General Characteristics and Classification of Chondrichthyes (Cartilaginous Fishes)
Chondrichthyes, or cartilaginous fishes, include sharks, rays, skates, and chimaeras. Unlike bony fishes, their skeleton is entirely made of cartilage, making them lighter and more flexible.
General Characteristics:
Habitat:
- Primarily marine, with a few species found in freshwater.
Body Structure:
- Typically fusiform (spindle-shaped) for efficient swimming.
- Skin covered with placoid scales (dermal denticles) that reduce friction.
Skeleton:
- Composed entirely of cartilage, often strengthened by calcification.
Jaws and Teeth:
- Powerful jaws with multiple rows of teeth that are continuously replaced.
Respiration:
- Possess 5 to 7 pairs of gill slits on the sides of their heads.
- Lack an operculum (gill cover), unlike bony fishes.
Buoyancy:
- Absence of a swim bladder; buoyancy is maintained through a large, oil-rich liver.
- Their pectoral fins help generate dynamic lift.
Reproduction:
- Internal fertilization via specialized structures (claspers) in males.
- Three reproductive modes:
- Oviparous: Lays eggs.
- Ovoviviparous: Eggs hatch inside the mother’s body.
- Viviparous: Gives birth to live young.Classification up to Order:
Chondrichthyes is divided into two main subclasses:
1. Subclass Elasmobranchii (Sharks, Rays, Skates, Sawfishes)
Key Characteristics:
- Multiple gill slits (usually five) on each side of the head.
- Lack a bony operculum.
Orders:
- Order Carcharhiniformes (Ground Sharks): Includes tiger sharks, bull sharks, and reef sharks.
- Order Lamniformes (Mackerel Sharks): Includes great white sharks and basking sharks.
- Order Rajiformes (Skates): Skates have a flattened body, fleshy tail, and lack stinging spines.
- Order Myliobatiformes (Rays and Manta Rays): Characterized by their wide, flat bodies and large pectoral fins.
2. Subclass Holocephali (Chimaeras or Ghost Sharks)
Key Characteristics:
- Single gill opening covered by a soft operculum.
- Cartilaginous skeleton.
Order:
- Order Chimaeriformes: Includes species like the rabbitfish and elephant fish.
