I’m assuming the same way we decide which animals are okay to eat or not - arbitrarily and by culture. For example, maybe some Pacific tribes of mermaids might be fine using starfish, but Atlantis area that would be considered barbaric, perhaps even with a touch of propaganda saying they use starfish for sexual reasons as well. I love where this is going, I’m gonna go write a controversial AO3 fiction about a Pacific mermaid fighting against Atlantic racism, but secretly in a degrading race-play relationship.