Preface World aquaculture is increasing day-by-day from its infancy of layman's culture to scientific culture, for the enhancement of production to meet the growing demand of the human being through optimum utilization of available water resources. This is possible due to the scientific knowledge on the fish and shellfish. Physiology is the scientific study on the functioning of the various systems of the organism which compliments with the anatomy. The present work is a handy book on fish and shellfish (decapod) physiology. The author also gives an informative prospective on the basic anatomical structures and functions of important systems of both finfish and shellfish. The author found that many a time the aqua-culturists had great difficulties in solving the common problems encountered during the culture. This is because of the fact that they lack basic knowledge on the physiology of cultured species. By providing the primary information on physiology through this book, the fundamental problems can be partly addressed and their culture can be efficiently managed. This book on Physiology of Finfish and Shellfish has been written after the author's 25 years of lecturer on the subject both at undergraduate and postgraduate level. Since this book is based on the prescribed syllabus on the subject, it will be useful for the students of Fishery Science. Many books are available on the Physiology of either finfish or shellfish. However not a single book is available in the market till now dealing both finfish and shellfish physiology. Therefore, the author had made an attempt to bridge the gap. Whatever physiology books are available they are very elaborative for the research or expensive only for the libraries. Thus, these are not suitable either for the ready reference or budget of the common aqua culturists and beginners of Fishery Science. This will be an informative handy book which will be easily referred and also affordable. So, this book will be used by a broad spectrum of fish and shellfish students, biologists, research workers and progressive aqua-culturists.

Aquaculture is the culture of aquatic organisms per unit water area per unit time for maximum profit. The economically important finfishes and shellfishes, whose body physiology is adjustable to varied environmental conditions, are considered as aquaculture candidate species. So, an overall knowledge on the physiology of such species is required to improve the culture practices. 1.1. Finfish A fish may be defined as a vertebrate, adapted for a purely aquatic life, propelling and balancing itself by means of fins, and obtaining oxygen  (O2) from the water for survival and life processes. As the fishes have fins, they are also known as finfishes. Fish describes a life from rather than a taxonomic group. They share certain features with other vertebrates. These are gill slits, a notochord, a dorsal hollow nerve cord and tail. In number of individuals, and may be also in number of species, fishes are presently superior to other vertebrates, including mammals. There are 20,000 different species of fishes. Fishes form largest group of vertebrates, Pisces. The various types of fishes differ so much in shape, color and size that it is difficult to believe they all belong to the same group of animals. Fish in general are cold-blooded and adjusted either to freshwater, brackishwater or marine environment.

The knowledge of food, feeding habit, anatomical adaptations and the physiology of digestion of the candidate aquaculture species is highly essential to develop cost-effective artificial feed for successful culture practices. The digestion of poikilothermic aquatic fishes and shellfishes is influenced by many factors, and is different from the digestion of terrestrial animals. 2.1.    Anatomy and Physiology of Fish Digestive System 2.1.1.  Feeding Behavior of Fishes Based on the feeding habit, the fishes are generally classified as herbivores, omnivores, carnivores and detritivores. Within these categories, the fishes further categorized as euryphagous, having a mixed diet, stenophagous, consuming a limited combination of food types and monophagous, accepting only one type of food. A majority of fishes are euryphagous carnivores, and their feeding mode and food types are associated with the body form and digestive system. Fish generally change their feeding habits depending upon the availability of food. Fishes may be classified as predators, grazers, strainers etc. Plankton is a collective term for variety of marine and freshwater plant or organisms that drift on or near the surface of the water. Most fishes eat the material that is smaller than them. The smallest fishes eat these planktons, which act as their main food and all smaller fishes are the food of the bigger fishes. Largest shark feed on plankton by straining these tiny marine plants and animals from the water. The zoo-planktons, comprise protozoa, small crustaceans, jelly fish, and mollusks, together with the eggs and larvae of many species of marine and fresh water.

Embryologically, the circulatory system is the first organ system to develop in the living animals. The form and function of this system vary considerably among the animals and are shaped by a variety of environmental factors i.e. phenotypic plasticity. Oxygen (O2) acquisition and carbon dioxide (CO2) elimination occur simultaneously at the gill site of fish and shrimp. The gases are transported around the body by the circulation of the blood. 3.1. Circulatory System of Fishes In fishes, heart is simply a muscular enlargement and modification of a part of the main blood vessels, lying between the hepatic capillaries and the gills. Fishes have a closed circulatory system with a heart that pumps blood in a single loop throughout the body. From the heart, blood goes to the gill capillary bed. Then via the systemic capillaries it returns back to the heart. Usually the gills account for approximately 30% of the total resistance to blood flow, the rest being in the visceral and somatic vasculature. Tuna requires more O2 and the gill area is large with gill resistance up to 57%.

Various activities of the body in different external environmental conditions are co-ordinated by the appropriate message from the neurons due to the stimuli from the environment. These messages pass in the form of impulses through the co-ordinated links between a number of neurons or nerve cells. The constant passage of messages require a very little energy but high level of O2 .There is interaction between the nervous and endocrine system with the areas of interchange among the two systems. Some neurons show the action of both the systems and thus termed as neurosecretory or neuro-endocrine. Fishes have a well-developed nervous system. Broadly the nervous system can be divided into central nervous system (CNS) and peripheral nervous system (PNS).The brain and spinal cord are the parts of the CNS. The motor and sensory nerves link the receptor and effector organs. All nerve tissues other than brain and spinal cord are grouped under PNS. Peripheral nervous system consists of nerves, ganglia and receptors. The nerves carry sensory information from special receptor organs like eyes, internal ear etc. to the integrating centers of the brain and spinal cord. The PNS also carries information via different nerve cells from the integrating centers of the brain and spinal cord. This coded information is carried to the various organs and body systems, such as the skeletal muscle system for appropriate action in response to the original external and internal stimulus. The PNS can be divided into somatic and visceral. The term visceral is sometimes used in referring to the autonomic or involuntary nervous system (ANS), controlling visceral functions. The ANS helps to co-ordinate the activities of many glands and organs, and it is closely connected to the integrating centers of the brain.

The respiration of animals includes the exchange of gases either at the surface of the body or through the internal processes involving circulatory system. The gases that are exchanged are oxygen (O2), carbon dioxide (CO2) and the inert gas like nitrogen. When the organism respires, the O2 that is taken up from the external environment is transported by the vascular system to individual cells within the tissues. At the same time this system picks up CO2, an end product of cellular metabolism, and releases it to the environment. Thus respiration is a term applicable either to a whole organism or one of its cells. The mechanical process, by which respiration is achieved, is called ventilation.  This ventilation rate of aquatic organisms is controlled directly by the concentration of dissolve oxygen (DO2) in water. Respiration in aquatic medium is produced by a water current passing in a single direction. The area of respiratory surface is thus an important limiting factor in the movement and growth of the animal. For aquatic animals, if the water is at all deficient in O2, the rate of breathing is naturally accelerated, and the animal appears to respire hurriedly. Other factors, which lead to an increased rate of respiration, are the stresses.

Excretion is the process of elimination of metabolic wastes that include toxic nitrogenous end products of the protein metabolism and the gas such as carbon dioxide (CO2). These nitrogenous wastes are excreted through urine. Fecal matter is not included in the excretory products as it is simply the undigested food materials, and are not metabolic end products. The membranes of aquatic animals, that is permeable to gases, pass water and solutes between environment and body. Most aquatic animals maintain in their bodies a reasonably constant proportion of water and solutes. When it is different from the surrounding water, it requires some mechanism for regulation, which is known as osmoregulation. Most invertebrates in the sea have nearly the same salt concentration (isosmotic or isotonic) as the environment. Sharks and rays (elasmobranches) tend to have a slightly higher concentration of salts and urea (excretory end products) than seawater but the body fluids of most teleosts (bony fishes) have much lower salt concentration than seawater. Freshwater fishes, which live in a medium of lower salt concentration (hyposmotic or hypotonic), have the threat of hydration, so they have special mechanisms to get rid of excess body water. Marine fishes that live in an environment of high salt concentration (hyperosmotic or hypertonic) have the regulatory mechanisms by which excessive loss of water from the body is prevented and body osmolarity (proper osmotic concentration) is maintained by retaining less toxic nitrogenous excretory products. Marine fishes, which lose water osmotically across the gills and must get rid of salt, tend to drink large volume of seawater and produce very small quantities of urine (Fig-6.1).

Most of the aquatic organisms spend much of their lives and energies for reproduction. The young ones mature, develop sperms or eggs, spawn, and then recover to repeat the breeding cycle that continues till their death. In a majority of fish species males and females are separate individuals, fertilization is external and large number of eggs are produced (on an annual basis, its fecundity) which develop, hatch and mostly grow without parental care. The sperm and eggs are usually produced in such quantities that they represent a large expenditure of energy. The spawning process may be preceded by migrations or nest building, and followed by care of the young. Thus the success of any fish is ultimately determined by the ability of its members to reproduce successfully in a fluctuating environment and thereby to maintain viable populations.  As each fish species occurs under an unique set of ecological conditions, it has an unique reproductive strategy with varied anatomical, physiological, behavioral and bioenergetics adaptations. Unlike the finfishes, in shellfishes sexes are separate. The shellfishes use various signals to attract the opposite sex.

The highly specialized organs that receive physical and chemical stimuli from the environment and various body parts, and transform them into stimuli are called sense organs or sensory receptors. These organs are closely associated with nervous system. The physical stimuli such as change of heat, light intensity and quality, acoustical stimuli and chemical stimuli like hear, taste, smell, touch, lateral line etc are received by the sense organs such as ear, eye, nose and others. These are more prominent sense organs. Less obvious ones in fishes are sensory crypts and papillae, ampullae of Lorenzini and Savi. The crustaceans have various sense organs whose action are mediated through the nervous system. Crustacean antennae are important sites for the reception of environmental information, mainly chemical, and aesthetases, the olfactory hairs on the first pair, are usually well developed. The aesthetases help not only in locating food but also in recognizing other individuals and their sexual state.  The frequent grooming of the antennae by many decapods prevents fouling of the receptor sites (Bauer, 1971).

Glossary Antenna : The long segmented appendages located behind the eyestalks. These allow the shellfish to interact with its environment by touch and chemoreception.  Acron : At the front (or anterior end) of the shellfish body there is an unsegmented or presegmented region called acron. Antennule : Shorter segmented appendages located between and below the eyestalks, sensory organs; these also use chemoreception to "smell" and "taste".  Appendages : Ten legs (five pairs) including a claw-bearing pair with spines used for feeding and defense, followed by three pairs of sharply pointed walking legs, and a pair modified as flat swimming paddles at the rear, swimming legs.  Appetite : It is the amount of food eaten voluntarily at one time, appears to be the increase of stomach fullness, although this does not explain the entire appetite phenomenon. Apron : Abdomen of a crab, which is folded under the body; male's is shaped like the Washington Monument or an inverted Y. An immature female's is triangular (pyramid shaped) and mature female's is semicircular, like the dome of the Capitol building.  Aquaporin : This is a protein present in the membranes of the renal collecting duct cells.