A stab in the dark here, but I remember coming across something like this before in my reading. I think it is a kind of Long Arm Shrimp, Macrobrachium species. Some of them get big, really big.
Thanks SnakeIce that is definitely my shrimp, and it turns out its a female as she was carrying eggs today but she has already dropped them. Just wish I had more info on her as I can't seem to find any....ho hum
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I can't find much about M. dux. It has a common name of Congo River Prawn. Many of the other Macrobrachium species larva require drifting into brackish or marine to develop into the shrimp form. So unless this one is unusual it would be a challenge to raise any.