Information about Torsion (gastropod)
Torsion is a gastropod synapomorphy which occurs in all gastropods during larval development. Torsion is the rotation of the visceral mass, Mantle and shell 180˚ with respect to the head and foot of the gastropod. This brings the mantle cavity and anus to an anterior position above the head. In some species there is a degree of secondary detorsion or rotation towards the original position, this may be only partial or full detorsion.
During torsion the visceral mass remains almost unchanged anatomically, there are however other important changes to other internal parts of the gastropod. Before torsion the gastropod has an euthyneural nervous system, where the two visceral nerves run parallel down the body. Torsion results in a streptoneural nervous system, where the visceral nerves cross over in a figure of eight fashion. As a result the parietal ganglions end up at different heights. Because of differences between the left and right hand sides if the body, there are different evolutionary pressures on left and right hand side organs and as a result in some species there are considerable differences. Some examples of this are: in the ctenidia (equivalent to lungs or gills) in some species, one side may be reduced or absent; or in some hermaphrodite species the right hand renal system has been transformed into part of the reproductive system.
One role which is seems a likely candidate for the original purpose is that of defence against predators in adult gastropods. By moving the mantle cavity over the head, the gastropod can retract its vulnerable head into its shell. Some gastropods close the entrance to their shell with a tough operculum which is attached to their foot. In evolutionary terms the appearance of an operculum occurred shortly after that of torsion, a possible link with the role of torsion, though there is not sufficient evidence either for or against this theory.
There are many other advantages torsion provided gastropods. For aquatic gastropods the anterior positioning may be useful for preventing sediment getting into the mantle cavity, which is more likely with a posterior positioning due to sediment being stirred up by the motion of the gastropod. In terrestrial species, ventilation is better with anterior positioning. This is due to the back and forth motion of the shell during movement which would tend to block the mantle opening against the foot if it was in a posterior position. Another possible advantage for aquatic species is the osphradium (olfactory sense organs) are moved to an anterior position and are able to sample the water the gastropod is moving into to rather than from, this may help the gastropod locate food or avoid predators.
Evolution of an asymmetrical conispiral shell allowed gastropods to grow larger but resulted in an unbalanced shell. Torsion allows repositioning of the shell, bringing the centre of gravity back to the middle of the gastropod’s body and so helps prevent it falling over.
What ever the original role of torsion, it is clear that further adaptations linked to torsion provide modern gastropods with many advantages.
Development
There are two different developmental stages which cause torsion. The first stage is caused by the development of the asymmetrical velar/foot muscle which has one end attached to the left side of the shell and the other end has fibres attached to the left side of the foot and head. At a certain point in larval development this muscle contracts, causing an anticlockwise rotation of the visceral mass and mantle of roughly 90˚. This process is very rapid taking from a few minutes to a few hours. After this transformation the second stage of torsion development is achieved by differential tissue growth of the left hand side of the organism compared to the right hand side. This second stage is a much slower stage and rotates the visceral mass and mantle a further 90˚. Detorsion is brought about by reversal of the above phases.During torsion the visceral mass remains almost unchanged anatomically, there are however other important changes to other internal parts of the gastropod. Before torsion the gastropod has an euthyneural nervous system, where the two visceral nerves run parallel down the body. Torsion results in a streptoneural nervous system, where the visceral nerves cross over in a figure of eight fashion. As a result the parietal ganglions end up at different heights. Because of differences between the left and right hand sides if the body, there are different evolutionary pressures on left and right hand side organs and as a result in some species there are considerable differences. Some examples of this are: in the ctenidia (equivalent to lungs or gills) in some species, one side may be reduced or absent; or in some hermaphrodite species the right hand renal system has been transformed into part of the reproductive system.
Evolutionary Roles
The original advantage torsion gave gastropods is unclear. It is further complicated by the fact that torsion brought with it a number of problems. A particular problem gastropods had to overcome come was the location where wastes were excreted – above the head which can potentially lead to fouling of the mouth and sense organs. Never the less, the diversity and success of the gastropods suggests torsion is very advantageous indeed.One role which is seems a likely candidate for the original purpose is that of defence against predators in adult gastropods. By moving the mantle cavity over the head, the gastropod can retract its vulnerable head into its shell. Some gastropods close the entrance to their shell with a tough operculum which is attached to their foot. In evolutionary terms the appearance of an operculum occurred shortly after that of torsion, a possible link with the role of torsion, though there is not sufficient evidence either for or against this theory.
There are many other advantages torsion provided gastropods. For aquatic gastropods the anterior positioning may be useful for preventing sediment getting into the mantle cavity, which is more likely with a posterior positioning due to sediment being stirred up by the motion of the gastropod. In terrestrial species, ventilation is better with anterior positioning. This is due to the back and forth motion of the shell during movement which would tend to block the mantle opening against the foot if it was in a posterior position. Another possible advantage for aquatic species is the osphradium (olfactory sense organs) are moved to an anterior position and are able to sample the water the gastropod is moving into to rather than from, this may help the gastropod locate food or avoid predators.
Evolution of an asymmetrical conispiral shell allowed gastropods to grow larger but resulted in an unbalanced shell. Torsion allows repositioning of the shell, bringing the centre of gravity back to the middle of the gastropod’s body and so helps prevent it falling over.
What ever the original role of torsion, it is clear that further adaptations linked to torsion provide modern gastropods with many advantages.
References
- Brusca, R.C. Brusca, G.J. (1990) Invertebrates. Sinauer Associates, Inc. Massachusetts
- Ruppert, E.E et al. (2004) Invertebrate Zoology. Seventh edition. Brooks/Cole – Thompson Learning. Belmont, California
Gastropoda
Cuvier, 1797
Subclasses
Eogastropoda (True Limpets and relatives)
Orthogastropoda
The gastropods, also previously known as gasteropods, or univalves
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Cuvier, 1797
Subclasses
Eogastropoda (True Limpets and relatives)
Orthogastropoda
The gastropods, also previously known as gasteropods, or univalves
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In evolutionary biology, a synapomorphy is a derived character state shared by two or more terminal groups (taxa included in a cladistic analysis as further indivisible units) and inherited from their most recent common ancestor.
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larva (Latin; plural larvae) is a juvenile form of animal with indirect development, undergoing metamorphosis (for example, insects or amphibians).
The larva can look completely different from the adult form, for example, a caterpillar differs from a butterfly.
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The larva can look completely different from the adult form, for example, a caterpillar differs from a butterfly.
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viscus (IPA: /ˈvɪskəs/) (plural: viscera /ˈvɪsərə/
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The mantle is an organ found in mollusks. It is the dorsal body wall covering the main body, or visceral mass. In many species, the epidermis of this organ secretes calcium carbonate to create a shell.
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gastropod shell is an animal shell which is part of the body of a gastropod or snail. It is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage.
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Anterior can refer to:
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- Anterior and posterior, both Anatomical terms of location
- Anterior (band), A Welsh Metal band
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Asymmetry is the absence of, or a violation of, a symmetry.
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Asymmetry in organisms
Due to how cells divide in organisms, asymmetry in organisms is fairly usual in at least one dimension, with biological symmetry also being common in at least one dimension...... Click the link for more information.
MUSCLE (multiple sequence comparison by log-expectation) is public domain, multiple sequence alignment software for protein and nucleotide sequences.
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Biological tissue is a collection of interconnected cells that perform a similar function within an organism.
The study of tissue is known as histology, or, in connection with disease, histopathology.
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The study of tissue is known as histology, or, in connection with disease, histopathology.
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nervous system of an animal coordinates the activity of the muscles, monitors the organs, constructs and also stops input from the senses, and initiates actions. Prominent parts of a nervous system include neurons and nerves, which are used in coordination.
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A nerve is an enclosed, cable-like bundle of axons (the long, slender projection of a neuron). Neurons are sometimes called nerve cells, though this term is technically imprecise since many neurons do not form nerves, and nerves also include the glial cells that
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Parallel is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. The existence and properties of parallel lines are the basis of Euclid's parallel postulate.
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Parietal may refer to:
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- Parietal lobe of the brain
- Parietal bone of the skull
- Parietal scales of a snake lie in the general region of the parietal bone.
- Parietal cell in the stomach
- Parietal pleura
- Parietal pericardium
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ganglion (pl. ganglia) is a tissue mass, which is composed mainly of somata and dendritic structures, which often interconnect with each other to form a complex system of ganglia known as a plexus.
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hermaphrodite is an organism that posses both male and female genetalia.[1] In many species, hermaphroditism is a common part of the life-cycle, particularly in some asexual animals and some plants.
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Nephridia are invertebrate organs which function similarly to kidneys. They remove metabolic wastes from an animal's body. They are present in many different invertebrate lines. There are two basic types, metanephridia and protonephridia.
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The reproductive system is the ensembles and interactions of organs and/or substances within an organism that strictly pertain to reproduction. As an example, this would include in the case of female mammals, the hormone estrogen, ova, and the uterus and the vagina, and the breasts.
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Excretion is the process of eliminating waste products of metabolism and other non-useful materials.[1] It is an essential process in all forms of life.
In single-celled organisms, waste products are discharged directly through the surface of the cell.
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In single-celled organisms, waste products are discharged directly through the surface of the cell.
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Fouling refers to the accumulation and deposition of living organisms (biofouling) and certain non-living material on hard surfaces, most often in an aquatic environment. This can be the fouling of ships, pilings, and natural surfaces in the marine environment (marine fouling),
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mouth, also known as the buccal cavity or the oral cavity, is the orifice through which an organism takes in food and water.
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Location
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sensory system: in this case, vision, for the visual system. ]]
A sensory system is a part of the nervous system responsible for processing sensory information.
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A sensory system is a part of the nervous system responsible for processing sensory information.
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predation describes a biological interaction where a predator organism feeds on another living organism or organisms known as prey.[1] Predators may or may not kill their prey prior to feeding on them.
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neutrality is disputed.
* It may contain original research or unverifiable claims.
Please help [ improve the article] or discuss these issues on the talk page. The term adult has three distinct meanings:
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* It may contain original research or unverifiable claims.
Please help [ improve the article] or discuss these issues on the talk page. The term adult has three distinct meanings:
- Grown man or woman; mature person.
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Vulnerable may refer to:
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- Vulnerability
- Vulnerable species
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operculum (plural : opercula or operculums) of gastropods is a corneous or calcareous structure which exists in some families of snails, and which, in its most typical condition, fits perfectly into the opening or aperture of the shell.
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The term aquatic refers to water and can be either a noun or an adjective. Dictionary definitions do not specify what kind of water, although in both general use and in the sciences, the implication is often that of fresh water.
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Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of water or other liquid. Sedimentation is the deposition by settling of a suspended material.
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Terrestrial animals are animals that live predominantly or entirely on land, as compared with aquatic animals, which live predominantly or entirely in the water (e.g., fish, lobsters, octopuses), or amphibians, which rely on a combination of aquatic and terrestrial habitats (e.g.
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