Characteristics of fishes

Origin and Evolution: Fishes first evolved during the Silurian period of the Palaeozoic era. They experienced a significant increase in diversity and population during the Devonian period.
Golden Age of Fishes: The Devonian Period is specifically referred to as the "Golden Age of Fishes" due to their rapid diversification.
Classification and Diversity:
* Fishes represent the first group of jawed vertebrates.
* They constitute approximately $40\%$ of all known vertebrate species.
Metabolism: They are cold-blooded vertebrates, also described as poikilothermic or ectotherms.
Size Variation: There is immense variation in size among species:
* Smallest Fish: Paedocypris progenetica, which measures approximately $7.9\,mm$ .
* Largest Fish: Rhinodon typus (commonly known as the Whale shark).
Ichthyology: This is the scientific study of fishes.

Body Divisions: The fish body is divided into three distinct regions: the head, the trunk, and the tail. A neck is absent in all fishes.
Body Shape: They possess a streamlined, spindle-shaped body, which is a structural adaptation that reduces resistance and facilitates efficient swimming.
Integumentary System:
    * Mucous Glands: The skin contains multicellular mucous glands that secrete a lubricant to reduce friction in water.
    * Scales (Exoskeleton): The body is covered by an exoskeleton consisting of dermal scales or bony plates. Common types include:
        * Placoid scales
        * Cycloid scales
        * Ctenoid scales
        * Ganoid scales
Specific External Features:
    * Maxillary and Mandible: Components of the jaw structures.
    * Operculum: A bony flap that covers and protects the gill slits in bony fishes.
    * Lateral Line: A visible line along the side of the body associated with sensory perception.

Types of Fins: Fishes utilize both paired and unpaired fins for balance and movement.
* Paired Fins: These include the pectoral and pelvic fins, which are supported by the pectoral and pelvic girdles.
* Unpaired Fins: These include the dorsal, anal (ventral), and caudal fins. The caudal fin is primarily used for propulsion, while others maintain balance.
Function: Fins are essential for locomotion and maintaining the animal's orientation in the water column.

Endoskeleton: The internal skeleton may be composed of either cartilage (in cartilaginous fishes) or bone (in bony fishes).
Vertebrae: The vertebrae are described as amphicoelous, meaning both the anterior and posterior sides of the centrum are concave.
Ribs: Ribs are present in fishes to provide structural support.
Girdles: The internal skeleton includes pectoral and pelvic girdles that provide the necessary support for the paired fins.

Brain and Spinal Cord: The nervous system is comprised of a central brain and a spinal cord. The brain is relatively small and does not occupy the entire cranial cavity.
Cranial Nerves: Fishes possess $10$ pairs of cranial nerves.
Lateral Line Sense Organs: These specialized organs are located on the lateral sides of the body. They are designed to detect pressure changes and vibrations in the water, serving as a "distant touch" mechanism.
Olfaction (Nostrils):
    * Typically, a pair of external nostrils is present on the head.
    * These nostrils usually open into blind nasal sacs.
    * Internal Nostrils: Generally absent in most fishes, but they are present in specific groups such as Osteolepids and Dipnoi (lungfishes).
Vision: A pair of eyes is present on the head for visual perception.

Respiratory Organs: Respiration is primarily conducted through gills, which are extensions of the pharynx.
* In Elasmobranches (Cartilaginous Fishes): The gill slits are separate and open directly to the outside.
* In Bony Fishes: The gill slits are covered by a protective operculum.
Accessory Respiration: Certain freshwater fishes possess accessory respiratory organs that allow them to perform aerial respiration (breathing atmospheric air).
Coelom Structure: The body cavity is divided into two parts separated by the Septum transversum:
* Pericardial Cavity: A small cavity housing the heart.
* Pleuroperitoneal Cavity: A larger cavity containing the viscera (internal organs), excluding the kidneys.

Dentition: Fish teeth are characterized as:
    * Homodont: All teeth are of the same shape and size.
    * Acrodont: Teeth are attached to the summit of the jawbone.
    * Polyphyodont: Teeth are continuously replaced throughout the animal's life.
Intestinal Adaptations: The digestive system is well-developed. In many species, such as sharks, a spiral valve (also known as a scroll valve) is present in the intestine to increase the surface area for nutrient absorption.
Excretory System:
* Kidneys: Fishes possess mesonephric kidneys.
* Urinary Bladder: A urinary bladder is absent in fishes.

Sexual Dimorphism: Sexes are typically separate (dioecious).
Reproductive Structures: In male sharks, specialized structures called claspers are present, which are used for internal fertilization.
Reproductive Strategies:
* Oviparous: Many fishes lay eggs.
* Viviparous: Some fishes give birth to live young.
Developmental Classification: Fishes are classified as anamniotes because they do not develop an amnion (an embryonic membrane) during their developmental stages.