For the sake of argument, let's take for granted your statement, that 'suffering should be reduced as much as possible'.
If the discomfort of a single crab can prevent worse discomfort/suffering/death of many other beings, and results in reduced net pain, then the utilitarian line of reasoning seems to be that we might actually be morally obligated to take blood from crabs.
I don't understand the point you're trying to make above.
In this case specifically, the outcome isn't unclear. Let's call the crab's pain one unit of pain. Assume that unit can directly alleviate 20 units of pain across a handful of other beings. The utilitarian ought to prefer avoiding 19 units of net pain, than allowing 19 units of net pain to occur.
I read your initial post to be some sort of utilitarian moral argument, roughly, that less pain is better. Or something like that. That argument, in this case in particular, leads in the opposite direction than I think you want.