Board Thread:Naruto Discussions/@comment-24847491-20140921145042/@comment-1512913-20140922180937

You would expect Hashirama to summon a slug or at least make a reference to slug. There's nothing. Call me crazy, but I think it may be even possible that slugs don't teach Senjutsu. Just because toad, snake and slug are a trio doesn't mean they all have to have the same.

Tsunade's equivalent of Sage Mode is Strength of the Hundred Technique. If slugs had Sage Mode, she would most certainly know it, but nothing indicates she does.