine production and decrease it when they go from the river to the sea. In addition, they drink water when they enter the sea and stop drinking in the river, to release or not exit through the gills.
Don't miss this article on fish that breathe out of water if you are more interested in this topic.
