The reason of all rules of Islam is based on getting the benefit and avoiding the harm. Allah (SWT) knows the secrets and real effects , and He ordered us to avoid things which are Haraam, and we believe that He does not prevent us from any thing for no reason, but for His absolute wisdom, whether we know it or not.
Crab, lobster and other sea and water animals except prawns are Haraam. Simply because they ( except prawns) are harmful for our health and He knows the details of the harm. We do trust His absolute wisdom and never argue with Him. Scientific researches are improving day by day and providing more evidence that every Haraam in Islam is in fact harmful. We need to submit to the Will of Allah (SWT) with surety that all His orders to us are to protect us from harm and provide us with benefits.