The common octopus (Octopus vulgaris) is a mollusk belonging to the class Cephalopoda. Octopus vulgaris is one of the most studied of all the octopus species, and also one of the most intelligent. It ranges from the eastern Atlantic, extends from the Mediterranean Sea, Black sea and the southern coast of England, to the southern coast of South Africa. It also occurs off the Azores, Canary Islands, and Cape Verde Islands. The species is also common in the Western Atlantic.
Characteristics
Octopus vulgaris grows to in mantle length with arms up to long.Norman, M.D. 2000. Cephalopods: A World Guide. ConchBooks. It lives for 1–2 years and may weigh up to . Mating may become cannibalistic. O. vulgaris is caught by bottom trawls on a huge scale off the northwestern coast of Africa. More than are harvested annually.
The common octopus hunts at dusk. Crabs, lobsters, and bivalve mollusks (such as cockles) are preferred, although the octopus eats almost anything it can catch. It is able to change colour to blend in with its surroundings, and is able to jump upon any unwary prey that wanders across its path. Using its beak, it is able to break into the shells of shelled mollusks. It also possesses venom to subdue its prey.
They have evolved to have large nervous systems and brains. An individual has about 500 million neurons in its body, almost comparable to dogs. They are intelligent enough to distinguish brightness, navigate mazes, recognize individual people, learn how to unscrew a jar or raid lobster traps. They have also been observed keeping "gardens", in which they collect various marine plant life and algae, alongside collections of shells and rocks; this behavior may have inspired the 1969 Beatles title, "Octopus' Garden". O. vulgaris was the first invertebrate animal protected by the Animals (Scientific Procedures) Act 1986 in the UK. Training experiments have shown the common octopus can distinguish the brightness, size, shape, and horizontal or vertical orientation of objects.
Species complex While traditionally described as a cosmopolitan species, recent genetic research has identified Octopus vulgaris as a species complex. There are currently six recognized members of the Octopus vulgaris species complex. Each member of the complex is associated with specific geographic regions, often associated with continental coastlines.
Octopus vulgaris Octopus tetricus Octopus americanus Octopus aff. vulgaris Octopus sinensis Octopus djinda
Physiology Habitat and demands The common octopus has worldwide distribution in tropical, subtropical and temperate waters throughout the world. They prefer the floor of relatively shallow, rocky, coastal waters, often no deeper than . Although they prefer around , salinity throughout their global habitat is found to be between roughly . They are exposed to a wide variety of temperatures in their environments, but their preferred temperature ranges from about . In especially warm seasons, the octopus can often be found deeper than usual 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. 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 roughly 2.4 times the amount 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, it uses a jet mechanism that involves creating a much higher pressure in its mantle cavity that allows it to propel itself through the water. As the common octopus' heart and gills are located within its 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.
Respiration The octopus uses gills as its respiratory surface. The gill is composed of branchial ganglia and a series of folded lamellae. Primary lamellae extend out to form demi branches and are further folded to form the secondary free folded lamellae, which are only attached at their tops and bottoms. The tertiary lamellae are formed by folding the secondary lamellae in a fan-like shape.Young, Richard E. & Vecchione, Michael. (2002). Evolution of the gills in the octopodiformes. Bulletin of marine science. 71(2): 1003–1017 Water moves slowly in one direction over the gills and lamellae, into the mantle cavity and out of the octopus' funnel.
The structure of the octopus' gills allows for a high amount of oxygen uptake; up to 65% in water at . The thin skin of the octopus accounted for a large portion of oxygen uptake in an in-vitro study; the estimate suggests around 41% of all oxygen absorption is through the skin when at rest. This number is affected by the activity of the animal – the oxygen uptake increases when the octopus is exercising due to its entire body being constantly exposed to water, but the total amount of oxygen absorption through skin is actually decreased to 33% as a result of the metabolic cost of swimming. When the animal is curled up after eating, its absorption through its skin can drop to 3% of its total oxygen uptake. The octopus' respiratory pigment, hemocyanin, also assists in increasing oxygen uptake. Octopuses can maintain a constant oxygen uptake even when oxygen concentrations in the water decrease to around or 31.6% saturation (standard deviation 8.3%). If oxygen saturation in sea water drops to about 1–10% it can be fatal for Octopus vulgaris depending on the weight of the animal and the water temperature. Ventilation may increase to pump more water carrying oxygen across the gills but due to receptors found on the gills the energy use and oxygen uptake remains at a stable rate. The high percent of oxygen extraction allows for energy saving and benefits for living in an area of low oxygen concentration.
Water is pumped into the mantle cavity of the octopus, where it comes into contact with the internal gills. The water has a high concentration of oxygen compared to the blood returning from the veins, so oxygen diffuses into the blood. The tissues and muscles of the octopus use oxygen and release carbon dioxide when breaking down glucose in the Krebs cycle. The carbon dioxide then dissolves into the blood or combines with water to form carbonic acid, which decreases blood pH. The Bohr effect explains why oxygen concentrations are lower in venous blood than arterial blood and why oxygen diffuses into the bloodstream. The rate of diffusion is affected by the distance the oxygen has to travel from the water to the bloodstream as indicated by Fick's laws of diffusion. Fick's laws explain why the gills of the octopus contain many small folds that are highly vascularized. They increase surface area, thus also increase the rate of diffusion. The capillaries that line the folds of the gill epithelium have a very thin tissue barrier (10 μm), which allows for fast, easy diffusion of the oxygen into the blood. In situations where the partial pressure of oxygen in the water is low, diffusion of oxygen into the blood is reduced, Henry's law can explain this phenomenon. The law states that at equilibrium, the partial pressure of oxygen in water will be equal to that in air; but the concentrations will differ due to the differing solubility. This law explains why O. vulgaris has to alter the amount of water cycled through its mantle cavity as the oxygen concentration in water changes.
The gills are in direct contact with water – carrying more oxygen than the blood – that has been brought into the mantle cavity of the octopus. Gill capillaries are quite small and abundant, which creates an increased surface area that water can come into contact with, thus resulting in enhanced diffusion of oxygen into the blood. Some evidence indicates that lamellae and vessels within the lamellae on the gills contract to aid in propelling blood through the capillaries.
Circulation The octopus has three hearts, one main two-chambered heart charged with sending oxygenated blood to the body and two smaller branchial hearts, one next to each set of gills. The circulatory circuit sends oxygenated blood from the gills to the atrium of the systemic heart, then to its ventricle which pumps this blood to the rest of the body. Deoxygenated blood from the body goes to the branchial hearts which pump the blood across the gills to oxygenate it, and then the blood flows back to the systemic atrium for the process to begin again. Three aortae leave the systemic heart, two minor ones (the abdominal aorta and the gonadal aorta) and one major one, the dorsal aorta which services most of the body. The octopus also has large blood sinuses around its gut and behind its eyes that function as reserves in times of physiologic stress.
The octopus' heart rate does not change significantly with exercise, though temporary cardiac arrest of the systemic heart can be induced by oxygen debt, almost any sudden stimulus, or mantle pressure during jet propulsion. Its only compensation for exertion is through an increase in stroke volume of up to three times by the systemic heart, which means it suffers an oxygen debt with almost any rapid movement. The octopus is, however, able to control how much oxygen it pulls out of the water with each breath using receptors on its gills, allowing it to keep its oxygen uptake constant over a range of oxygen pressures in the surrounding water. The three hearts are also temperature and oxygen dependent and the beat rhythm of the three hearts are generally in phase with the two branchial hearts beating together followed by the systemic heart. The Frank–Starling law also contributes to overall heart function, through contractility and stroke volume, since the total volume of blood vessels must be maintained, and must be kept relatively constant within the system for the heart to function properly.Hill, Richard W., Gordon A. Wyse, and Margaret Anderson. Animal Physiology. 3rd ed. Sunderland, MA: (635–636, 654–657, 671–672) Sinauer Associates, 2012. Print
The blood of the octopus is composed of copper-rich hemocyanin, which is less efficient than the iron-rich hemoglobin of vertebrates, thus does not increase oxygen affinity to the same degree. Oxygenated hemocyanin in the arteries binds to , which is then released when the blood in the veins is deoxygenated. The release of into the blood causes it to acidify by forming carbonic acid. The Bohr effect explains that carbon dioxide concentrations affect the blood pH and the release or intake of oxygen. The Krebs cycle uses the oxygen from the blood to break down glucose in active tissues or muscles and releases carbon dioxide as a waste product, which leads to more oxygen being released. Oxygen released into the tissues or muscles creates deoxygenated blood, which returns to the gills in veins. The two brachial hearts of the octopus pump blood from the veins through the gill capillaries. The newly oxygenated blood drains from the gill capillaries into the systemic heart, where it is then pumped back throughout the body.