Editing Sea spider (section)

=== 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" />