Octopus is the largest genus of octopuses, comprising more than 100 species. These species are widespread throughout the world’s oceans. Many species formerly placed in the genus Octopus are now assigned to other genera within the family Octopodidae.
The word is derived from the Greek number eight, “oktō” (ὀκτώ) and “pous, podos” (πούς, ποδός) which means feet.
The common octopus is typically found in tropical waters throughout the world, such as the Mediterranean Sea and East Atlantic. They prefer the floor of relatively shallow, rocky, coastal waters, often no deeper than 200 meters. Although they prefer around 36 grams per liter, salinity throughout their global habitat is found to be between roughly 30 and 45 grams of salt per liter of water. They are exposed to a wide variety of temperatures in their environments, however their preferred temperature ranges from about 15 °C to 16 °C. In especially warm seasons, the octopus can often be found deeper than usual in order to escape the warmer layers of water. In moving vertically throughout the water, the octopus is subjected to various pressures and temperatures which affect the concentration of oxygen available in the water. This can be understood through Henry’s Law, which states that the concentration of a gas in a substance is proportional to pressure and solubility, which is influenced by temperature. These various discrepancies in oxygen availability introduce a requirement for regulation methods.
Primarily, the octopus situates itself in a shelter where a minimal amount of its body is presented to the external water, which would pose a problem for an organism which breathes solely through its skin. When it does move, most of the time it is along the ocean or sea floor, in which case the underside of the octopus is still obscured. This crawling increases metabolic demands greatly, requiring they increase their oxygen intake by approximately 2.4 times the amount that is required for a resting octopus. This increased demand is met by an increase in the stroke volume of the octopus’ heart.
The octopus does sometimes swim throughout the water, exposing itself completely. In doing so, the octopus uses a jet mechanism that involves creating a much higher pressure in their mantle cavity that allows them to propel themselves through the water. As the common octopus’ heart and gills are located within their mantle, this high pressure also constricts and puts constraints on the various vessels that are returning blood to the heart. Ultimately, this creates circulation issues and is not a sustainable form of transportation, as the octopus cannot attain an oxygen intake that can balance the metabolic demands of maximum exertion.