Why is there a spider in my tank?

[new but handy]

Well it might not be a spider but all I have seen of it are its legs and it looks exactly like a spider. It even moves like one. Judging by the size of its legs it is probably 3" across its leg span. I am going to try to get a pic now. Are there marine spiders? Either way it's creepy looking.

Oh I finally saw what I think is a bristle worm. It is about 10" long. It was just out roaming around. By the time I fumbled my camera out it was gone