the seabed. Water loss by evaporation has to be tolerated (by means of the operculum which holds water in and prevents its escape as vapour), or avoided (by moving into water or a shaded area). The peak in dog whelk population density is approximately coincidental with the mid-tidal zone. It lives in the middle shore. In general it can be said that at high vertical heights on the shoreline the dog whelk is most threatened by biotic factors such as predation from birds and interspecific competition for food, but abiotic factors are the primary concern, creating a harsh environment in which it is difficult to survive. At low vertical heights it is biotic factors, such as predation from crabs and intraspecific competition, which cause problems. The upper limit of the range in which the dog whelk is generally found is approximately coincidental with the mean high water neap tide line, and the lower limit of the range is approximately coincidental with the mean low water neap tide line, so that the vast majority of dog whelks are found on the mid-tidal zone. Tidal pools and comparable microhabitats extend the vertical range of organisms such as the dog whelk as they provide a more constant environment, but they are prone to increased salinity because evaporation concentrates dissolved substances. This can create toxic conditions for many species. The dog whelk can only survive out of water for a limited period, as it will gradually become desiccated and die if emersed for too long. Metabolic processes within cells take place in solution, and a decrease in water content makes it impossible for the organism to function properly. In experiments it has been shown that 50% of dog whelks die at 40 °C, and it can be assumed that at temperatures lower than this a smaller proportion will be killed off. Furthermore, the dog whelk has to excrete ammonia directly into water, as it does not have the adaptation possessed by many upper shore species which would allow it to produce uric acid for excretion without loss of water. When kept emersed for seven days at a temperature of 18 °C, 100% of dog whelks die, in contrast to many periwinkle species which can lose even more water than the dog whelk (i.e. more than 37% of their total body mass) but survive as a result of their ability to excrete toxic waste products