Scientist capturing a shark.
(© Ken Hoppen, oceannotions@primus.com.au)

Introduction
The scientific name for the group of animals that includes the sharks and rays is called the Chondrichthyes. This group form only a small proportion of all living fishes. Sharks and rays share many features with other fish - known as bony fish - but they also have several features that separate them from bony fish.

Skeletal Structure
One key feature of the sharks and rays that separate them from the bony fishes and other vertebrates (amphibians, reptiles, birds and mammals) is their skeleton. Most vertebrates have a skeleton made of hard, dense bone. Sharks and rays have a skeleton made of cartilage, which is softer and more flexible.

• Skeletal Structure
• Teeth
• Fins and Movement
• Gills and Spiracles
• Skin
• Sensory Systems
• Reproduction

Teeth
The teeth of sharks and rays also vary in size and shape. Both the upper and lower jaws of sharks and rays have teeth that are embedded in the gums, rather than attached to the jaw. This means they can be continually replaced.

ome sharks have small sharp teeth for grasping prey, and more flattened back teeth for crushing hard shells. Other sharks may have large sharp teeth, more suited to cutting their prey. The whale shark, basking shark and manta ray do not have teeth at all. Instead, they have evolved gill rakers, and use these to strain out plankton from the water as the shark breathes.

Gills and Spiracles
In common with the bony fishes, sharks and rays have gills to extract oxygen from the water in which they live. Most shark species have five pairs of gill openings, but there are several that have six or seven pairs of gill openings. Rays have either five or six pairs, located on the underside of the body.

The rays and many of the bottom-dwelling sharks have spiracles behind the eyes. Spiracles are holes that allow the rays and sharks to take in water for breathing while they rest on the sea floor. Fast-swimming sharks lack spiracles and depend on water entering the mouth and passing through the gill openings to breathe, so they must swim continuously to breathe.

Fins and Movement
Generally, sharks and rays have three types of unpaired fins (dorsal, anal and caudal) and two types of paired fins (pectoral and pelvic). However, some groups do not have all of these features, or the features have been modified (over millions of years) to form long tails or flattened body discs. Body shape and locomotion vary between different animals in the group.