acidification results from increases in atmospheric carbon dioxide. Oceans absorb around one–third of the increase. The dissolved gas reacts with the water to form carbonic acid, and thus acidifies the ocean. This decreasing pH is another issue for coral reefs. Ocean surface pH is estimated to have decreased from about 8.25 to 8.14 since the beginning of the industrial era, and a further drop of 0.3–0.4 units is expected. This drop has made it so the amount of hydrogen ions have increased by 30%. Before the industrial age the conditions for calcium carbonate production were typically stable in surface waters since the carbonate ion is at supersaturated concentrations. However, as the ionic concentration falls, carbonate becomes under-saturated, making calcium carbonate structures vulnerable to dissolution. Corals experience reduced calcification or enhanced dissolution when exposed to elevated . This causes the skeletons of the corals to weaken, or even not be made at all. Ocean acidification may also have an effect of 'gender discrimination' as spawning female corals are significantly more susceptible to the negative effects of ocean acidification than spawning male coral Bamboo coral is a deep water coral which produces growth rings similar to trees. The growth rings illustrate growth rate changes as deep sea conditions change, including changes due to ocean acidification. Specimens as old as 4,000 years have given scientists "4,000 years worth of information about what has been going on in the deep ocean interior". Rising carbon dioxide levels could confuse brain signaling in fish. In 2012, researchers reported on their results after studying the behaviour of baby clown and damselfishes for several years in water with elevated levels of dissolved carbon dioxide, in line with what may exist by the end of the century. They found that the higher carbon dioxide disrupted a key brain receptor in the fish, interfering with neurotransmitter functions. The damaged central nervous systems affected fish behaviour and diminishing their sensory capacity to a point "likely to impair their chances of survival". The fishes were less able to locate reefs by smell or "detect the warning smell of a predator fish". Nor could they hear the sounds made by other reef fish, compromising their ability to locate safe reefs and avoid dangerous ones. They also lost their usual tendencies to turn to the left or right, damaging their ability to school with other fish. Although previous experiments found several detrimental effects on