So,I'm doing the dawn-guard quest line and have given serana the sanguine rose.The thing is,every time she see's a mudcrab she activates it.Fighting a dragon?Nope,better not use it.A mudcrab?FEEL MY WRATH PUNY MORTAL.

Any ideas why she does this?

Perhaps it has something to do with the dragon being airborne when Serana goes to attack it? Does she also neglect to use the Rose on the dragon when it has landed?

Otherwise... got me. Something to do with the dragon's level versus that of a puny mudcrab's?

The mudcrabs are more dangerous...

I've seen a pair of them wipe out an imperial patrol, three wolves, and a bear... so fear the mudcrabs, these ain't your oblivion/morrowind nuisance pests

Yes, mudcrabs are a lot more dangerous than most people realize. I've seen one mudcrab kill two sabre cats with hardly any trouble at all.

As to why Serana killed the mudcrab first instead of focusing on the dragon, it's hard to know for sure, but it probably has something to do with priority in the game programming.

Serana is the most glitchy NPC in the game, I have no answer for you, sorry.

I've seen a dragon fight a mudcrab...it was very disappointing. Took the dragon about 10 minutes to kill him.

The dragon intended to fight me at Meko's shack, but as it flew around, it spotted a mudcrab and went for the kill. The dragon used fire breathe and sometimes landed to give the thing a few bites. I got so frustrated that I went down there to kill the damn mudcrab myself.

I wonder how hard it would be for someone to make a summon giant mudcrab spell... just imagine it a 10x normal sized mudcrab to fight your foes for you with adjusted damage and armor..

She clearly has a phobia of mudcrabs. Why else would she react in such a way?

Just like Farkas.

She tried eating them this one time and it gave her the squirts.
so yeah....

Because all followers love staves. Give them a summoning staff, and they will rain dremoras and atronachs over skeevers and mudcrabs. Same with reanimation. Serana likes to reanimate ANYTHING (as you might have noticed). Not just dead bandits and draugr. No. Unarmed skeletons, deer, goats, bunnies - they can all be members of her personal army. And if you give any follower a reanimating staff, you'll notice the same behavior. (I recall Lydia had a tendency to resurrect male bandits with those bare-chested armors all the time - I'm not sure, I almost got the feeling she was trying to tell me something... )

Serana may hate mudcrabs but she definitely loves rabbits. She keeps resurrecting them instead of something useful in combat.

A real Monty Python moment...

She hasn't done this with me, yet. I'm still waiting for it. XD She resurrected a couple Death hounds and dead vampires though.