Class Pisces

Class Pisces includes all vertebrate animals that are adapted to an aquatic life and have a skeleton made of cartilage or bone, gills for respiration, and paired fins. They are the first vertebrates to evolve jaws (except jawless fishes, which are primitive members of Agnatha). Pisces is a class of aquatic, cold-blooded vertebrates that live in water and breathe through gills. This class includes all types of true fishes, both bony and cartilaginous.

• Class Pisces meaning refers to a group of aquatic, cold-blooded vertebrates that belong to the Subphylum Vertebrata under Phylum Chordata. 
• Fishes represent one of the earliest and most diverse vertebrate classes, adapted for life in water through features like gills, fins, and streamlined bodies.

1.0Characteristics of Class Pisces

Subphylum Vertebrata characteristics are well represented in fishes, with adaptations specific to aquatic life. The main subphylum vertebrata characteristics in Pisces include:

1. Body Structure – Fishes have a streamlined body that reduces friction and enables efficient swimming.
2. Skeleton – Can be cartilaginous (Chondrichthyes) or bony (Osteichthyes).
3. Respiration – Gills serve as the primary respiratory organs. Some species have adaptations to breathe air occasionally.
4. Fins – Paired and median fins aid in locomotion, balance, and direction control.
5. Skin – Covered with scales and mucus to reduce friction and prevent infection.
6. Heart and Circulation – Two-chambered heart with single circulation; blood passes once through the heart in one cycle.
7. Nervous System – Well-developed brain and sense organs for detecting movement, vibrations, and changes in water currents.
8. Reproduction – Mostly external fertilisation; development is often oviparous.

2.0Classification of Class Pisces

• The superclass Pisces is not a single, unified group but is traditionally divided into three distinct classes. 
• This classification reflects major evolutionary splits and different skeletal structures.

Class Placodermi (Armoured Fishes - Extinct)

• These were ancient, extinct fishes that were prominent during the Devonian Period, often called the "Age of Fishes." 
• They were characterised by heavy, bony plates that covered their heads and trunks. Placoderms were among the first jawed vertebrates, and their extinction marked a significant shift in aquatic ecosystems. 
• A well-known example is Dunkleosteus, a massive apex predator.

Class Chondrichthyes (Cartilaginous Fishes)

• This class includes the sharks, rays, skates, and chimaeras. They are defined by a skeleton made entirely of cartilage rather than bone. 
• This lighter, more flexible skeleton is an adaptation for a fast, predatory lifestyle.

Characteristics:

• Cartilaginous Skeleton: The lack of an actual bony skeleton is its most notable feature.
• Placoid Scales: Their skin is covered in dermal denticles or placoid scales, which are tooth-like and give their skin a rough, sandpaper-like texture.
• No Swim Bladder: They lack a swim bladder and maintain buoyancy with a large, oily liver and continuous swimming.
• Multiple Gill Slits: They typically have 5 to 7 gill slits on each side of the head, which are not covered by an operculum.
• Internal Fertilisation: Reproduction is through internal fertilisation, often involving claspers in males.
• Examples: Great White Shark, Manta Ray, and Sawfish.

Class Osteichthyes (Bony Fishes)

This is the largest and most diverse group of vertebrates, comprising over 96% of all fish species. Their bony skeleton distinguishes them.

• Characteristics:
• • Bony Skeleton: The presence of a hard, bony skeleton.
  • Scales: Their bodies are covered in various types of scales, such as cycloid, ctenoid, or ganoid scales.
  • Swim Bladder: Most bony fishes possess a swim bladder, a gas-filled sac that helps them control their buoyancy in the water column without expending much energy.
  • Operculum: The gills are covered by a bony plate called the operculum, which protects the gills and helps pump water over them for efficient respiration.
  • External Fertilisation: Reproduction is typically through external fertilisation, with females laying eggs that are then fertilised by males in the water.

Examples: Tuna, Salmon, Goldfish, and Clownfish. This class is further subdivided into two subclasses:

• Subclass Sarcopterygii (Lobe-finned fishes): These fishes have fleshy, lobed fins supported by bones, which are homologous to the limbs of land vertebrates. This group includes lungfishes and the coelacanths. They are considered the direct ancestors of amphibians and all other tetrapods.
• Subclass Actinopterygii (Ray-finned fishes): This is the vast majority of bony fishes. Bony spines or rays support their fins. Examples include almost all common fish species.

3.0Some Characteristic Features of Class Pisces

• Aquatic Adaptation – Streamlined body reduces water resistance.
• Respiratory System – Gills with extensive capillary networks for efficient oxygen exchange.
• Circulatory System – Closed system with single circulation; the heart pumps deoxygenated blood to the gills.
• Sensory Adaptation – The Lateral line system detects pressure changes and waterborne vibrations.
• Locomotory Adaptation – Fins and tail enable swift swimming and manoeuvring.
• Excretion – Kidneys regulate osmotic balance; marine fishes excrete salts, freshwater fishes retain salts.
• Reproduction – Mostly sexual, with fertilisation typically external; eggs are often laid in large numbers.

4.0Habitat and Distribution

Fishes inhabit a wide range of aquatic environments:

1. Freshwater Habitats – Rivers, lakes, ponds, and streams. Examples: Rohu, Catla, Clarias.
2. Marine Habitats – Oceans, seas, and coral reefs. Examples: Shark, Torpedo, Trygon.
3. Brackish Water – Estuaries and mangroves, where freshwater meets the sea.

Fishes play a crucial role in aquatic ecosystems as predators, prey, and contributors to the food chain.

5.0Adaptations and Organ Systems

The success of fish is due to a remarkable set of biological systems and adaptations.

Respiratory System

• Fishes use gills to breathe. Water enters the mouth and flows over the gill filaments. 
• Oxygen diffuses from the water into the blood, and carbon dioxide diffuses from the blood into the water. 
• In bony fishes, the operculum's movement creates a pressure gradient that draws water over the gills, a highly efficient process known as a buccal pump.

Circulatory System

• Fishes have a single-circuit circulatory system. The heart has two chambers: an atrium and a ventricle. 
• The heart pumps deoxygenated blood to the gills, where it becomes oxygenated. 
• This oxygenated blood then flows directly to the body tissues and organs before returning to the heart. 
• This is a less efficient system than the double-circuit system found in birds and mammals, but is ideally suited for aquatic life.

Nervous System and Sensory Organs

• Fishes have well-developed brains, spinal cords, and sensory organs. They possess excellent senses of sight, smell, and taste. 
• Their most unique sensory organ is the lateral line system. 
• This system consists of a row of small pores and canals running along the sides of the fish's body, which are connected to a series of sensory cells called neuromasts. 
• The lateral line detects water movement, pressure changes, and vibrations, allowing the fish to navigate, locate prey, and avoid predators in murky or dark waters.

Reproduction

• Reproduction in fishes is incredibly diverse. Most species are oviparous, meaning they lay eggs. 
• These eggs are often fertilised externally. Some species are ovoviviparous (eggs develop inside the mother but hatch internally, as in some sharks) or viviparous (live birth, with the young developing internally and receiving nourishment from the mother, as in some sharks and guppies). 
• Parental care can range from none to complex behaviours like nest building and guarding.

Body Shape and Locomotion

• Fish body shapes are a direct result of their lifestyle and habitat. 
• The streamlined, fusiform shape is common among open-water swimmers like tuna and marlins, enabling them to swim at high speeds. 
• Other shapes include:
• Compressed: Flattened from side to side (e.g., Angelfish), common in fishes that live among coral reefs.
• Depressed: Flattened from top to bottom (e.g., Rays and Skates), ideal for bottom-dwelling life.
• Eel-like: Long and snake-like (e.g., Eels), suited for navigating tight spaces and crevices.
• Locomotion is primarily achieved by the powerful caudal fin (tail fin) pushing against the water, while the paired fins are used for stability and manoeuvring.

Comparison Between Chondrichthyes and Osteichthyes