Legislations differ on this from country to country, but the general gist is indeed that already by creating something you instantly own the copyright to it (provisions like a certain amount of originality in the work notwithstanding). There is no need to register copyright, nor to even publish the work. So everything we write here is technically copyrighted (again, if it meets certain thresholds, but whole sequences surely do that) and not fit to be used by anybody else except with consent by the "copyright holder". These are the rules of the Berne Convention, and almost all countries (a dozen or so exceptions, but even North Korea for once is part of it) have signed them.
When it comes to practically getting damages or having the right to de-publish a work that stole from your work it gets way more complicated, since you have to proof that it was your idea, that the author of the other work had knowledge of your idea and that it was still copyrighted (since copyrights expire). Also, other stuff may happen. This depends heavily of the legislation of the country in question. In the US it is comparatively lenient with "fair use" rights and the necessity to indeed meet some requirements if you want more than just "yeah, that's your copyright, alright", like e. g. compensation, but in other countries not so much, and the country of the (presumed) original author counts.
So in practice, nothing would probably happen, especially since it's not worth much, since you have a burden of proof, etc. So it should be fair game. But the fact that it was just some throwaway ideas on a forum, even on one with a heavy dose of pirating, doesn't invalidate the technical idea and the author indeed has automatic copyright. However, it can make for some pretty ugly scenes in forums.
