Seizures, comas, and even death can result with a decrease in blood pH of 0.2–0.3. Similarly, even little changes in seawater pH can have a negative influence on marine life, affecting chemical communication, reproduction, and growth. The construction of skeletons in marine organisms is especially susceptible to acidity. For example, the shells of some mollusks are made up of calcium carbonate that has a neutral or slightly alkaline pH (8.1-8.4). However, if the water in which these creatures live becomes too acidic, the shells will be degraded by erosion. Marine animals can adapt to varying degrees of pH change. Some species are able to regulate their body temperature when exposed to low pH waters, while others migrate to more alkaline regions of the ocean. Very few studies have examined how various marine organisms respond to changes in pH, but evidence suggests that many species would be likely affected by a widespread increase in ocean acidity. Changes in pH could cause problems for certain marine animals by interfering with their ability to communicate using chemical signals (i.e., odors), reproduce successfully, or grow to maturity. Ocean acidification is caused by the absorption of atmospheric carbon dioxide into the world's oceans. Because carbon dioxide molecules are less dense than water molecules, this process causes water to become more acidic. Scientists predict that if current trends continue, the average depth of the ocean will decline by about 200 meters (660 feet) by 2100.