wow that's a lot of bitshares. It happened to me once in bittrex I think, luckily the amount bounced back to me since it did not leave their wallet for some reason and I was able to email their support and get my money back.
Try contacting poloniex email / chat support if you can get help from them.
Good luck and I hope you get it back...
P.S. Why are you transferring to polo?? You can just trade on the DEX, although the GUI is not as good as polo, it will grow on you eventually. Anyway just asking though..