Pisces

Division - Gnathostomata (bears jaw)

  1. Super-class - Pisces (bear fins)

(i) Class - Chondrichthyes

  • Class Chondrichthyes, also known as cartilaginous fishes, is a group of jawed fishes that have a skeleton made primarily of cartilage instead of bone.
  • They are divided into two subclasses: Elasmobranchii (sharks, rays, and skates) and Holocephali (chimaeras).

 

General Characteristics:

  • Chondrichthyes are marine fishes found in both saltwater and freshwater habitats.
  • Their skeleton is made of cartilage, which provides flexibility and lighter body weight.
  • They have paired fins and well-developed jaws.
  • They are cold blooded i.e. (poikilothermous)
  • Their gill slits are separate and without operculum (gill cover).
  • Air bladder is absent.
  • Most species have placoid scales (tiny tooth-like structures) covering their body.

 

Respiration:

  • Chondrichthyes respire through gills, and most species require constant swimming to ensure oxygen flow over their gills.
  • Spiracles, located behind the eyes in some species, allow water to enter the gills even when the mouth is closed.

 

Feeding Habits:

  • Most chondrichthyans are carnivorous and have well-developed teeth.
  • Sharks have several rows of teeth, which are continuously replaced throughout their lives.
  • They have a variety of feeding strategies, including filter feeding, scavenging, and predation.

 

Circulatory System:

  • Chondrichthyans have a closed circulatory system with a two-chambered heart.
  • The heart pumps deoxygenated blood to the gills for oxygenation and then distributes oxygenated blood to the rest of the body.

 

Excretion:

  • They excrete nitrogenous wastes primarily in the form of urea, which helps them maintain osmotic balance in seawater.
  • Chondrichthyes possess specialised structures called rectal glands to excrete excess salt.

 

Reproduction:

  • Most chondrichthyans have separate sexes and exhibit internal fertilisation.
  • Males have modified pelvic fins called claspers, which are used to transfer sperm to the female during mating.
  • Reproduction can be oviparous (laying eggs), ovoviviparous (eggs hatch internally, and young are born live), or viviparous (young develop inside the female and are nourished by a placenta-like structure).

 

Economic Importance:

  • Chondrichthyans have significant economic value as a food source in many cultures.
  • Shark fins are used in the production of shark fin soup, while their liver oil is used in various industries.
  • Some species of rays and skates are harvested for their meat and skin.

 

Examples:

Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), some have electric organs (e.g., Torpedo), some possess poison sting (e.g., Trygon)

 

 

 

(ii) Class - Osteichthyes

  • Class Osteichthyes, also known as bony fishes, is a diverse group of fishes that have a skeleton made primarily of bone.
  • They are the largest class of vertebrates and include both freshwater and marine species.

 

General Characteristics:

  • Osteichthyes have a bony endoskeleton, which provides structural support and protection.
  • They have paired fins and well-developed jaws with teeth.
  • Most species have scales covering their body, which may be cycloid, ctenoid, or ganoid scales.
  • Air bladder is present which regulates buoyancy.

 

Respiration:

  • Osteichthyes respire through gills, which are protected by an operculum (a bony flap).
  • Water is drawn over the gills, and oxygen is extracted for respiration.

 

Feeding Habits:

  • Bony fishes exhibit a wide range of feeding habits, including herbivory, carnivory, omnivory, and filter feeding.
  • They have a variety of mouth shapes and teeth adaptations to suit their feeding preferences.

 

Circulatory System:

  • Osteichthyes have a closed circulatory system with a two-chambered heart, consisting of one atrium and one ventricle.
  • Blood is pumped from the heart to the gills for oxygenation and then distributed to the rest of the body.

 

Excretion:

  • They excrete nitrogenous wastes primarily in the form of ammonia, which is released across the gills.
  • Some bony fishes also have a specialised structure called the kidney to regulate water and ion balance.

 

Reproduction:

  • Osteichthyes can exhibit various reproductive strategies, including external fertilisation and internal fertilisation.
  • Some species are oviparous (laying eggs), while others are ovoviviparous (eggs hatch internally, and young are born live) or viviparous (young develop inside the female and are nourished by a placenta-like structure).

 

Economic Importance:

  • Bony fishes are of significant economic importance as a food source worldwide.
  • They are also popular in the aquarium trade due to their diverse colours, shapes, and behaviours.
  • Some species of bony fishes, such as carp and tilapia, are cultivated in aquaculture for commercial purposes.

 

Examples:

  • Marine – Exocoetus (Flying fish), Hippocampus (Sea horse).
  • Freshwater – Labeo (Rohu), Catla (Katla), Clarias (Magur).
  • Aquarium – Betta (Fighting fish), Pterophyllum (Angel fish).