Reduced absorption of Calcium in fishes

Biological function of Calcium: The principal biological functions of calcium may be summarised as follows:

• Calcium is an essential component of bone, cartilage and the crustacean exoskeleton.
• Calcium is essential for the normal clotting of blood, by stimulating the release of thromboplastin from the blood platelets.
• Calcium is an activator for several key enzymes, including pancreatic lipase, acid phosphatase, cholinesterase, ATPases, and succinic dehydrogenase.
• Through its role in enzyme activation, calcium stimulates muscle contraction (ie. promotes muscle tone and normal heart beat) and regulates the transmission of nerve impulses from one cell to another through its control over acetylcholine production.
• Calcium, in conjunction with phospholipids, plays a key role in the regulation of the permeability of cell membranes and consequently over the uptake of nutrients by the cell.
• Calcium is believed to be essential for the absorption of vitamin B12 from the gastrointestinal tract.

Calcium is readily absorbed through the gastro-intestinal tract (through vitamin D3 action), gills, skin and fins of fish. In general, dietary calcium absorption is facilitated by dietary lactose (by forming a soluble sugar-calcium complex) and by high gastric acidities (by aiding solubilization of the calcium salt).

In fishes, four organs and their respective hormones are involved in calcium absorption and its homeostasis:

1. The ultimobranchial bodies secreting the hormone calcitonin
2. The corpuscles of Stannius secreting the hormone stanniocalcin
3. The female gonads secreting the hormone estrogen
4. The pituitary gland secreting the hormones prolactin and corticotrophin.