Your crustacean eyes give you blindsight for 30 feet, as well as the ability to see in the UV spectrum of light. Crabs use these abilities to track and maneuver in dark places and to hunt prey.

You are equipped with a set of gills under your carapace, as well a membrane lining your mouthparts that absorbs oxygen for breathing. This evolutionary trait allows your kind to thrive both above and below the water.

Your chitinous claws are harder than stone and have the crushing force of 4000 psi, dealing out monstrous damage to any creature unlucky enough to find itself within range of your mighty grasp. 

You can use DEX instead of STR for the attack and damage rolls of your unarmed strikes. Your unarmed strikes count as a weapon with the Finesse property for all effects which require it, such as Sneak Attack

Nature has blessed you with a nigh impregnable shell, which fortifies your fleshy true self. It is sought after by many creatures for use as armor, or even dwellings for smaller kind. Most creatures wouldn't dare combat your kind to obtain such a prize, but the bodies of your deceased are frequently picked clean, much to the distress of the crabfolk, who hold profound reverence for the dead and firm, ritualistic beliefs in an afterlife.

Without apposable thumbs, or multiple digits to manipulate objects, crabfolk are unable to utilize tools or weapons. This is the one factor that prohibits crabs from becoming the ultimate apex creature. Well that and, bar magical intervention, they exist in too low a degree of sentience.

Through a combination of strenuous practice and magical enhancements to your brain and mouthparts, you have been gifted the ability of speech by the being or thing that has awoken you.

Your brain has been reformed through impossible thaumaturgical means, creating new neural networks, reforming and expanding your sensorimotor cortex. You distinctly remember consciously feeling the contents of your head shifting and changing, your cognitive abilities overflowing beyond what you could previous conceptualize as "being" and "doing". Communicatory functions and concepts for which you had no point of reference from which to grasp, suddenly able able to process and execute as naturally as breathing. It was like being folded through reality. 

On top of this, the range of articulation of your mouthbits has been painfully, and extensively modified to allow for speech. Don't worry about how crab can hear.