Division - Gnathostomata (bears jaw)
- 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).
