What exactly is a professional mermaid?

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I have just come across somebody on Twitter who says that they are writing a book on "professional mermaids". Does this strike you as just a trifle odd? Did you realise that there are apparently both amateur and professional mermaids, and that - presumably - one can progress from one status to the other? Do you think it's like golf - you can only play in the big mermaid tournaments if you have achieved professional status? And how exactly does a mermaid earn her living so that she can call herself a professional? Do you think that some of them become super rich? And if they do, where exactly do they put their credit cards when they're out shopping? These are clearly deep and far-from-serious questions that deserve shallow and entirely ridiculous answers!