There are 7 Size Tiers and 28 sharks in Hungry Shark World.
Tier XS
The Blacktip Reef Shark is the sole member of this group. It is also the weakest shark in the game.
Tier S
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.
Tier M
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.
Tier L
This group of sharks is made up of the Bull Shark, the Goblin Shark, and the Mako Shark and Echo. They can destroy Steel Barriers and Oil Pipe barriers.
Tier XL
The sharks in this trio are the Megamouth Shark, the Great Hammerhead, Tiger Shark and Drago. These sharks can not only break Polished Stone Barriers, but also eat Blue Jellyfish.
Tier XXL
The group is made of the Basking Shark, the Whale Shark, and the Great White Shark. These sharks can break Crystal Barriers and can eat Green Jellyfish and whales.
Tier !!
The eight sharks in this Tier are Megalodon, Big Momma, Killer Whale, Atomic Shark, Zombie Shark, Buzz (Helicoprion), Mr Snappy, Robo Shark and dark magic shark
Atomic Shark
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.