Rounded and tapering at both ends. The very shape of a fish is a major factor in its movement. If the shape of the fish is blocky it will have alot of drag and will move throught the water very slowly.
Fusiform bodies are bodies whose shape is optimized for peak laminar flow.
Laminar flow is when water or air flows smoothly around an object with little or no drag. Creatures like tunas and bluefish have elongated, and tapered bodies that allow them to move very quickly, and this is fusiform bodies. Jets, and boats are also designed with fusiform shape to allow maximum speed. The fusiform adaptation allows fish like the albacore tuna to swim at such high speeds that very few predators can catch them. The fusiform shape also allows predators to catch prey.