Did you know that the maybe most dangerous shark out there is called after a kitchen utensil? The cookiecutter shark (also known as cigar shark after its shape) has the hunt down to a fine art, and it doesn’t discriminate: everything in the sea is prey. It is named after its method to feed: by cutting out circular chunks of skin and flesh.
The cookiecutter shark uses its bright green bioluminescence (the strongest known of any shark) as lure by day and as camouflage by counter-illumination at night – together with a little non-luminescent patch as bait. Since the light conditions change with time and weather conditions it migrates vertically up to 1.9 mi -3 km- every day to retain this advantage, while its big liver (up to 35% of its weight), filled with low-density oils, renders it nearly neutrally buoyant over a wide range of depths.
After attracting its prey (which it then sees using its large eyes that are placed forward on the head and have got neurons concentrated in a concentric area, unlike other sharks) the shark uses its short, broad caudal fin to propels itself forward. Then the shark latches onto its prey with its suctorial lips while closing its spiracles and retracting its tongue to provide underpressure (which makes it possible to consume whole prey such as squid in size to the shark itself, too).
Now its hook-like upper teeth grasp onto the prey, while its lower teeth do the cutting. They interlock and form a single saw-like cutting edge. To ensure a sharp-edged saw at all times, they are regularly replaced altogether (instead of successively like with other sharks) that’s why the shark swallows its old sets of teeth, so that it can recycle the calcium and and phosphates content. Due to a specialized jaw, the cookiecutter shark can vibrate this teeth-row like a electric carving knife and cuts a conical plug of flesh out of its prey. At last, the shark twists and writhes, looses the outer suction and wrenches its meal from the body, in the process benefiting from the laminar flow of water created by its larger prey’s momentum, and swims away.
Thank god this ruthless predator is only up to 20 in -50 cm- long and leaves craters averaging 2 in -5 cm- across and 2.5 in -7 cm- deep on every type of medium to large-sized oceanic animal (like whales, sharks, tunas, porpoises and seals) and divers and swimmers, but also on fishing, research and naval equipment, even the neoprene hull or rubber cables of submarines (solved by installing fiberglass covers).
I think that a bigger cookiecutter shark would not remain unseen (at least after the attack), and risks that the big predators retaliate and eat it. Additionally, to wrench a bigger chunk of meat out of the prey the shark would need more force and a larger laminar flow of water (thus prey swimming faster as existent). Therefore the cookiecutter shark stays small and is just a parasite: annoying but not deadly.