Gill Slits:
There are six or seven pairs of gills in cartilaginous fishes while four pairs in bony fishes due to the loss of spiracle (Fig. 5.1 a & b).
Gill slits of bony fishes are covered by operculum while operculum is absent in cartilaginous fishes. In sharks gill slits are laterally situated while in rays they are ventrally placed. A pair of spiracle is present in Elasmobranchii anterior to first gill which corresponds to a vestigeal primitive first gill slit.
Although spiracle is absent in bony fishes, in Actinopterygii it is replaced by a pseudo- branch which is free in some fishes but skin covered in others.
Pseudo Branch:
In carp and rainbow trout the pseudo branch is embedded in submucosal connective tissue of pharyngeal wall and shows a glandular appearance due to complete conglutination of branchial filaments. (Fig. 5.1a & Fig. 5.2).
In some species, a pseudo branch with hemibranchs structure is located inside the operculum. However, in eel the pseudo branch is not present, it is also absent in cat fishes (Siluroidae) and feather back (Notopteridae).
In glandular pseudo-branch, 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. According to
Whittenberg and Haedrich (1974), the pseudo-branch regulates the flow of the arterial blood to the opthalmic artery to increase the amount of blood carbon dioxide.
Parry and Holliday (1960) found that in rainbow trout extirpation of pseudo-branch induced melanophore expansion and body colour change, suggesting the secretion of a melanophore-aggregating hormone from tissue. It also helps in metabolic gas exchange of retina and filling of gas bladder. Because of its direct vascular connection with the choroid gland on the eyeball, the pseudo-branch has been implicated in the regulation of intracellular pressure.
The structure of gills has been studied extensively in Indian fishes by light transmission and scanning electron microscopy. The gill comprises of gill rakers, gill arch, gill filaments (Primary gill lamellae and lamellae) (Fig. 5.3a & b).
A complete gill is known as holobranch. It consists of a bony or cartilaginous arches. 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) hemi-branch (Fig. 5.4 a, b, c, d).
The architectural plan of teleostean gills shows heterogeneity in their functional unit which is due to varied osmoregulatory, feeding and respiratory behaviour and to the physicochemical status of their environment.
In teleost fishes, five pairs of branchial arches are present of which first four bear gill lamellae (Fig. 5.4a, b) 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, hypo-branchials 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 of gaseous exchange. It encloses afferent and efferent branchial arteries and veins (Fig.
5.4a and Fig. 5.6a, b, c).
It is also provided by nerves. The branches of 9th (glossopharyngeal) cranial nerve innervate the first gill, while II, III, & IV arches are supplied by the branches of vagus
(10th cranial nerve). It also contains abductor and adductor muscles. Inside it contains gill rakers, taste buds, mucous gland cells and sensory papillae.
Gill Raker:
It occurs in two rows on the inner margin of each gill arch. Each gill arch is short stumpy structure supported by bony elements (Fig. 5.3a & b). The gill arch projects across the pharyngeal opening. They are modified in relation to food and feeding habits.
The mucous cells of the epithelium help to remove sediments from the covering epithelium in order to enable the taste buds to function effectively and to sense the chemical nature of food passing through the gill sieve.
Gill Filaments (Primary Gill Lmellae):
Each hemi-branch consists of both primary and secondary lamellae (Fig. 5.5).
The primary gill filaments remain separated from the branchial septum at their distal end making two hemi-branch in opposition which direct the water flow between the gill filaments. Amongst dual breathers the heterogeneity in the gill system is more pronounced particularly in the swamp eel, Monopterus, Amphipnous cuchia and climbing perch, Anabas testudineus.
In Monopterus, gill filaments are stumpy and are present only in second pair of gill and lack gill lamellae. According to Munshi and Singh (1968) and Munshi (1990), the remaining three pairs are without functional lamellae. It is the modification for another way of exchange of gases.
The gill filaments are blade-like structures supported by gill rays. The heads of the gill rays of both the hemibranchs are connected by ligaments. They are provided with two types of adductor muscle units in teleosts. The gill filaments are also lined by epithelium referred to as primary epithelium. The epithelium has glandular and non-glandular part.
Lamellae (Secondary Lamella):
The each gill filament is made up of secondary gill lamellae which are actual seat of exchange of gases. They are generally semicircular and lined up along both sides of the gill filaments. The lamellae frequency is directly proportional to the dimension and resistance of the gill sieve.
The secondary lamellae are having two sheets of epithelium which are separated by space and through these spaces blood circulates. The epithelial sheets are separated by a series of pillar cells. Each cell consists of central body and is provided with extensions at each end (Fig. 5.6d).
Branchial Glands:
These are specialized cells of the epithelium. They are glandular in nature and perform different functions in normal and experimental conditions. The most common specialized branchial glands are the mucous glands and acidophilic granular cells
(chloride cells).
Mucous Glands:
These gland cells are unicellular. They may be oval or pear shaped with a neck through which they open outside the epithelium. The nucleus lies at the bottom of the cells. They are typical goblet cells. They are present throughout the epithelium, i.e., gill arch, gill filament and secondary lamellae.
Respiratory Mechanism: The continuous undulational flow of water over gill surface is accomplished by respiratory pump. It is now unanimously accepted that the respiratory pumps of a teleost consist of buccal cavity and two opercular cavities, caused by movements of bone of arches and operculi, resulting in the pumping action of the system.
In the beginning, the water enters into the mouth by expansion of buccal cavity. The water is then accelerated over the gill by the simultaneous contraction of buccal cavity and cavity contracts, expelling the water out through the opercular opening, the cycle begins again (Fig. 5.7).
The respiratory cycle is a complex mechanism and involves a large number of muscles, bones, ligaments and articulations. Several authors from time to time have described this system and made an attempt to understand its working.
Ram Ventilation:
It is carried by strong abduction of the two hemibranchs of a holobranch towards each other. Interruption of this cycle causes reversal of flow or cough which fish uses to clear foreign matter or excess mucus from the gills. Drummond (1973) stated that frequency of cough in Salvelinus fontinalis can be a sub-lethal indicator of excessive copper concentration in freshwater.
There is an active and passive ventilation during the swimming movement. The transition to ram gill ventilation in fish is a graded process as swimming picks up from rest. The first indication that a critical swimming speed has been reached is signaled by the drop-out of a single cycle.
The drop-out continues until only occasionally ventilatory movements and ‘cough’ are noticed. Return of active movements with gradual reductions in swimming velocity to below critical shows nearly the same sequence but in reverse order.
The additional respiratory mechanism of sharks and rays can be divided into three phases, which are as follows:
(i) When coracohyoid and coracobranchial muscles contracts to enlarge the angle enclosed by the gill arches and to increase the oropharyngeal cavity for entrance of water through mouth or spiracle during which the gill slits are kept closed.
(ii) When the contraction between the upper and lower parts of each gill occurs with the relaxation of abductors of lower jaw and gill arches, which causes mouth to serve as pressure pump. During this phase the forward flow of water through mouth is prevented by the oral valve and the water is directed backward towards the internal gill clefts. The inter-septal spaces are enlarged by the contraction of the inter-opercular adductors to reduce the hydrostatic pressure at the inner surface of the gill and water is drawn into the gill cavities, which remain closed at the outside.
(iii) The third phase includes relaxation of inter-opercular muscle and contraction of their sets of muscle which cause the internal gills to narrow, and the water is forced through the gill lamellae. This follows the opening of gill clefts, and the water is forced to the outside. The mackerel sharks (Limmidae) take the sufficient respiratory water during swimming and do not show pronounced breathing movements.
Mechanism of Entry of Water:
In all bony fishes the pressure and flow of water in the oral cavity is regulated by the muscles, which move the bases of holobranchs. “Coughing” is the process of a voilent sweeping of water over the gill lamellae to make them free from accumulated detritus.
It is carried by strong abductions of the two hemibranchs of a holobranch towards each other. The diffusion of oxygen from the respiratory water is added not only by the gill filaments but also by the direction of blood and water flow.
These works a counter-current system in which the oxygenated water flows from the oral to aboral side of the gills, but the blood in lamellae flows in opposite direction, i.e., from aboral lamellar afferent to oral lamellar efferent blood vessels.
During this counter-flow of oxygenated water and blood, the oxygen enters the gill and carbon dioxide leaves it. In the tench (Tinea) this counter-current system is so modified that the 51% of oxygen from incoming respiratory water diffuses in and 9% oxygen from outgoing water currents.
In the osteichthyes fishes, in which gills are covered by operculum, the water is propelled over the gills by suction pressure. In the beginning of inspiration, when the operculum is closed forcefully, and mouth is opened while several muscles contract including sternohyoid and elevator of palatine.
The branchiostegal rays are spread and lowered simultaneously and the mouth cavity is enlarged creating negative water pressure in it. Thus water is drawn into the mouth and after a short interval the space between the gills and the operculum is enlarged as the gill covers are abducted anteriorly although the opercular skin flaps are still closed posteriorly by outside water pressure.
A negative pressure develops in the gill cavity resulting flow of water over gills. This is followed by reduction of buccal and opercular cavities. At this time the oral valves presents the backflow of water out of the mouth and mouth cavity begins to function as pressure pump instead of a suction pump.
Then each operculum is immediately brought towards the body, the gill flaps open and water is expelled, being prevented from flowing backwards by high pressure in the buccal cavity as compared to the epibranchial cavity.