The Quranic prohibition is precise and clear; only pig meat, blood and dead animal are haraam.
[2:173] He only prohibits for you the eating of animals that die of themselves (without human interference), blood, the meat of pigs, and animals dedicated to other than GOD. If one is forced (to eat these), without being malicious or deliberate, he incurs no sin. GOD is Forgiver, Most Merciful.
[5:3] Prohibited for you are animals that die of themselves, blood, the meat of pigs, and animals dedicated to other than GOD. (Animals that die of themselves include those) strangled, struck with an object, fallen from a height, gored, attacked by a wild animal - unless you save your animal before it dies - and animals sacrificed on altars. Also prohibited is dividing the meat through a game of chance; this is an abomination. Today, the disbelievers have given up concerning (the eradication of) your religion; do not fear them and fear Me instead. Today, I have completed your religion, perfected My blessing upon you, and I have decreed Submission as the religion for you. If one is forced by famine (to eat prohibited food), without being deliberately sinful, then GOD is Forgiver, Merciful.
[6:145] Say, "I do not find in the revelations given to me any food that is prohibited for any eater except: (1) carrion, (2) running blood, (3) the meat of pigs, for it is contaminated, and (4) the meat of animals blasphemously dedicated to other than GOD." If one is forced (to eat these), without being deliberate or malicious, then your Lord is Forgiver, Most Merciful.
[16:115] He only prohibits for you dead animals, blood, the meat of pigs, and food which is dedicated to other than GOD. If one is forced (to eat these), without being deliberate or malicious, then GOD is Forgiver, Most Merciful.
The Quran is the complete book of guidance. So, based on the Quranic teaching, the crabs are halaal.
If someone quotes a hadith and tells you that crabs are haraam, then quote the following verses of the Quran:
[10:59] Say, "Did you note how GOD sends down to you all kinds of provisions, then you render some of them unlawful, and some lawful?" Say, "Did GOD give you permission to do this? Or, do you fabricate lies and attribute them to GOD?"
[16:116] You shall not utter lies with your own tongues stating: "This is lawful, and this is unlawful," to fabricate lies and attribute them to GOD. Surely, those who fabricate lies and attribute them to GOD will never succeed.
Thank you and may God guide us.
[33:62] God's Sunnah is unchangeable
[39:23] The Quran is the best Hadith