the shell or shell plating on your body allows you to move in specific ways and to make you harder to damage 

the way your large exoskeleton is made it allows for a more unique fighting style and gives you a passive +2 to your AC  

you have one or 2 large claws or claw like appendages that allow you too attack with greater damage than you would normally happen  

your claws act as a strength based melee weapon that deals 2D6 +strength, your claws can come in a variety of shapes and sizes to reflect the style of crustation you inherit traits from 

you hunker down and build yourself up in your shell or retract into your shell

in doing you loose all movement and actions other than removing from the shell in exchange for your ac becoming 15 + DEX + any other armor advantages, you are till able to see in a limited field and any non-action spells can still be used if applicable 

your body is specially designed to work under water allowing you to breath both air and water as well as swim under water at a speed of 25 ft per turn 

you gain a bonus +2 to your CON score and a +1 to your STR score 

Decasapiens unlike humans live for a a modest 50-60 years on average with most decasapiens dying of old age and natural causes 

you gain a walking speed of 30 ft as well as a swimming speed of 25ft

as a decasapiens you gain the ability to speak both common and aquan