Editing Sea spider (section)

== Morphology ==
[[File:Callipallene brevirostris (YPM IZ 077244) 003.jpeg|thumb|''[[Callipallene brevirostris]]'']]
Many sea spiders are recognised by their enormous walking legs in contrast to a reduced body region,  resulting into the so-called "all legs" or "no body" appearance. The body segments ([[somite]]s) are generally interpreted as three main sections ([[Tagma (biology)|tagma]]): cephalon (head, aka cephalosoma), trunk (aka thorax) and abdomen.<ref name=":9" /><ref name=":3" /> However, the definition of cephalon and trunk might differ between literature (see text), and some studies might follow a prosoma (=cephalon+trunk)–opisthosoma (=abdomen) definition, aligning to the tagmosis of other chelicerates.<ref name=":22">{{Cite journal |last1=Vilpoux |first1=Kathia |last2=Waloszek |first2=Dieter |date=2003-12-01 |title=Larval development and morphogenesis of the sea spider ''Pycnogonum litorale'' (Ström, 1762) and the tagmosis of the body of Pantopoda |url=https://www.sciencedirect.com/science/article/abs/pii/S1467803903001154 |journal=Arthropod Structure & Development |volume=32 |issue=4 |pages=349–383 |doi=10.1016/j.asd.2003.09.004 |pmid=18089018 |bibcode=2003ArtSD..32..349V |issn=1467-8039|url-access=subscription }}</ref><ref name=":2" /> The [[exoskeleton]] of the body is tube-like, lacking the dorsoventral division ([[tergite]] and [[sternite]]) seen in most other arthropods.<ref name=":3">{{Cite journal |last1=Dunlop |first1=Jason A. |last2=Lamsdell |first2=James C. |date=2017 |title=Segmentation and tagmosis in Chelicerata |url=https://www.academia.edu/28212892 |journal=Arthropod Structure & Development |volume=46 |issue=3 |pages=395–418 |doi=10.1016/j.asd.2016.05.002 |pmid=27240897 |bibcode=2017ArtSD..46..395D |issn=1467-8039}}</ref>
<gallery mode="packed" heights="160">
20200205 Pycnogonida Pantopoda morphology.png|Generalized morphology of a pantopod pycnogonid
1937 Smithsonian miscellaneous collections Snodgrass 1936 p24 Fig. 07.jpg|Ventral view and leg base of ''[[Nymphonidae|Chaetonymphon spinosum]]''
</gallery>
The cephalon is formed by the fusion of ocular somite and four anterior segments behind it (somite 1–4). It consists of an anterior [[proboscis]], a dorsal ocular tubercle with [[eye]]s, and up to four pairs of [[appendages]] ([[Chelicerae|chelifores]], [[Pedipalp|palps]], {{linktext|oviger}}s and first walking legs). Although some literature might consider the segment carrying the first walking leg (somite 4) to be part of the trunk,<ref name=":1" /> it is completely fused to the remaining head section to form a single cephalic tagma.<ref name=":22" /><ref name=":3" /> The proboscis has three-fold symmetry, terminating with a typically Y-shaped mouth (vertical slit in [[Austrodecidae]]<ref name=":8">{{Cite journal |last1=Arango |first1=Claudia P. |last2=Wheeler |first2=Ward C. |date=2007 |title=Phylogeny of the sea spiders (Arthropoda, Pycnogonida) based on direct optimization of six loci and morphology |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1096-0031.2007.00143.x |journal=Cladistics |language=en |volume=23 |issue=3 |pages=255–293 |doi=10.1111/j.1096-0031.2007.00143.x |pmid=34905863 |issn=0748-3007}}</ref>). It usually has fairly limited dorsoventral and lateral movement. However, in those species that have reduced chelifores and palps, the proboscis is well developed and flexible, often equipped with numerous sensory bristles and strong rasping ridges around the mouth.<ref name="2017proboscis">{{Cite journal |last1=Wagner |first1=Philipp |last2=Dömel |first2=Jana S. |last3=Hofmann |first3=Michaela |last4=Hübner |first4=Jeremy |last5=Leese |first5=Florian |last6=Melzer |first6=Roland R. |date=2017-03-01 |title=Comparative study of bisected proboscides of Pycnogonida |url=https://link.springer.com/article/10.1007/s13127-016-0310-6 |journal=Organisms Diversity & Evolution |language=en |volume=17 |issue=1 |pages=121–135 |doi=10.1007/s13127-016-0310-6 |bibcode=2017ODivE..17..121W |issn=1618-1077|url-access=subscription }}</ref> The proboscis is unique to pycnogonids, and its exact [[Homology (biology)|homology]] with other arthropod mouthparts is enigmatic, as well as its relationship with the absence of [[Labrum (arthropod mouthpart)|labrum]] (preoral upper lip of ocular somite) in pycnogonid itself.<ref name=":3" /> The ocular tubercle has up to two pairs of simple eyes ([[ocelli]]) on it, though sometimes the eyes can be reduced or missing, especially among species living in the deep oceans. All of the eyes are median eyes in origin, homologous to the median ocelli of other arthropods, while the lateral eyes (e.g. [[compound eyes]]) found in most other arthropods are completely absent.<ref>{{Cite journal |last1=Miether |first1=Sebastian T. |last2=Dunlop |first2=Jason A. |date=2016 |title=Lateral eye evolution in the arachnids |url=http://www.bioone.org/doi/10.13156/arac.2006.17.2.103 |journal=Arachnology |language=en |volume=17 |issue=2 |pages=103–119 |doi=10.13156/arac.2006.17.2.103 |issn=2050-9928|url-access=subscription }}</ref>
<gallery mode="packed" heights="130">
Pseudopallene pachycheira.jpeg|''[[Pseudopallene]] pachycheira'', showing robust chelifores and the absence of palps.
Pycnogonum littorale (YPM IZ 030249).jpeg|''[[Pycnogonum litorale]]'', showing the absence of both chelifores and palps. Ovigers are absent in female.
Colossendeis (MNHN-IU-2013-2065).jpeg|''[[Colossendeis]]'' sp., showing the absence of chelifores but otherwise elongated proboscis, palps and ovigers.
Nymphon maculatum 2011-721 (1) (cropped).jpg|''[[Nymphon]] maculatum'', showing the presence of both chelifores, palps and ovigers.
</gallery>
In adult pycnogonids, the chelifores (aka cheliphore<ref name=":9" />), palps and ovigers (aka ovigerous legs<ref name=":4">{{Cite journal |last1=Bain |first1=Bonnie A. |last2=Govedich |first2=Fredric R. |date=2004 |title=Courtship and mating behavior in the Pycnogonida (Chelicerata: Class Pycnogonida): a summary |url=http://www.tandfonline.com/doi/abs/10.1080/07924259.2004.9652607 |journal=Invertebrate Reproduction & Development |language=en |volume=46 |issue=1 |pages=63–79 |doi=10.1080/07924259.2004.9652607 |bibcode=2004InvRD..46...63B |issn=0792-4259|url-access=subscription }}</ref>) are variably reduced or absent, depending on taxa and sometimes sex. [[Nymphonidae]] is the only family where all of three pairs are always functional. The ovigers can be reduced or missing in females, but are present in almost all males.<ref>[https://academic.oup.com/mbe/article/38/2/686/5904272?login=false Phylogenomic Resolution of Sea Spider Diversification through Integration of Multiple Data Classes]</ref> In a functional condition, the chelifores terminate with a [[chelae|pincer]] (chela) formed by two segments (podomeres), like the chelicerae of most other chelicerates. The scape (peduncle) behind the pincer is usually unsegmented, but could be bisegmented in some species, resulting into a total of three or four chelifore segments.<ref name=":11">{{Cite journal |last1=Siveter |first1=Derek J. |last2=Sutton |first2=Mark D. |last3=Briggs |first3=Derek E. G. |last4=Siveter |first4=David J. |date=2004 |title=A Silurian sea spider |url=https://www.nature.com/articles/nature02928 |journal=Nature |language=en |volume=431 |issue=7011 |pages=978–980 |doi=10.1038/nature02928 |pmid=15496921 |bibcode=2004Natur.431..978S |issn=1476-4687|url-access=subscription }}</ref><ref name=":1" /> The palps and ovigers have up to 9 and 10 segments respectively, but can have fewer even when in a functional condition.<ref>{{Cite journal |last1=Cano-Sánchez |first1=Esperanza |last2=López-González |first2=Pablo J. |date=2016-12-15 |title=Basal articulation of the palps and ovigers in Antarctic Colossendeis (Pycnogonida; Colossendeidae) |journal=Helgoland Marine Research |volume=70 |issue=1 |pages=22 |doi=10.1186/s10152-016-0474-7 |doi-access=free |issn=1438-3888}}</ref><ref name=":13">{{Cite journal |last1=Sabroux |first1=Romain |last2=Edgecombe |first2=Gregory D. |last3=Pisani |first3=Davide |last4=Garwood |first4=Russell J. |date=2023 |title=New insights into the sea spider fauna (Arthropoda, Pycnogonida) of La Voulte-sur-Rhône, France (Jurassic, Callovian) |url=https://onlinelibrary.wiley.com/doi/10.1002/spp2.1515 |journal=Papers in Palaeontology |language=en |volume=9 |issue=4 |pages=e1515 |doi=10.1002/spp2.1515 |bibcode=2023PPal....9E1515S |issn=2056-2802}}</ref> The palps are rather featureless and never have claws in adult Pantopoda, while the ovigers may or may not possess a terminal claw and rows of specialised spines on its curved distal segments (strigilis).<ref name=":13" /> The chelifores are used for feeding and the palps are used for sensing and manipulating food items,<ref name=":5">{{Cite journal |last1=Dietz |first1=Lars |last2=Dömel |first2=Jana S. |last3=Leese |first3=Florian |last4=Lehmann |first4=Tobias |last5=Melzer |first5=Roland R. |date=2018-03-15 |title=Feeding ecology in sea spiders (Arthropoda: Pycnogonida): what do we know? |journal=Frontiers in Zoology |volume=15 |issue=1 |pages=7 |doi=10.1186/s12983-018-0250-4 |doi-access=free |issn=1742-9994 |pmc=5856303 |pmid=29568315}}</ref> while the ovigers are used for cleaning themselves, with the additional function of carrying offspring in males.<ref name=":4" /> 
{| class="wikitable sortable mw-collapsible mw-collapsed"
|+Conditions of chelifores, palps, and ovigers by family<ref name=":8" /><ref name=":0" /><ref name=":13" /><ref name=":14" />
!{{Diagonal split header|families|appendages}}
!chelifores
!palps
!ovigers
|-
![[Austrodecidae]]
|absent
|functional
|functional<br>(absent in some male of ''[[Austrodecus]]'')
|-
![[Rhynchothoracidae]]
|absent
|functional
|functional
|-
![[Pycnogonidae]]
|absent
|absent
|absent in female<br>(both sexes in ''[[Pycnogonum|Nulloviger]]'')
|-
![[Colossendeidae]]
|absent<br>(functional in polymerous genera)
|functional
|functional
|-
![[Endeidae]]
|absent
|absent
|absent in female
|-
![[Phoxichilidiidae]]
|functional
|absent
|absent in female
|-
![[Pallenopsidae]]
|functional
|reduced
|functional 
|-
![[Ammotheidae]]
|reduced
|functional
|functional
|-
![[Ascorhynchidae]]
|reduced
|functional
|functional
|-
![[Callipallenidae]]
|functional
|absent<br>(functional in some male)
|functional
|-
![[Nymphonidae]]
|functional
|functional
|functional
|-
|}
[[File:1909 The Cambridge natural history volume IV p531 Fig. 282.jpg|thumb|''[[Decolopoda]] australis'', showing 10 legs and four-segmented chelifores (upper left).]]
[[File:1909 The Cambridge natural history volume IV p509 Fig. 274.jpg|thumb|Tibia 2 (distal portion), tarsus, propodus and claws of various pantopod pycnogonids.]]
[[File:NMNH-Sexanymphon mirabilis1-000001.jpg|thumb|''[[Sexanymphon]] mirabilis'', a species with six pairs of legs]]
The leg-bearing somites (somite 4 and all trunk somites, the alternatively defined "trunk/thorax") are either segmented or fused to each other, carrying the walking legs via a series of lateral processes (lateral tubular extension of the somites). In most species, the legs are much larger than the body in both length and volume, only being shorter and more slender than the body in [[Rhynchothoracidae]]. Each leg is typically composed of eight tubular segments, commonly known as coxa 1,  2 and 3, femur, tibia 1 and 2, tarsus, and propodus.<ref name=":2" /> This terminology, with three coxae, no trochanter, and using the term "propodus", is unusual for arthropods. However, based on [[muscular system]] and [[serial homology]] to the podomeres of other chelicerates, they are most likely coxa (=coxa 1), trochanter (=coxa 2), prefemur/basifemur (=coxa 3), postfemur/telofemur (=femur), patella (=tibia 1), tibia (tibia 2) and two tarsomeres (=tarsus and propodus) in origin.<ref>{{Cite journal |last=Shultz |first=Jeffrey W. |date=1989 |title=Morphology of locomotor appendages in Arachnida: evolutionary trends and phylogenetic implications |url=https://academic.oup.com/zoolinnean/article-lookup/doi/10.1111/j.1096-3642.1989.tb00552.x |journal=Zoological Journal of the Linnean Society |language=en |volume=97 |issue=1 |pages=1–55 |doi=10.1111/j.1096-3642.1989.tb00552.x|url-access=subscription }}</ref> The leg segmentation of Paleozoic taxa is a bit different, noticeably they have annulated coxa 1 and are further divided into two types: one with flattened distal (femur and beyond) segments and first leg pair with one less segment than the other leg pairs (e.g. ''[[Palaeoisopus]]'',  ''[[Haliestes]]''), and another one with an immobile joint between the apparently fourth and fifth segment which altogether might represent a divided femur (e.g. ''[[Palaeopantopus]]'',  ''[[Flagellopantopus]]'').<ref name=":1" /> Each leg terminates with a main claw (aka pretarsus/apotele, the true terminal segment), which may or may not have a pair of auxiliary claws on its base. Most of the joints move vertically, except the joint between coxa 1–2 (coxa-trochanter joint) which provide lateral mobility (promotor-remotor motion), and the joint between tarsus and propodus did not have muscles, just like the subdivided tarsus of other arthropods.<ref name=":2" /><ref name=":13" /> There are usually eight (four pairs) legs in total, but a few species have five to six pairs. These are known as polymerous (i.e., extra-legged) species, which are distributed among six genera in the families [[Pycnogonidae]] (five pairs in ''[[Pentapycnon]]''), [[Colossendeidae]] (five pairs in ''[[Decolopoda]]'' and  ''[[Pentacolossendeis]]'', six pairs in ''[[Dodecolopoda]]'') and [[Nymphonidae]] (five pairs in ''[[Pentanymphon]]'', six pairs in ''[[Sexanymphon]]'').<ref>{{cite book |last=Ruppert |first=Edward E. |year=1994 |title=Invertebrate Zoology |edition=6th |others=Barnes, Robert D. |place=Fort Worth, TX |publisher=Saunders College Pub |isbn=0-03-026668-8 |language=English |oclc=30544625 |url=https://www.worldcat.org/oclc/30544625}}</ref><ref name=":2">{{cite encyclopedia |last=Crooker |first=Allen |title=Sea Spiders (Pycnogonida) |year=2008 |encyclopedia=Encyclopedia of Entomology |pages=3321–3335 |editor-last=Capinera |editor-first=John L. |place=Dordrecht, NL |publisher=Springer Netherlands |lang=en |doi=10.1007/978-1-4020-6359-6_4098 |isbn=978-1-4020-6359-6 <!-- redundant --- |url=https://doi.org/10.1007/978-1-4020-6359-6_4098 |access-date=2022-02-17 --> }}</ref>

Several alternatives had been proposed for the position homology of pycnogonid appendages, such as chelifores being protocerebral/homologous to the labrum (see text)<ref name="Maxmen" /> or ovigers being duplicated palps.<ref name=":16">{{Cite journal |last1=Manuel |first1=Michaël |last2=Jager |first2=Muriel |last3=Murienne |first3=Jérôme |last4=Clabaut |first4=Céline |last5=Guyader |first5=Hervé Le |date=2006-07-01 |title=Hox genes in sea spiders (Pycnogonida) and the homology of arthropod head segments |url=https://link.springer.com/article/10.1007/s00427-006-0095-2 |journal=Development Genes and Evolution |language=en |volume=216 |issue=7 |pages=481–491 |doi=10.1007/s00427-006-0095-2 |pmid=16820954 |issn=1432-041X|url-access=subscription }}</ref> Conclusively, the classic, morphology-based one-by-one alingment to the prosomal appendages of other chelicerates was confirm by both [[neuroanatomic]] and [[Genetics|genetic]] evidences.<ref name=":16" /><ref name="Jager" /> Noticeably, the order of pycnogonid leg pairs are mismatched to those of other chelicerates, starting from the ovigers which are homologous to the first leg pair of [[arachnid]]s. While the fourth walking leg pair was considered aligned to the variably reduced first opisthosomal segment (somite 7, also counted as part of the prosoma based on different studies and/or taxa) of euchelicerates, the origin of the additional fifth and sixth leg pairs in the polymerous species are still enigmatic.<ref name=":12" /><ref name=":3" /> Together with the cephalic position of the first walking legs, the anterior and posterior boundary of pycnogonid leg pairs are not aligned to those of euchelicerate prosoma and opisthosoma, nor the cephalon and trunk of pycnogonid itself.<ref name=":22" />

{|class="wikitable
!{{Diagonal split header|taxa|somites}}
!0<br>(ocular somite)
!1
!2
!3
!4
!5
!6
!7
|-
!Euchelicerates
|[[Labrum (arthropod mouthpart)|labrum]]
|[[chelicerae]]
|[[pedipalps]]
|leg 1
|leg 2
|leg 3
|leg 4
|chilarium in [[Xiphosura|horseshoe crabs]], appendage absent in arachnids
|-
!Pycnogonids
|?
|chelifores
|palps
|ovigers
|leg 1
|leg 2
|leg 3
|leg 4
|}

The abdomen (aka trunk end<ref name=":11" />) does not have any appendages. In Pantopoda it is also called the anal tubercle,<ref name=":12" /><ref name=":17" /> which is always unsegmented, highly reduced and almost vestigial, simply terminated by the [[anus]]. It is considered to be a remnant of opisthosoma/trunk of other chelicerates, but it is unknown which somite (s) it actually aligned to. So far only Paleozoic species have segmented abdomens (at least up to four segments, presumably somite 8–11 which aligned to opisthosomal segment 2–5 of euchelicerates), with some of them even terminated by a long [[telson]] (tail).<ref name=":9">{{Cite journal |last1=Bergström |first1=Jan |last2=Stürmer |first2=Wilhelm |last3=Winter |first3=Gerhard |date=1980 |title=''Palaeoisopus'', ''Palaeopantopus'' and ''Palaeothea'', pycnogonid arthropods from the Lower Devonian Hunsriick Slate, West Germany |url=https://www.academia.edu/5146832 |journal=Paläontologische Zeitschrift |volume=54 |issue=1–2 |pages=7 |doi=10.1007/BF02985882 |bibcode=1980PalZ...54....7B |issn=0031-0220}}</ref><ref name=":19">{{Cite journal |last1=Poschmann |first1=Markus |last2=Dunlop |first2=Jason A. |date=2006 |title=A New Sea Spider (arthropoda: Pycnogonida) with a Flagelliform telson from the Lower Devonian Hunsrück Slate, Germany |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1475-4983.2006.00583.x |journal=Palaeontology |language=en |volume=49 |issue=5 |pages=983–989 |doi=10.1111/j.1475-4983.2006.00583.x |bibcode=2006Palgy..49..983P |issn=1475-4983}}</ref><ref name=":1" />