Class Pisces

Pisces is a class of aquatic vertebrates that includes all fish.

• The term "Pisces" is derived from the Latin word meaning "fish."
• This group represents the most diverse class of vertebrates, with more than 32,000 species distributed across freshwater and marine habitats.
• Fishes play an important ecological role, serving as a food source for many animals and helping maintain water quality by feeding on algae and aquatic plants.
  

Characteristics of Class Pisces

• Aquatic Vertebrates: Pisces are vertebrates adapted for life in water.
• Respiration through Gills: They respire by extracting oxygen from water through specialised organs called gills.
• Fins for Locomotion: Pisces have fins, which they use for movement and steering in water.
• Covered in Scales: Their bodies are typically covered in scales, which provide protection and reduce friction in water.
• Varies in Size and Habitat: Fish come in several sizes and inhabit various aquatic environments, including freshwater, saltwater, and brackish water habitats.
• Classification: Pisces are broadly classified into two main groups: bony fish (Osteichthyes) and cartilaginous fish (Chondrichthyes).
• Ecological Roles: Pisces play essential roles in aquatic ecosystems as predators, prey, and contributors to nutrient cycling and habitat structure.
• Economic Importance: Fish are economically significant as a major food source for humans and serve as the basis for many commercial fisheries worldwide.
• Threats and Conservation: Fish face threats from habitat destruction, overfishing, pollution, and climate change. Conservation efforts aim to protect fish populations and their habitats to ensure their survival and maintain healthy aquatic ecosystems.

Scientific Classification of Pisces

The systematic classification of Pisces is given below:

Classification of Pisces

Pisces are categorised into the following three types:

1. Placodermi (Aphstohyoids)

Placodermi is an extinct class of fish that lived during the Silurian and Devonian periods, from around 430 to 360 million years ago. Here's a brief overview of Placodermi:

• Placoderms were characterised by their heavily shielded bodies, with thick bony plates covering their heads and thoraxes.
• Unlike earlier jawless fish, Placoderms had true jaws, making them some of the earliest jawed vertebrates.
• Placoderms typically had paired pectoral and pelvic fins, as well as dorsal and anal fins.
• Placoderms came in several sizes, from small species measuring a few centimetres to large predators reaching several meters in length.
• Placoderms inhabited various aquatic environments, including marine, freshwater, and estuarine habitats.
• Despite their success during the Devonian period, Placoderms eventually went extinct by the end of the Devonian around 360 million years ago.

2. Chondrichthyes

Chondrichthyes is a class of fish characterised by their skeletons made of cartilage rather than bone. This class includes some of the most well-known aquatic predators, such as sharks, rays, and chimaeras. Here's an overview of Chondrichthyes:

• Chondrichthyes have skeletons made primarily of cartilage, a flexible and lightweight connective tissue.
• Chondrichthyes have powerful jaws and several rows of replaceable teeth.
• The skin of Chondrichthyes is covered in dermal denticles, small tooth-like scales that provide protection and reduce drag as they move through the water.
• Chondrichthyes breathe through gills, which are located on either side of their bodies.
• Many Chondrichthyes have a mode of reproduction known as ovoviviparity, where fertilised eggs develop within the body of the female and hatch internally before being born as live young. However, some species lay eggs (oviparity), while others give birth to fully developed live young (viviparity).
• Chondrichthyes play important roles in marine ecosystems as top predators, helping to regulate populations of prey species and maintain ecosystem balance.
• Many species of Chondrichthyes are facing threats from overfishing, habitat degradation, and climate change.

3. Osteichthyes

Osteichthyes is a class of fish characterised by their skeletons made primarily of bone. Here's an overview of Osteichthyes:

• Osteichthyes have skeletons made primarily of bone, providing them with a more rigid and supportive structure compared to cartilaginous fish like sharks and rays.
• Most Osteichthyes have well-developed jaws equipped with teeth.
• Osteichthyes typically have paired pectoral and pelvic fins, as well as dorsal, anal, and caudal (tail) fins.
• Osteichthyes breathe through gills, which are located behind their heads.
• The skin of Osteichthyes is covered in scales, which vary in size, shape, and structure depending on the species.
• Osteichthyes have various reproductive strategies. Many species lay eggs, which are fertilised externally in the water. Some species show parental care, where one or both parents protect the eggs and/or young after they are laid.
• They can be found in both freshwater and saltwater habitats, as well as in various depths and temperatures.
• Osteichthyes represent the largest class of vertebrates, with over 30,000 species identified to date. This class includes familiar groups such as salmon, trout, bass, tuna, and angelfish, as well as many lesser-known species.

Types of Scales

Fish belonging to the class Pisces, have various types of scales. The scales provide protection, reduce friction in water, and play a role in osmoregulation.

Here are the main types of scales found in fish:

• Cosmoid Scales: Found in ancient fish groups like coelacanths, these scales consist of a bony base overlaid with a layer of dentine, enamel, and cosmine. They are thick and heavy, offering strong protection.
• Ganoid Scales: Typically found in primitive bony fish such as gars and bowfin, ganoid scales are thick, rhomboidal-shaped, and coated with hard enamel-like material. They often overlap, providing a tough armour-like covering.
• Cycloid Scales: These scales are thin, round or oval-shaped, and have a smooth surface. They are found in most bony fish, including salmon, carp, and trout. Cycloid scales are flexible and overlap like roof shingles.
• Ctenoid Scales: Similar to cycloid scales but with comb-like projections along their edges, ctenoid scales are found in many bony fish, including perch, bass, and wrasses. These projections provide additional protection and help in reducing drag.

Here are examples of fish from the class Pisces:

• Sharks: Great white shark (Carcharodon carcharias)
• Rays: Manta ray (Manta birostris)
• Skates: Big skate (Beringraja binoculata)
• Chimaeras: Elephant fish (Callorhinchus milii)
• Lungfish: African lungfish (Protopterus spp.)
• Tuna: Yellowfin tuna (Thunnus albacares)
• Angelfish: Emperor angelfish (Pomacanthus imperator)