## ABSTRACT

Hydrostatic pressure affects all organisms as barometric pressure in air and hydrostatic pressure in water. It is normally close to 1 atmosphere (atm) (101.325 kPa) on land with a reduction for height e.g. around 50 kPa at a height of 5800m. As hydrostatic pressure in water, it is the natural proxy for depth and in simple terms changes by around 100 kPa or 1 atm for every 10m of descent down through the water. By convention the additional 1 atm provided by the weight of air above the water surface is taken as the starting value so that at a depth of 10m we consider the hydrostatic pressure to be 100kPa although the absolute value would be 100 kPa due to the water plus 100 kPa due to air pressure giving a combined value of 200 kPa (Hills, 1972). In addition fluid systems including hydraulic skeletons in many animals increase hydrostatic pressure above ambient for instance due to hearts pumping or in microenvironments including skeletal load bearing joints of vertebrates where transients occur. Measurements up to 10–20 MPa have been made from human hips (Macdonald, 1997). Hydrostatic pressure acts all around an organism and is in balance with no net force unlike a differential pressure. There has been considerable interest and uncertainty about how animals, especially those with no gas compartment, respond to hydrostatic pressure changes. A particular question is how do animals sense very small changes in pressure around 1 kPa corresponding to depths of 10 cm of water or less. The intention here is to present a selection of evidence to show that animals do sense hydrostatic pressure and make use of the 144information as a proxy for depth in their normal lives. There is also now some limited information from known hydrostatic pressure sensors which are in fact also angular acceleration receptors in vestibular systems of crabs and sharks.