Octopuses breathe underwater using specialized organs called gills. These aquatic creatures rely on water for their respiratory processes.
The Mechanics of Octopus Respiration
Octopuses breathe by drawing water into a muscular chamber called the mantle cavity. This occurs as radial muscles in the mantle wall contract, pulling in seawater through an opening. The water then flows over the octopus’s two gills.
The gills are comb-like and feathery, with numerous thin-walled filaments. This extensive surface area, covered in tiny capillaries, is where gas exchange takes place. Dissolved oxygen from the seawater diffuses across these delicate membranes into the octopus’s bloodstream, while carbon dioxide moves from the blood into the water.
To maintain continuous water flow, the octopus uses muscular contractions of its mantle. After passing over the gills, water is expelled through a muscular tube called the siphon. This expulsion is also utilized for jet propulsion.
An octopus’s circulatory system supports gas exchange with three hearts. Two branchial hearts pump blood through the gills to pick up oxygen. This oxygenated blood then returns to a systemic heart, which circulates it throughout the body. The blood contains hemocyanin, a copper-based protein that binds with oxygen, giving the blood a bluish tint. Octopuses can also absorb some oxygen through their skin, a process known as cutaneous respiration.
Why Water is Crucial for Octopus Gills
Octopus gills are adapted to extract dissolved oxygen from water, not gaseous oxygen from the air. Their delicate, feather-like structure, with numerous thin filaments and a large surface area, is designed for efficient gas exchange in water. These filaments require the buoyancy and support of water to remain spread out and functional.
When an octopus is exposed to air, the lack of water causes gill filaments to collapse and stick together. This collapse reduces the surface area available for oxygen absorption. Without the necessary surface area, the gills become ineffective at transferring oxygen into the bloodstream.
Furthermore, the thin membranes of the gills need to remain moist for diffusion to occur. In air, these membranes quickly dry out, further impeding the transfer of oxygen. Unlike lungs, which are internal and adapted to a gaseous environment, octopus gills are external (within the mantle cavity) and specialized for a liquid medium, making them unsuitable for air breathing.
Octopuses Out of Water: Limits to Survival
While octopuses are aquatic creatures, they can briefly emerge from water for specific purposes, such as moving between tide pools during low tide or escaping a predator. However, their ability to survive out of water is severely limited. This terrestrial excursion is typically short-lived, lasting from a few minutes to, in some cases, up to an hour, depending on factors like species, humidity, and temperature.
The immediate consequence of being out of water is the collapse of their gills. This prevents them from efficiently absorbing oxygen from the air, quickly leading to respiratory failure. The delicate gill structures are not designed to function without the support and moisture of water.
Additionally, octopuses face the threat of rapid desiccation, or drying out, when exposed to air. Although their skin can absorb a small amount of oxygen, this cutaneous respiration is insufficient to meet their metabolic needs for extended periods. Therefore, prolonged absence from their watery habitat results in suffocation and dehydration.