Crabs aren't very sneaky. You have a -4 to Sleight of Hand Checks

You are proficient in using your claw as a weapon. When you make a successful attack on an enemy with your claw they'll take 1d6+strength slashing damage and have to make a grapple check.

When an opponent makes an unarmed strike against you they must succeed a con save (DC: 10+Proficiency+Constitution) on a failed save they take 1d4+con damage.

You have an AC of 15

Your Strength score increases by 3, and your Constitution score increases by 2. Your Dexterity, Intelligence, and Charisma decrease by 1

Crabs can theoretically live infinitely as long as they are not killed. They do not die of old age

Crabs aren't very knowledgeable about the laws of the world they are usually inclined to do whatever they want without knowledge or respect for laws. 

Crabs that are over a 100 years old are typically 4 feet wide and 4 feet tall. Your size is Medium.

Your base walking speed is 35 feet. You have a swim speed of 15 feet. You have a climbing speed of 15. You have a dig speed of 5 feet for sand and dirt. You have 8 legs and two claws. You can lose them at will or due to injury. You lose five feet of movement for every leg lost If you lose for than 5 legs you cannot walk without the aid of your claws. If you lose your arm you cannot use abilities that require two hands and you lose half of your strength modifier until it grows back. Arms and legs grow back after shedding your shell during a long rest it'll take 2d4 days.

You gain proficiency in the Intimidation skill. You gain proficiency in the Survival skill.

You can speak, read, and write Common and Seafolk.