although most would just post that he spit water, tobirama has always used a hand seal for using a suiton jutsu [
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] and his techniques have always been named
In this case it was neither as madara's rods prevented him from moving his body and the jutsu wasnt named as a suiton jutsu
Also hashirama says the rods have been pushed to their pressure points which prevent them from molding chakra
So it is most likely that it wasnt a jutsu but simple needles like shizune
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probably poisoned, he is not shown to be using poison but like madara says tobirama is a shrewd man and he might have prepared for situations like this
as to why madara used susanoo to defend, i am not sure but if sasuke can use susanoo to defend against kunai from danzo then it isnt strange for madara to use susanoo, he didnt have any other weapon to defend anyway