2. Shrimp and crab foods contain a certain amount of pentavalent arsenic compounds, and watermelon contains vitamin C, which has oxidation effect, and both of them may also produce toxicity and poisoning reactions. Therefore, it is generally recommended not to eat watermelon immediately after eating crabs, and to eat it at least 2 to 3 hours apart, which can effectively absorb the nutritional value of both without causing toxic side effects.