Sinistral and dextral

Last updated August 28, 2023

Sinistral and dextral, in some scientific fields, are the two types of chirality ("handedness") or relative direction. The terms are derived from the Latin words for "left" (sinister) and "right" (dexter). Other disciplines use different terms (such as dextro- and laevo-rotary in chemistry, or clockwise and anticlockwise in physics) or simply use left and right (as in anatomy).

Relative direction and chirality are distinct concepts. Relative direction is from the point of view of the observer; a completely symmetric object has a left side and a right side, from the observer's point of view, if the top and bottom and direction of observation are defined. Chirality, however, is observer-independent: no matter how one looks at a right-hand screw thread, it remains different from a left-hand screw thread. Therefore, a symmetric object has sinistral and dextral directions arbitrarily defined by the position of the observer, while an asymmetric object that shows chirality may have sinistral and dextral directions defined by characteristics of the object, regardless of the position of the observer.

Biology

Gastropods

Because the coiled shells of gastropods are asymmetric, they possess a quality called chirality–the "handedness" of an asymmetric structure.

Over 90%^[1] of gastropod species have shells in which the direction of the coil is dextral (right-handed). A small minority of species and genera have shells in which the coils are almost always sinistral (left-handed). Very few species show an even mixture of dextral and sinistral individuals (for example, Amphidromus perversus ).^[2]

Flatfish

The most obvious characteristic of flatfish, other than their flatness, is their asymmetric morphology: both eyes are on the same side of the head in the adult fish. In some families of flatfish, the eyes are always on the right side of the body (dextral or right-eyed flatfish), and in others, they are always on the left (sinistral or left-eyed flatfish). Primitive spiny turbots include equal numbers of right- and left-sided individuals, and are generally more symmetric than other families.^[3]

Geology

In geology, the terms sinistral and dextral refer to the horizontal component of the movement of blocks on either side of a fault or the sense of movement within a shear zone. These are terms of relative direction, as the movement of the blocks is described relative to each other when viewed from above. Movement is sinistral (left-handed) if the block on the other side of the fault moves to the left, or if straddling the fault the left side moves toward the observer. Movement is dextral (right-handed) if the block on the other side of the fault moves to the right, or if straddling the fault the right side moves toward the observer.^[4]

Related Research Articles

Body relative directions are geometrical orientations relative to a body such as a human person's body. The most common ones are: left and right; forward(s) and backward(s); up and down. They form three pairs of orthogonal axes.

A flatfish is a member of the ray-finned demersal fish order Pleuronectiformes, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the head, one or the other migrating through or around the head during development. Some species face their left sides upward, some face their right sides upward, and others face either side upward.

A mirror image is a reflected duplication of an object that appears almost identical, but is reversed in the direction perpendicular to the mirror surface. As an optical effect it results from reflection off from substances such as a mirror or water. It is also a concept in geometry and can be used as a conceptualization process for 3-D structures.

Asymmetry is the absence of, or a violation of, symmetry. Symmetry is an important property of both physical and abstract systems and it may be displayed in precise terms or in more aesthetic terms. The absence of or violation of symmetry that are either expected or desired can have important consequences for a system.

Handedness is a human attribute reflecting the unequal distribution of fine motor skill between the left and right hands.

Left and right or left–right may refer to:

Dakshinavarti shankha, also referred to as Valampuri shankhu and Sri Lakshmi shankha is a sacred Hindu conch. It refers to the shell of a large sea snail from the Indian Ocean, but one that has a rare reverse-turning spiral.

In structural geology, vergence refers to the direction of the overturned component of an asymmetric fold. In simpler terms, vergence can be described as the horizontal direction in which the upper-component of rotation is directed. Vergence can be observed and recorded in folds to help a geologist determine characteristics of larger fold areas. Vergence is used to provide an overall characterization, in the symmetry of folds, and can be used to observe changes in small-scale structures in relation to the axis of a large fold. The vergence of a fold lies parallel to the surrounding surfaces of a fold, so if these surrounding surfaces are not horizontal in nature, the vergence of the fold will be inclined. For a fold, the direction, as well as the extent to which vergence occurs, can be calculated from the strike and dip of the axial surfaces, along with that of the enveloping surfaces. These calculations can be very useful for geologists in determining the overall elements of larger areas.

The gastropod shell is part of the body of a gastropod or snail, a kind of mollusc. The shell is an exoskeleton, which protects from predators, mechanical damage, and dehydration, but also serves for muscle attachment and calcium storage. Some gastropods appear shell-less (slugs) but may have a remnant within the mantle, or in some cases the shell is reduced such that the body cannot be retracted within it (semi-slug). Some snails also possess an operculum that seals the opening of the shell, known as the aperture, which provides further protection. The study of mollusc shells is known as conchology. The biological study of gastropods, and other molluscs in general, is malacology. Shell morphology terms vary by species group.

Amphidromus is a genus of tropical air-breathing land snails, terrestrial pulmonate gastropod mollusks in the family Camaenidae. The shells of Amphidromus are relatively large, from 25 mm (0.98 in) to 75 mm (3.0 in) in maximum dimension, and particularly colorful. During the 18th century, they were among the first Indonesian land snail shells brought to Europe by travelers and explorers. Since then, the genus has been extensively studied: several comprehensive monographs and catalogs were authored by naturalists and zoologists during the time period from the early 19th to the mid 20th centuries. Modern studies have focused on better understanding the evolutionary relationships within the group, as well as solving taxonomic problems.

Turbinella pyrum, common names the chank shell, sacred chank or chank, also known as the divine conch or referred to simply as a conch, is a species of very large sea snail with a gill and an operculum, a marine gastropod mollusk in the family Turbinellidae. This species occurs in the Indian Ocean.

The term chiral describes an object, especially a molecule, which has or produces a non-superposable mirror image of itself. In chemistry, such a molecule is called an enantiomer or is said to exhibit chirality or enantiomerism. The term "chiral" comes from the Greek word for the human hand, which itself exhibits such non-superimposeability of the left hand precisely over the right. Due to the opposition of the fingers and thumbs, no matter how the two hands are oriented, it is impossible for both hands to exactly coincide. Helices, chiral characteristics (properties), chiral media, order, and symmetry all relate to the concept of left- and right-handedness.

Two-dimensional rotation can occur in two possible directions. Clockwise motion proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back up to the top. The opposite sense of rotation or revolution is anticlockwise (ACW) or counterclockwise (CCW).

Amphidromus perversus is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Camaenidae.

Port and starboard are nautical terms for watercraft, aircraft and spacecraft, referring respectively to the left and right sides of the vessel, when aboard and facing the bow (front).

Chirality is a property of asymmetry important in several branches of science. The word chirality is derived from the Greek χειρ (kheir), "hand", a familiar chiral object.

Ayna is a monotypic genus of air-breathing land snails, terrestrial gastropod mollusks in the family Enidae. The sole species in the genus is Ayna mienisi. It was described as Zebrina (Ramusculus) mienisi. The genus Ramusculus however is a monotypic genus only distantly related to Ayna. The closest relative of Ayna is probably Clausilioides.

Jeremy was a left-coiled garden snail investigated by biologists. The snail had a rare condition which caused its shell to coil to the left; in most snails the shell coils to the right. At first it was thought to be a rare genetic mutation, although later work revealed that it was likely due to an accident in early development.

Left-right asymmetry is the process in early embryonic development that breaks the normal symmetry in the bilateral embryo. In vertebrates, left-right asymmetry is established early in development at a structure called the left-right organizer and leads to activation of different signalling pathways on the left and right of the embryo. This in turn cause several organs in adults to develop LR asymmetry, such as the tilt of the heart, the different number lung lobes on each side of the body and the position of the stomach and spleen on the right side of the body. If this process does not occur correctly in humans it can result in the syndromes heterotaxy or situs inversus.

References

↑ Schilthuizen M. & Davison A. (2005). "The convoluted evolution of snail chirality". Naturwissenschaften 92(11): 504–515. doi : 10.1007/s00114-005-0045-2.
↑ Amphidromus perversus (Linnaeus, 1758).
↑ Chapleau, Francois & Amaoka, Kunio (1998). Paxton, J.R. & Eschmeyer, W.N. (eds.). Encyclopedia of Fishes. San Diego: Academic Press. xxx. ISBN 0-12-547665-5.
↑ Park, R.G. (2004). Foundation of Structural Geology (3 ed.). Routledge. p. 11. ISBN 978-0-7487-5802-9.

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[1] Schilthuizen M. & Davison A. (2005). "The convoluted evolution of snail chirality". Naturwissenschaften 92(11): 504–515. doi : 10.1007/s00114-005-0045-2.

[2] Amphidromus perversus (Linnaeus, 1758).

[EoF-3] Chapleau, Francois & Amaoka, Kunio (1998). Paxton, J.R. & Eschmeyer, W.N. (eds.). Encyclopedia of Fishes. San Diego: Academic Press. xxx. ISBN 0-12-547665-5.

[Park-4] Park, R.G. (2004). Foundation of Structural Geology (3 ed.). Routledge. p. 11. ISBN 978-0-7487-5802-9.

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