biology

List of animals featuring external asymmetry

List of animals featuring external asymmetry is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand List of animals featuring external asymmetry rather than just read about it. In short: This is a list of animals that markedly feature external asymmetry in some form. They are exceptions to the general pattern of symmetry in biology.

Key takeaways

List of animals featuring external asymmetry belongs to biology; place it in that map before memorising details.
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Reproduce the core statement of List of animals featuring external asymmetry from memory before moving on to harder problems.

Reference excerpt

This is a list of animals that markedly feature external asymmetry in some form. They are exceptions to the general pattern of symmetry in biology. In particular, these animals do not exhibit bilateral symmetry, which permits streamlining and is common in animals.

Birds The crossbill has an unusual beak in which the upper and lower tips cross each other. The wrybill is the only species of bird with a beak that is bent sideways (always to the right). Many owl species, such as the barn owl, have asymmetrically positioned ears that enhance sound positioning.

Fish

Many flatfish, such as flounders, have eyes placed asymmetrically in the adult fish. The fish has the usual symmetrical body structure when it is young, but as it matures and moves to living close to the sea bed, the fish lies on its side, and the head twists so that both eyes are on the top. The jaws of the scale-eating cichlid Perissodus microlepis occur in two distinct morphological forms. One morph has its jaw twisted to the left, allowing it to eat scales more readily on its victim's right flank. The other morph has its jaw twisted to the right, which makes it easier to eat scales on its victim's left flank. The relative abundance of the two morphs in populations is regulated by frequency-dependent selection.

Mammals The narwhal has a helical tusk on its upper left jaw. Odobenocetops, an extinct toothed whale, may have possessed similar asymmetrical dentition, though it differed from the narwhal in possessing two erupted, rear-facing tusks with the right significantly longer than the left. The sperm whale (Physeter macrocephalus) has a single nostril on its upper left head. The right nostril forms a phonic lip. The source of the air forced through the phonic lips is the right nasal passage. While the left nasal passage opens to the blow hole, the right nasal passage has evolved to supply air to the phonic lips. It is thought that the nostrils of the land-based ancestor of the sperm whale migrated through evolution to their current functions, the left nostril becoming the blowhole and the right nostril becoming the phonic lips. The fin whale (Balaenoptera physalus) has complex and asymmetrical coloration on its head, with the jaw dark grey on one side and white on the other. The caribou or reindeer (Rangifer tarandus) has asymmetrical antlers. Adult males, in particular, usually possess one brow tine formed into a "shovel" shape. Suggestions for its function include eye protection during antler-threshing courtship displays or as an offensive weapon during the mating season. Honey badgers of the subspecies signata have a second lower molar on the left side of their jaws, but not the right. Humans show a systematic aurofacial asymmetry, meaning that the face (eyes, nose and mouth) are displaced to the left with respect to the midplane between the ears. In young children this asymmetry is on average 4 degrees and is easily recognized (See also: Axial twist theory).

Reptiles

Iwasaki's snail-eater snake (Pareas iwasakii) is a snail-eating specialist; even newly hatched individuals feed on snails. It has asymmetric jaws, which facilitates feeding on snails with dextral (clockwise coiled) shells. A consequence of this asymmetry is that this snake is much less adept at preying on sinistral (counterclockwise coiled) snails.

Invertebrates Fiddler crabs and hermit crabs have one claw much larger than the other, which in hermit crabs is used as a makeshift "door" to block the opening of the shell when it retracts inside. If a male fiddler loses its large claw, it grows another on the opposite side after molting. A soft abdomen is also present in a hermit crab as an asymmetrical modification due to the asymmetrical nature of the snail shells they inhabit. All gastropods are asymmetrical. This is easily seen in snails and sea snails, which have helical shells. At first glance slugs appear externally symmetrical, but their pneumostome (breathing hole) is always on the right side. The origin of asymmetry in gastropods is a subject of scientific debate. Other gastropods develop external asymmetry, such as Glaucus atlanticus that develops asymmetrical cerata as they mature. Histioteuthis is a genus of squid, commonly known as the cock-eyed squid, because in all species the right eye is normal-sized, round, blue and sunken; whereas the left eye is at least twice the diameter of the right eye, tubular, yellow-green, faces upward, and bulges out of the head. Sponges are asymmetrical. Corals build colonies that are not symmetrical, but the individual polyps exhibit radial symmetry. Alpheidae feature asymmetrical claws that lack pincers, the larger of which can grow on either side of the body, and if lost can develop on the opposite arm instead. Certain polyopisthocotylean monogeneans are asymmetrical, as an adaptation to their attachment to the gill of their fish hosts. Certain parasitic copepods that live inside the gill chamber of their fish hosts are asymmetrical. Lobsters of the genera Homarus, Nephrops, and Homarinus have dimorphic claws, a crushing one and a cutting one. Thrips have asymmetrical mouthparts, unique among insects.

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Illustrations

Worked examples

Example 1 — a first encounter with List of animals featuring external asymmetry

Start with the simplest possible case. Write down what List of animals featuring external asymmetry claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to List of animals featuring external asymmetry before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about List of animals featuring external asymmetry ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of List of animals featuring external asymmetry

In research: List of animals featuring external asymmetry appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses List of animals featuring external asymmetry in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: List of animals featuring external asymmetry is common in secondary-school and first-year university syllabi. It links to neighbouring topics Animal anatomy, Asymmetry, Lists of animals, so understanding it makes those chapters shorter.
In everyday life: Look for List of animals featuring external asymmetry outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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Frequently asked questions

What is List of animals featuring external asymmetry in simple terms?

This is a list of animals that markedly feature external asymmetry in some form. They are exceptions to the general pattern of symmetry in biology.

Why does List of animals featuring external asymmetry matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study List of animals featuring external asymmetry?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on List of animals featuring external asymmetry.