If you're expecting any member of the Royals as a guest at your dinner party, pay special attention to the menu. There are some situations where a great shellfish dish can impress your guests, but you'll find your Royal friend turning it away. Royals will not eat shellfish in any scenario.
Some people believe this to be a strict adherence to one of the teachings of the Old Testament, but it's actually a practical strategy. Of all the foods out there, shellfish is the single most likely to cause food poisoning and therefore lead to illness. Royal appearances and functions are planned for months, and in some cases, years in advance. Having to cancel appearances due to illness throws the calendar into turmoil and means that a senior Royal might have to let down a cause or charity who might be depending on them to turn up. It's better for them to minimize risk wherever possible.