How do fish breathe?

It’s probable that you know that most of the fishes that inhabit in the Earth breathe due to the presence of gills. However, this is not the only respiratory system present in fishes. In this post, we will review different types of breathing in fishes. 

INTRODUCTION

The respiratory system of fishes have to be adapted to two important limitations of underwater life. On the one hand, the amount of dissolved oxygen is smaller in the water than in the air: at 23ºC, air has 210 ml of oxygen per litre of air, while in freshwater is about 6,6 ml/l and in salt water is 5,3 ml/l. On the other hand, water is much more dense and viscous than air. These limitations explain the adaptations in the breathing of this group of animals.

BREATHING WITH GILLS

The oral cavity of teleostei fishes (modern ray-finned fishes) is communicated with the exterior through the mouth and pharyngeal pouches, lateral openings present in the pharynx in which the gills develop. Thanks to the opercle (or gill cover), a hard structure placed in each side of the head, gills are protected.

The structure of the gills is complex. From branchial arches, curved structures that pierce through pharyngeal pouches in each side of the head, two gill filaments grow forming a V. These filaments produce the gill lamellas, folds of the wall’s filaments with a perpendicular disposition. In each side of the filament, we may find between 10 to 40 lamellas per mm. So, it is in these lamellas where the gas exchange happens because they are a very thin wall of tissue and are well supplied with blood.

Structure of the fishes’ gills (Picture: AS Biology Ms Timms).

So, the oxygenated water that passes through the mouth cross the gills and finally abandon the oral cavity through the opercle, while the blood flows in the opposite direction across lamellas to catch the oxygen.

The larva of many fishes have external gills in each side of the head. In the rest of phases, gills become internal. Fishes with a respiration with gills are hagfishes, lampreys, elasmobranchii and bony fishes.

The hagfish is a species with a breathing with gills (Picture: Natureduca).

BREATHING WITH LUNGS

About 400 bony fish species are known to have the ability of breathing from air, most of them living in freshwater ecosystems. Anyway, most of them have both gills and lungs. These species with the two mechanisms usually use the air in certain occasions:

• When the oxygen level in the water goes down.
• When the temperature increases, so the higher the temperature is, the higher are the oxygen necessities.

Lungfishes (Dipnoi) are among the species with the most advanced system. Their lungs have crests and septums similar to those in the lungs of amphibians. The Australian lungfish (Neoceratodus) can breathe with both gills and a lung. African lungfishes (Protopterus) and the South American lungfish (Lepidosiren) breathe with a complex lung and single gills. These fishes need to compulsorily breathe air, as in the contrary they die.

Lungfishes: Australian lungfish (Neoceratodus forsteri), African lungfish (Protepterus annectens), South American lungfish (Lepidosiren paradoxa) and Devonian lungfish (Dipterus) (Picture: Encyclopaedia Britannica).

OTHER BREATHING MECHANISMS IN FISHES

Many fishes have the capacity of breathing through the skin, specially when they are born because they are so small that they do not have specialised organs. As the animal is growing, gills or/and lungs are developing because the diffusion through the skin is not enough. Anyway, skin may be responsible of a 20% or more of gas exchange in some adult individuals. Others can do it through the mouth, the pharynx, the oesophagus, the intestine or the rectum, as is the case of Hoplosternum.

Hoplosternum has the ability of breathing through the digestive tube (Picture: Free Pet Wallpapers).

Some species have developed cavities beyond the gills, the suprabranchial chambers, which can be filled of air. In other, complex organs developed from a very irrigated branchial arch can be formed and act as a lung. This is the case of the catfish and Electrophorus .

Some fishes have the ability to breathe air without a specific adaptation. This is the case of the American eel (Anguilla rostrata), that cover the 60% of the oxygen requests through the skin and the 40% swallowing air from the atmosphere.

REFERENCES

• Notes of the subject Chordates of the Degree in Biology (University of Barcelona).
• Hickman, Roberts, Larson, l’Anson & Eisenhour (2006). Principios integrales de Zoología. Ed. McGraw Hill (13 ed)
• Hill, Wyse & Anderson (2006). Fisiología animal. Ed. Medica Panamericana