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Fish with their amazing diversity and adaptability are more than just inhabitants of aquatic ecosystems; they are key players in the intricate web of life that sustains our planet. Fish provides both inner joy (beauty, recreation, wonder, and religious symbolism) and practical values (ecology, food, and commerce). The study of fish is a condensed and expanding record of knowledge of humanity of various fish species, incorporating discoveries, archaeological data and databases. Each species of fish from the smallest guppies to the mighty marlins, contributes to the complex mosaic of ecological balance.

The term “fish” is usually a convenient description for a group of cold-blooded, ectothermic, aquatic non-tetrapod craniates that breathe with gills (Nelson, 2006). Fishes exploit every nook and niche of the domain of water. They are especially noted for their unimaginable numbers and bewildering forms. Most of them have a stream-lined body covered by scales and pose fins supported by fin rays and breathed by gills throughout life. They constitute the first group of animals that have developed biting jaws in the phylogenetic history of vertebrates.

2.1 Introduction Life exists in almost all diverse habitats, ranging from high mountain peaks of more than 20,000 feet to deep sea bottoms of up to 10,000m. Living organisms can be found in ponds, pools, ditches, sulphur springs, hot springs, the cold of the Polar Regions, the scorching heat of deserts, dense tropical forests, and anywhere else life is possible. However, not all organisms occupy all possible life-supporting areas.

3.1 Introduction Fish are the only true aquatic vertebrates. These exploit every nook and cranny of the water domain. They are notable for their vast numbers and perplexing forms. In the phylogenetic history of vertebrates, this is the first group of animals to develop biting jaws. The origin, evolution and interrelationships of fishes are not fully understood due to incomplete fossil records and a scarcity of primitive types. The early appearance of bone, cartilage and enamel-like substances was crucial in the evolution of fish.

While describing fish, we need complete general information about the different specimens of fish included in the particular class or grade. A flow sheet of the general characters used in this concern may be tabulated as: 1. Origin of the term: The term Pisces comes from the old version of the plural of the Latin word piscis or singular word piscus meaning “fish.” 2. Literal meaning and definition: It describes either the creature or the meat from the creature. It is a streamlined, cold-blooded aquatic vertebrate with scales and gills, 3. Origin and evolution: The first fish appeared around 530 million years ago during the Cambrian period and then underwent a long period of evolution so that, today, they are by far the most diverse group of vertebrates. The Devonian Period is known as the “Age of Fishes”.

The term “fish” most commonly refers to Agnatha (jawless fishes), Chondrichthyes (sharks and rays), Sarcopterygii (lobe-finned fishes) and Actinopterygii (ray f inned fishes). Although the ichthyologist Regan (1910) defined a fish species as a product of interrelated communities with common “morphospecies”, in the early 20th century, it should be emphasized that the concept of species classification varied across scientists. Aristotle (335-332 BC) provided the earliest classification of fish and described 117 Mediterranean fish species obtained from the Aegean Sea close to Greece.

Agnatha consists of cyclostomes, conodonts and ostracoderms. Among recent animals, cyclostomes are sister to all jawed vertebrates known as gnathostomes. They have little economic importance. It includes the oldest known craniate fossils and has many primitive characteristics. The oldest fossil of agnathans appeared in the Cambrian period and two groups the lampreys (Hyperoartii) and the hagfish (Myxini or Hyperotreti) comprising about 120 species still survive today (Fig. 6.1). Hagfishes are considered members of the subphylum Vertebrata, because they secondarily lost vertebrae; before this event was inferred from molecular and developmental data, the group Craniata was created by Linnaeus to reference hagfish and vertebrates.

7.1 Introduction Placoderms (plate-skinned fishes) (Fig. 7.1) flourished and evolved in divergent lines during the Devonian period and became extinct by the end of the Permian period. They were bottom-dwelling, mud-sucking Palaeozoic fishes that may have been nectonic. Their bodies were divided into three parts: the head, the trunk and the tail. They ranged in size from 10 cm to 9 meters. The head and trunk are surely covered by bony plates.

8.1 Introduction Cartilaginous fishes have ventrally placed mouths with teeth in several series. They possess rigid dorsal fins and placoid scales. They have five pairs of gill clefts opening individually to the exterior, a lower jaw articulated with the upper jaw and upper jaw not fused to the skull, no swim bladder, an operculum, an endolymphatic duct and a heterocercal or homocercal tail. It shows internal fertilization, meroblastic cleavage and direct development. Elasmobranchii includes a wide range of cartilaginous fishes. Dogfish sharks are commonly studied all over the world probably because of their suitable size for dissection.

9.1 Introduction Holocephalians are a group of highly specialised cartilaginous fishes that live in the sea. It is structurally similar to Elasmobranchii and Teleostomi. It exists in both extinct and living forms. Previously considered a class of Pisces, Nelson (1984) classified Holocephali as a sub-class. 9.2 Systematic Position Phylum: Chordata  

10.1 Introduction Bony fishes (perfect mouth fishes) have a distinct mechanism for opening their mouths by lowering the mandible through the hyoid apparatus, skull either hyostylic or amphistylic. They have a single pair of respiratory openings, a movable maxilla and premaxilla, four-gill arches, a swim bladder, an operculum and a homocercal tail. They are both freshwater and marine. They vary greatly in shape but are built on the same basic plan.

11.1 Introduction The crossopterygians, primitive, lobe-finned, bony fishes, represent a very interesting group of fish showing many features of amphibians and other tetrapods. They possess paired fins with a large median lobe, fins covered by scales, a cosmine layer, swim bladder as lungs, nostrils that open into the mouth cavity and autostylic jaw suspension. 11.2 Taxonomic position

12.1 Introduction Dipnoans are commonly referred to as ‘lungfish’. Dipnoi owes its name to the presence of two internal nostrils. Dipnoans, which stand on the basic piscine platform, have many interesting features that the early fishes went through to become land vertebrates, particularly amphibians. As a result, the group’s systematic position becomes contentious. Dipnoans evolved during the middle Devonian period and thrived during the Permian and Triassic periods. They became scarce after the Triassic period and are now represented by three specialised genera. According to Young (1981), dipnoans have acquired special characteristics through paedomrphosis.

13.1 Introduction The fish’s distinguishing features are its fins. The fins constitute the major propulsive organs in fishes. These are either folds of skin or projections from the body surface. The fins lack coelom but are supported by muscles and fin rays (lepidotrichia). These supporting rays may be bony, cartilaginous, fibrous or horny. The diversity in the fin system is due to its adaptive responsiveness. A great variety of fins is observed in fishes. However, they are primarily of two types:

14.1 Introduction The integument is an envelope to separate and protect a fish from its environment; it also provides the means through which most contacts with the outer world are made (Fig. 14.1, 14.2 and 14.3). It is a large organ and is continuous with the linings of all body openings, and also covers the fins. The fish integument is dotted with various kinds of unicellular and multicellular glands along with abundant mucus-secreting glands. Poison glands of many cartilaginous fishes and some bony fishes are frequently associated with spines on the fins, tails, and gill covers. Photophores, light-emitting organs found especially in deep-sea forms, maybe modified mucous glands.

15.1 Introduction An endoskeleton is a skeleton that is on the inside of the body of an animal. It is an internal supporting structure composed of mineralized tissue. The endoskeleton develops within the skin or the deeper body tissues. It participates in protection, helps in movement, provides shape and support and stores minerals like calcium. Gerringer et al. (2021) suggested that changes in skeletal structure are non linear and are driven by hydrostatic pressure, other environmental factors and evolutionary ancestry.

16.1 Introduction Fish are primarily designed to swim in the water. Their fusiform body allows for efficient swimming. Swimming is the most cost-effective mode of fish locomotion because fish bodies are supported by water and thus do not require energy to counter gravity. To swim, a fish uses its fins. The caudal fin is primarily used for propulsion, while the remaining fins are used for balance control and f ine maneuvering. However, some Plectognathi fish (Trunk-fish, Parrot-fish, Butterfly-fish, Porcupine-fish, and Trigger-fish, among others) have lost their ability to swim using body flexure and can only move with their other fins.

17.1 Introduction Mechanical irritability is a general phenomenon exhibited by a cell. Apart from a general response to mechanical deformation, there are specialised receptors distributed internally as well as externally that act as mechanoreceptors. It includes the following: 1. Algesireceptor (Nociceptors): For pain. 2. Georeceptor: A gravity and acceleration receptor. 3. Phonoreceptor: Used for vibratory contact and sensation of hearing and also for equilibrium.

18.1 Introduction The digestive system of fish shows remarkable diversity in its morphology and function, related to both taxonomy and different feeding habits (Abdulhadi, 2005). Fish eat a wide variety of foods. Some feed on plants, others on small fish, and the remainder are omnivorous. The digestive system is drastically altered as a result of different feeding habits. In response to feeding habits, the structural plan of the buccal cavity, pharynx, and rest of the gut has changed.

19.1 Introduction Fish are the most common aquatic vertebrates. Fishes’ primary aquatic adaptation is the presence of gills. They use the oxygen that remains dissolved within the water via their gills. The gills use up to 80% of the oxygen dissolved in the water that passes over them. In addition to the gills, skin, the air bladder and accessory respiratory organs perform gas exchange functions in some fishes. Fish gills are essentially the same. The gills have been specially modified to use up to 80% of the oxygen dissolved in the water that passes over them. Cyprinus and Carassius gills excrete six to ten times more nitrogenous waste than kidneys.

20.1 Introduction Physiological adaptations for air-breathing in bony fishes have fascinated researchers for well over a century. The metabolic rate of some fish is so high that dissolved oxygen in water becomes insufficient. Prolonged drought causes some f ish to develop devices to survive in the absence of water for a short period. To overcome these challenges, fishes have developed accessory respiratory organs that can function in both aquatic and aerial environments. As an adaptation to specific environmental conditions, accessory respiratory organs develop in addition to the gills. The development of these organs is dependent on the availability of extra oxygen. The development of an accessory respiratory organ is most common in tropical freshwater fishes.

21.1 Introduction The air (=gas or swim) bladder, also known as the fish maw, is a tough sac-like structure with an overlying capillary network. It develops as an outgrowth from the oesophageal region and is found in bony fish. It demonstrates a wide range of development, structure, and function in various fish. It is usually a hydrostatic organ that controls buoyancy, allowing the body to remain at the current water depth without having to expend energy swimming. It is similar to a diver’s buoyancy compensation device (BCD). It also serves as a resonating chamber for producing or receiving sound and is used as an air-breathing organ.

22.1 Introduction The blood vascular system of fishes is modified to suit the aquatic mode of life. Fish contains the simplest single-circuit closed blood vascular system. The structure of the heart in fish varies little. In fish, all of the chambers of the heart are essentially the same. The size and shape of the component chambers may differ. A large heart is found in extremely sluggish fishes with low oxygen requirements. The number and arrangement of valves in the conus arteriosus are the most distinguishing features in various fishes.

23.1 Introduction Excretion is the elimination of nitrogenous metabolic wastes from the gut and skin of the body. Through excretion, certain substances are removed while others are conserved. Fish are either ammonotelic or ureotelic animals. Kidneys play the most important role in excretion to maintain water-salt balance. Fish gill ammonia excretion has been investigated for 80 years, but its mechanisms are still up for debate. A new paradigm has been established by the relatively recent identification of the ammonia-transporting role of the Rhesus (Rh) proteins, a family related to the Mep/Amt family of methyl ammonia and ammonia transporters in many living organisms. These proteins also occur as glycosylated proteins in fish gills (Wright and Wood, 2009).

24.1 Introduction Water is a universal biological solvent and the medium in which most of the cellular reactions of metabolism occur. Fish contain a large amount of water in their cells and extracellular fluids. It is important to a fish that its water content be nearly constant. This more or less steady state of water content in the body can only be maintained when there is a quality difference between the amount of water entering and leaving the body. The process of regulating the movement of water and its volume in the body is known as osmoregulation.

25.1 Introduction All fishes have a similar nervous system organisation. Meninx primitive is a single protective covering that covers the brain and spinal cord. The brain varies in size about the body in different species and does not occupy the entire cranial cavity (Fig. 25.1). 25.2 The Brain Except for elasmobranchs, which have larger brains, the relative ratio of the brain has been observed to be fairly constant in all fishes. The brain-body weight ratio, also known as the cerebro-somatic index (CSI), is calculated as follows. 

Endocrine glands secrete their products into the bloodstream and body tissues in conjunction with the central nervous system to control and regulate a variety of body functions (Fig. 26.1). Various endocrine glands described below have been discovered in fish that are associated with various tasks and functions. 26.1 Pituitary Gland 26.1.1 Introduction The pituitary gland (=Hypophysis) is a central and discrete organ of the neuro endocrine system in fish playing critical roles in various physiological processes such as stress response and behaviour.

Fishes use their sense organs to detect their bodies and their environment. Sense organs include the eyes, the lateral lines, the nostrils, the olfactory organs and the taste organs. Each of these sense organs is equipped with sensory nerve endings. Most fish possess highly developed sense organs to detect smell, sight, hearing, taste, touch, temperature, etc. 27.1 Olfaction in Fishes 27.1.1 Introduction

28.1 Introduction The use of biological light in active transport, nerve transmission, and muscle contraction is a feature shared by all living organisms. Surprisingly, a large number of living organisms have evolved the ability to convert chemical energy into light, which is manifested as bioluminescence. Bioluminescence is a fascinating biological phenomenon found in both terrestrial and aquatic living organisms. The light emitted by living organisms as a result of chemical reactions in their bodies is known as bioluminescence. Cold light is produced by this type of chemiluminescence.

Fishes are generally unisexual and reproduce by several methods. Some families (Sparidae and Serranidae) are hermaphrodite also. Alz (1964) reported that in certain species, hermaphrodism becomes a normal way of life. In hermaphroditic f ishes, self-fertilisation is a common process. Parthenogenesis could have been produced experimentally (Austin and Walton 1960). There maybe a few secondary organs that increase reproductive fitness. The genital papilla is a small, fleshy tube behind the anus in some fishes, from which the sperm are released; the sex of a fish can often be determined by the shape of its papilla.

30.1 Introduction Sexual dimorphism is a condition in which sexes of the same species exhibit morphological differences that are not directly related to reproduction. It is a common and noticeable feature of the animal world. It is extremely important in biodiversity assessments, as well as in biometry, breeding biology, induced breeding, breeding, pheromone biology and other related fields.

31.1 Introduction Courtship is the behavioural interaction that occurs between males and females before, during and just after mating. It encompasses all expressive movements associated with pair formation and mating. “Courtship is the heterosexual communication system that leads to the consummatory sexual act,” says DeMorris. Arthropods, mollusks, fish, reptiles, and mammals are the most affected. It consists of various activities and serves a specific purpose. Many courtship activities ensure that people only mate with people of their species. The distinction between species’ courtship behaviour allows females to identify males of their species. It is concerned with reproductive fitness and provides opportunities for survival

32.1 Introduction Development or embryogenesis refers to the events that occur after fertilisation. Embryogenesis in fish includes the formation of stages up to fingerlings. The process is similar in all fishes, but there are some differences between teleosts and elasmobranchs. The development of telolecithal eggs in fish begins soon after sperm fertilisation.

33.1 Introduction World fisheries face the threats of overfishing and climate change that have resulted in the extinction of genetically unique stocks and loss of genetic diversity. Genomics has brought new tools that can help address fundamental questions in fisheries management such as stock identification, population structure, and adaptive response to environmental change (Bernatchez et al, 2017).

34.1 Introduction Fishes are the first aquatic vertebrates whose life cycle is largely affected by the aquatic habitat. They exhibit much variation in their social life. Some species show no attraction between males and females, conversely, some provide high social attraction and parental care. Therefore, parental care is developed for their social life and habitat.

35.1 Introduction Age and growth studies are vital aspects of modern fisheries because this information is pivotal for fish population and management policies. Age and growth are important factors in the study of the natural history of fish. Age determination is an ancient practice that aids in systematic studies of fish and f isheries. Age denotes the activities of the organism about its size, whereas growth can be defined as an increase in any dimension of an organism from time or age until maturation. A fish ages as it grows, and vice versa. Fish age determination is important to marine resource management.

36.1 Introduction Migration refers to the long-distance movement of fish from one location to another in search of food or breeding. It involves a temporary or permanent absence from one’s home range and the establishment of a residence in another. It is one of the most remarkable and fascinating characteristics of fish. It is the mass movement of fish from one body of water to another. It entails longer-duration movements of schools of fish than those seen during normal daily activities.

37.1 Introduction Temperature describes the kinetic energy of the gases that make up air. As gas molecules move more quickly, air temperature increases. Among several environmental changes, increased air temperature affects the water cycle by increasing water temperature, which causes changes in local weather, water quality and river bodies (Gossiaux et al., 2019). This is because the water temperature is one of the most important factors for physicochemical properties of water quality (Szumiriska et al., 2020), aquatic habitats (Duggals et al., 2018), spawning rates (Besson et al., 2016) and fish growth (Collas et al., 2019) (Fig. 37.1).

38.1 Introduction To fishery management, understanding the abundance of a given species population is critical to the success of that species’ fishery. In this context, a fish population is a naturally occurring stock of a race of fish that occupy a specific area of space and act as a biological unit. The density of the fish population, on the other hand, is the population size per unit of space.

39.1 Introduction Like all other animals, fish are also subjected to certain diseases. This is one of the most important problems confronting a fish culturist. Diseases at times may take the form of epidemics. The occurrence and magnitude of infections and diseases are closely related to the sanitary conditions prevalent in the water and also the general health of the fish.

40.1 Introduction India has an 8,118-kilometer-long coastline that stretches through nine Maritime States (Gujarat, Tamil Nadu, Kerala, West Bengal, Maharashtra, Odisha, Andhra Pradesh, Karnataka, and Goa) and four Union Territories (Daman and Diu, Puducherry, Lakshadweep Islands, and Andaman and Nicobar Islands). The country also has a Continental Shelf of 0.53 million km2 and an Exclusive Economic Zone (EEZ) of 2.02 million km2. The EEZ’s annual potential yield is estimated to be 3.93 million tonnes of fish.

41.1 Introduction Fishes adapted to live at a temperature ranging from 10-200C constitute coldwater f ish. Upland water at high altitudes in the mountains and spring water at low elevations in temperate regions remain cooler than the rest, allowing coldwater f isheries to thrive in these areas. Such bodies of water, which include several hill streams, pools, lakes and reservoirs, are abundant in the Himalayan and Deccan plateau regions of peninsular India. These are fed by either melting snow or springs in the north or rain on the Deccan plateau.

42.1 Introduction Several fish species including their seeds, eggs and spores have been imported either occasionally or artificially and introduced into the water of our nation. These fishes are not native to our country and are called exotic fishes. Based on utility, these fishes include food fishes, game fishes and larvicidal fishes. In other words, the fish species that are not of Indian origin but transplanted into Indian water for cultural purposes are called exotic fishes. Exotic fish are an alien species that are non-indigenous having their origin in another country

43.1 Introduction Game fish (also known as sport fish or quarry) are freshwater or saltwater fish that are pursued by recreational anglers or fished with a rod and line from clear running water (Hora 1943). Some game fish (such as salmon and tuna) are commercially targeted. In other words, game fish are fish caught for sport or subsistence. Trophy f ish are game fish that are larger and heavier than the average for the species.

44.1 Introduction Ornamental fish are a diverse and colourful species benefiting millions of aquarium enthusiasts. The term "ornamental fish" was first coined in the early 19th century to describe fish kept solely for decorative purposes by ancient civilizations such as the Sumerians and Egyptians. They are known for their shapes, sizes, vibrant colours, intricate patterns and distinct behaviours with hobbyists and collectors. They are widely used to distinguish between fish kept for food or other practical purposes.

45.1 Introduction Mosquitoes of various species breed in ponds, swamps, gutters, and other bodies of stagnant water, carrying parasites that cause malaria, filaria, dengue fever, chikungunya, Japanese encephalitis, Zika and yellow fever. Various approaches have been used to control mosquito populations to reduce the incidence of vector borne diseases, including chemical, physical, and biological measures. In all types of habitats, biological control against mosquito vectors is safe, friendly, and cost effective. Mosquito biological control entails introducing natural enemies such as parasites, disease organisms, and predatory animals into the environment.

46.1 Introduction Since humans settled and flourished adjacent to the aquatic bodies and therefore f ishes might have attracted their attention. The increasing population has threatened mankind variously amongst which, the most important one is the food problem. Fish being a valuable and easily available source of food can be used to overcome the food problem to a certain extent. Indian rivers are the major source of irrigation systems, drinking water and fish as food. The fish industry of West Bengal and Orissa is some 1500 years old and in West Bengal, every family traditionally has at least one small or moderate-sized pond.

47.1 Introduction Fish are one of the most important groups of vertebrates and have always been a significant species in man’s life, influencing it in a variety of ways. Fish are highly valued in human civilization for their food, cooking, and nutritional value. They have significant economic, nutritional, medicinal, industrial, aesthetic and religious values, and they employ millions of people worldwide. Several species are sought after as luxury foods in high-end restaurants. They help to ensure food security in many parts of the world by serving as a valuable supplement to varied and nutritious diets.

1. Class Placodermi includes   (a) Acanthodi  (b) Coccostei   (c) Pterichthys  (d) All.  2. Division Siluri lacks   (a) Channa sp.  (b) Clarias sp.   (c) Mystus sp.  (d) Wallago sp  3. Agnatha appeared in ………… period   (a) Craboniferous  (b) Devonian   (c) Ordovician  (d) Silurian