Anatomy of finfish and shellfish

Unit 6 - Respiratory system in fishes

6.2.3. Pseudobranch
In carp and rainbow trout the pseudobranch is embedded in submucosal connective tissue of pharyngeal wall and shows a glandular appearance due to complete conglutination of branchial filaments. In some species, a pseudobranch with hemibranchs structure is located inside the operculum. However, in eel the pseudobranch is not present. it is also absent in cat fishes (Siluroidae) and featherback (Notopteridae).
In glandular pseudobranch, abundant distribution of blood capillaries is found in the parenchyma enclosed by connective tissue. It contains acidophilic cells in mitochondria and endoplasmic reticulum and is rich in enzyme carbonic anhydrase. The pseudobranch regulates the flow of the arterial blood to the ophthalmic artery to increase the amount of blood carbon dioxide. In rainbow trout extirpation of pseudobranch indused melanophore expansion and body colour change, suggesting the secretion of a melanophore-aggregating hormone frome tissue. It also helps in metabolic gas exchange of retina and filling of gas bladder. Because of its direct vascular connection with choroid gland on the eyeball, the pseudobranch has been implicated in the regulation of intercellular pressure. The structure of gills has been studied extensively in Indian fishes by light, transmission and scanning electron microscopy. The gill comprises gill rakers, gill filaments (primary gill lamellae) and lamellae (Secondary gill lamellae).
A complete gill is known as holobranch. It consists of a bony or cartilaginous arch. The anterior and posterior part of each gill arch possesses plate like gill filaments. Each holobranch consists of an anterior (oral) and a posterior (aboral) hemibrnch. The architectural plan of teleostean gills shows hetergenity in their functional unit which is due to varied osmoregulatory, feeding and respiratory behaviour and to the physcochemical status of their environment.
In teleost fishes, five pairs of branchial arches are present of which first four bear gill lamellae but the fifth is devoid of gill lamellae and transformed into the pharyngeal bone for mastication of food. It does not play any role in respiration. The gill arch is an important unit and bears primary (gill filament) and secondary lamellae. The branchial arch typically consists of paired pharyngobranchials, epibranchials, ceratobranchials, hypobranchials and a median unpaired basibranchial. The epibranchial and the ceratobranchial elements of each branchial arch bears two rows of gill filaments of the two hemibranchs of the holobranch., which are the seat gaseous exchange. It encloses afferent and efferent branchialarteries and veins. It is also provided by nerves. The branches of 9th (glossopharyngeal) cranial nerve innervate the first gill, while II, III and IV arches are supplied by the branches of vages (10th cranial nerve). It also contains abductor and abductor muscles. Inside it containes gill rakers, taste buds, mucous gland cells and sensory papillae.