Editing Tusk shell

{{short description|Class of elephant tusk shell molluscs}}
{{Automatic taxobox
| taxon = Scaphopoda
| name = Scaphopods
| fossil_range = [[Carboniferous]]–[[Neogene]] ({{geological range|Mississippian|earliest=Middle Ordovician|Recent|ref=<ref name=Steiner1992/><!-- Steiner says earliest are Mid Ord--><ref>{{Cite book
| chapter = The Scaphopoda
| volume = 42
| pages = 137–236
| doi = 10.1016/S0065-2881(02)42014-7
| series = Advances in Marine Biology
| last1 = Reynolds
| first1 = Patrick D.
| title = Molluscan Radiation - Lesser-known Branches
| pmid = 12094723
| isbn = 9780120261420
| year = 2002
}}</ref><!-- Earliest concrete fossils accepted here are Mississippian-->) }}
| image = Scaphopoda (10.3897-zookeys.707.13042) Figure 23.jpg
| image_caption = Various Scaphopoda, from left to right: ''[[Fissidentalium]]'', [[Gadilida]], ''[[Gadila (mollusc)|Gadila]]'', and [[Gadilida]].
| authority = [[Heinrich Georg Bronn|Bronn]], 1862
| subdivision_ranks = Orders
| subdivision = * [[Dentaliida]] 
* [[Gadilida]]
}}
{{Sea shell topics}}

'''Scaphopoda''' {{IPAc-en|s|k|æ|ˈ|f|ɒ|p|ə|d|ə}} (plural '''scaphopods''' {{IPAc-en|ˈ|s|k|æ|f|ə|p|ɒ|d|z}}, from Ancient Greek σκᾰ́φης ''skáphē'' "boat" and πούς ''poús'' "foot"), whose members are also known as '''tusk shells''' or '''tooth shells''', are a [[class (biology)|class]] of shelled [[Marine life|marine]] [[invertebrate]]s belonging to the [[phylum]] [[Mollusca]] with worldwide distribution and are the only class of exclusively [[infaunal]] marine molluscs. Shells of species within this class range in length {{convert|0.5-18|cm|abbr=on}} (with  ''Fissidentalium metivieri'' as the longest).<ref>{{cite journal | pmc=4714876 | year=2016 | last1=Vermeij | first1=G. J. | title=Gigantism and Its Implications for the History of Life | journal=PLOS ONE | volume=11 | issue=1 | pages=e0146092 | doi=10.1371/journal.pone.0146092 | pmid=26771527 | bibcode=2016PLoSO..1146092V | doi-access=free }}</ref> Members of the order [[Dentaliida]] tend to be larger than those of the order [[Gadilida]].

These molluscs live in soft [[substrate (biology)|substrates]] offshore (usually not [[intertidal]]ly). Because of this subtidal habitat and the small size of most species, many [[beachcombing|beachcombers]] are unfamiliar with them; their shells are not as common or as easily visible in the beach drift as the shells of [[sea snail]]s and [[clam]]s.

Molecular data suggest that the scaphopods are a sister group to the [[cephalopod]]s, although higher-level molluscan phylogeny remains unresolved.<ref>{{Cite journal| last3=Lindgren, A.R.| last1=Giribet | first3=A. R. | first1=G.| last4=Huff, S.W. | first4=S. W. | first2=A.| last5 = Schrödl, M | first5=M.| last6 = Nishiguchi, M.K. | first6=M. K.| title = Evidence for a clade composed of molluscs with serially repeated structures: monoplacophorans are related to chitons| last2= Okusu, A| journal = Proceedings of the National Academy of Sciences of the United States of America| volume = 103| issue = 20| pages = 7723–7728| date=May 2006 | pmid = 16675549| pmc = 1472512| doi = 10.1073/pnas.0602578103|bibcode = 2006PNAS..103.7723G | doi-access=free }}</ref>

==Classification==
The group is composed of two subtaxa, the [[Dentaliida]] (which may be paraphyletic) and the monophyletic [[Gadilida]].<ref name=Steiner1992>{{Cite journal| doi = 10.1093/mollus/58.4.385| title = Phylogeny and Classification of Scaphopoda| year = 1992| last1 = Steiner | first1 = G. .| journal = Journal of Molluscan Studies| volume = 58| issue = 4| pages = 385–400 }}</ref>
The differences between the two orders is subtle and hinges on size and on details of the radula, shell, and foot. Specifically, the Dentaliids are the physically larger of the two families, and possess a shell that tapers uniformly from anterior (widest) to posterior (narrowest); they also have a foot which consists of one central and two lateral lobes and which bends into the shell when retracted.  The Gadilids, on the other hand, are much smaller, have a shell whose widest portion is slightly posterior to its aperture, and have a foot which is disk-like and fringed with tentacles which inverts into itself when retracted (in this state resembling a pucker rather than a disk).
[[File:Calliodentalium semitracheatum (MNHN-IM-2012-13993).jpeg|thumb|Shell of ''Calliodentalium semitracheatum'' <small>(Boissevain, 1906)</small> (specimen at MNHN, Paris)]]
According to the [[World Register of Marine Species]]:
* [[Dentaliida]] <small>da Costa, 1776</small>  
** family [[Anulidentaliidae]] <small>Chistikov, 1975</small> – 3 genera
** family [[Calliodentaliidae]] – 1 genus
** family [[Dentaliidae]] <small>Children, 1834</small> – 14 genera
** family [[Fustiariidae]] <small>Steiner, 1991</small> – 1 genus
** family [[Gadilinidae]] <small>Chistikov, 1975</small> – 3 genera
** family [[Laevidentaliidae]] <small>Palmer, 1974</small> – 1 genus
** family [[Omniglyptidae]] <small>Chistikov, 1975</small> – 1 genus
** family [[Rhabdidae]] <small>Chistikov, 1975</small> – 1 genus
* [[Gadilida]] <small>Starobogatov, 1974</small>
** sub-order [[Entalimorpha]] <small>Steiner, 1992</small>
*** family [[Entalinidae]] <small>Chistikov, 1979 </small> – 9 genera
** sub-order [[Gadilimorpha]] <small>Steiner, 1992</small>
*** family [[Gadilidae]] <small>Stoliczka, 1868</small> – 8 genera
*** family [[Pulsellidae]] <small>Scarabino in Boss, 1982</small> – 3 genera
*** family [[Wemersoniellidae]] <small>Scarabino, 1986</small> – 2 genera

== Evolution ==

=== Fossil record ===
[[File:05898 Grand Canyon Mollusca Fossils (4739033749).jpg|thumb|[[Internal mold]] of a fossil scaphopod, [[Kaibab Formation]] ([[Grand Canyon]])]]
There is a good fossil record of scaphopods from the [[Mississippian (geology)|Mississippian]] onwards,<ref name="Yochelson20072">{{cite journal|author=Ellis L. Yochelson|author2=Royal H. Mapes|author3=Doris Heidelberger|year=2007|title=An enigmatic molluscan fossil from the Devonian of Germany: scaphopod or cephalopod?|journal=Paläontologische Zeitschrift|volume=81|issue=2|pages=118–122|doi=10.1007/BF02988386|bibcode=2007PalZ...81..118Y |s2cid=85124687}}</ref> making them the youngest molluscan class.

The Ordovician ''[[Rhytiodentalium]] kentuckyensis'' has been interpreted as an early antecedent of the scaphopods, implying an evolutionary succession from [[ribeirioid]] [[rostroconch]] molluscs such as ''[[Pinnocaris]]''.  However, a competing hypothesis suggests a Devonian/Carboniferous origin from a non-mineralized ancestor, or from a more derived, Devonian, [[conocardioid]] rostroconch.<ref name="Peel20042">{{Cite journal|author=Peel, J.S.|year=2004|title=''Pinnocaris'' and the origin of scaphopods.|journal=Acta Palaeontologica Polonica|volume=49|issue=4|pages=543–550}}</ref>

=== Phylogeny ===
The scaphopods are largely agreed to be members of the [[Conchifera]], however their phylogenetic relationship with the other members of this subphylum remains contentious. The Diasoma concept proposes a clade of scaphopods and bivalves based on their shared infaunal lifestyle, burrowing foot, and possession of a mantle and shell. Pojeta and Runnegar proposed the extinct [[Rostroconchia]] as the stem group of the Diasoma.<ref>{{cite book |doi=10.3133/pp968 |chapter=The paleontology of rostroconch mollusks and the early history of the phylum Mollusca |series=Professional Paper |year=1976 |last1=Pojeta |first1=John |last2=Runnegar |first2=Bruce }}</ref> An alternative hypothesis proposes the cephalopods and gastropods as sister to the scaphopods with [[helcionellid]]s as the stem group.<ref>{{Cite journal|last1=Stöger|first1=I.|last2=Sigwart|first2=J. D.|last3=Kano|first3=Y.|last4=Knebelsberger|first4=T.|last5=Marshall|first5=B. A.|last6=Schwabe|first6=E.|last7=Schrödl|first7=M.|date=2013|title=The Continuing Debate on Deep Molluscan Phylogeny: Evidence for Serialia (Mollusca, Monoplacophora + Polyplacophora)|journal=BioMed Research International|language=en|volume=2013|pages=407072|doi=10.1155/2013/407072 |pmc=3856133|pmid=24350268|doi-access=free}}</ref> A review of deep molluscan phylogeny in 2014 found more support for the scaphopods, gastropods, or cephalopods than for scaphopods and bivalves, thus the shared body features of scaphopods and bivalves may be convergent adaptations due to similar lifestyles.<ref>{{Cite journal|last1=Schrödl|first1=Michael|last2=Stöger|first2=Isabella|date=2014-12-25|title=A review on deep molluscan phylogeny: old markers, integrative approaches, persistent problems|journal=Journal of Natural History|volume=48|issue=45–48|pages=2773–2804|doi=10.1080/00222933.2014.963184|bibcode=2014JNatH..48.2773S |s2cid=84697563|issn=0022-2933}}</ref> Analysis of the scaphopod nervous system demonstrated that both scaphopods and cephalopods share a similar nervous system structure, with ventrally shifted pedal nerves and lateral nerves that extend dorsally. These similarities led to the conclusion that scaphopods are sister to the cephalopods with gastropods as sister to them both.<ref name=":0" /> More recent research, including the sequenced genome of tusk shells, support the Diasoma model with bivalves as the sister group.<ref>[https://www.eurekalert.org/news-releases/1001805 Genomes of enigmatic tusk shells provide new insights into early Molluscan evolution]</ref>

==Orientation==
The morphological shape of the scaphopod body makes it difficult to orient it satisfactorily.  As a result, researchers have often disagreed as to which direction is anterior/ posterior and which is ventral/ dorsal.  According to [[Ronald Shimek|Shimek]] and Steiner, "[t]he apex of the shell and mantle are anatomically dorsal, and the large aperture is ventral and anterior.  Consequently, the concave side of the shell and viscera are anatomically dorsal.  The convex side has to be divided into anteriorly ventral and dorsally posterior portions, with the anus as the demarcation.  Functionally, as in [[cephalopod]]s, the large aperture with the foot is anterior, the apical area posterior, the concave side dorsal and the convex side ventral."<ref>{{cite book|title=Microscopic anatomy of invertebrates|volume=6B: Mollusca II|last1=Shimek|first1=Ronald|last2=Steiner|first2=Gerhard|chapter=Chapter 6|publisher=Wiley-Liss, Inc.|date=1997|page=719}}</ref>

== Anatomy ==
===Shells===
{{multiple image
|align=right
|perrow=1
|total_width=250

|image1=Antalis inaequicostata 01.JPG
|caption1=Shell of ''[[Antalis|Antalis inaequicostata]]''

|image2=Dentalium octangulatum 01.JPG
|caption2=Shell of ''[[Dentalium (genus)|Dentalium octangulatum]]''

}}
The shells of the members of the Gadilida are usually glassy-smooth and narrow, with a reduced aperture. This along with other structures of their anatomy allows them to move with surprising speed through loose sediment to escape potential bottom-dwelling predators.

The Dentalids, by contrast, tend to have strongly ribbed and rough shells. When they sense vibrations anywhere around them, their defensive response is to freeze. This makes them harder to detect by animals such as [[ratfish]], which can sense the electrical signals given off by the most minute muscle movement.

=== Mantle ===
The [[mantle (mollusc)|mantle]] of a scaphopod is entirely within the shell. The foot extends from the larger end of the shell, and is used to burrow through the substrate. The scaphopod positions itself head down in the substrate, with the apical end of the shell (at the rear of the animal's body) projecting upward. This end seldom appears above the level of the substrate, however, as doing so exposes the animal to numerous predators.  Most adult scaphopods live their lives entirely buried within the substrate.

Water enters the mantle cavity through the apical aperture, and is wafted along the body surface by [[cilia]]. There are no [[gill]]s; the entire surface of the mantle cavity absorbs [[oxygen]] from the water. Unlike most other molluscs, there is no continuous flow of water with a separate exhalant stream. Instead, deoxygenated water is expelled rapidly back through the apical aperture through muscular action once every ten to twelve minutes.

=== Feeding and digestion ===
[[File:Scaphopoda.svg|thumb|upright=2|Anatomical diagram of ''[[Rhabdus rectius]]'']]
A number of minute [[tentacle]]s around the foot, called ''captacula'', sift through the sediment and latch onto bits of food, which they then convey to the mouth. The mouth has a grinding [[radula]] that breaks the bit into smaller pieces for digestion. The radulae and cartilaginous oral bolsters of the Gadilidae are structured like zippers where the teeth actively crush the prey by opening and closing on it repeatedly, while the radulae and bolsters of the Dentaliidae work rachet-like to pull the prey into the esophagus, sometimes whole.

The massive radula of the scaphopods is the largest such organ relative to body size of any mollusc (among whom, except for the [[Bivalvia|bivalves]], the presence of which is a defining characteristic). The remainder of the digestive system consists of a digestive [[diverticulum]], [[esophagus]], [[stomach]], and [[intestine]]. A digestive gland secretes enzymes into the stomach, but, unlike some other molluscs, does not digest the food directly itself. The [[anus]] opens on the ventral/ underside of the animal, roughly in the middle of the mantle cavity.

=== Vascular system ===
The scaphopod vascular system is rudimentary lacking heart [[Atrium (heart)|auricles]] as well as corresponding [[Ctenidium (mollusc)|ctenidia]] (gills) and blood vessels; the blood is held in sinuses throughout the body cavity, and is pumped through the body by the rhythmic action of the foot. The [[heart]], a characteristic feature of all other groups of mollusca, has been considered totally lost or reduced to a thin fold of the [[pericardium]]; however, according to more recent studies, the muscular, regularly beating [[perianal]] blood sinus is [[Homology (biology)|homologous]] to the [[Ventricle (heart)|ventricle]] and is therefore considered the scaphopod heart.<ref>{{cite book |first=Patrick D. |last=Reynolds |contribution=Scaphopoda: The Tusk Shells |editor1-first=Charles F. |editor1-last=Sturm |editor2-first=Timothy A. |editor2-last=Pearce |editor3-first=Ángel |editor3-last=Valdés |title=The Mollusks: A guide to their study, collection, and preservation |place=Boca Ratón, FL |publisher=Universal Publishers |year=2006 |pages=229–238, esp. 231}}</ref>

Metabolic waste is excreted through a pair of [[nephridium|nephridia]] close to the anus.  The tusk shells appear to be the only extant molluscs which completely lack the otherwise standard molluscan [[reno-pericardial aperture]]s.  Furthermore, they also appear to be the only molluscs with openings that directly connect the hemocoel with the surrounding water (through two "water pores" located near the nephridial openings).  These openings may serve to allow the animal to relieve internal pressure by ejecting body fluid (blood) during moments of extreme muscular contraction of the foot.<ref name=KhannaYadav2004>{{cite book |author1=Khanna, D.R. |author2=Yadav, P.R. |title=Biology of Mollusca |url=https://books.google.com/books?id=FlGw93oZocoC&pg=PA193 |date=1 January 2004 |publisher=Discovery |isbn=978-81-7141-898-5|page=198}}</ref>

=== Nervous system ===
The nervous system is generally similar to that of [[cephalopod]]s.<ref name=":0">{{Cite journal |last1=Sumner-Rooney |first1=Lauren H. |last2=Schrödl |first2=Michael |last3=Lodde-Bensch |first3=Eva |last4=Lindberg |first4=David R. |last5=Heß |first5=Martin |last6=Brennan |first6=Gerard P. |last7=Sigwart |first7=Julia D. |date=2015 |title=A neurophylogenetic approach provides new insight to the evolution of Scaphopoda: A neurophylogenetic approach in Scaphopoda|journal=Evolution & Development |language=en |volume=17 |issue=6 |pages=337–346 |doi=10.1111/ede.12164 |pmid=26487042|s2cid=37343813 }}</ref> One pair each of cerebral and pleural [[ganglion|ganglia]] lie close to the oesophagus, and effectively form the animal's [[brain]].

A separate set of pedal ganglia lie in the foot, and a pair of visceral ganglia are set further back in the body, and connect to pavilion ganglia via long connectives. Radular and sub-radular ganglia are also present, as are [[statocysts]] with [[staticonia]]. Scaphopods have no [[mollusc eye|eye]]s, no [[osphradia]],<ref name=SturmPearce2006>{{cite book |editor1-first=Charles F. |editor1-last=Sturm |editor2-first=Timothy A. |editor2-last=Pearce |editor3-first=Ángel |editor3-last=Valdés |title=The Mollusks: A guide to their study, collection, and preservation |year=2006 |publisher=Universal Publishing |place=Boca Raton, FL |isbn=978-1-58112-930-4 |page=2 |url=https://books.google.com/books?id=-NbmHx93s8gC&pg=PA2}}</ref> or other distinct sensory organs.<ref name=IZ>{{cite book |last=Barnes |first=Robert D. |year=1982 |title=Invertebrate Zoology |publisher=Holt-Saunders International |location=Philadelphia, PA |pages=432–434 |isbn=0-03-056747-5}}</ref> However, scaphopods do possess genes involved in photoreceptor formation and function implying scaphopods may have had eyes that degenerated over evolutionary time.<ref>{{cite journal |last1=Wollesen |first1=Tim |last2=McDougall|first2=Carmel |last3=Arendt |first3=Detlev |date=2019-10-19 |title=Remnants of ancestral larval eyes in an eyeless mollusk? Molecular characterization of photoreceptors in the scaphopod Antalis entalis |journal=EvoDevo |volume=10 |issue=1 |page=25 |doi=10.1186/s13227-019-0140-7 |issn=2041-9139 |pmc=6800502 |pmid=31641428 |doi-access=free }}</ref>

==Reproduction and development==
Scaphopods have [[dioecious|separate sexes]], and [[external fertilisation]]. They have a single [[gonad]] occupying much of the posterior part of the body, and shed their [[gamete]]s into the water through the nephridium.

Once fertilized, the eggs hatch into a free-living [[trochophore]] larva, which develops into a [[veliger]] larva that more closely resembles the adult, but lacks the extreme elongation of the adult body.<ref name=IZ/> The three-lobed foot originates prior to metamorphosis while the cephalic tentacles develop post metamorphosis. Scaphopods remain univalved throughout their morphogenesis contrary to bivalves.<ref>{{Cite journal|last1=Wanninger|first1=Andreas|last2=Haszprunar|first2=Gerhard|date=2001|title=The expression of an engrailed protein during embryonic shell formation of the tusk-shell, Antalis entalis (Mollusca, Scaphopoda)|journal=Evolution & Development|language=en|volume=3|issue=5|pages=312–321|doi=10.1046/j.1525-142X.2001.01034.x|pmid=11710763|s2cid=8936294|issn=1525-142X}}</ref>

== Ecology ==
Tusk shells live in [[seafloor sediment]], feeding primarily on [[foraminifera]]ns; some supplement this with vegetable matter.<ref>{{Cite journal
 | doi = 10.1002/(SICI)1097-4687(199908)241:2<175::AID-JMOR7>3.0.CO;2-0
 | last1 = Guralnick | first1 = R.
 | last2 = Smith | first2 = K.
 | title = Historical and biomechanical analysis of integration and dissociation in molluscan feeding, with special emphasis on the true limpets (Patellogastropoda: Gastropoda)
 | journal = Journal of Morphology
 | volume = 241
 | issue = 2
 | pages = 175–195
 | year = 1999
 | pmid = 10420163 
| s2cid = 14497120 }}</ref>

== Human use ==
The shells of ''[[Dentalium hexagonum]]'' and ''[[Dentalium pretiosum]]'' were strung on thread and used by the natives of the [[Pacific Northwest]] as [[shell money]]. ''Dentalium'' shells were also used to make belts and headdresses by the [[Natufian]] culture of the [[Middle East]], and are a possible indicator of early social stratification.<ref>{{cite book|title=Exploring Prehistory: How Archaeology Reveals Our Past|edition= 2nd |editor1-first=Pam J. |editor1-last=Crabtree|editor2-first= Douglas V. |editor2-last=Campana|pages=233|isbn=0-07-297814-7|date= 2005-06-21 |publisher= McGraw-Hill Education }}</ref>

<gallery class="center">
File:Necklace PRE.2009.0.238.1-a.jpg|Tusk shell necklace from Bronze Age ([[MHNT]])
File:Necklace PRE.2009.0.238.1-IMG 1851.jpg|Tusk shell necklace from Bronze Age (MHNT)
File:Necklace PRE.2009.0.238.1-IMG 1992.jpg|Tusk shell necklace from Bronze Age (MHNT)
File:Necklace PRE.2009.0.238.1-IMG 2001.jpg|Tusk shell necklace from Bronze Age (MHNT)
</gallery>

== References ==
{{reflist}}

== Further reading ==
{{EB1911 Poster|Scaphopoda}}
* For a comprehensive overview, see {{Cite book | last1 = Reynolds | first1 = P. D. | chapter = The scaphopoda | doi = 10.1016/S0065-2881(02)42014-7 | title = Molluscan Radiation - Lesser-known Branches | series = Advances in Marine Biology | volume = 42 | pages = 137–236 | year = 2002 | isbn = 9780120261420 | pmid =  12094723}}
* Scarabino V., (1995) [https://biostor.org/reference/252556 Scaphopoda of the tropical Pacific and Indian Oceans, with description of 3 new genera and 42 new species] P. Bouchet (ed) ''Résultats des Campagnes MUSORSTOM'', Volume 14 Mémoires du Muséum National d'Histoire Naturelle, 167 189-379.
:* Cited by —
::: {{ Cite journal
   | date = 2001
   | issue = 3
   | volume = 23
   | pages = 433–460
   | s2cid = 81442755
   | title = Catalogue of supraspecific taxa of Scaphopoda (Mollusca)
   | url = https://sciencepress.mnhn.fr/en/periodiques/zoosystema/23/3/catalogue-des-taxons-supraspecifiques-de-scaphopoda-mollusca
   | journal = [[Zoosystema]]
 }}
* Steiner G. & Kabat A. 2004. ''Catalog of species-group names of Recent and fossil Scaphopoda (Mollusca).'' Zoosystema 26 (4): 549-726
* [https://web.archive.org/web/20070811162831/http://www.mnhn.fr/publication/zoosyst/z01n3a4.pdf Steiner, G.; Kabat, A. R. (2001). Catalogue of supraspecific taxa of Scaphopoda (Mollusca). Zoosystema. 23(3): 433-460]

{{Mollusc}}

{{taxonbar|from=Q238314}}
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[[Category:Mollusc shells]]
[[Category:Scaphopods| ]]
[[Category:Mississippian first appearances]]
[[Category:Extant Carboniferous first appearances]]
[[Category:Taxa named by Heinrich Georg Bronn]]
[[Category:Conchifera]]