Over the years various classification systems have been developed to describe orientation, and the terminology used within these systems is confusing. Popular systems in which orientation can be described are kinesis and taxis. Orientation behavior represents an example of the type of behavior referred to as maintenance behavior.
The simplest response through which invertebrates find a suitable location to live is referred to as kinesis. The response is not directed toward or away from a stimulus, but nevertheless places the animal in an optimum location. Changes in activity, rate of movement, and/or turning is non-directional and directly related to the intensity of the stimulus from moment to moment. Kinetic responses often occur when the source of the stimulus cannot be sensed at a distance. Several types of kinesis are recognized including: barokinesis, hy-grokinesis, orthokinesis, photokinesis, thigmokinesis, and klinokinesis. Theories regarding kinesis are made more con fusing when describing orientation using two kineses. For example, an animal that changes its rate of movement under illumination is said to exhibit "photo-orthokinesis." The words negative and positive also can be added to these terms in order to further adapt their meaning; an animal that is active under little or no illumination is said to exhibit "photo-negative kinesis."
Examples of kinesis: BAROKINESIS
Various classes of invertebrates react to pressure changes, including increased locomotion because of changes in barometric pressure. Larvae of the crab genus Carcinus swim toward the water's surface when pressure increases. Copepods, adult and larval polychaetes, and jellyfish medusae are other examples of animals that increase their activity in response to pressure changes.
Increased locomotion in reaction to changes in humidity is referred to as hygrokinesis. Some species of nematoda are stimulated to move due to conditions of low humidity and are less active when there are high degrees of humidity. The increase in activity under dry conditions increases the chances of finding a suitable damp environment. Increasing locomotion based on fluctuations in humidity levels is important among terrestrial invertebrates (e.g., planaria) because, with the exception of insects, very few have developed methods of conserving significant amounts of water.
Increased locomotion resulting from changes in levels of light is called photokinesis. Flatworms (e.g., Dugesia doroto-cephala, Dugesia tigrina) all increase their activity depending on the intensity of illumination that surrounds them. Other examples of organisms that exhibit photokinesis include gill and skin fluke larvae (monogenea), jellyfishes, and rotifers. Not all invertebrates respond to increases in illumination. When conducting studies on the effect of light on activity levels it is important to separate the role of illumination from the temperature increases produced by light.
This form of kinesis is defined as increased locomotion in response to changes in contact with the immediate physical environment. Some invertebrates are more active in open spaces than in closed spaces. Examples of closed spaces include cracks and crevices. For example, the contraction of longitudinal muscles in nematodes produces a whiplike un-dulatory motion that relies on environmental substrata for the body to push against; when they are placed in fluid without substrata they thrash around.
Orthokinesis refers to changes in the speed or frequency of movement in reaction to changes in the intensity of a stimulus. Stimuli that produces a change in direction (such as turning) is known as klinokinesis. Movement influenced by gravity is known as geokinesis, and changes of movement caused by water currents is known as rheokinesis. Kinetic responses also can be in reaction to chemical and temperature stimuli (chemokinesis and thermokinesis, respectively). The oncomi-ricidia (larvae) of the Monogenea have been shown to change speed and direction in reaction to gravity, current, and light stimuli.
Taxis is a more complex response through which invertebrates find a suitable location to live. The response is directed toward or away from a stimulus to place the animal in an optimum location to inhabit. Changes in activity, rate of movement, and/or direction are related to the intensity of the stimulus gradient from moment to moment. Taxis differs from kinesis in that taxic responses allow invertebrates to engage in specific activity as opposed to general activity, relative to a stimulus source.
Taxis can be characterized by:
1. Whether the animal moves toward or away from a stimulus.
2. The way in which the animal moves.
3. The complexity of the sensory structures used to detect the stimulus.
An invertebrate with a single visual receptor can determine the direction of a light source simply by moving the receptor (such as turning its head) and sampling the stimulus gradient produced by the light. If the animal is attracted to light, its receptor becomes more active the closer it moves toward the light source. The majority of invertebrates have at least two receptors; the second receptor allows the animal to make simultaneous comparisons from each side of its body from moment to moment as it moves through a stimulus gradient.
Several different types of taxis are recognized including, phototaxis, klinotaxis, phototropotaxis, and phototelotaxis. Moreover, movements toward the source of stimulation are called positive, and movements away from the source are called negative. For example, movements toward a source of light is called "positive phototaxis," while movement away from light is referred to as "negative phototaxis."
Examples of various form of taxis include: PHOTOTAXIS
An animal that moves toward (positive phototaxis) or away (negative phototaxis) from light is exhibiting phototaxis. Movement is parallel to the direction of light. Examples of animals that exhibit this type of behavior are jellyfishes, on-comiricidia (monogenea larvae), and some echinoderms (sea stars and sea urchins). Planarians are negatively phototoxic in that they seek less illuminated areas.
A change in directed movement based on successive comparisons of a stimulus is referred to as klinotaxis. The larvae of many flies, including the common house fly, Musca domes-
tica, find the location of a light source by moving their head left and right in order to compare the relative intensity of a stimulus. Derivatives of this behavior also occur to many lower invertebrates, including planaria and echinoderms.
Animals that display phototropotaxis undergo movement toward a source of illumination based on a comparison of information gathered by their eyes. The animal orientates toward the direction of light (assuming it exhibits positive phototropotaxis) and moves in a direction that keeps the eyes equally stimulated. Phototropotaxis is best demonstrated experimentally in what is known as a "two-light experiment." In this design, an animal is placed between two light sources. Pho-totropotaxis is indicated if the animal follows a path between the two light sources thereby stimulating the photoreceptors equally. Phototropotaxis also can be detected by preventing visual information reaching one eye. This test can be done by removing or painting an eye. Phototropotaxis is indicated if the animal begins to engage in "circus movements." For example, when the honey bee (Apis mellifera) is blinded in one eye it will perform "circus movements" (sideways movements) toward a light source. Positive phototropotaxis can be detected if the animals continuously turns toward a light source; if an animal is negatively phototropotaxic it will continuously turn away from a light source (i.e., it will turn so that its blind side faces the light).
Movement directed toward one of two sources of illumination is known as phototelotaxis. This form of behavior is dependent upon the type of sophistication present in the visual receptors. If the eye is capable of forming an image that will allow the animal to identify the source of illumination, the animal can move directly toward the source without the need for comparing two sources of illumination. Pho-totelotaxis is found in invertebrates possessing compound eyes (arthropods such as insects and crustaceans) including hermit crabs, isopods, and mysid crustaceans. There are no known examples of this behavior among the lower invertebrate and invertebrate deuterostomes.
Geotaxis refers to movement along lines of gravitational force. As with all forms of taxic behavior, the direction of movement can be either positive or negative. Geotaxis is observed on surfaces (especially inclines), in water, air, sand, or mud. The most pronounced examples of geotaxis are found in invertebrates that live in sand or mud. Many examples of geotatic behavior occur in animals with statocysts, although there are examples where statocysts are not involved. The sta-tocyst is a heavy object (statolith) located in a fluid-filled chamber used to detect gravitational forces. When the stato-cyst is moved, the statolith induces movement by activating various sensory and motor systems that return the animal back to its normal balance. Geotaxis is most readily studied in invertebrates by having an animal crawl on a vertical glass plate that is gently rotated or inclined. In order to test burrowing animals such as polychaetes the animal can be sandwiched
between two glass plates filled with sand, or on a rotating table or centrifuge. Examples of geotactic behavior can be found in medusa (e.g., Cotylorhiza tuberculata), planaria (e.g., Convoluta roscoffensis), and polychaetes (e.g., Arenicola grubei). There are many cases of invertebrates that exhibit geotatic behavior without statocysts, including Helix, Limax, the sea anemone Cerianthus, monogeneans, starfishes, and sea urchins.
Rheotactic behavior involves movement directed by water flow, and can be found in most classes of invertebrates that inhabit water. Examples of organisms that display rheotaxis include anemones, planarians, monogeneans, and many protostomes (gastropods, crustaceans, and both nymphs and larvae of insects).
Thigmotaxis is defined as movement when direction is determined by a stimulus making contact with an animal's body. Turbellarians are positively thigmotactic on their ventral sides, and negatively thigmotactic on their dorsal sides, which keeps their ventral side against the substrate. Movement influenced by air currents is known as Anemotaxis. Taxic responses also are created by chemical and temperature stimuli (referred to as chemotaxis and thermotaxis respectively).
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