Lab Paper - Spiny Dogfish Dissection

The Spiny Dogfish

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Figure 1: Courtesy of NOAA

INTRODUCTION:

There are three major kinds of fishes in the world: jawless fishes, bony fishes and cartilaginous fishes which are the sharks, rays and chimaeras. Cartilaginous fishes are members of the class Chondrichthyes. Their skeletons are made of cartilage which is a tough, flexible material that is also found in the human joints such as knees and elbows. Sharks do not have a swim bladder which helps them to move up and down the water very quickly without any risk. They use their oil-filled livers to help in maintaining neutral buoyancy. An interesting exception is the sand tiger shark, which maybe on the first rung of the evolutionary ladder towards developing its own bony-fish-like swim bladder. It goes to the surface to swallow air, then holds it inside its stomach (Carwardine & Watterson, 2002). Shark fins are also very different from bony fish. Their fins are thick and stiffened by cartilaginous rods and internal fibers of collagen. The skin of a shark feels very smooth when stroked in one direction, but stroked in the opposite direction it can scrape the skin of a human raw. The skin is covered in tooth-like dermal denticles. They help the shark become hydrodynamic by breaking up the interface between skin and water, thus reducing turbulence when the shark is swimming.

Sharks breathe by inhaling water through the mouth and then extracting oxygen as it passes over the gills. Sharks also have holes behind their eyes (one on each side of the head) called spiracles. They are larger in bottom-dwelling sharks and may be used as intake valves for breathing. In most other species, the spiracles are barely visible and serve no known purpose.

Sharks and rays are further grouped together in the subclass Elasmobranchii. There are roughly 350 species of sharks that can be found worldwide. Sharks come in all shapes, sizes and colors. They range from tiny lantern and spined...