Classification of Fish (pisces) – Systematic Classification and Based on feeding habit, habitat and manner of reproduction

What is Fish?

• Fish are fascinating aquatic creatures classified as gill-bearing vertebrates that thrive in water. They are characterized by their unique anatomical features, such as swimming fins, a hard skull, and the absence of limbs with digits. The diversity among fish can be broadly categorized into two primary groups: jawless fish, which represent a more basal lineage, and jawed fish, encompassing both living species and extinct groups such as placoderms and acanthodians. This distinction highlights the evolutionary advancements that have taken place over millions of years.
• Most fish are ectothermic, meaning their body temperature fluctuates with the surrounding environment. However, certain species, including large predatory fish like the great white shark and tuna, possess the ability to maintain a higher core temperature. This physiological adaptation enables them to be more active and efficient hunters in their aquatic habitats. Furthermore, fish are known for their complex communication methods, which often involve acoustic signals, particularly during mating rituals or territorial displays.
• The evolutionary history of fish dates back to the Cambrian period, when the first fish emerged as small filter feeders. During the Paleozoic era, fish diversified significantly, adapting to various ecological niches. Early fish, known as ostracoderms, were equipped with heavy bony plates that provided protection against predation. The advent of jaws, marked by the emergence of placoderms in the Silurian period, marked a crucial evolutionary step, allowing these creatures to exploit new feeding strategies and significantly increasing their diversity during the Devonian, often referred to as the “Age of Fishes.”
• The lineage of bony fish, distinguished by features such as swim bladders and ossified endoskeletons, became predominant after a mass extinction event at the end of the Devonian period, which decimated the placoderm populations. Bony fish are further divided into two primary categories: lobe-finned and ray-finned fish. Teleosts, a subgroup of ray-finned fish, account for approximately 96% of all extant fish species. These fish possess a remarkable ability to protrude their jaws, a feature that enhances their predatory capabilities. Notably, the evolutionary transition from lobe-finned fish to tetrapods, which led to the emergence of terrestrial vertebrates, underscores the complex evolutionary history linking aquatic and land-dwelling species.
• Fish play an integral role in human society and culture. From prehistoric times, they have been a vital source of nutrition, with both commercial and subsistence fishing practices flourishing across the globe. Fish are harvested from wild populations or cultivated in aquaculture settings, reflecting their significance as a natural resource. Additionally, recreational fishing and fishkeeping have become popular pastimes, with many individuals maintaining aquaria or garden ponds to appreciate these creatures in a domestic setting.
• Throughout history, fish have also held cultural significance, featuring prominently in religious iconography, literature, and art. The term “fish” itself has etymological roots in Proto-Germanic, closely related to terms in various languages such as German (“Fisch”), Latin (“piscis”), and Old Irish (“īasc”). While the exact origins of the word remain uncertain, some linguists trace it back to a reconstructed Proto-Indo-European root, *peysk-, which suggests a longstanding relationship between humans and these remarkable aquatic animals.

General description of fish

Fish are fascinating aquatic vertebrates characterized by their unique adaptations for life in water. They are distinguished by the absence of limbs with digits, opting instead for fins that facilitate swimming. As anamniotes, fish embryos develop without an amniotic membrane, a trait that differentiates them from tetrapods such as amphibians, reptiles, birds, and mammals. Furthermore, their respiratory system relies on gills, which enable them to extract oxygen from water. The following points provide a comprehensive overview of the key characteristics and functions of fish:

• Diversity and Size: Fish comprise a staggering array of over 32,000 bony species, more than 1,100 cartilaginous species, and over 100 jawless species, such as hagfish and lampreys. This incredible diversity allows fish to inhabit various ecological niches. They vary dramatically in size, with the whale shark, reaching lengths of up to 16 meters, contrasting sharply with small teleosts that measure a mere 8 millimeters.
• Evolutionary History: The evolutionary lineage of fish dates back to approximately 530 million years ago during the Cambrian Period. Early fish developed from ancestors possessing notochords and forward-facing eyes. Over time, fish diversified extensively, adapting to distinct habitats and developing essential anatomical features such as jaws, bony skeletons, and swim bladders.
• Habitats: Fish thrive in a myriad of aquatic ecosystems, occupying both freshwater and marine environments. Approximately half of all fish species are found in freshwater, primarily in large river basins within tropical rainforests, while coral reefs serve as hotspots for marine diversity. Remarkably, some fish can endure extreme habitats, including the depths of the ocean and temporary terrestrial environments.
• Anatomy and Physiology: Fish are equipped with numerous adaptations suited for their aquatic existence. Their streamlined bodies enhance swimming efficiency, while scales provide protection and camouflage. Fish possess a closed circulatory system, wherein blood is pumped from the heart to the gills for oxygenation before being distributed throughout the body. Additionally, fish utilize a complex array of sensory systems, such as the lateral line system, which detects water vibrations, and some species possess the ability to sense electrical fields.
• Behavior: Fish exhibit a diverse range of behaviors, notably shoaling and schooling. Shoaling refers to loosely organized groups, while schooling involves synchronized movements, both strategies serving as defense mechanisms against predators and enhancing foraging efficiency. Communication among fish occurs through various acoustic signals, produced via mechanisms like grinding teeth or vibrating swim bladders, especially during mating, feeding, and displays of aggression.
• Conservation: Global fish populations face significant threats due to overfishing, habitat degradation, pollution, and the introduction of invasive species. The Food and Agriculture Organization reported that, as of 2017, 34% of the world’s marine fish stocks were classified as overfished. These challenges underscore the critical need for sustainable fishing practices and conservation initiatives to preserve these vital aquatic organisms.
• Importance to Humans: Throughout history, fish have served as a crucial source of protein, underpinning vast global fishing industries. Beyond their economic value, fish hold cultural significance across various societies, appearing in religious symbolism, art, literature, and film. Recreational fishing and the popularity of aquariums further illustrate the multifaceted roles that fish play in human culture and economy.

Position of Fishes in Phylum Chordata

History of Classification of Fishes

The history of fish classification reflects the evolving understanding of ichthyology and the relationships among different fish groups. This classification process has undergone significant changes from early fossil discoveries to contemporary systematic approaches. Below is a detailed examination of key milestones in the classification of fishes.

• Early Fossil Discoveries: The first fossil vertebrates, known as Ostracoderms, emerged during the Ordovician period. These small, jawless forms were bony and exhibited characteristics that relate them to modern cyclostomes, marking an important step in vertebrate evolution.
• Devonian Period: Often referred to as “The Golden Age of Fishes,” the Devonian period witnessed an extraordinary diversification of fish species. This era was crucial for the development of various fish lineages, which laid the foundation for future classifications.
• Peter Artedi (1705-1735): Recognized as the “Father of Ichthyology,” Artedi made significant contributions to the systematic study of fishes. He established methods and principles for classifying fish that were so thorough that even Carl Linnaeus, a pioneer of modern taxonomy, could not modify them effectively. Artedi’s work remains influential in the field of ichthyology.
• Georges Cuvier: Cuvier was the first to propose a classification of fishes based on both external and internal anatomical features. His approach laid the groundwork for future classification systems, emphasizing the importance of anatomical structure in taxonomy.
• Müller’s Classification: Müller proposed a classification system dividing fishes into six subclasses under the class Pisces:
  1. Dipnoi: Lungfishes that can breathe air.
  2. Teleostei: The diverse group of bony fishes.
  3. Ganoidei: Includes genera such as Polypterus, Amia, and Lepidosiren.
  4. Elasmobranchi: Sharks, rays, and related species.
  5. Marsipobranchii: Cyclostomes, or jawless fishes.
  6. Leptocardii: Represents amphioxus and similar forms.
• Berg’s Classification (1940): L.S. Berg expanded the classification of fishes into seven distinct classes within the series Pisces:
  1. Pterichthys
  2. Coccostei
  3. Acanthodii
  4. Elasmobranchii
  5. Holocephali
  6. Dipnoi
  7. Teleostomi
• Romer’s Classification (1971): A.S. Romer arranged fishes into four classes under the super-class Pisces:
  • Agnatha: Jawless vertebrates, further divided into two subclasses: Monorhina and Diplorhina.
  • Placodermii: Armored or plate-skinned fishes, encompassing several orders, such as Petalichthyida and Arthrodira.
  • Chondrichthyes: Cartilaginous fishes, including sharks and rays.
  • Osteichthyes: Bony fishes, which further include the Crossopterygii subclass.
• Nelson’s Classification (1994): J.S. Nelson presented a comprehensive classification of fishes, which was aligned with a cladistic approach reflecting evolutionary relationships. His hierarchical system included:
  • Subphylum: Vertebrates
  • Superclasses:
    • Agnatha (Jawless)
    • Gnathostomata (Jawed)
  • Within Gnathostomata, he detailed several subclasses and classes:
    • Elasmobranchomorphii: Includes Elasmobranchii (sharks and rays) and Holocephali (chimaeras).
    • Teleostomi: Divided into further classes such as Osteichthyes, which includes subdivisions like Actinopterygii (ray-finned fishes) and Crossopterygii (lobe-finned fishes).

Systematic Classification of fishes (pisces)

The classification of fishes, or Pisces, encompasses a diverse array of organisms that exhibit various anatomical and physiological characteristics. Fishes are defined as gill-bearing aquatic craniates that lack limbs with digits, encompassing living species as well as several extinct groups. The following points outline the classification of fishes based on their taxonomy and key attributes.

• Taxonomic Hierarchy: Fishes belong to the Animal Kingdom, classified under Phylum Chordata and Subphylum Vertebrata. This classification highlights key features such as the presence of a notochord, a tubular nerve cord, paired gills, a post-anal tail, a ventral heart, and an endoskeleton.
• Superclass Gnathostomata: This superclass includes craniates where one pair of visceral arches has evolved into jaws. These organisms also possess an internal ear with three semi-circular canals and paired appendages, which can be fins or limbs. The superclass is divided into two primary groups: Pisces and Tetrapoda.
• Pisces: This group includes three main classes:
  1. Placodermi
  2. Chondrichthyes (Cartilaginous Fishes)
  3. Osteichthyes (Bony Fishes)

Class 1: Placodermi (Aphstohyoids)

Placodermi, also referred to as Aphstohyoids, represents a significant group of extinct fishes that played an essential role in the evolutionary history of vertebrates. This class is characterized by several unique anatomical features and adaptations, which provide insights into the early development of jawed fishes. The following points elucidate the defining characteristics and evolutionary importance of Placodermi.

• Extinction Status: Placodermi includes only extinct species, highlighting a significant chapter in the history of fish evolution.
• Armored Structure: These fishes are distinguished by their robust armor, consisting of bony plates or shields that form a protective exoskeleton. This anatomical feature likely served as a defense mechanism against predators.
• Bony Endoskeleton: The endoskeleton of Placodermi is composed of bone, which supports the body structure and provides rigidity.
• Gill Slits: The hyoidean gill slits in these fishes are complete and well-developed, indicating a sophisticated respiratory system for filtering oxygen from water. Unlike some other fish groups, these gill slits were not reduced, allowing for effective gas exchange.
• Jaw Suspension: Placodermi exhibit autodiastylic jaw suspension. This means the jaws are connected directly to the skull, allowing for efficient feeding strategies, which is a notable advancement in jaw structure compared to earlier fish.
• Caudal Fin Structure: A heterocercal caudal fin is present in these fishes. This fin structure, characterized by unequal lobes, aids in propulsion and maneuverability in the aquatic environment.
• Hyoid Arch: Unlike in many modern fishes, the hyoid arch in Placodermi does not support the jaws. This feature indicates an evolutionary divergence in jaw mechanics compared to other fish classes.
• Primitive Jaws: Placodermi possess primitive jaws that suggest a transitional form in the evolution of more advanced jaw structures found in subsequent fish classes. This primitive nature reflects the evolutionary stage of these organisms.
• Survival Period: Placodermi thrived during the Paleozoic era, specifically up to the Permian period, indicating their successful adaptation to various ecological niches in ancient aquatic environments.
• Notable Examples: Representative species of this class include Climatius and Bothriolepis, which provide valuable fossil records that contribute to the understanding of early fish evolution.

Class 2: Chondrichthyes or Elasmobranchi

Chondrichthyes, commonly known as Elasmobranchii, encompasses a diverse group of fishes primarily characterized by their cartilaginous skeletons and unique adaptations for life in marine environments. This class includes familiar species such as sharks, rays, and skates, which exhibit several distinctive anatomical features and behaviors. The following points detail the key characteristics and classifications within this group.

• Marine Habitat: Chondrichthyes are exclusively marine, occupying various oceanic environments from shallow coastal waters to the deep sea. Their adaptation to marine life includes specialized feeding and locomotion strategies.
• Exoskeleton Composition: These fishes possess an exoskeleton made of placoid scales, which are small, tooth-like structures that provide protection and reduce drag as they swim. The placoid scales are an important adaptation for efficient movement in water.
• Cartilaginous Endoskeleton: Unlike bony fishes, Chondrichthyes have a skeleton composed of cartilage, which is lighter and more flexible than bone. This cartilaginous structure contributes to buoyancy and allows for a more agile swimming capability.
• Jaw Suspension Mechanisms: The jaw structure in Chondrichthyes is characterized by either amphistylic or hyostylic jaw suspension. These configurations facilitate a wide range of feeding behaviors, allowing these fishes to capture prey effectively.
• Gill Structures: Chondrichthyes typically have 5 to 7 pairs of gills located on the sides of their bodies. These gills are vital for respiration, extracting oxygen from the water as it flows over them.
• External Gill Openings: The external gill openings in these fishes are separate and not covered by an operculum, a feature that distinguishes them from many bony fishes. This anatomical arrangement allows for more efficient water flow over the gills.
• Caudal Fin Structure: The heterocercal caudal fin, where the upper lobe is larger than the lower, provides enhanced thrust and maneuverability, aiding in swimming and stability.
• Reproductive Adaptations: Male Chondrichthyes exhibit specialized structures called claspers, which are used for copulation. These claspers facilitate internal fertilization, a significant reproductive adaptation.
• Absence of Air Bladder: Chondrichthyes lack an air bladder, a common feature in bony fishes that helps with buoyancy control. Instead, these fishes rely on their large livers, which are rich in oil, to maintain buoyancy.

Class 2: Chondrichthyes is further divided into two primary subclasses:

• Sub-class Selachi: This subclass includes sharks and their relatives. The pectoral fins in Selachi are supported by cartilaginous rods and are well developed, contributing to their agility in the water. This subclass is divided into two main orders:
  • Order Proto Selachi: This order features jaws equipped with numerous pointed teeth and paired nasal openings, showcasing primitive characteristics. Few living species represent this order today.
  • Order Euselachi: Members of this order possess well-developed placoid scales and exhibit separate openings for their five pairs of gill slits. This order contains many of the modern sharks.
• Sub-class Bradyodonti: This subclass consists of fossil and modern chimeras, which exhibit distinct morphological features.
  • Mouth Structure: Bradyodonti possess a small mouth bounded by lips, with a holostylic jaw suspension, which provides additional stability for feeding.
  • Gill Covering: In this subclass, gill openings are enclosed in a boneless operculum, which provides protection and aids in respiration.
  • Male Claspers: Males of this subclass also have a frontal clasper on the head for reproductive purposes.
  • Orders within Bradyodonti:
    • Order Eubradyodonti: This order includes the species Helodus, known for its unique jaw structure.
    • Order Holocephali: Often referred to as chimeras or “devil fishes,” this order is characterized by its distinct morphology and adaptations.

Class 3: Osteichthyes (Telostomi)

Osteichthyes, also referred to as Telostomi, represents a significant group of vertebrates known for their bony skeletons and diverse adaptations to various aquatic environments. This class includes both freshwater and marine species, showcasing a wide range of morphological and physiological traits that enable them to thrive in different habitats. The following points elucidate the key features and subdivisions of Osteichthyes.

• Habitat Diversity: Osteichthyes are found in marine, freshwater, and brackish water environments. This adaptability to multiple habitats is a crucial factor in their evolutionary success and ecological diversity.
• Exoskeleton Composition: The exoskeleton of these fishes is formed from cycloid, ctenoid, or ganoid scales, which serve both protective and hydrodynamic functions. These scale types vary in structure and contribute to the efficiency of swimming.
• Bony Endoskeleton: Osteichthyes possess a fully bony endoskeleton, which provides structural support and facilitates various adaptations for locomotion and buoyancy. This bony framework distinguishes them from the cartilaginous fishes in class Chondrichthyes.
• Jaw Suspension Mechanism: The jaw suspension in Osteichthyes is autostylic, allowing for greater jaw mobility and more complex feeding behaviors. This adaptation enables them to exploit a wide range of food sources.
• Presence of Operculum: The operculum, a bony flap covering the gills, is present in Osteichthyes. This feature aids in respiration by allowing the fish to create a water flow over the gills, enhancing gas exchange.
• Absence of Claspers: Unlike Chondrichthyes, Osteichthyes do not possess claspers, reflecting differences in reproductive strategies. Most species exhibit external fertilization.
• Air Bladder: Typically, Osteichthyes possess an air bladder, an organ that aids in buoyancy control. This adaptation allows them to maintain their position in the water column with minimal energy expenditure.

Class 3: Osteichthyes is further divided into two primary subclasses:

• Sub-class I: Crossopterygii: This subclass includes bony fishes characterized by lobed, fleshy fins. Crossopterygii encompasses two significant orders:
  • Order Rhipidistia: This order consists of extinct fishes, although a living representative, Latimeria, was discovered in 1938. This coelacanth is considered a “living fossil,” providing insight into ancient vertebrate forms.
  • Order Dipnoi: This order includes modern lungfishes, with only three genera remaining today:
    • Neoceratodus (Australian lungfish)
    • Protopterus (African lungfish)
    • Lepidosiren (South American lungfish). These fishes demonstrate discontinuous distribution and possess adaptations for both aquatic and aerial respiration.
• Sub-class II: Actinopterygii: Comprising a vast number of species, Actinopterygii includes bony fishes that inhabit both freshwater and marine environments. This subclass is divided into three superorders:
  • Superorder I: Chondrostei: This superorder includes three orders, but only the Polypteriformes order remains extant. Members of this order exhibit ganoid scales, symmetrical caudal fins, and dorsal fins with multiple finlets. An example is Polypterus.
  • Superorder II: Holostei: Comprising two orders, this superorder features diverse body forms and adaptations:
    • Order Amiiformes: Characterized by a heterocercal caudal fin, with Amia (bowfin) serving as a representative.
    • Order Lepidoteiformes: Notable for their elongated snouts and also possessing a heterocercal caudal fin. An example is Lepidosteus (gar pike).
  • Superorder III: Teleostei: This is the most important superorder, containing nearly 25,000 species divided into multiple orders, such as:
    • Order Clupeiformes: Includes fishes like Hilsa (herrings) and salmon, characterized by a homocercal caudal fin.
    • Order Cypriniformes: Features Labeo (carps) with Weberian ossicles connecting the ear to the air bladder, facilitating enhanced hearing.
    • Order Anguilliformes: Contains eel-like fishes such as Anguilla, which have an air bladder connected to the intestine.
    • Order Beloniformes: Comprising physoclistic fishes like Exocoetus (flying fish) that lack fin spines.
    • Order Syngnathiformes: Includes species like Hippocampus (seahorse) and Syngnathus (pipefish), characterized by their unique body shapes and reproductive adaptations.
    • Order Pleuronectiformes: Contains flatfishes like Cynoglossus, where both eyes are positioned on one side, and the body exhibits asymmetry.
    • Order Gadiformes: Includes fishes such as Gadus (cod), characterized by their fin structure and lack of spine.

Classification based on feeding habit

The classification of fish based on feeding habits provides valuable insights into their ecological roles and dietary preferences. Understanding these categories not only aids in the study of fish biology but also helps in the conservation and management of aquatic ecosystems. Below is a detailed overview of the various feeding classifications among fishes.

• Herbivores: These fish primarily consume plant materials. They play a crucial role in controlling algae growth and maintaining the health of aquatic ecosystems. Herbivorous fish often have specialized digestive systems to break down fibrous plant matter effectively.
• Carnivores: This category includes fish that feed exclusively on animal matter. They are typically predatory and can be further classified based on their hunting strategies:
  • Predators: These fish actively hunt macroscopic animals. Their feeding behavior can vary, as they may either pursue their prey or remain stationary, waiting for prey to come within reach.
    • Visual Hunters: Diurnal predators, such as many species of fish, rely heavily on their acute vision to detect and capture prey.
    • Nocturnal Feeders: Fish like eels have heightened senses that include olfactory capabilities and lateral line systems, which allow them to navigate and hunt in low-light conditions.
    • Dental Adaptations: Predators typically possess caniniform teeth, designed for grasping and holding onto slippery prey.
• Omnivores: These fish obtain nutrients from both plant and animal sources, demonstrating flexibility in their diets. Their adaptability allows them to thrive in diverse environments.
• Planktivores: This group feeds primarily on plankton, which consists of microscopic plant and animal life found in aquatic environments, including bacteria. Planktivorous fish possess specialized adaptations to capture tiny organisms efficiently.
• Detritivores: Fish in this category feed on decaying organic matter. They play an essential role in nutrient cycling within their ecosystems.

In addition to dietary classifications, fish can also be categorized based on their specific feeding behaviors:

1. Grazers: Grazing fish feed on bottom-dwelling organisms or selectively consume plankton. They often graze on algae or coral polyps, using bites to take in food while continuously browsing.
   • Dental Adaptations: Grazers typically possess incisor-shaped teeth.
   • Specialized Teeth: For example, parrotfish have teeth that are fused into a beak, along with pharyngeal teeth that aid in grinding larger food items into smaller pieces for easier digestion.
2. Strainers: These fish filter small organisms, such as diatoms and crustaceans, from the water. They swim through areas rich in plankton, using specialized structures to extract food from the water.
   • Anatomical Features: Strainers possess numerous fine and elongated gill rakers that help them filter food particles efficiently.
3. Suckers: Suckers are adapted to ingest mud or substrate materials that contain food. They often separate food items from sediment before swallowing.
   • Mouth Adaptations: They have plicate and papillose lips to enhance their suction capabilities.
   • Dental Features: Many suckers possess pharyngeal teeth, while some species may be edentulous (toothless).
4. Parasites: Fish such as lampreys and hagfishes exhibit unique feeding habits by obtaining nutrients through the consumption of body fluids from host fish.
   • Feeding Mechanism: These parasites utilize specialized plicate and papillose lips to maximize suction efficiency.
   • Adaptations: Parasitic lampreys have salivary glands that produce anticoagulants, which prevent blood clotting and facilitate feeding, similar to the function found in leeches.

Classification of fish based on habitat

The classification of fish based on their habitat is essential for understanding the ecological roles and adaptations of various species. Fish are primarily categorized into three major habitats: freshwater, brackish water, and marine environments. Each of these habitats presents distinct conditions that shape the physiological and behavioral characteristics of the fish residing within them.

1. Freshwater Fish:
   • These species are predominantly found in rivers, lakes, and streams where the salinity is less than 0.5 parts per thousand (ppt).
   • Approximately 40% of all known fish species inhabit freshwater ecosystems, showcasing a remarkable diversity.
   • Freshwater fish can be further divided into:
     • Coldwater Fish: Thrive in cooler temperatures, typically ranging from 5 to 20 degrees Celsius.
       • Examples: Mahseer and trout are commonly associated with coldwater environments.
     • Warmwater Fish: Prefer warmer temperatures, generally between 25 and 35 degrees Celsius.
       • Examples: Carp, catfish, snakeheads, and featherbacks are representative of warmwater fish.
2. Brackish Water Fish:
   • These species are capable of tolerating varying levels of salinity, ranging from 0.5 to 30.0 ppt.
   • They inhabit environments such as estuaries, coastal waters, and backwaters, where freshwater mixes with saltwater.
   • Brackish water habitats are critical transition zones that support unique biological communities.
   • Examples: Mullet, milkfish, seabass, pearlspot, and mudskippers exemplify the diversity of brackish water fish.
3. Marine Fish:
   • Marine fish reside in seawater environments, characterized by salinities greater than 30 ppt.
   • This category encompasses a vast array of species, with about 240 known species contributing to marine fisheries.
   • Marine fish are adapted to a wide range of habitats, from shallow coastal waters to the deep sea.
   • Examples: Sardines, mackerel, ribbonfish, anchovies, grouper, cobia, and various species of tuna illustrate the diversity and ecological significance of marine fish.

Classification of fish based on manner of reproduction

The classification of fish based on their manner of reproduction is vital for understanding their life cycles, breeding behaviors, and ecological strategies. Fish can be categorized into several groups depending on their reproductive strategies, which include oviparity, viviparity, and ovoviviparity.

1. Oviparous Fish: These species lay eggs that develop and hatch outside the mother’s body.
   • Characteristics:
     • Eggs are usually fertilized externally, though some species exhibit internal fertilization.
     • Oviparous fish often produce a large number of eggs to increase the chances of survival.
     • Many species exhibit parental care, such as guarding the eggs or caring for the fry after hatching.
   • Examples: Most bony fish, including salmon, trout, and goldfish, are oviparous.
2. Viviparous Fish: These fish give birth to live young instead of laying eggs.
   • Characteristics:
     • Fertilization occurs internally, and the developing embryos are nourished directly by the mother.
     • Viviparous fish usually have fewer offspring compared to oviparous species but provide a higher level of care and protection.
     • Some species possess a specialized structure, like a placenta, to facilitate nutrient transfer to the developing young.
   • Examples: Guppies, sharks, and certain species of rays are known to be viviparous.
3. Ovoviviparous Fish: These species exhibit a reproductive strategy that combines elements of oviparity and viviparity.
   • Characteristics:
     • Eggs are fertilized internally and develop within the mother, but the embryos obtain nourishment from the egg yolk rather than from the mother.
     • The young are born live after developing in the protective environment of the mother’s body, which provides some protection against external threats.
     • This strategy often results in a moderate number of offspring.
   • Examples: Some species of sharks and the Atlantic mackerel are ovoviviparous.