Fact: in D&D 5th edition, there is a fifth level spell available to bards and druids called Awaken, which permanently grants an intelligence score of 10 and the ability to speak 1 language the caster knows to a beast with an intelligence score of 3 or lower.
Fact: in D&D 5th edition, most spells require a vocal and/or somatic component, where the caster must speak certain magical phrases clearly and with correct pronunciation, and they must accompany these words with certain hand gestures which can still be made while holding a spellcasting focus in the hand with which the gestures are made. For instance, you might wave a wand to fulfill a spell’s somatic and/or material components.
Fact: in D&D 5th edition, crabs have an intelligence score less than or equal to 3, and can hold a wand in their little crab claws.
Fact: if you were to use Awaken on a crab, it would gain the ability to speak languages utterable by humanoid characters, and could therefore theoretically fulfill the verbal components of spells.
Conclusion: An awakened crab who became a wizard is a fantastic character concept that I intend to use in the future as, at minimum, an NPC.