Physiology of Finfish and Shellfish (2+1)

2.1.12. Bony fish

In bony fish , the gas exchange organs of fish are called gills . The gills lie in a branchial chamber covered by a bony operculum. The great majority of bony fish species have five pairs of gills.

Though all bony fish lack a spiracle, the pseudobranch associated with it often remains, being located at the base of the operculum.

From each gill arch arise two rows of filaments. From each gill arch arise two rows of filaments to form a complete gill or holobranch, which are arranged in a V-shaped manner.

(hemibranch- Each row forms a half gill)

The gill arch along with the filaments is called a gill. Each gill is composed of many filaments that are each covered in many lamellae . The lamellae are very fine structures and contain blood capillaries , which have blood flowing in the opposite direction to the water. This is very important for getting all of the available oxygen out of the water and into the blood. This is called “counter current” mechanism and helps in the maximum exchange of gases i.e., intake of oxygen and release of carbon dioxide. If the blood flowed in the same direction as the water passing it, then the blood would only be able to get half of the available oxygen from the water. The blood and water would reach equilibrium in oxygen content and diffusion would no longer take place. By having the blood flow in the opposite direction, the gradient is always such that the water has more available oxygen than the blood, and oxygen diffusion continues to take place after the blood has acquired more than 50% of the water's oxygen content. The countercurrent exchange system gives fish 80-90% efficiency in acquiring oxygen.

Oxygen passes from the water into the blood at the gills. Removal of carbon dioxide also occurs, as the blood containing high concentrations of the waste gas goes to the gills, and the carbon dioxide diffuses out into the water down a diffusion gradient (external water has lower concentrations of carbon dioxide than levels in the blood, so this sets up a diffusion gradient).