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Octopus Adaptations in Coral reefs: Looking at multiple adaptations for survival
Octopods have been a focus of scientific and public interest throughout history. Cephalopods have inspired stories of sea monsters and other folklore, are a delicate cuisine item in many countries, as well as boggling scientists with intelligence unparalleled in the invertebrate world as well as camouflage abilities unseen anywhere else in the animal kingdom. Octopods are part of the class of cephalopods, phylum of Mollusca, related to squids and nautiluses. Octopi range around the world.
There is an estimated 289 species of octopus worldwide. Octopuses have been recorded up to ten feet long, but most are much smaller. Predators include maray and conger eels, dolphins sharks and other large fish. Octopi have an ink sac that releases ink spurts when threatened. In addition, they use a second secretion to dull the predatorsÕ sense of smell. The lifespan is short, varying from six months to three years depending on species (Boyle 1987). In this paper, I will discuss a few adaptations octopi posses. Many other adaptations exist but will not be discussed.
Habitat Ð Dens
Found in all the oceans of the world at almost all depths, the octopod order must be diverse enough to survive in almost any environment. One of the most diverse habitats is a coral reefs containing open waters as well as colorful backgrounds of rock and coral. This rock and coral serves as shelter. Most of the larger species of octopi live in deeper waters while smaller octopi are more diverse in coastal, tropical waters (WikiAnswers 2008).
Octopi species with less mass are more abundant than larger octopus species and are located in tropical regions. One clear advantage to being smaller in size is the availability of more habitats. As discussed in May 1988, for a 10-fold decrease in characteristic length, it is safe to expect a 180-fold increase in total number of individuals. The habitat increases as fractal dimensions increase. In other words, the smaller of scale used to measure the space, the larger the measurement will be. A 10-fold decrease in physical size produces a roughly 3-fold increase in apparently available space (May 1988). Thus, smaller octopi have a greater selection of habitats than a larger octopus. This may be an advantage if den availability is scarce. Also, coral reefs offer large amounts of small cervices that make size crucial.
Because cephalopods lack a hard outer covering which all other Molluscs have, they must reside in cracks or holes in the rock or other substrate for protection from predators. The loss of a shell allows the octopus to be very mobile, a huge advantage for any predator. Each den is used for a few days as the octopus move often with the exception of brooding females. Octopi rest inside the den with the mantle and head facing away from the entrance and the arms towards the entrance. Studies have shown that larger octopus require larger dens proportional to volume. However, length and diameter (for gill ventilation) taken separately are more crucial for selection. A small entrance to the den provides better protection from predators. Dens are sometimes hidden by the octopus with stones, shells and other solid objects. This serves to minimize the size of the entrance for protection as well as camouflage from predation. (Aronson 1986). This illustrates the octopi ability to change its environment to best suit its needs.
Some species of Octopus show strong territorial characteristics based on food, mating and dens but these studies have been inconclusive as the behaviors are not constant. For example, Octopus dofleini, a species found off the island of Vancouver, has been known for intraspecific fighting and even cannibalism over evenly distributed dens in nature. However, they have been also seen sharing the same plastic tube in captivity. It has been shown that Octopus Cyanea, off the coast of Hawaii, has a requirement for large feeding territories causing over-distribution, while the dens are not defended. Size based dominance is common, at least in laboratory conditions. Octopus bimaculoides are tolerant of other octopus sharing the same ÒdenÓ (Aronson 1986). Many questions are still unanswered regarding territories of O. dofleini and other species.
Feeding
Many Octopi adaptations are specifically suited for assistance to being a predator. These adaptations range from having three separate hearts to maximize blood flood in a closed circulatory system, two rows of suckers on each of the eight arms, hard beak used for crushing, advanced eyesight, advanced nervous system as well as motility. The prey ranges from amphieuran, lamellibranch, gastropod and bivalvia mollusk shells, nautilus and crustaceans. Octopus mimus (as most octopus are) is an opportunistic predator eating whatever it can consume, often preying on bivalves or crabs (Cortez et al 1998).
Many octopi consume sessile bivalves requiring some skill in preparing the bivalve for digestion. O. dofleini opens shells by drilling or by pulling the shells apart. Shells are opened by traction and pulling the valves apart with the powerful suckers while injecting salivary substances to relax the adductor muscles and weaken the prey. Smaller shells were digested partially and not drilled while larger shells were drilled before opening. At least seven other octopi species are known to drill holes in shells of their prey. These species include Octopus vulgaris, Octopus bimaculoides, Octopus bimaculatus, Octopus micropyrsus, Octopus dolfleini, Eledone cirrhosa and Eledone moschata. Drilling is least used and resorted to only when other methods were unsuccessful(Cortez et al 1998).
Drilling is done by a salivary papilla that is below the radula. The papilla also acts as assistant radula. Some chemical dissolution of the shell occurs during the drilling by salivary substances released. Composition and structure of shell affects the size, shape and form of cavity formed while the size of the octopus plays no role in the size of the hole. O. mimus drills holes right into the adductor muscles on most shells which hold the shell together. Also, the adductor muscle is a place for digestive juices to get in and start weakening the bivalve. One hypothesis is that the nervous system is strong at the adductor muscles and can take up the salivary juices and circulate them easily weakening the bivalve more efficiently. No one is sure how O. mimus knows where that spot is on the Protothaca thaca, a common bivalve prey. No other species is that specific in its drilling habits. Evidence has shown that even though they have a very highly developed nervous system and elaborate behavior, they are unable to differentiate basic features of what they handle, shape, weight or size. These features seem critical for finding the right spot to drill on a shell. It is thought to be chemo-tactical O. vulgaris had no preference of location of drilling for a species (Cortez et al 1998).
McQuaid interprets the optimal foraging theory as an optimal feeding behavior that produces the maximum energy yield per unit of time. Bigger prey yields more energy for the predator but time to consume is also greater. Also, energy required to consume the prey is higher and energetically unproductive for the predator. Smaller prey requires less time and energy to consume but also yield less energy and benefit to the consumer. Applied to octopi consumption of bivalves, if given equal availability of a range of prey sizes, octopi will choose the largest prey that it can by opening via traction and pulling; octopus will not consume the smallest shells. When only larger shells are present, drilling will occur. This is because the only way to get energy and eat is to exert the energy to consume the larger shells. Drilling opens the larger shells, as it is more efficient than traction. Medium sized shells are not drilled, as traction is more efficient for medium sized shells. It was found that O. vulgaris drilling of bivalves was dependent on bivalve size; only the large specimens were drilled. The specimens that could be eaten without drilling were preferred (Cortez et al 1998).
Cryptic behaviors Ð
Octopi are known for their ability to change color and texture almost instantly to blend into almost every background to escape predation. This is done by crypsis or mimicry, a common adaptation among the animal kingdom. Octopi achieve this adaptive trait with an advanced chromatophore system, which allows them to change skin appearance with great speed and diversity using pigmented organs in skin (Hanlon 1999).
Chromatophore organs are a combination of many muscles, nerve, glial and sheath cells collectively making it a neuromuscular organ. Pigment granules are inside of an elastic sac called the cytoelastic sacculus. When changing color, the nerve impulses instruct striated muscle cells to change the sacculus form or size. When the muscle is contracted, the chromatophore expands. Upon relaxation, energy is stored in the elastic sacculus and the chromatophore is retracted. This alters the translucency, reflectivity or opacity of the skin and thus the animal. Size and density of chromatophore varies between habitat and lifestyle (Messenger 2001.) The nerves that control the chromatophore are thought to be located in the brain in a similar order to the chromatophore they control. Thus, the pattern of color alteration matches the pattern of neuronal activation often creating a wavelike affect. (Wikipedia 2008) Different brain lobes are organized for the input of visual stimulation and are located in a different area than the motoneurons used to transfer specific information directly to the chromatophore muscles. The entire chromatophore system operates without any feedback system, boggling the scientist (Messenger 2001).
The primary defense of shallow water octopi is crypsis defined by Edmunds 1974 as Óanimals which are camouflaged to resemble part of that environment are said to be crypticÓ. Endler 199 explains ÒÉ a color or pattern is cryptic if it resembles a random sample of the visual background as perceived by the predator at the time and place at which the prey is most vulnerable to predationÓ . Common examples of crypsis in octopus include general background resemblance, disruptive coloration, deceptive resemblance and counter shading. (Hanlon 1999)
O. cyanea lives in the Indo-Pacific oceans and is often the most common on coral reefs, foraging during daylight. Predators include large fish and sharks. O. cyanea are often studied due to their abundance. They have been recorded to change skin patterning up to 4.56 times per minute on a diverse coral reef habitat while foraging for food. In a dead coral reef, the rate of change was 2.57 times per minute. This demonstrates that the diversity of habitat may influence the requirement of change of the skin to remain cryptic, with less diverse habitats requiring less change. It was also noted that up to 46% of the time, the octopi were conspicuous instead of being cryptic (Hanlon 1999).
A researcher, Dr. Hanlon from Woods Hole, found that O. cyanea spends 22-50% of its time outside of a den conspicuous or obviously not blending in to the background. The study was done while obvious predators were present. This illustrates that instead of remaining camouflaged most of the time, the octopus was obviously displaying its self to possible prey and predators. This is inconsistent with previously assumed predator elusion by crypsis. It is unknown what advantages being conspicuous might have in a coral reef environment. It is normally thought of octopuses blending in to get away from predators, but new evidence suggests otherwise. Although crypsis is thought to remain primary defense against predators in shallow water cephalopods, less crypsis is being used than previously thought. Because much of the prey is sessile, crypsis has little benefit to attacking prey. However, in some species, octopi do stalk motile prey and cryptic behaviors are used. Some scientists hypothesize that octopi use keen eyesight to counter their lack of camouflage while being conspicuous. The eyesight could be used to scan for predators, determine foraging path, memorize seascape, and assessing surroundings for future crypsis. It is also thought that crypsis might be a neurophysiologic ally expensive to operate and so it is only used when needed to elude a predator. Also, when foraging, the predator might notice movement (thus crypsis is pointless) and looking like an octopus might have an unknown advantage (Hanlon 1999).
Secondary tactics are used when the primary defense of crypsis fails and escape from predator is necessary either by quick movement or by startling the predator with a conspicuous coloration. In Dr. HanlonÕs study, some secondary defense tactics have been found in O. cyanea when no predator is present. The acute skin patterns were conspicuous and are possibly used as communication among other octopus.
Dr. Hanlon categorized six chronic body patterns and nine acute body patterns although many more chronic patterns exist and are not described. The six mechanisms of crypsis or chronic body patterns are general background resemblance, counter shading /concealment of the shadow, deceptive resemblance- appearing like something else not moving, rarity through rapid neutrally controlled polyphenism, disruptive coloration and cryptic behavior while exhibiting vigilance (Hanlon & Messenger 1996). All dark body patterns (brown, black and other dark colors) are generally used for alarm in octopi, cuttlefishes and most squid in a conspicuous acute pattern. However, the O. cyanea was recorded remaining in the all- dark body pattern for long periods of time, especially in deep sand habitats with dead coral while not exhibiting alarmed behaviors. The all-dark coloration was seen as a cryptic as well as a conspicuous pattern in different situations. This indicated that, at least for O. cyanea, dark coloration is used for more than warning or alarm (Hanlon 1999).
Mimicry has not been described in cephalopod behaviors until recently. Some octopi have been seen trying to imitate parrotfish or other animals. Squid, Sepioteuthis sepioidea , have been documented to swim backwards with two false eyespots and undulating the arms in mimic of parrotfish. This behavior was done to approach small prey (Hanlon & Messenger 1996). ÒMoving RockÓ is a relatively new description in which an octopus mimics nearby rocks and moves on the tips of its arms at the same speed as the light reflection and wave patterns across wide openings between rock ridges. Achieving this complicated task requires assessment of the habitat and an obvious decision to mimic the shape, texture, pattern, brightness and color of nearby rocks. The rocks may not be immediately next to the octopus, forcing the octopus to remember the rock formation as it moves across the open ocean floor. The octopus must remain in a specific position while its eight arm tips ÒtiptoeÓ across the substrate at the same speed as light patterns and wave action appear to move simultaneously (Hanlon 1999). This is a demonstration of learning, memory and adaptation to new environments while receiving new visual and mechanical information.
Overall, the octopiÕs strategy is to use a diverse set of body patterns, often outside of a den, using multiple techniques to remain undetected by its predators. In a complex ecosystem, the refined sensory (vision) , effectors (chromatophore system) capabilities, decision-making capabilities and a well developed central nervous system combine to create a evolutionary well suited predator in the coral reef system.
Movement-
Octopods are related to gastropods and bivalves in the Mollusc phylum, in which the musculature of these organisms is limited at best. However, the cephalopod class has a much more developed musculature system allowing for quicker, more accurate movements as well as an infinite range of motion. Together, these features allow octopi to be well adapted to a predatory niche in the open ocean as well as the coral reef ecosystem. The musculature of the eight limbs gives infinite range of motion of any limb independently (Huffard 06).
Common movements of octopus include stretching, fetching and grabbing. At least for O. vulgaris, marine biologists stereotype such movements for stretching or extension of an arm. When stretching occurs, the muscles in the arms form vertebrate-like joints in which two opposing waves of muscles activation meet during the fetching and reaching movements. The nervous control of this movement is poorly understood but the action could be done simply. In theory, the nerve control could be done by a decentralized nervous system, bypassing the brainÕs motor control center. This allows for simple feed-forward movements without the muscle and nerve actions being processed in the brain, which would take up time. Thus quicker movements are possible (Huffard 06).
In many studies, four types of locomotion in octopus are analyzed; jetting, swimming, crawling or bipedal walking. In these types of movement, different gaits have been observed. During jet propulsion, water is brought into the mantle cavity then expelled through the funnel very rapidly. This jets the organism very quickly backward away from danger. This is physiologically inefficient and leads to oxygen debt inside the body. The oxygen debt leads to high internal mantle pressure, high enough to stop a heart. As a result, jet propulsion is dangerous and used only when extreme speed is needed. During jet propulsion, there is little or no variation of body skin pattern or cryptic posture indicating that a fast escape is more important than predator deception during escape. However, the complicated patterns and positions lacking may be due to mechanical restraints. For example, to reduce drag and friction while jetting, the skin must be smooth and not textured. Smooth skin requires relaxed papillae muscles, which are controlled by the same brain lobe that controls chromatophore expansion. There is a coupling of relaxation of papilla muscles and reduction of pigment in the chromatophores. This results in smooth, pale skin as a result of coupled biomechanical brain activity regardless of behavioral preference (Hufford 2006).
Jetting octopuses are described to be elongated, smooth and possess few color patterns and a streamlined form as camouflage no longer plays an important role. These features are consistent across many species. In addition, some octopi swim dorsoventrally compressed with its arms held out to the side. This might be a position of reduction of drag or a threatening posse. T. mimicus swims dorsoventrally flattened while appearing to look like a similarly colored toxic flatfish. This is a combination of camouflage (mimicry) and escape. A. aculeatus is known for the most complex skin patterning and camouflage capabilities of any octopus often using Òflatfish mimicryÓ also (Hufford2006).
Swimming is quick but as not as fast as jetting. It is used during quick movements such as attacking prey, during mating or territorial defense. Water is moved underneath the body by most or all of the eight arms. Crawling has been seen in both deep sea and shallow water octopi the shallow water Abdopus aculeatus is mostly seen bipedal walking. Crawling is the most common mode of transportation in benthic habitats. It is also the most diverse type of motion. Crawling is done by pushing off the substrate with irregular and intermittent movements, using multiple arms moving ÒbackwardsÓ. Crawling is done for slow movements away from the den in foraging or non-predatory or fleeing movements. This saves energy when speed is not required (Huffard 2006).
Bipedal locomotion is often slower than crawling or walking and leaves six arms free to posse in cryptic postures or pick up any prey. It is used when the octopus must remain camouflaged as an alternative to crawling (Huffard 2006.
Remembering that the main defense of an octopus is camouflage, motion plays an opposing role in defense. To remain camouflaged, the octopus often needs to remain still presenting a conflict of hiding or escaping. Some octopi species get around this problem by frequent shape changing or polyphenism such as the O. cyanea. It often moves while looking like an inedible animal Ðdynamic mimicry also seen in Thaumocotpus mimicus. Another example is a Òmoving rock. During bipedal locomotion, Amphioctopus marginatus and Abdopus aculeatus, also employ predator deception while moving (Huffard 06).
Breeding-
Octopi are sexual and mate polygamous. Breeding occurs seasonally. Male octopi send waves of spematophore (sperm mixed with protective liquids) down one of his arms, the hectocotyluÕs, into the female to fertilize her eggs internally. Then the female will lay her fertilized eggs, (between 10,000 to 500,000 eggs) in a den and guard them until hatching. After hatching, the female octopus will die, leaving her offspring to complete their direct development into adulthood on their own. Boyle (1987) explains, ÒIt is generally thought that cephalopods are fast growing animals that reproduce once and then die.ÕÓ It is thought that the maturation hormone is coupled with natural death in both male and female octopi (Wood 2008)
Conclusion-
Octopi are among the most evolutionary advanced invertebrates with a large number of adaptations for survival and success. Each unique adaptation further increases the octopus productivity in the marine ecosystem. Without a thorough understanding of the delicate organism and its adaptations, humans will lose not only a great learning opportunity but the knowledge and ability of protecting the octopods forever.
Bibliography
Aronson R B, 1986 Life History and Den ecology of Octopus Briareus (Robson) in a Marine Lake. Journal of Experimental Marine biology and Ecology Vol. 95 pp. 37-56
Boyle, P.R. 1987. Cephalopod Life Cycles. Vol 2. Academic Press, London. 441pp.
Cortez T, Castro B G, Guerra A. Drilling behavior of Octopus mimus Gould. Journal of Experimental Marine Biology and Ecology. 224 1998 pages 193-203
Endler JA. 1991. Interactions between predators and prey. In: Krebs JR, Davis NB, eds. Behavioral
Ecology. An Evolutionary Approach. Oxford:Blackwell Scientific Publications, 169Ð196.
Edmunds M. 1974. Defense in Animals. A Survey of Anti-Predator Defenses. New York: Longman Group
Ltd.
Hanlon RT, Messenger JB. 1996. Cephalopod Behaviour. Cambridge: Cambridge University Press.
Hanlon R T, Forsythe J W, Joneschild DE, 1999 Crypsis, conspicuousness, mimicry and polyphenism as antipredator defenses of foraging octopuses on Indo-Pacific coral reefs, with a method of quantifying crypsis from video tapes. Biological Journal of the Linnean Society. 66: pages 1-22
Hufford C L. Locomotion by Abdopus aculeatus (Cephalopda: Octodidae): walking the line between primary and secondary defense. The Journal of Experimental Biology 209, pages 3697-3707. 2006
May R. M. How Many Species are there on Earth? Science, New series Vol. 241 No. 4872 pp. 1441-1449. American Association for the Advancement of Science. Sep. 16, 1988
McQuaid, C.D. 1994. Feeding behavior and selection of bivalve prey by Octopus vulgaris Cuvier. Journal of Experimental Marine Biology and Ecology 177, 187-202.
Messenger J B. Cephalopod chromatophores: neurobiology and natural history. June 2001
Wood, J B, The Cephalopod Page(TCP), Bermuda Institute of Ocean Sciences. 2008
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