The fundamental difference is that these are two, separate endocrine which secrete different hormones, the CS secreting Stanniocalcin (STC) and and the ultimobranchials secreting calcitonin (CT).

Although a lot of research was devoted to these hormones in the 70's-80's, I'd say that there are a lot of questions still unresolved, e.g. for the function of CT in fish.

However, if we do accept your assumption that both are hypocalcemic, then we can move to your follow-up question about the need to duplicate "the same function".

I think there are several physiological reasons for such "duplication":

1. Stability of plasma Ca ion levels is of fundamental importance for vital physiological processes such as muscle and nerve function, so that the endocrine regulation needs to be very robust. In mammals, we see similar duplication in PTH and 1,25OH2D3 both being hypercalcemic.

2. Generally speaking, any physiological process is affected by many hormones, and most hormones have several functions. If you look at how many hormones regulate e.g. blood sugar levels, or metabolism and growth, you'll see a lot of "duplication of function".

3. The change in Ca levels (e.g. hypocalcemia) is the end result of many different physiological processes in the body, i.e. there are several target organs for regulating the calcium balance. In fish these include bone, scales, gills, intestine and kidneys. Two hormones may have completely different target organs although the end-result of their action is hypocalcemia in both cases. This allows for the regulation to be differenciated. In some cases, hypercalcemia should perhaps be counteracted by changing kidney function, sometimes by changing intestinal function. In our research, we e.g. saw that STC could decrease intestinal uptake of Ca.

Thank You very much for giving such a detailed description and to clear my doubts.

I'm happy to provide answers on the subject. My early research was focused on the endocrine regulation of Ca balance in fish. Cheers