Editing Clam shrimp

{{Short description|Suborder of arthropods}}
{{Automatic taxobox
| name = Clam shrimp
| fossil_range = {{fossil range|Devonian|Recent}}
| image = California Clam Shrimp (Cyzicus californicus).jpg
| image_caption = California clam shrimp, ''Cyzicus californicus''
| taxon = Diplostraca
| subdivision_ranks = Orders of clam shrimp
| subdivision =
* Cyclestherida <small>Sars, 1899</small>
* Laevicaudata <small>Linder, 1945</small>
* Spinicaudata <small>Linder, 1945</small>
}}

'''Clam shrimp''' are a group of bivalved [[branchiopod]] [[crustacean]]s that resemble the unrelated [[Bivalvia|bivalved molluscs]].<ref name=Webb1979/> They are extant and also known from the fossil record, from at least the Devonian period and perhaps before.<ref name=Woolfe1990/> They were originally classified in the former [[order (biology)|order]] '''Conchostraca''', which later proved to be [[paraphyly|paraphyletic]], because [[Diplostraca|water fleas]] are nested within clam shrimps. Clam shrimp are now divided into three orders, '''Cyclestherida''', '''Laevicaudata''', and '''Spinicaudata''', in addition to the fossil family Leaiidae.<ref name=VanDamme2016/><ref name=worms/>

== Characteristics ==
Both valves of the shell are held together by a strong closing [[muscle]]. The animals react to danger by contracting the muscle, so that the valves close tightly and the crustacean, as if dead, lies motionlessly at the bottom of the pool.

In most species the head is [[anatomical terms of location#Dorsal and ventral|dorsoventrally]] compressed. The [[Sessility (zoology)|sessile]] [[compound eye]]s are close together and located on the forehead; in the genus ''Cyclestheria'' they are truly fused. In front of them is a simple [[Nauplius (larva)|naupliar]] eye. The first pair of [[antenna (biology)|antennae]] is reduced and unsegmented. The second pair of antennae, however, is long and biramous. Both branches are covered with numerous bristles. The crustaceans swim primarily by swooping the antennae. In the common genus ''Lynceus'', which can open its spherical valves wide, the [[thoracic]] legs move in an oar-like manner along with the antennae.

The number of segments constituting the thorax varies from 10 to 32, and the number of legs varies accordingly. They are similar in structure to the legs of [[tadpole shrimp]], and similarly, their size decreases from front to back. In females, the outer lobes of several middle legs are modified into long, upward-bending threadlike outgrowths, used to hold the eggs on the dorsal side of the body under the shell. However, the main functions of the thoracic legs are respiration and carrying food forward to the mouth. The gills are basically the outer lobes of all thoracic legs that are closest to the base of the leg. The legs are in constant movement, and the water between the valves of the [[carapace]] is quickly renewed. The body ends in a large [[chitin]]ised [[telson]], which is either [[Anatomical terms of location#Directional terms|laterally]] compressed and bears a pair of large hooks, or dorsoventrally compressed, with short hooks.

== Reproduction and development ==
=== Reproduction ===
Clam shrimp have different reproductive strategies. For example, within the family [[Limnadiidae]] are found [[dioecy|dioecious]] (male-female), [[hermaphroditic]] (only hermaphrodites), and [[androdioecious]] (male-hermaphrodite) species.

=== Life cycle ===
The eggs are surrounded by a tough shell and can withstand drying out, freezing and other hostile conditions. In some species these eggs can hatch after as long as 7 years.

When the egg arrives in a suitable pool, a larva hatches out at the [[Nauplius (larva)|nauplius]] stage (the nauplius stage is absent in Cyclestherida).<ref>{{cite journal | url=https://pubmed.ncbi.nlm.nih.gov/26486940/ | pmid=26486940 | year=2015 | last1=Fritsch | first1=M. | last2=Richter | first2=S. | title=How the cladoceran heterogonic life cycle evolved--insights from gamogenetic reproduction and direct development in Cyclestherida | journal=Evolution & Development | volume=17 | issue=6 | pages=356–366 | doi=10.1111/ede.12163 | s2cid=1932950 }}</ref> Clam shrimp nauplii are distinguished by very small front antennae. At the second stage ([[metanauplius]]), the larva develops the small shell. They develop very quickly. For instance, ''Cyzicus'' reaches sexual maturity in 19 days after hatching.

== Taxonomy ==
Extant clam shrimp belong to three orders, divided into five families;<ref name="M&D">{{cite book |url=http://atiniui.nhm.org/pdfs/3839/3839.pdf |title=An Updated Classification of the Recent Crustacea |author=Joel W. Martin & George E. Davis |year=2001 |pages=132 |publisher=[[Natural History Museum of Los Angeles County]] |access-date=2011-04-04 |archive-date=2013-05-12 |archive-url=https://web.archive.org/web/20130512091254/http://atiniui.nhm.org/pdfs/3839/3839.pdf |url-status=dead }}</ref> some notable genera and [[prehistoric]] [[taxa]] are also listed:<!-- genus list is very incomplete but no complete list sems to exist online -->
{{Col-begin|width=70%}}
{{Col-break}}
'''Spinicaudata''' <small>Linder, 1945</small>
* [[Cyzicidae]] <small>Stebbing, 1910</small>
** ''[[Caenestheria]]''
** ''[[Caenestheriella]]''
** ''[[Cyzicus (crustacean)|Cyzicus]]''
** ''[[Eocyzicus]]''
* [[Eosestheriidae]] <small>Zhang & Chen, 1976</small> ([[fossil]])
** ''[[Bairdestheria]]''
** ''[[Menucoestheria]]'' <small>Gallego & Covacevich, 1998</small>
* [[Euestheridae]] <small>Defretin, 1965</small> ([[fossil]])
** ''[[Laxitextella]]'' <small>[[Heinz Kozur|Kozur]], 1982</small>
* [[Leptestheriidae]] <small>Daday, 1923</small>
** ''[[Eoleptestheria]]''
** ''[[Leptestheria]]''
** ''[[Leptestheriella]]''
** ''[[Maghrebestheria]]''
** ''[[Sewellestheria]]''
* [[Limnadiidae]] <small>Baird, 1849</small> (including Imnadiidae)
** ''[[Afrolimnadia]]''
** ''[[Calalimnadia]]'' 
** ''[[Eulimnadia]]''
** ''[[Imnadia]]''
** ''[[Limnadia]]''
** ''[[Limnadiopsis]]''
** ''[[Limnadiopsidum]]''
** ''[[Metalimnadia]]''
* [[Palaeolimnadiidae]] <small>Tasch, 1956</small> ([[fossil]])
** ''[[Krasiestheria]]'' <small>Olempska, 2004</small> (tentatively placed here)
{{Col-break}}
'''Cyclestherida''' <small>Sars, 1899</small>
* Cyclestheriidae <small>Sars, 1899</small>
** ''[[Cyclestheria]]''
** ''[[Paracyclestheria]]''
'''Laevicaudata''' <small>Linder, 1945</small>
* [[Lynceidae]] <small>Baird, 1845</small>
** ''[[Leptestheriella]]''
** ''[[Limnetis (crustacean)|Limnetis]]'' 
** ''[[Lynceiopsis]]''
** ''[[Lynceus (crustacean)|Lynceus]]''
** ''[[Paralimnetis]]''
{{col-end}}

== Geological history ==
Modern clam shrimp have little significance to humans. However, extinct species of these crustaceans are often studied by [[geologist]]s. In [[freshwater]] deposits, generally poor in fossils, the well-preserved clam shrimp shells are found quite often. They help identify the age of the corresponding [[Stratum|strata]].<ref>{{cite journal |last1=Scholze |first1=Frank |last2=Shen |first2=Shu-Zhong |last3=Backer |first3=Malte |last4=Wei |first4=Hai-Bo |last5=Hübner |first5=Marcel |last6=Cui |first6=Ying-Ying |last7=Feng |first7=Zhuo |last8=Schneider |first8=Joerg W. |date=June 2020 |title=Reinvestigation of conchostracans (Crustacea: Branchiopoda) from the Permian–Triassic transition in Southwest China |url=https://www.sciencedirect.com/science/article/abs/pii/S1871174X18301367 |journal=[[Palaeoworld]] |volume=29 |issue=2 |pages=368–390 |doi=10.1016/j.palwor.2019.04.007 |s2cid=189973963 |access-date=15 July 2023|url-access=subscription }}</ref>

During the past geological periods clam shrimp were apparently more numerous and diverse than they are now. 300 extinct species are known, and half as many living species. The oldest clam shrimp, such as ''[[Asmussia murchisoniana]]'', were found in [[Devonian]] deposits. Many extinct species, mostly Triassic specimens, once lived in marine environments, where no extant clam shrimp inhabit today.

== See also ==
*[[Isoxys]]

== References ==
{{Reflist|32em|refs=
<ref name=Webb1979>{{cite journal | author = J. Webb | year = 1979 | title = A reappraisal of the palaeoecology of conchostracans (Crustacea: Branchiopoda) |journal=[[Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen]] | volume = 158 | issue = 2 | pages = 259–275| doi = 10.1127/njgpa/158/1979/259 }}</ref>

<ref name=Woolfe1990>{{cite journal | author = K. J. Woolfe | year = 1990 | title = Trace fossils as paleoenvironmental indicators in the Taylor Group (Devonian) of Antarctica | journal = [[Palaeogeography, Palaeoclimatology, Palaeoecology]] | volume = 80 | issue = 3–4 | pages = 301–310 | doi = 10.1016/0031-0182(90)90139-X| bibcode = 1990PPP....80..301W }}</ref>

<ref name=VanDamme2016>{{Cite journal|last1=Van Damme|first1=Kay|last2=Kotov|first2=Alexey A.|date=December 2016|title=The fossil record of the Cladocera (Crustacea: Branchiopoda): Evidence and hypotheses|url=https://linkinghub.elsevier.com/retrieve/pii/S0012825216303701|journal=Earth-Science Reviews|language=en|volume=163|pages=162–189|doi=10.1016/j.earscirev.2016.10.009|bibcode=2016ESRv..163..162V |url-access=subscription}}</ref>

<ref name=worms>
{{Cite web| title=Diplostraca
| url=http://marinespecies.org/aphia.php?p=taxdetails&id=155670
| website=WoRMS, World Register of Marine Species
| access-date=2021-11-23
}}</ref>
}}

== External links ==
{{Portal|Crustaceans|Arthropods}}
* [http://www.sacsplash.org/critter/clam-shrimp]
* [http://www.ucmp.berkeley.edu/arthropoda/crustacea/branchiopoda.html Introduction to the Branchiopoda]
*{{Wikispecies-inline|Cyclestherida}}
*{{Wikispecies-inline|Laevicaudata}}
*{{Wikispecies-inline|Spinicaudata}}

{{Branchiopoda}}
{{Taxonbar|from=Q500917}}

[[Category:Branchiopoda]]
[[Category:Extant Devonian first appearances]]
[[Category:Arthropod common names]]