This article says that hemocyanins differ from hemoglobins in that they do not bind oxygen co-operatively. On the contrary many hemocyanins are extremely co-operative, in fact, some are much more co-oerative than most hemoglobins. Some are the most co-operative binding protiens known. (see van Holde, K.E., and KI Miller, 1995, Hemocyanin. Academic Press, NY.)