Respiration in Fish and the Role of Gills

Fish is an aquatic (freshwater or marine water) animal, and the study of fish is known as Ichthyology. The scientific classification is as follows:

NOTE:

Animalia: It is the highest taxonomic group into which organisms are grouped.

Chordata: It is a large phylum of animals that includes vertebrates together with sea squirts.

Vertebrates: A large group of animals which have a backbone or spinal column are classified as vertebrates.

**All vertebrates are chordates but all chordates are not vertebrates**

Pisces: It is a class in the vertebrate subphylum within the phylum Chordata. 

Now that you know that fish belongs to the class Pisces, mentioned below are the characteristics of fish:

• Fishes are cold-blooded (poikilothermic) animals.

• Their body is streamlined and is spindle-shaped.

• The body is covered with dermal scales, which is waterproof. Scales can be placoid, cycloid, ctenoid, organoid type.

• The locomotory organs of fishes are fins.

• They don't possess eyelids, and only internal ears are present.

• The heart is two-chambered, which is also known as a venous heart. Blood circulation is unicircuit.

• Fishes are unisexual (i.e., internal or external fertilization).

• Fishes respire through Gills. There are usually 4 to 7 pairs of Gills present in most fish species.

Respiration:

Respiration is a process of taking in oxygen and removing out carbon-di-oxide to produce energy from the oxidation of complex organic substances.

Fishes respire through GILLS.

Gills:

• Branchia (plural: Branchiae) is the Greek word for Gills.

• Similar to lungs in humans, gills are the respiratory organs present in many aquatic organisms.

• Gills are present near the head and are made of a feathery texture, which lets them allow water to enter and facilitate the exchange of gasses.

• Gills are located next to the mouth cavity of the fish.

• Gills have a red color because they're filled with blood vessels

• Most fish exchange oxygen and carbon dioxide that's dissolved in water.

• Gills absorb oxygen, which is dissolved in water and releases co₂ (carbon dioxide).

• Gills usually consist of lamellae (plates), tissues that are like thin filaments (protein structures), which have highly folded surfaces in order to increase the overall surface area.

• The filaments perform many functions like ion and water exchange, along with the exchange of ammonia, acids, co₂, and oxygen.

• The capillary network of these filaments gives a large surface area for the exchange of oxygen (o₂) and co₂.

• A large surface area is needed because water contains less amount of dissolved oxygen than air. For example, 250 grams of oxygen is present in one cubic meter of air at STP (standard temperature and pressure), but in water, the concentration of oxygen is lower than in air, and thus diffusion of oxygen happens very slowly.

• The gaseous exchange in fish happens through highly vascularised gills. The unidirectional water flow is facilitated by a special pumping mechanism that is present in most fish.

Concurrent Exchange Mechanism: 

In some molluscs and fishes, this mechanism greatly enhances the efficiency of gills as water flows across the gills, in the opposite direction of blood flow. This, in turn, increases the efficiency of gills by 90%, through which the dissolved oxygen in the water can be recovered.

• Gills are protected by gill covers on both sides of the throat (pharynx).

• Fishes exchange gasses by pulling the oxygen-rich water content through their mouth and by pumping it across the gills. Once the oxygen has been absorbed by the gills, the remaining water is thrown out.

• Bony fishes have only one-gill openings, but some fishes like lampreys and sharks have multiple gill openings.

• Gill openings are also known as gill slits.

• Bony fishes have three pairs of gills, Cartilaginous fishes have 5 to 7 pairs of gills and jawless primitive fishes have 7.

• There are two types of fishes categorized on their breathing techniques:

• Obligate Air Breathers: Fishes who breathe air periodically or they suffocate. E.g., African lungfish.

• Facultative Air Breathers: Fishes who only breathe air if they need to and can otherwise depend upon their gills for oxygen. E.g., Catfish (Hypostomus Plecostomus).

Respiration in Bony Fish:

• In bony fishes, the gills are present in a branchial chamber, which is covered by the operculum.

• The Operculum helps put water pressure inside the throat in order to allow proper ventilation. 

• In bony fishes, the gill arches don't have septum. Instead, the gills project from the arch, which in turn is supported by gill rays.

• In some bony fishes, external gills are also present.

• Fishes take oxygen-rich water through their mouth, pump it across the gills, after which the oxygen enters their bloodstream. Once the oxygen is absorbed, the remaining water is thrown out through the gill slits.

Respiration in Lampreys and Hagfish:

• Gill slits are absent in these fishes. Instead, they possess a spherical pouch-like structure in which the gills are present.

• They have a circular opening on the outside.

• Like other fishes which have gill slits, these fishes have two gills in each pouch.

• The openings are sometimes covered and form an operculum.

• Hagfish have six to fourteen pairs of pouches, while Lampreys have seven pairs of pouches. 

Respiration in Cartilaginous Fishes:

• Five pairs of gill slits are present in cartilaginous fishes that open directly to the outside.

• They have a long septum, which looks like a sheet and is supported by the gill array.

• Lamellae is present along the gill septum.

• Fishes breathe by sucking water through the opening known as a spiracle instead of their mouths. This spiracle contains a small pseudobranch, which takes only oxygenated blood from the gills.

How Do Some Fishes Respire Without Gills?

Although most fishes in the ecosystem employ gills for respiration, some others adapt other methods to facilitate the same. Below mentioned are the most significant among them:

• Cutaneous respiration (gas exchange occurs across the skin of an organism instead of gills or lungs). E.g., Reedfish and mudskippers.

• An electric eel can respire through buccal cavities.

• Some fishes like Scolopacidae respire through digestive tracts.

• Some fishes have accessory breathing organs, like labyrinth organs above gills in labyrinth fish, etc.

Summary

• Water has dissolved oxygen and carbon dioxide. Most fish use gills to facilitate the exchange of gasses and most important respire in water. 

• Gills are present beside the pharynx and are covered by the operculum.

• Most fish use the countercurrent exchange mechanism of breathing, in which the water flows opposite the bloodstream, which in turn increases the efficiency of the exchange of gasses by 90%. 

• The large surface area provided by the folded structures of lamellae helps in the exchange of gasses.

• Gills are red in color because of the presence of the blood vessels.

• Some fishes have a swim bladder ( a hydrostatic organ), which is located between the kidney and stomach, which can control their water depth by controlling the concentration of gas inside the bladder.