Velella

{{short description|Species of cnidarian}} {{Speciesbox | image = Velella Bae an Anaon.jpg | image_caption = Close-up of a ''V. velella'' colony | parent_authority = Lamarck, 1801<ref>{{cite book |last1=Lamarck |first1=J. B. |year=1801 |title=Système des animaux sans vertèbres |url=https://www.biodiversitylibrary.org/page/14117719#page/381/mode/1up |location=Paris, France |publisher=by the author and Deterville |page=555 |via=Biodiversity Heritage Library |archive-date=2023-06-07 |access-date=2023-06-16 |archive-url=https://web.archive.org/web/20230607045937/https://www.biodiversitylibrary.org/page/14117719#page/381/mode/1up |url-status=live }}</ref> | taxon = Velella velella | authority = (Linnaeus, 1758) | synonyms = {{hidden begin|title = List}} (Genus) * ''Armenista'' Haeckel, 1888 * ''Rataria'' Eschscholtz, 1829 * ''Velaria'' Haeckel, 1888 (Species) * ''Armenista sigmoides'' Haeckel, 1888 * ''Holothuria spirans'' Forsskål, 1775 * ''Medusa pocillum'' Montagu, 1815 * ''Medusa velella'' Linnaeus, 1758 * ''Rataria cordata'' Eschscholtz, 1829 * ''Rataria mitrata'' Eschscholtz, 1829 * ''Velella antarctica'' Eschscholtz, 1829 * ''Velella aurora'' Eschscholtz, 1829 * ''Velella australis'' de Haan, 1827 * ''Velella caurina'' Eschscholtz, 1829 * ''Velella cyanea'' Lesson, 1826 * ''Velella emarginata'' Quoy & Gaimard, 1824 * ''Velella indica'' Eschscholtz, 1829 * ''Velella lata'' Chamisso & Eysenhardt, 1821 * ''Velella limbosa'' Lamarck, 1816 * ''Velella meridionalis'' Fewkes, 1889 * ''Velella mutica'' Lamarck, 1801 * ''Velella oblonga'' Chamisso & Eysenhardt, 1821 * ''Velella oxyothone'' Brandt, 1835 * ''Velella pacifica'' de Haan, 1827 * ''Velella patellaris'' Brandt, 1835 * ''Velella pyramidalis'' Cranch, 1818 * ''Velella radackiana'' de Haan, 1827 * ''Velella sandwichiana'' de Haan, 1827 * ''Velella scaphidia'' Peron & Lesueur, 1807 * ''Velella septentrionalis'' Eschscholtz, 1829 * ''Velella sinistra'' Chamisso & Eysenhardt, 1821 * ''Velella tentaculata'' Lamarck, 1801 * ''Velella tropica'' Eschscholtz, 1829 {{hidden end}} }}

'''''Velella''''' is a genus of hydrozoa (a group of small, predatory marine cnidarians) in the family Porpitidae. Its only known species is '''''Velella velella''''',<ref>{{cite WoRMS |year=2013 |title=''Velella velella'' (Linnaeus, 1758) |db=Hydrozoa |id=117832 |access-date=23 April 2013}}</ref> a widely-distributed free-floating colonial animal that lives on the surface of the open ocean. It is commonly known by the names '''sea raft''', '''by-the-wind sailor''', '''purple sail''', '''little sail''', or simply ''Velella''.<ref>Harrington Wells (1937). ''Seashore Life''. Wagner Publishing Company, USA (see pages 138 and 144 in the 1942 edition)</ref>

It is a member of a specialised ocean surface community that includes the better-known Portuguese man o' war. Specialized predatory molluscs prey on these cnidarians. Such predators include nudibranchs (sea slugs) in the genus ''Glaucus''<ref name="Gosliner 1987">Gosliner, T.M. (1987). ''Nudibranchs of Southern Africa'' page 127, {{ISBN|0-930118-13-8}}</ref> and purple snails in the genus ''Janthina''.<ref name="Branch 2010">Branch, G.M., Branch, M.L, Griffiths, C.L. and Beckley, L.E. (2010). ''Two Oceans: a guide to the marine life of southern Africa.'' Cape Town:Struik Nature. page 188. {{ISBN|9781770077720}}.</ref>

Each apparent individual is a hydroid colony, and most are less than about {{Convert|7|cm|abbr=on}} long. They are usually deep blue in colour, with a small stiff sail that catches the wind and propels them over the surface of the sea. Under certain wind conditions, they may be stranded by the thousands on beaches.<ref>{{Cite web |date=2024-04-22 |title=By-the-wind-sailors: Creatures wash up on Guernsey beaches |url=https://www.bbc.com/news/articles/cer34mg0j80o |access-date=2024-04-23 |website=BBC News |language=en-gb |archive-date=2024-04-22 |archive-url=https://web.archive.org/web/20240422165325/https://www.bbc.com/news/articles/cer34mg0j80o |url-status=live }}</ref><ref>{{cite news |last1=Galloway |first1=Kayla |title=These blue sea creatures are washing up on San Francisco beaches — they're not jellyfish |url=https://www.ktvu.com/news/blue-sea-creatures-velella-velella-san-francisco |access-date=9 May 2026 |agency=Fox 2 KTVU |date=28 April 2026}}</ref>

Like other cnidarians, ''V. velella'' are carnivorous. They catch their prey, generally plankton, by means of tentacles that hang down in the water and bear stinging nematocysts (also called cnidocysts). The toxins in their nematocysts are effective against their prey. While cnidarians all possess nematocysts, in some species the nematocysts and toxins therein are more powerful than other species. The nematocysts of ''V. velella'' are relatively benign to humans, although each person may respond differently to contact with the nematocyst toxin; irritation may occur to skin exposed to ''V. velella'' nematocysts.

==Distribution and habitat== {{see also|Ocean surface ecosystem}} left|thumb|Chirality ('handedness') affects the direction of drift in ''V. velella''<ref name="NJP"/> thumb|Stranded ''V. velella'' [[File:Vellelawholebeach.jpg|thumb|''V. velella'' stranded on Rodeo Beach, Marin County, California.]] ''V. velella'' lives in warm and temperate waters in all the world's oceans. They live at the water/air interface, with the float above the water, and polyps hanging down about a centimetre below. Organisms that live partly in and partly out of the water like this are known as neuston.<ref>{{cite news|title=Uncovering the Neuston, a Mysterious Living Island of Sea Creatures|work=NPR|date=2021-05-09|url=https://www.npr.org/2021/05/09/995173043/uncovering-the-neuston-a-mysterious-living-island-of-sea-creatures|access-date=2023-05-21|language=en|archive-date=2023-05-22|archive-url=https://web.archive.org/web/20230522023901/https://www.npr.org/2021/05/09/995173043/uncovering-the-neuston-a-mysterious-living-island-of-sea-creatures|url-status=live}}</ref> Offshore boaters sometimes encounter thousands of ''V. velella'' on the water surface.{{citation needed|date=May 2023}}

The small rigid sail projects into the air and catches the wind. However, ''V. velella'' sails always align along the direction of the wind where the sail may act as an aerofoil, so that the animals tend to sail downwind at a small angle to the wind.<ref>{{cite book |title = Principles of Animal Locomotion |first=R. |last= McNeill Alexander |author-link=R. McNeill Alexander |publisher = Princeton University Press|year=2002|isbn= 0-691-08678-8}}</ref> Having no means of locomotion other than its sail, ''V. velella'' is at the mercy of prevailing winds for moving around the seas, and are thereby also subject to mass-strandings on beaches throughout the world. For example, a mass stranding occurs most years in the spring along the West Coast of North America, from British Columbia to California, beginning in the north and moving south over several weeks' time. In some years, so many animals are left at the tide line by receding waves, that the line of dying (and subsequently rotting) animals may be many centimetres deep, along hundreds of kilometres of beaches.{{Citation needed|date=June 2015}} Mass strandings have been reported also on the west coast of Ireland,<ref>{{Cite journal |first=M. J. |last=Delap |year=1921 |title=Drift on the Kerry coast |journal=The Irish Naturalist |volume=30 |issue=3 |page=40 |jstor=25525026}}</ref> and in Hayle, on the west coast of Cornwall in England.<ref>{{cite web | url=https://www.cornwalllive.com/news/cornwall-news/thousands-jellyfish-like-creatures-swamp-7288276 | title='Thousands' of jellyfish-like creatures swamp beach | date=4 July 2022 | access-date=4 July 2022 | archive-date=4 July 2022 | archive-url=https://web.archive.org/web/20220704195233/https://www.cornwalllive.com/news/cornwall-news/thousands-jellyfish-like-creatures-swamp-7288276 | url-status=live }}</ref>

The individual colonies are asymmetrical, being either 'diagonal' or 'antidiagonal' (effectively, 'left-handed' or 'right-handed'; mirror images of each other). This chirality affects the direction of their dispersal; in the northern Pacific Ocean, 'diagonal' individuals form tend to drift east to the west coast of North America, while 'antidiagonal' individuals form tend to drift west to the coasts of eastern Asia, such as Japan.<ref name="NJP">{{cite journal |last1=Cameron |first1=Robert P |last2=McArthur |first2=Duncan |last3=Yao |first3=Alison M |title=Strong chiral optical force for small chiral molecules based on electric-dipole interactions, inspired by the asymmetrical hydrozoan ''Velella velella'' |journal=New Journal of Physics |volume=25 |issue=8 |date=2023-08-01 |issn=1367-2630 |doi=10.1088/1367-2630/ace7ee |page=083006 |url=https://iopscience.iop.org/article/10.1088/1367-2630/ace7ee |access-date=2026-05-25|doi-access=free }}</ref>

==Life cycle== Like many Hydrozoa, ''V. velella'' has a bipartite life cycle, with a form of alternation of generations. The deep blue, by-the-wind sailors that are recognized by many beach-goers are the polyp phase of the life cycle. Each "individual" with its sail is really a hydroid colony connected by a canal system that enables the colony to share whatever food is ingested by individual polyps. Each by-the-wind sailor is a colony of all-male or all-female polyps. The colony has several different kinds of polyps, some of which are both feeding and reproductive, called gonozooids, and others protective, called dactylozooids.<ref name="Brinckmann-Voss">{{cite book |author=A. Brinckmann-Voss |year=1970 |title=Anthomedusae/Athecatae (Hydrozoa, Cnidaria) of the Mediterranean. Part I. Capitata |series=Fauna e Flora del Golfo di Napoli |volume=39 |pages=1–96, 11 pls |publisher=Stazione Zoologica}}</ref>

The gonozooids each produce numerous tiny jellyfish by an asexual budding process, so that each ''V. velella'' colony produces thousands of tiny jellyfish (medusae), each about {{Convert|1|mm|abbr=on}} high and wide, over several weeks. The tiny medusae are each provided with many zooxanthellae, single-celled endosymbiotic organisms typically also found in corals and some sea anemones, that can utilize sunlight to provide energy to the jellyfish. Curiously, although a healthy captive ''V. velella'' will release many medusae under the microscope, and are expected to do the same in the sea, the medusae of ''V. velella'' are rarely captured in the plankton and very little is known about their natural history. The medusae develop to sexual maturity within about three weeks in the laboratory and their free-spawned eggs and sperm develop into a planktonic larva called a "conaria," which develops into a new floating ''V. velella'' hydroid colony.<ref name="Brinckmann-Voss" />

==Systematics== The Porpitidae is a family of the Hydrozoa erected for two genera of hydroids that live floating free at the surface of the open ocean: ''Velella'' and ''Porpita''. The systematic position of these peculiar genera has long been a topic of discussion among taxonomists who work with pelagic Cnidaria. The three genera{{clarify|reason=which three?|date=July 2023}} were put in with athecate hydroids in the mid-to-late 19th century by some, whereas other authors at the time included them in the Siphonophorae. A new order was established for these genera by Totton,<ref>{{cite journal |last1=Totton |first1=A. K. |title=Siphonophora of the Indian Ocean: Together with Systematic and Biological Notes on Related Specimens from Other Oceans |date=1954 |journal=Discovery Reports |volume=27 |pages=1–162 [30, 33] |url=https://www.biodiversitylibrary.org/page/5611213 |oclc=14100651 |archive-date=2023-04-10 |access-date=2023-04-10 |archive-url=https://web.archive.org/web/20230410124214/https://www.biodiversitylibrary.org/page/5611213 |url-status=live }}</ref> in 1954, called the Chondrophora, while at the same time, other authors favored again placing them in the Anthomedusae/Athecatae.<ref name="Brinckmann-Voss" /> Most authors in the past 40 years have accepted interpretation of these animals as unusual floating colonial athecate hydroids, which produce medusae clearly belonging in the Anthomedusae. Although the exact position of the family Porpitidae within the Athecatae/Anthomedusae is not yet clear, the order Chondrophora is no longer used by hydrozoan systematists.{{citation needed|date=June 2017}}

==See also== *''Porpita porpita'' *''Glaucus atlanticus''

==References== {{reflist|25em}}

== External links == {{commons category|Velella velella}} * [http://animaldiversity.ummz.umich.edu/site/accounts/information/Velella_velella.html Animal Diversity Web: ''Velella''] * {{cite news |url=http://news.nationalgeographic.com/2015/04/150420-velella-jellyfish-pictures-california-beach-ocean-animals-science.html |title=Billions of Blue Jelly Fish Wash Up on American Beaches |publisher=National Geographic Society |date=20 April 2015}}{{dead link|date=January 2025|bot=medic}}{{cbignore|bot=medic}} * [https://web.archive.org/web/20040409101104/http://jellieszone.com/velella.htm Jellieszone.com: ''Velella''] *[https://www.marlin.ac.uk/species/detail/1802 Marine Life Information Network] * {{cite news |url=http://news.bbc.co.uk/1/hi/wales/south_west/6187954.stm |title=Millions of jelly creatures land |publisher=BBC News |date=27 November 2006}} * [https://web.archive.org/web/20120112073512/http://www.planktonchronicles.org/en/episode/16 ''Plankton Chronicles''] Short documentary films and photos

Category:Porpitidae Category:Monotypic Cnidaria genera Category:Cnidarians of the Atlantic Ocean Category:Cnidarians of the Indian Ocean Category:Cnidarians of the Pacific Ocean Category:Taxa named by Jean-Baptiste Lamarck