Mammals (Class Mammalia) are one of the six basic groups of animals. Mammals are a group of vertebrates that includes about 5,000 species that range in size from the minute Bumblebee Bat which measures a mere 4cm to the collasal Blue Whale which can reach lengths of 29m and weights of 176 tons. Mammals have successfully adapted to and colonized a wide variety of habitats including land, air, and water. Their widespread success is largely due to their ability to regulate their own body temperature. But mammals have also evolved many other unique adaptations to help them succeed in numerous environments, as is illustrated by their many forms of locomotion.

Sharks are odd fishes. These sleek predators differ in a number of ways from other fishes and as a result, they are not tucked into the same clade that contains the 20,000-plus species of ray-finned fishes. Instead, sharks branched off from the ray-finned fish lineage some 420 million years ago. The two groups have been forging their own distinct paths ever since.

So what features set sharks apart from the ray-finned fishes? What evolutionary innovations have they come up with in all that time since they diverged from other fish? Here are a few ways sharks differ from other fishes:

• No bones - Sharks lack a bony skeleton, their body frame consists instead of cartilage. Ray-finned fishes have a skeleton made of true bone.
• No operculum - The gill slits of sharks are exposed while the gills of ray-finned fishes are protected by a bony plate called an operculum.
• No swim bladder - Sharks lack a swim bladder. To maintain neutral bouyancy, they rely instead on low-density cartilage, liver oils, and hydrodynamic planing. Ray-finned fishes regulate their bouyancy by contracting and expanding their swim bladder.
• Mobile upper jaw - Most modern sharks have the ability to dislodge their upper jaw from its seat against the skull. This means the shark's jaw has extra mobility and power. In contrast, the upper jaw of ray-finned fishes is attached to its skull.
• Placoid scales (or denticles) - Sharks have skin that is covered with denticles, tiny tooth-like structures composed of dentine. Shark skin is therefore rough in texture, much like sandpaper. Ray-finned fishes, in contrast, have scales and lack denticles.
• Inflexible fins - Sharks have thick, inflexible fins that lack the fine bones and mauneuverability characteristic of the fins of ray-finned fishes.

Despite these differences, sharks do share a number of characteristics with the ray-finned fishes including the arrangement of their fins and many aspects of their internal anatomy (circulatory, digestive, reproductive, and nervous systems).

Refs: Tricas, T. 1997. Sharks & Rays. New York: Time-Life Books. 288p.

Photo © Engamon / iStockphoto.

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