Water covers about three fourth portion of earth, occurring in the oceans, seas, lakes, streams, ponds etc. All these water bodies contain wide variety of flora as well as fauna. One such animal that is most easily found in the oceans is the Octopus. Octopus is commonly known as the devil fish as its external appearance is terrific enough to frighten a human being and other animals. Octopus belongs to the phylum Mollusca and class Cephalopoda. The order to which it belongs is Octopoda and is completely marine in habitat.
The octopus bears two eyes and four pairs of arms and like other cepahlopods its body is bilaterally symmetrical. It has a beak and the mouth is situated at the centre position of the arms. Exoskeleton is absent so it can squeeze itself in small crevices. The intelligence power is very well developed and it perhaps is the most intelligent animal among all invertebrates. It dwells in wide regions in the oceans but chiefly found occurring among the coral reefs. For their defense from the enemies they basically hide themselves or expel an inky material or show colour changing activity called camouflage. All octopuses are generally venomous but only the small blue ringed octopuses are harmful to humans and lead them to heavenly abode. About 300 species of octopus are known today.
Octopuses are very clearly identified by their eight dangerous arms each bearing suckers. Their arms are very different from those found in other cephalopods like squids and the cuttlefish. They have entirely soft bodies with no sign of exoskeleton like the shell found in Nautilus and cuttlefish. They have a beak like structure similar to that of a parrot but very small in size and it is the only hard structure present in their bodies. Their soft bodies enable them to hide under small areas when attacked. They have a very short life span but some species may live up to six months. The North Pacific Giant Octopus can live up to five years under certain conditions. Their life span is basically affected by reproduction. The males live only for few months after mating and die later on and the females die as early their eggs hatch. The females show parental care towards their eggs and do not feed until their eggs hatch but starvation is not the cause of their death. The two optic glands secrete endocrine substances that are responsible for the programmed cell death. It has been found that if these optic glands are removed surgically the octopus survives many months after reproduction and if then starved then may die due to starvation.
There are three hearts in octopuses. The two are responsible for pumping the blood from the two gills and the third one is responsible for pumping the blood from whole of the body. Their blood carries a copper containing protein haemocyanin which is the oxygen carrier as well as the blood pigment. Haemocyanin is much more efficient oxygen carrier in comparison to the vertebrate oxygen carrier haemoglobin. The haemocyanin is found dissolved in the plasma and is not carried within the red blood cells of the vertebrates and gives a blue colour to the blood. Octopuses extract water from their mantle cavity and like other molluscs have finely divided and highly vascularized gills present either on the outer or inner surface of body.
Octopuses are very intelligent and this is a matter of debate among the scientists. Experiments have shown that they have short term and the long term memory also. Although they have very short life span but they learn and it has not been proved till now that whether the young ones have innate behaviours or not. They have a very complex nervous system. Two third of the neurons are located in the arms and the arms have remarkable powers of autotomy. The arms also show complex reflex actions. The laboratory experiments have shown that they learn to distinguish between shapes and sizes and also break the aquariums in search of food.
The defense mechanism of the octopuses basically includes hiding under any suitable place so that they may not be detected by their predators. Other mechanism includes rapid escape from the predators and they also secrete inky fluids, show camouflage and breaking of their arms which also forma a part of their defense mechanism. Majority of the octopuses love to secrete a inky fluid in water as a part of defence from their foes and this inky fluid forms a cloud and confuses the prey and the octopuses run away. This inky fluid consists of a chemical called as melanin which is responsible for giving colour to human skin and hair. The inky fluid reduces the smelling efficiency of the predators and gives octopuses a chance to escape. Chromatophores present in the epidermal layer of the skin of octopuses contains red, yellow, orange and black pigments which makes the colouration of the skin similar to the background and the animal becomes protected from the enemies. Iridophores and leucophores are the reflective cells and are also responsible for the appearance of warning colouration. They also break-off their arms when attacked and their broken arms regenerate later on.
The act of reproduction in octopuses is very different. During the act of reproduction the male uses its special arm designated as heterocotylus and transfers the spermatophores through it into the mantle cavity of female. In some benthic octopuses the third arm is heterocotylus arm. The males usually die after sometime few months when they have mated. In some species the females store the sperms until their eggs have not become mature. When the eggs have been fertilized the females lay about 200,000 eggs and their number varies among species. The females generally attach the eggs in the form of long strings to any solid substratum. Females also show parental care. They guard their eggs from the predators and also provide air currents to the eggs so that they may get proper amount of oxygen. The mother octopuses do not hunt and do not feed until their eggs hatch and they become too weak at that time when their eggs hatch and can be easily killed by the predators. The young larvae that hatch from the eggs move towards the upper surface of sea and feed on planktons like copepods. This time is very critical as they may be easily killed by their enemies when they are grown enough they come back to the bottom and the cycle repeats.
The sense of vision is very much demarcated in octopuses and they can easily distinguish the polarization of light. The colour detection is variable among species. The two sense organs of the brain called the statocysts help the octopus to detect its body orientation. The autonomic response keeps the pupil of the eye in horizontal position. They have a very good sense of touch. The suckers present in the arms are generally the chemoreceptors which help the octopus to taste the substance it is actually touching. The arms also contain tension sensors so they act as proprioreceptors but their actual role needs further attention by the researchers. They move either by crawling or by swimming. They crawl slowly. The jet propulsion performed by them is their fastest mode of locomotion which is followed by swimming and crawling. Octopuses were worshiped by ancient people. They are often consumed as food by humans of many cultures. The Japanese and the Hawaiian people like octopus very much as it forms a very important part of their food. They are also used as pets by many people.
Nature has provided a number of special tactics to octopus to enjoy its life comfortably in its habitat.