Pisces

Pisces, belonging to the phylum Chordata, are vertebrates equipped with gills, fins, and streamlined bodies. As some of the earliest vertebrates, they exhibit extensive diversity in both form and behaviour. Their adaptations enable them to survive across freshwater and marine ecosystems. Classification of Pisces hinges on structural and functional features. Primitive jawless fishes contrast sharply with advanced bony fishes in morphology and physiology. 

1.0Pisces Examples and Their Role in Aquatic Ecosystems

• Fish, or Pisces, are incredible underwater creatures. 
• They draw breath through gills, sail the waters with fins, and call rivers, oceans, and brackish estuaries home.
• Even without a “true” spine, their sleek bodies and clever adaptations make them perfect for aquatic life.
• Whether paired or unpaired, fins give fish the power to move, explore, and master their aquatic realms. 
• Ichthyology studies fish and shows modern species descended from primitive sea squirts.

2.0Pisces Description

• Fish, or Pisces, have jaws and special systems for breathing, excretion, and circulation. 
• They can’t control their body heat, so they depend on the water around them for energy. 
• Basically, this group includes every “true fish,” from tiny goldfish in bowls to massive sharks in the oceans.
• Fish need to breathe and move to survive. Gills pull oxygen from the water, and fins, sometimes paired, sometimes solo, keep them steady and moving. 
• From catfish to sharks, and even some big creatures people call fish, each Pisces is a little miracle of evolution.

3.0General Characteristics of Pisces

Let’s explore what makes Pisces so distinctive:

• Aquatic Vertebrates: Found in freshwater or marine habitats, Pisces can be herbivorous, carnivorous, or omnivorous. They are cold-blooded and may lay eggs externally (oviparous) or internally (ovoviviparous).
• Body Shape: Most have a streamlined, spindle-shaped body for easy movement through water. Some species are elongated like snakes, while others are flattened top to bottom (dorsoventrally compressed). The body is generally divided into head, trunk, and tail.
• Fins: Dermal fin-rays support pectoral, pelvic, dorsal, and caudal fins; tail musculature drives locomotion.
• Exoskeleton: Scales, denticles, ossified plates. Protects fish and reduces drag, adapted for water.
• Endoskeleton: The Internal skeleton may be cartilaginous or ossified; vertebrae replace notochord; cranium contains visceral arches, the first forming jaws.
• Musculature: Muscles are segmented into myotomes, which are divided into dorsal and ventral sections.
• Digestive System: A complete alimentary canal with a distinct stomach and pancreas leads to the cloaca or anus.
• Respiration: Gills are the organs of breathing.
• Circulation: A two-chambered heart with one atrium and one ventricle pumps venous blood.

• Excretion: Mesonephros kidneys help regulate water and salts.
• Nervous System: The brain has five regions, and there are ten pairs of cranial nerves.
• Sensory Organs: No ear cavity or ossicles; three inner semicircular canals for balance. The lateral line senses water vibrations.
• Reproduction: Pisces reproduce with separate sexes, each possessing paired gonads. Fertilisation may be internal or external, depending on the species.

4.0Types of Scales

Fish scales protect the body and help it move through water. They vary in shape, size, and texture.

• Placoid Scales: Sharks and rays have these tiny, tooth-like scales.
• Ganoid Scales: Sturgeon and similar fish sport diamond-shaped, interlocking scales.
• Cycloid Scales: Carp, cod, and bowfin have smooth, round, see-through scales.
• Ctenoid Scales: Ctenoid scales are the armour with springy teeth for sunfish and flounder.
• Cosmoid Scales: Cosmoid scales, extinct relics, were the heavy shields of prehistoric aquatic warriors.

5.0Classification of Fishes

Pisces are classified into three main classes, each with unique features:

Placodermi (Armoured Fishes – Extinct)

• Covered in bony plates or shields
• Had a simple bony skeleton and jaws
• Tail fin: upper lobe bigger than lower
• Examples: Bothriolepis and Climatius
• Lived a long time ago until the Permian epoch

Elasmobranchii / Chondrichthyes (Cartilaginous Fishes)

• Found exclusively in marine environments.
• Cartilaginous skeleton and placoid scales.
• Amphistylic or hyostylic jaw suspension.
• Five to seven pairs of gills, exposed externally without an operculum.
• Heterocercal caudal fin and claspers in males for reproduction.
• No swim bladder.
• Examples include Scoliodon (dogfish shark) and Pristis (sawfish).

Osteichthyes (Bony Fishes)

• Found in freshwater, marine, and brackish waters.
• Bony endoskeleton with cycloid, ctenoid, or ganoid scales.
• Self-adjusting jaw suspension and a swim bladder for buoyancy.
• Absence of claspers; fertilisation may be external.
• Examples include Salmon and Neoceratodus (lungfish).