List of animals featuring external asymmetry

Short description: none

The beak of a wrybill is bent toward the right

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.[1]

This list is incomplete; you can help by expanding it.

## Contents

* 1 Birds
* 2 Fish
* 3 Mammals
* 4 Reptiles
* 5 Invertebrates
* 6 Gallery
* 7 See also
* 8 References

## Birds

The crossbill has an unusual beak in which the upper and lower tips cross each other.[2]

The wrybill is the only species of bird with a beak that is bent sideways (always to the right).[2]

Many owl species, such as the barn owl, have asymmetrically positioned ears that enhance sound positioning.[3]

## Fish

Fish: Dorsal view of right-bending (left) and left-bending (right) jaw morphs[4]

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.[5]

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.[4][6][7]

## 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.[8]

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.[9]

The caribou or reindeer (Rangifer tarandus) has asymmetrical antlers. Adult males, in particular, usually possess one brow tine formed into a "shovel" shape.[10] Suggestions for its function include eye protection during antler-threshing courtship displays[11] or as an offensive weapon during the mating season.[12]

Honey badgers of the subspecies signata have a second lower molar on the left side of their jaws, but not the right.[13]

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).[14]

## Reptiles

Skull of Pareas iwasakii

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. [15] [16] [17] [18] [19] [20] [21] [22]

## 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.[23] 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.[24]

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.[25] 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.[1]

Corals build colonies that are not symmetrical, but the individual polyps exhibit radial symmetry.[26]

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.[27]

Certain parasitic copepods[which?] 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.[28]

Thrips have asymmetrical mouthparts, unique among insects.[29]

## Gallery

* Head of a male crossbill

* Side and top view of a wrybill's beak

* A winter flounder

* A fiddler crab

* A hermit crabs different sized claws

* A Roman snail

* Chicoreus palmarosae, a sea snail

* A red slug, clearly showing the pneumostome

* A cock-eyed squid with the bottom eye larger than the top

* A sponge

* Coral

* Protocotyle euzetmaillardi, a polyopisthocotylean monogenean

* Heteromicrocotyloides megaspinosus, a polyopisthocotylean monogenean

* Silhouettes of bodies of 8 species of polyopisthocotylean monogeneans, all asymmetrical

* Asymmetric mouthparts of Heliothrips

* Plates of Stegosaurus stenops

* Frill of Lokiceratops rangiformis

## See also

* Symmetry in biology

## References

1. ↑ 1.0 1.1 Symmetry, biological, cited at FactMonster.com from The Columbia Electronic Encyclopedia (2007).
2. ↑ 2.0 2.1 Ngutuparore, the wrybill. Narena Olliver, New Zealand Birds Limited. Retrieved 8 February 2012.
3. ↑ Krings, Markus; Castanhinha, Rui; Müller-Limberger, Elena; Wipfler, Benjamin; Wagner, Hermann (August 2020). "Ear asymmetry in Tengmalm's owl (Aegolius funereus): Two phases of asymmetrical development of the squamoso-occipital wing". Zoology 141. doi:10.1016/j.zool.2020.125814. PMID 32629330. Bibcode: 2020Zool..14125814K.
4. ↑ 4.0 4.1 Lee, H. J.; Kusche, H.; Meyer, A. (2012). "Handed Foraging Behavior in Scale-Eating Cichlid Fish: Its Potential Role in Shaping Morphological Asymmetry". PLOS ONE 7 (9). doi:10.1371/journal.pone.0044670. PMID 22970282. Bibcode: 2012PLoSO...744670L.
5. ↑ American Plaice, Canadian Fisheries. Retrieved 8 February 2012.
6. ↑ Hori, M. (1993). "Frequency-dependent natural selection in the handedness of scale-eating cichlid fish". Science 260 (5105): 216–219. doi:10.1126/science.260.5105.216. PMID 17807183. Bibcode: 1993Sci...260..216H.
7. ↑ Stewart, T. A.; Albertson, R. C. (2010). "Evolution of a unique predatory feeding apparatus: functional anatomy, development and a genetic locus for jaw laterality in Lake Tanganyika scale-eating cichlids". BMC Biology 8 (1): 8. doi:10.1186/1741-7007-8-8. PMID 20102595.
8. ↑ Muizon, Christian de; Domning Fls, Daryl P. (April 2002). "The anatomy of Odobenocetops (Delphinoidea, Mammalia), the walrus-like dolphin from the Pliocene of Peru and its palaeobiological implications". Zoological Journal of the Linnean Society 134 (4): 423–452. doi:10.1046/j.1096-3642.2002.00015.x.
9. ↑ Clark (2008). "Fin Whale". https://www.adfg.alaska.gov/static/education/wns/fin_whale.pdf.
10. ↑ Goss, Richard J. (1990-06-01). "Interactions between asymmetric brow tines in caribou and reindeer antlers". Canadian Journal of Zoology 68 (6): 1115–1119. doi:10.1139/z90-165. ISSN 0008-4301. Bibcode: 1990CaJZ...68.1115G. https://www.nrcresearchpress.com/doi/pdf/10.1139/z90-165.
11. ↑ William O. Pruitt, Jr (1966-01-01). "The Function of the Brow-Tine in Caribou Antlers" (in en). Arctic 19 (2): 110–113. doi:10.14430/arctic3419. ISSN 1923-1245. https://journalhosting.ucalgary.ca/index.php/arctic/article/view/66469.
12. ↑ Bubenik, A. B. (1975). "Significance of antlers in the social life of barren ground caribou" (in English). Biol Pap Univ Alaska Spec Rep. https://agris.fao.org/agris-search/search.do?recordID=US201302990970.
13. ↑ *Rosevear, D. R. (1974). The Carnivores of West Africa. London: British Museum (Natural History). pp. 114–16. ISBN 978-0-565-00723-2. https://archive.org/details/carnivoresofwest00rose.
14. ↑ de Lussanet, M.H.E. (2019). "Opposite asymmetries of face and trunk and of kissing and hugging, as predicted by the axial twist hypothesis". PeerJ 7. doi:10.7717/peerj.7096. PMID 31211022.
15. ↑ Götz M, (2002). "The feeding behavior of the snail-eating snake Pareas carinatus Wagler 1830 (Squamata: Colubridae)". Amphibia-Reptilia 23: 487-493.
16. ↑ Hirata T, Ota H, (1993). "Predation on snails by the pareatine snake Pareas iwasakii". Japanese Journal of Herpetology. 15: 90-91.
17. ↑ Hori M, (1993). "Frequency-dependent natural selection in the handedness of scale-eating cichlid fish". Science. 260: 216-219
18. ↑ Hoso M, (2007). "Oviposition and hatchling diet of a snail-eating snake Pareas iwasakii (Colubridae: Pareatinae)". Current Herpetology. 26: 41-43.
19. ↑ Hoso, M., T. Asami, and M. Hori. (2007). "Right-handed snakes: convergent evolution of asymmetry for functional specialization". Biology Letters. 3: 169-172.
20. ↑ Hoso M, Hori M, (2006). "Identification of molluscan prey from feces of Iwasaki's slug snake, Pareas iwasakii". Herpetological Review. 37: 174–176.
21. ↑ Hoso M, Kameda Y, Wu SP, Asami T, Kato M, Hori M, (2010). "A speciation gene for left-right reversal in snails results in anti-predator adaptation". Nature Communications. 1 :133.
22. ↑ Ota H, Lin JT, Hirata T, Chen SL, (1997). "Systematic review of colubrid snakes of the genus Pareas in the East Asian Islands". Journal of Herpetology. 31 :79-87.
23. ↑ Reaney, Leeann T.; Milner, Richard N.C.; Detto, Tanya; Backwell, Patricia R.Y. (April 2008). "The effects of claw regeneration on territory ownership and mating success in the fiddler crab Uca mjoebergi". Animal Behaviour 75 (4): 1473–1478. doi:10.1016/j.anbehav.2007.09.021.
24. ↑ Schwab, I R; Nilsson, Dan-E (June 2007). "A stranger in his own home". The British Journal of Ophthalmology 91 (6): 709. ISSN 0007-1161. PMID 17563928.
25. ↑ Louise R. Page (2006). "Modern insights on gastropod development: Reevaluation of the evolution of a novel body plan". Integrative and Comparative Biology 46 (2): 134–143. doi:10.1093/icb/icj018. PMID 21672730.
26. ↑ Gateño, D.; Rinkevich, B. (May 2003). "Coral polyp budding is probably promoted by a canalized ratio of two morphometric fields". Marine Biology 142 (5): 971–973. doi:10.1007/s00227-003-1009-8. Bibcode: 2003MarBi.142..971G.
27. ↑ Pereira, Ariane; Tracey, Erica; Cooney, Patricia C.; Korey, Christopher A.; Hughes, Melissa (4 May 2014). "Post-autotomy claw regrowth and functional recovery in the snapping shrimp Alpheus angulosus". Marine and Freshwater Behaviour and Physiology 47 (3): 147–159. doi:10.1080/10236244.2014.928460. Bibcode: 2014MFBP...47..147P.
28. ↑ Angermeier, W. F. (1 April 1991). "Behavioral expression of the asymmetry in lobster claws" (in en). Bulletin of the Psychonomic Society 29 (4): 311–312. doi:10.3758/BF03333928. ISSN 0090-5054.
29. ↑ Childers, Carl C.; Achor, Diann S. (1991). "Structure of the mouthparts of Frankliniella bispinosa (Morgan) (Thysanoptera: Thripidae)" (in en). https://www.nrs.fs.fed.us/pubs/6370.

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