Welp, he should have known...
He spent all day searching oysters for more pearls, found lots of pretty little pearls too, none as beautiful or large as his precious black one, but lovely none the less, along with many marvelous shells. He was going to make something, jewelry perhaps, out of this.
However, for every oyster he opened holding a pearl, there were two that did not, and the mer happily gobbled down every one of them before moving on to the next.
In between searching of course he was swim spinning right out of the water, then quickly returning to his search, greedily opening and devouring oyster after oyster.
Is it any wonder, that now the poor mer has a terrible tummy ache?