Editing Sea spider (section)

==Distribution and ecology==
[[File:Nymphon-leptocheles.jpg|thumb|''Nymphon leptocheles'' grazing on a [[Hydrozoa|hydroid]]]]
Sea spiders live in many different oceanic regions of the world, from [[Australia]], [[New Zealand]], and the [[Pacific]] coast of the [[United States]], to the [[Mediterranean Sea]] and the [[Caribbean Sea]], to the north and south poles. They are most common in shallow waters, but can be found as deep as {{convert|7000|m}}, and live in both marine and estuarine habitats. Pycnogonids are well camouflaged beneath the rocks and among the [[algae]] that are found along shorelines.

Sea spiders are [[benthic]] in general, using their stilt-like legs to walk along the bottom, but they are also capable of swimming by using an umbrella pulsing motion,<ref>{{cite web|last=McClain|first=Craig|date=August 14, 2006|title=Sea Spiders|url=http://deepseanews.blogspot.com/2006/08/sea-spiders.html|url-status=dead|archive-url=https://web.archive.org/web/20070709121802/http://deepseanews.blogspot.com/2006/08/sea-spiders.html|archive-date=9 July 2007|website=Deep Sea News Info}} </ref> and some Paleozoic species with flatten legs might even have a [[nektonic]] lifestyle.<ref name=":9" /><ref name=":1" /> Sea spiders are mostly [[carnivorous]] [[predator]]s or [[scavenger]]s that feed on soft-bodied invertebrates such as [[cnidarian]]s, [[sea sponge|sponges]], [[polychaete]]s, and [[bryozoa]]ns, by inserting their proboscis into targeted prey item. Although they are known to feed on [[sea anemone]]s, most sea anemones survive this ordeal, making the sea spider a [[parasite]] rather than a predator of sea anemones.<ref name=":5" /> A few species such as ''[[Nymphonella tapetis]]'' are specialised endoparasites of [[bivalve]] [[mollusk]]s.<ref>{{Cite journal |last1=Miyazaki |first1=Katsumi |last2=Tomiyama |first2=Takeshi |last3=Yamada |first3=Katsumasa |last4=Tamaoki |first4=Masanori |date=2015-07-01 |title=18S Analysis of the Taxonomic Position of an Endoparasitic Pycnogonid, Nymphonella Tapetis (Arthropoda: Pycnogonida: Ascorhynchidae) |url=https://academic.oup.com/jcb/article/35/4/491/2547795 |journal=Journal of Crustacean Biology |volume=35 |issue=4 |pages=491–494 |doi=10.1163/1937240X-00002348 |bibcode=2015JCBio..35..491T |issn=0278-0372}}</ref><ref>{{Cite journal |last1=Yamada |first1=Katsumasa |last2=Miyazaki |first2=Katsumi |last3=Tomiyama |first3=Takeshi |last4=Kanaya |first4=Gen |last5=Miyama |first5=Yoshifumi |last6=Yoshinaga |first6=Tomoyoshi |last7=Wakui |first7=Kunihiro |last8=Tamaoki |first8=Masanori |last9=Toba |first9=Mitsuharu |date=June 2018 |title=Impact of sea spider parasitism on host clams: susceptibility and intensity-dependent mortality |url=https://www.cambridge.org/core/journals/journal-of-the-marine-biological-association-of-the-united-kingdom/article/abs/impact-of-sea-spider-parasitism-on-host-clams-susceptibility-and-intensitydependent-mortality/DA4547DECD7996BE63B58B576B7F656E |journal=Journal of the Marine Biological Association of the United Kingdom |language=en |volume=98 |issue=4 |pages=735–742 |doi=10.1017/S0025315417000200 |bibcode=2018JMBUK..98..735Y |issn=0025-3154|url-access=subscription }}</ref>

Not much is known about the primary predators of sea spiders, if any. At least some species have obvious defensive methods such as [[autotomy|amputating]] and [[Regeneration (biology)|regenerating]] body parts,<ref>{{Cite journal |last1=Brenneis |first1=Georg |last2=Frankowski |first2=Karina |last3=Maaß |first3=Laura |last4=Scholtz |first4=Gerhard |date=2023-01-31 |title=The sea spider Pycnogonum litorale overturns the paradigm of the absence of axial regeneration in molting animals |journal=Proceedings of the National Academy of Sciences |language=en |volume=120 |issue=5 |pages=e2217272120 |doi=10.1073/pnas.2217272120 |doi-access=free |issn=0027-8424 |pmc=9946000 |pmid=36689663|bibcode=2023PNAS..12017272B }}</ref><ref>{{Cite journal |last1=Petrova |first1=Maria |last2=Bogomolova |first2=Ekaterina |date=2023-11-01 |title=Walking leg regeneration in the sea spider Nymphon brevirostre Hodge, 1863 (Pycnogonida) |url=https://linkinghub.elsevier.com/retrieve/pii/S1467803923000774 |journal=Arthropod Structure & Development |volume=77 |pages=101310 |doi=10.1016/j.asd.2023.101310 |bibcode=2023ArtSD..7701310P |issn=1467-8039|url-access=subscription }}</ref> or being unpleasant meals via high levels of [[ecdysteroids]] (ecdysis [[hormone]]).<ref>{{Cite journal |last=Tomaschko |first=K-H |date=1994-07-01 |title=Ecdysteroids fromPycnogonum litorale (Arthropoda, Pantopoda) act as chemical defense againstCarcinus maenas (Crustacea, Decapoda) |url=https://link.springer.com/article/10.1007/BF02059872 |journal=Journal of Chemical Ecology |language=en |volume=20 |issue=7 |pages=1445–1455 |doi=10.1007/BF02059872 |pmid=24242643 |bibcode=1994JCEco..20.1445T |issn=1573-1561|url-access=subscription }}</ref> On the other hand, sea spiders are known to be infected by parasitic [[gastropod]] mollusks<ref>{{Cite journal |last1=Lehmann |first1=Tobias |last2=Gailer |first2=Juan P. |last3=Melzer |first3=Roland R. |last4=Schwabe |first4=Enrico |date=2007-01-01 |title=A scanning-electron microscopic study of Dickdellia labioflecta (Dell, 1990) (Gastropoda, Littorinoidea) on Colossendeis megalonyx megalonyx Fry and Hedgpeth, 1969 (Pycnogonida, Colossendeidae): a test for ectoparasitism |url=https://link.springer.com/article/10.1007/s00300-006-0178-6 |journal=Polar Biology |language=en |volume=30 |issue=2 |pages=243–248 |doi=10.1007/s00300-006-0178-6 |issn=1432-2056|url-access=subscription }}</ref><ref>{{Cite journal |last1=Schiaparelli |first1=Stefano |last2=Oliverio |first2=Marco |last3=Taviani |first3=Marco |last4=Griffiths |first4=Huw |last5=Lörz |first5=Anne-Nina |last6=Albertelli |first6=Giancarlo |date=2008 |title=Short Note: Circumpolar distribution of the pycnogonid-ectoparasitic gastropod |url=https://www.cambridge.org/core/journals/antarctic-science/article/abs/short-note-circumpolar-distribution-of-the-pycnogonidectoparasitic-gastropod-dickdellia-labioflecta-dell-1990-mollusca-zerotulidae/C7C035ECB89CA05CCF62B80E82BABCC1 |journal=Antarctic Science |language=en |volume=20 |issue=5 |pages=497–498 |doi=10.1017/S0954102008001302 |issn=1365-2079|url-access=subscription }}</ref> or hitch‐rided by [[Sessility (motility)|sessile]] animals such as [[goose barnacle]]s, which may negatively affect their locomotion and respiratory efficiency.<ref>{{Cite journal |last1=Lane |first1=Steven J. |last2=Tobalske |first2=Bret W. |last3=Moran |first3=Amy L. |last4=Shishido |first4=Caitlin M. |last5=Woods |first5=H. Arthur |date=2018-08-01 |title=Costs of epibionts on Antarctic sea spiders |url=https://link.springer.com/article/10.1007/s00227-018-3389-9 |journal=Marine Biology |language=en |volume=165 |issue=8 |pages=137 |doi=10.1007/s00227-018-3389-9 |bibcode=2018MarBi.165..137L |issn=1432-1793|url-access=subscription }}</ref>

=== Reproduction and development ===
[[File:Tanystylum californicum 104577432.jpg|thumb|''Tanystylum californicum'' with eggs, ventral view.]]All sea spiders have separate sexes, except the only known [[hermaphroditic]] species ''Ascorhynchus corderoi'' and some extremely rare [[gynandromorph]] cases.<ref name=":4" /> Among all extant families, the [[Colossendeidae]] and [[Austrodecidae]] are the only two that still lack any observations on their reproductive behaviour and life cycle.<ref name=":4" /><ref name=":17" /> Reproduction involves [[external fertilization|external fertilisation]] when male and female stack together (usually male on top), exuding sperm and eggs from the gonopores of their respective leg coxae.<ref name=":4" /> After fertilisation, males glue the egg cluster with cement glands and using their ovigers (the oviger-lacking ''[[Nulloviger]]'' using only the ventral body wall) to take care of the laid eggs and young.<ref name=":4" />[[File:Achelia spinosa (YPM IZ 077366) 004.jpeg|left|thumb|200x200px|Protonymphon larva of ''Achelia spinosa'']]
In most cases, the offsprings hatch as a distinct larval stage known as protonymphon. It has a blind gut and the body consists of a cephalon and its first three pairs of cephalic appendages only: the chelifores, palps and ovigers. In this stage, the chelifores usually have attachment glands, while the palps and ovigers are subequal, three-segmented appendages known as palpal and ovigeral larval limbs. When the larvae [[Moulting|moult]] into the postlarval stage, they undergo transitional [[metamorphosis]]: the leg-bearing segments develop and the three pairs of cephalic appendages further develop or reduce. The postlarva eventually metamorphoses into a juvenile that looks like a miniature adult, which will continue to moult into an adult with a fixed number of walking legs.<ref name=":17">{{Cite journal |last1=Brenneis |first1=Georg |last2=Bogomolova |first2=Ekaterina V. |last3=Arango |first3=Claudia P. |last4=Krapp |first4=Franz |date=2017-02-07 |title=From egg to "no-body": an overview and revision of developmental pathways in the ancient arthropod lineage Pycnogonida |journal=Frontiers in Zoology |volume=14 |issue=1 |pages=6 |doi=10.1186/s12983-017-0192-2 |doi-access=free |issn=1742-9994 |pmc=5297176 |pmid=28191025}}</ref><ref>{{Cite journal |last1=Alexeeva |first1=Nina |last2=Tamberg |first2=Yuta |last3=Shunatova |first3=Natalia |date=2018-05-01 |title=Postembryonic development of pycnogonids: A deeper look inside |url=https://linkinghub.elsevier.com/retrieve/pii/S146780391830029X |journal=Arthropod Structure & Development |volume=47 |issue=3 |pages=299–317 |doi=10.1016/j.asd.2018.03.002 |pmid=29524544 |bibcode=2018ArtSD..47..299A |issn=1467-8039|url-access=subscription }}</ref> In [[Pycnogonidae]], the ovigers are reduced in juveniles but reappear in oviger-bearing adult males.<ref name=":22" /> 

These kind of "head-only" larvae and its [[Anamorphosis (biology)|anamorphic]] metamorphosis resemble [[crustacean]] [[Crustacean larva|nauplius]] larvae and [[megacheira]]n larvae, all together might reflects how the larvae of a common ancestor of all arthropods developed: starting its life as a tiny animal with a few head appendages, while new body segments and appendages were gradually added as it was growing.<ref name=":22" /><ref>{{Cite journal |last1=Liu |first1=Yu |last2=Melzer |first2=Roland R. |last3=Haug |first3=Joachim T. |last4=Haug |first4=Carolin |last5=Briggs |first5=Derek E. G. |last6=Hörnig |first6=Marie K. |last7=He |first7=Yu-yang |last8=Hou |first8=Xian-guang |date=2016-05-17 |title=Three-dimensionally preserved minute larva of a great-appendage arthropod from the early Cambrian Chengjiang biota |journal=Proceedings of the National Academy of Sciences |language=en |volume=113 |issue=20 |pages=5542–5546 |doi=10.1073/pnas.1522899113 |doi-access=free |issn=0027-8424 |pmc=4878483 |pmid=27140601|bibcode=2016PNAS..113.5542L }}</ref>

Further details of the postembryonic developments of sea spiders vary, but their categorization might differ between literatures. As of the 2010s, there are five types identified as follows:<ref name=":17" />
{| class="wikitable"
|+
!{{Diagonal split header|Characteristics|Type}}
!1
!2
!3
!4
!5
|-
!Also known as
|typical protonymphon
|attaching larva (partially), lecithotrophic protonymphon
|atypical protonymphon
|encysted larva
|attaching larva (partially)
|-
!Hatch as
|protonymphon
|protonymphon
|protonymphon
|protonymphon
|postlarva
|-
!Palpal and ovigeral larval limbs
|functional, claw-like
|functional, claw-like
|functional, claw-like
|functional, filament-like
|variably reduced or absent
|-
!Hatching with walking leg buds
|no
|no
|no
|no
|at least leg 1–2 present
|-
!Walking leg development
|sequential
|sequential
|synchronized for all legs
|synchronized for leg 1–3
|remaining legs sequential
|-
!Instar leaving father
|protonymphon
|postlarva with at least leg 1–2
|protonymphon
|protonymphon
|postlarva with at least leg 1–2
|-
!Postlarval life cycle
|parasite of [[cnidarian]]s and rarely [[mollusk]]s
|[[lecithotrophic]] on ovigers, thereafter free living
|ectoparasites of mollusks and [[polychaete]]s
|endoparasite of [[hydrozoan]]s
|lecithotrophic on oviger, thereafter free living
|-
!Occurred taxa
|[[Ammotheidae]], [[Ascorhynchidae]], [[Endeidae]], [[Nymphonidae]],  [[Pallenopsidae]], [[Pycnogonidae]]
|Ammotheidae, Nymphonidae
|Ammotheidae
|Ammotheidae, [[Phoxichilidiidae]]
|[[Callipallenidae]], Nymphonidae, Pallenopsidae
|}
The type 1 (typical protonymphon) is the most common and possibly an ancestral one. When the type 2 and 5 (attaching larva) hatches it immediately attaches itself to the ovigers of the father, where it will stay until it has turned into a small and young juvenile with two or three pairs of walking legs ready for a free-living existence. The type 3 (atypical protonymphon) have limited observations. The adults are free living, while the larvae and the juveniles are living on or inside temporary hosts such as [[polychaete]]s and [[clam]]s. The type 4 (encysted larva) is a parasite that hatches from the egg and finds a host in the shape of a polyp colony where it burrows into and turns into a cyst, and will not leave the host before it has turned into a young juvenile.<ref name=":21">{{cite journal |last1=Bain |first1=B. A. |date=2003 |title=Larval types and a summary of postembryonic development within the pycnogonids |journal=Invertebrate Reproduction & Development |volume=43 |issue=3 |pages=193–222 |bibcode=2003InvRD..43..193B |doi=10.1080/07924259.2003.9652540 |s2cid=84345599}}</ref><ref name=":4" /><ref name=":17" />