{{Short description|Species of cephalopod}} {{Speciesbox | fossil_range = {{fossil range|Miocene|Recent|ref=<ref>{{cite journal|title=''Spirula'' (Sepioidea: Cephalopoda) from the Lower Miocene of Kaipara Harbour, New Zealand (note)|author=Hayward, B.W. |url=http://www.kaiparaharbour.net.nz/publications/documents/Hayward1976Spirula%20from%20lower%20miocene%20of%20kaipara%20hrb%20NZ.pdf|journal= New Zealand Journal of Geology and Geophysics|year= 1977|doi=10.1080/00288306.1976.10423557|volume=19|pages=145–147|doi-access=free}}</ref>}} | image = Spirula spirula1.jpg | image_caption = Dorsal view of female | image2 = Spirula spirula2.jpg | image2_caption = Ventral view of female<br />(chromatophores of mantle missing) | status = LC | status_system = IUCN3.1 |status_ref =<ref name=IUCN>Barratt, I. & Allcock, L. (2012). [http://www.iucnredlist.org/details/162587/0 ''Spirula spirula'']. The IUCN Red List of Threatened Species. Version 2014.2.</ref> | grandparent_authority = Owen, 1836 | genus = Spirula | parent_authority = Lamarck, 1799 | species = spirula | authority = (Linnaeus, 1758) | synonyms = *''Nautilus spirula'' <small>Linnaeus, 1758</small> }}
'''''Spirula spirula''''' is a species of deep-water squid-like cephalopod mollusk. It is the only extant member of the genus '''''Spirula''''', the family '''Spirulidae''', and the order Spirulida. Because of the shape of its internal shell, it is commonly known as the '''ram's horn squid'''<ref>Norman, M. 2000. ''Cephalopods: A World Guide''. Hackenheim, ConchBooks.</ref> or the '''little post horn squid'''. Because the live animal has a light-emitting organ, it is also sometimes known as the '''tail-light squid'''.
Live specimens of this cephalopod are very rarely seen because it is a deep-ocean dweller. The small internal shell of the species is, however, quite a familiar object to many beachcombers. The shell of ''Spirula'' is extremely light in weight, very buoyant, and surprisingly durable; it very commonly floats ashore onto tropical beaches (and sometimes even temperate beaches) all over the world. This seashell is known to shell collectors as the '''ram's horn shell''' or simply as ''Spirula''.
{{gallery |align = centre |footer = |height = 200 |width = 150 |file: Spirula spirula lateral-view.jpg |Side view of a ''Spirula'' shell (with a centimetre ruler for scale) |alt1 = |file: Spirula spirula dorsal-view.jpg |Ventral view; the siphuncle and the last septum of the phragmocone are visible |alt2 = }}
== Taxonomy == Swedish botanist Carl Linnaeus described ''Nautilus spirula'' Linnaeus, 1758 in his book ''Systema Naturae''.<ref>{{Cite book |last=Linné |first=Carl von |url=https://www.biodiversitylibrary.org/item/10326 |title=Systema naturae, per regna tria naturae: secundum classes, ordines, genera, species cum characteribus, differentiis, synonymis, locis |publisher=Typis Ioannis Thomae |year=1758 |edition=Ed. 13. |volume=v. 1, pt. 2 |location=Vindobonae [Vienna] |page=1163 |language=Latin}}</ref> In 1799, French naturalist Jean-Baptiste Lamarck described the genus ''Spirula'' and transferred this species to it, and ''Spirula spirula'' is the name still used today for the ram's horn squid.<ref name=":0">{{Cite web |title=Critter of the week: Spirula spirula {{!}} NIWA |url=https://niwa.co.nz/coasts/critter-week/critter-week-spirula-spirula |access-date=2024-04-25 |website=niwa.co.nz}}</ref><ref>{{Cite book |last=Lamarck |first=Jean-Baptiste |url=https://www.biodiversitylibrary.org/item/48402 |title=Mémoires de la Société d'Histoire Naturelle de Paris |year=1799 |volume=1 |page=80 |language=French}}</ref> ''S. spirula'' is the only species in the monotypic genus ''Spirula.'' A morphometric study published in 2010 showed that shell characteristics of ''S. spirula'' vary with geography, but no subspecies or additional species were proposed.<ref>{{Cite journal |last1=Neige |first1=Pascal |last2=Warnke |first2=Kerstin |date=2010 |title=Just how many species of Spirula are there? A morphometric approach |url=https://www.wikidata.org/wiki/Q125590378 |journal=Cephalopods - Present and Past |language=English |pages=77–84}}</ref>
== Description == ''S. spirula'' has a squid-like body between 35 mm and 45 mm long. It is a decapod, with eight arms and two longer tentacles, all with suckers. The arms and tentacles can all be withdrawn completely into the mantle.
The species lacks a radula<ref> {{The Mollusca|12 |author = Nixon, M. |year = 1988 |title = Paleontology and Neontology of Cephalopods |chapter= The buccal mass of fossil and recent Cephalopoda |pages = 103–122 }}</ref>{{rp|110}}<ref> {{cite book |author1=Landman, Neil H. |author2=Tanabe, Kazushige |author3=Davis, Richard Arnold |year=1996 |title=Ammonoid Paleobiology |publisher=Springer |isbn=978-0-306-45222-2 |url=https://books.google.com/books?id=hKbkB4MzUIkC }}</ref>{{rp|26}} (or, at most, has a vestigial radula).<ref name=Warnke2005/>
{{gallery |align = centre |height = 150 |width = |file: Spirula spirula dissected.jpg |Female with dissected mantle cavity |alt1 = |file: Spirula spirula larva 2.jpg |Immature specimens at various stages of development |alt2 = |file: Spirula spirula larva.jpg |Immature specimen |alt3 = }}
===Shell=== The most distinctive feature of this species is its buoyancy organ, an internal, chambered, endogastrically coiled shell in the shape of an open planispiral (a flat spiral wherein the coils do not touch each other), and consisting of two prismatic layers. The shell functions to osmotically control buoyancy.<ref name=Warnke2005> {{cite journal |last1=Warnke |first1=K. |last2=Keupp |first2=H. |year=2005 |title=''Spirula'' – a window to the embryonic development of ammonoids? Morphological and molecular indications for a palaeontological hypothesis |journal=Facies |volume=51 |issue=1–4 |page=60 |s2cid=85026080 |doi=10.1007/s10347-005-0054-9 |bibcode=2005Faci...51...60W |url=http://doc.rero.ch/record/13832/files/PAL_E731.pdf }} </ref> Unlike the nautilus, which exchange air and liquid only in the three most adoral chambers (the remaining chambers always being gas-filled), spirula can bring cameral fluid into all of their chambers.<ref>[https://books.google.com/books?id=HPzSEAAAQBAJ&dq=Nautilus+adoral+chambers+spirula+squid+cameral+fluid&pg=SA7-PA1 Evolution of the Ammonoids]</ref> Another trait is that it is mineralized, a feature only seen in cuttlefish and the nautilus amongst extant species of cephalopods.<ref> {{cite journal |last1=Lemanis |first1=R. |last2=Korn |first2=D. |last3=Zachow |first3=S |last4=Rybacki |first4=E. |last5=Hoffmann |first5=R. |year=2016 |title=The evolution and development of cephalopod chambers and their shape |journal=PLOS ONE |volume=11 |issue=3 |article-number=e0151404 |pmc=4786199 |pmid=26963712 |bibcode=2016PLoSO..1151404L |doi=10.1371/journal.pone.0151404 |doi-access=free }} </ref>
The siphuncle is marginal, on the inner surface of the spiral.<ref>{{cite web |title=Spirula spirula |website=Tree of Life (tolweb.org) |url=http://tolweb.org/tolarchive/19989/20060309/Spirula_spirula.html}}</ref>
{{clear}} thumb|left|475px| Illustration showing the position of the shell inside the mantle thumb|center|240px| End of mantle showing the photophore {{clear}}
== Behaviour == thumb|Dated illustration of a live animal, but shown ''upside down''; the photophore on a live animal points ''downward''.<ref name="diversity"/> ''S. spirula'' is capable of emitting a green light from a photophore located at the tip of its mantle, between the ear-shaped fins.<ref name=Warnke2005/> Evidently this seems as a counter-illumination strategy, as ''in situ'' observations have captured footage of animals in a vertical stance, with photophore pointing downward and head up.<ref name="diversity"> {{cite journal |last1=Lindsay |first1=Dhugal |last2=Hunt |first2=James |last3=McNeil |first3=Mardi |last4=Beaman |first4=Robin |last5=Vecchione |first5=Michael |date=27 November 2020 |title=The first ''in situ'' observation of the Ram's Horn squid ''Spirula spirula'' turns "common knowledge" upside down |journal=Diversity |volume=12 |issue=449 |page=449 |doi=10.3390/d12120449 |bibcode=2020Diver..12..449L |doi-access=free }} </ref>
== Habitat and distribution == thumb|Oral view of the left tentacular club By day, ''Spirula'' lives in the deep oceans, reaching depths of 1,000 m. At night, it rises to 100–300 m.<ref name="Clarke2009"> {{cite journal |last=Clarke |first=M.R. |year=2009 |title=Cephalopoda collected on the SOND Cruise |journal=Journal of the Marine Biological Association of the United Kingdom |volume=49 |issue=4 |pages=961–976 |doi=10.1017/S0025315400038042 |s2cid=86329056 }} </ref><ref name=":0" /> Its preferred temperature is around 10 °C, and it tends to live around oceanic islands, near the continental shelf.<ref name=Warnke2005/>
Most sources cite this species as tropical and they are observed to be plentiful in the subtropical seas around the Canary Islands. Shells are regularly found along the western coasts of South Africa. In 2022, records of the species have also been confirmed in the Arabian Sea.<ref>{{Cite web |last1=Sajikumar |first1=K.K |last2=Rajeeshkumar |first2=M.P |last3=Vellathi |first3=Venkatesan |date=June 2022 |title=Rediscovery of Ram's horn squid, Spirula spirula (Cephalopoda: Spirulidae), from the Arabian Sea |url=https://www.researchgate.net/publication/361528995}}</ref> However, significant quantities of shells from dead spirula are washed ashore even in temperate regions, such as coasts of New Zealand. Because of the great buoyancy of the shells, these may possibly have been carried long distances by ocean currents.
Much of the organism's life history has not been observed; for instance, they are thought to spawn in winter in deeper water, yet no spawnlings have been directly seen. They must occasionally venture into the upper 10 m of the sea, for they are sometimes found in albatross guts.<ref> {{cite journal |last1=Price |first1=G.D. |last2=Twitchett |first2=R.J. |last3=Smale |first3=C. |last4=Marks |first4=V. |year=2009 |title=Isotopic analysis of the life history of the enigmatic squid ''Spirula spirula'', with implications for studies of fossil Cephalopods |journal=PALAIOS |volume=24 |issue=5 |pages=273–279 |doi=10.2110/palo.2008.p08-067r |bibcode=2009Palai..24..273P |s2cid=131523262 }} </ref>
The species was observed for the first time in its natural habitat in 2020, when an ROV of the Schmidt Ocean Institute recorded it in the depths near the northern Great Barrier Reef.<ref name="diversity"/><ref> {{cite magazine |last=Fox |first=Alex |date=3 November 2020 |title=See strange squid filmed in the wild for the first time |website=Smithsonian Magazine |lang=en |url=https://www.smithsonianmag.com/smart-news/strange-squid-filmed-wild-first-time-180976192/ |access-date=2021-04-25 }} </ref>
==Evolutionary relationships== The order Spirulida also contains two extinct suborders: Groenlandibelina (including extinct families Groenlandibelidae and Adygeyidae), and Belopterina (including extinct families Belemnoseidae and Belopteridae).
''Spirula'' is likely the closest living relative of the extinct belemnites and aulacocerids. These three groups as a unit are closely related to the cuttlefish, as well as to the true squids.
==See also== * ''Jellyella''
==References== {{reflist}}
==External links== * {{tolweb|19989|title=''Spirula Spirula''}} * {{cite web |title=''Spirulidae'' forum |website=TONMO.com |url=http://www.tonmo.com/forums/forumdisplay.php?f=63 |archive-url=https://web.archive.org/web/20060316152536/http://www.tonmo.com/forums/forumdisplay.php?f=63 |archive-date=2006-03-16 |via=Internet Archive (archive.org) }}
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Category:Monotypic cephalopod genera Category:Cenozoic cephalopods Category:Miocene molluscs Category:Pliocene molluscs Category:Extant Miocene first appearances Category:Taxa named by Jean-Baptiste Lamarck Category:Taxa described in 1758 Category:Animal taxa named by Carl Linnaeus Category:Bioluminescent molluscs Category:Spirulida