There are 7 Size Tiers and 28 sharks in Hungry Shark World.
The Blacktip Reef Shark is the sole member of this group. It is also the weakest shark in the game.
This group composes of the Whitetip Reef Shark, the Porbeagle, and the Blue Shark. The special shark is Spike. These sharks can break Bone Barriers and are the stepping stones to getting better sharks.
This group is formed by the Sand Shark, the Thresher Shark, the Smooth Hammerhead, and Heidi. These sharks can destroy Cobblestone Barriers and Glaciers (the ice on the surface) in the Arctic. They can eat Dolphins, who can be very annoying for weak sharks.
The eight sharks in this Tier are Megalodon, Big Momma, Killer Whale, Atomic Shark, Zombie Shark, Buzz (Helicoprion), Mr Snappy, and Robo Shark. They can break any barrier and can eat Pink Jellyfish and the Single Mines. The Atomic Shark can eat Red Jellyfish.
The Atomic Shark unlike the other tier "!!" sharks so far is the only shark that can be only bought after the shark before the Big Momma (Dunkleosteus) is at its maximum level (level 30) just like in Hungry Shark Evolution. It is the only shark that does not rely on healing via. eating due to the fact it can be healed by toxic waste barrels. It also will gain a short gold-rush-like effect when over barrels, however this will render the barrel unsuable again. It can also eat everything except for mines.