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Another large polar species of sea angel, ''[[Clione antarctica]]'', defends itself from predators by synthesizing a previously unknown molecule, ''pteroenone''. Because of this secretion, predators will not eat the sea angel. A species of [[amphipod]] takes advantage of this trait: The amphipod will seize an individual of ''C. antarctica'' out of the water column, and carry it around for protection. Local population density of ''C. antarctica'' may reach extraordinary levels; up to 300 animals per cubic metre have been recorded.
Another large polar species of sea angel, ''[[Clione antarctica]]'', defends itself from predators by synthesizing a previously unknown molecule, ''pteroenone''. Because of this secretion, predators will not eat the sea angel. A species of [[amphipod]] takes advantage of this trait: The amphipod will seize an individual of ''C. antarctica'' out of the water column, and carry it around for protection. Local population density of ''C. antarctica'' may reach extraordinary levels; up to 300 animals per cubic metre have been recorded.


==Reproduction and development==
Like many gastropods, sea angels are simultaneous [[hermaphrodite]]s with internal fertilization. A fertilized animal later releases a gelatinous egg mass, and the eggs float freely until hatching. Their embryonic shells are lost within the first few days after hatching.
Like many gastropods, sea angels are simultaneous [[hermaphrodite]]s with internal fertilization. A fertilized animal later releases a gelatinous egg mass, and the eggs float freely until hatching. Their embryonic shells are lost within the first few days after hatching.


The gymnosomes, like other shell-less opisthobranchs,{{efn|Opisthobranchs with no shells are the gymnosomes (sea angels), the [[sacoglossa]], and [[nudibranch]]s.}} discard their shells at metamorphosis, with the retractor muscles being severed and the shell lost.<ref name=Gibson2003>
==Evolutionary relationships among Pteropods==
{{cite journal
| author = Gibson, Glenys D.
| year = 2003
| title = Larval Development and Metamorphosis in ''Pleurobranchaea maculata'', with a Review of Development in the Notaspidea (Opisthobranchia)
| journal = [[Biological Bulletin]]
| volume = 205 | issue = 2 | pages = 121–132
| pmid = 14583510 | jstor = 1543233 | doi = 10.2307/1543233
| url = http://www.biolbull.org/cgi/content/abstract/205/2/121
| access-date = 2010-06-04 | url-status = live
| archive-url = https://web.archive.org/web/20110616191909/http://www.biolbull.org/cgi/content/abstract/205/2/121
| archive-date = 2011-06-16
}}
</ref>
The group does not truly, therefore, lack a shell. Few larval shells have been described (and consequently an understanding of their fossil record is as yet unknown).<ref>
{{cite journal
| author = Janssen, A.W.
| year = 2003
| title = Notes on the systematics, morphology and biostratigraphy of fossil holoplanktonic Mollusca, 13. Considerations on a subdivision of Thecosomata, with the emphasis on genus group classification of Limacinidae
| journal = Cainozoic Research
| volume = 2 | issue = 1–2 | pages = 163–170
}}</ref>

== Distribution ==
[[File:Sea angels .jpg|right|thumb|250px|Sea angels in Australian waters]]
These organisms have a wide geographic range, from polar regions, under sea ice, to equatorial (tropic) seas.<ref name=Seibel2007/>

== Behavior ==
Gymnosomata are carnivorous, feeding only on their fellow pteropods, the [[Thecosomata]].<ref name=Seibel2007>{{Cite journal | last1 = Seibel | first1 = B. A. | last2 = Dymowska | first2 = A. | last3 = Rosenthal | first3 = J. | doi = 10.1093/icb/icm089 | title = Metabolic temperature compensation and coevolution of locomotory performance in pteropod molluscs | journal = [[Integrative and Comparative Biology]] | volume = 47 | issue = 6 | pages = 880–891 | year = 2007 | pmid = 21669767| doi-access = free }}</ref> Their lifestyles have coevolved with those of their prey, with their feeding strategy adapting to the morphology and consistency of the thecosome shell.<ref name=Seibel2007/>

Their hunting strategies are variable; some forms are ambush predators, sitting and waiting for their prey; whilst others actively pursue their prey; their metabolic rate is closely linked to that of their prey species.<ref name=Seibel2007/> Even the size of the gymnosomes is correlated to the size of their prey,<ref name=Seibel2007/> which they recognize by means of touch and grab using their sometimes-suckered buccal cones.<ref name=Seibel2007/> A combination of hooks and a toothed radula are employed to scour the flesh from the thecosomes' shells.<ref name=Seibel2007/>

Gymnosomes slowly beat their wing-like parapodia<ref name=Seibel2007/> in a rowing motion<ref name=Morton1958>{{Cite journal | last1 = Morton | first1 = J. E. | title = Observations on the Gymnosomatous Pteropod Clione Limacina (Phipps) | doi = 10.1017/S0025315400023687 | journal = [[Journal of the Marine Biological Association of the United Kingdom]] | volume = 37 | issue = 2 | pages = 287–297 | year = 2009 | url = http://plymsea.ac.uk/1927/1/Observations_on_the_gymnosomatous_pteropod_Clione_limacina_%28Phipps%29.pdf | access-date = 2019-07-12 | archive-date = 2017-09-22 | archive-url = https://web.archive.org/web/20170922013055/http://plymsea.ac.uk/1927/1/Observations_on_the_gymnosomatous_pteropod_Clione_limacina_%28Phipps%29.pdf | url-status = live }}</ref> to propel their "perfectly streamlined"<ref name=Morton1958/> bodies through the upper 20 m of the water column. Although usually slow-moving, beating their wings once or twice per second, they are capable of bursts of speed when they need to pursue their prey, calling a separate suite of muscles into action to obtain the higher beat frequency.<ref name=Seibel2007/>

==Taxonomy==
{{cladogram
{{cladogram
|align=right
|align=right
Line 77: Line 112:
}}
}}
The other suborder of pteropods, [[Thecosomata]], is superficially similar to sea angels, but are not closely related; some authorities include both [[Thecosomata]] and [[Gymnosomata]] as separate branches of the [[order (biology)|order]] [[Pteropoda]], whereas others list them as distinct orders within [[subclass (biology)|the subclass]] [[Heterobranchia]]. They have larger, broader parapodia, and most of that species retain a shell; they are commonly known as [[sea butterflies]].
The other suborder of pteropods, [[Thecosomata]], is superficially similar to sea angels, but are not closely related; some authorities include both [[Thecosomata]] and [[Gymnosomata]] as separate branches of the [[order (biology)|order]] [[Pteropoda]], whereas others list them as distinct orders within [[subclass (biology)|the subclass]] [[Heterobranchia]]. They have larger, broader parapodia, and most of that species retain a shell; they are commonly known as [[sea butterflies]].
{{clear}}


== Development ==
The gymnosomes, like other shell-less opisthobranchs,{{efn|Opisthobranchs with no shells are the gymnosomes (sea angels), the [[sacoglossa]], and [[nudibranch]]s.}} discard their shells at metamorphosis, with the retractor muscles being severed and the shell lost.<ref name=Gibson2003>
{{cite journal
| author = Gibson, Glenys D.
| year = 2003
| title = Larval Development and Metamorphosis in ''Pleurobranchaea maculata'', with a Review of Development in the Notaspidea (Opisthobranchia)
| journal = [[Biological Bulletin]]
| volume = 205 | issue = 2 | pages = 121–132
| pmid = 14583510 | jstor = 1543233 | doi = 10.2307/1543233
| url = http://www.biolbull.org/cgi/content/abstract/205/2/121
| access-date = 2010-06-04 | url-status = live
| archive-url = https://web.archive.org/web/20110616191909/http://www.biolbull.org/cgi/content/abstract/205/2/121
| archive-date = 2011-06-16
}}
</ref>
The group does not truly, therefore, lack a shell. Few larval shells have been described (and consequently an understanding of their fossil record is as yet unknown).<ref>
{{cite journal
| author = Janssen, A.W.
| year = 2003
| title = Notes on the systematics, morphology and biostratigraphy of fossil holoplanktonic Mollusca, 13. Considerations on a subdivision of Thecosomata, with the emphasis on genus group classification of Limacinidae
| journal = Cainozoic Research
| volume = 2 | issue = 1–2 | pages = 163–170
}}</ref>

== Distribution ==
[[File:Sea angels .jpg|right|thumb|250px|Sea angels in Australian waters]]
These organisms have a wide geographic range, from polar regions, under sea ice, to equatorial (tropic) seas.<ref name=Seibel2007/>

== Behavior ==
Gymnosomata are carnivorous, feeding only on their fellow pteropods, the [[Thecosomata]].<ref name=Seibel2007>{{Cite journal | last1 = Seibel | first1 = B. A. | last2 = Dymowska | first2 = A. | last3 = Rosenthal | first3 = J. | doi = 10.1093/icb/icm089 | title = Metabolic temperature compensation and coevolution of locomotory performance in pteropod molluscs | journal = [[Integrative and Comparative Biology]] | volume = 47 | issue = 6 | pages = 880–891 | year = 2007 | pmid = 21669767| doi-access = free }}</ref> Their lifestyles have coevolved with those of their prey, with their feeding strategy adapting to the morphology and consistency of the thecosome shell.<ref name=Seibel2007/>

Their hunting strategies are variable; some forms are ambush predators, sitting and waiting for their prey; whilst others actively pursue their prey; their metabolic rate is closely linked to that of their prey species.<ref name=Seibel2007/> Even the size of the gymnosomes is correlated to the size of their prey,<ref name=Seibel2007/> which they recognize by means of touch and grab using their sometimes-suckered buccal cones.<ref name=Seibel2007/> A combination of hooks and a toothed radula are employed to scour the flesh from the thecosomes' shells.<ref name=Seibel2007/>

Gymnosomes slowly beat their wing-like parapodia<ref name=Seibel2007/> in a rowing motion<ref name=Morton1958>{{Cite journal | last1 = Morton | first1 = J. E. | title = Observations on the Gymnosomatous Pteropod Clione Limacina (Phipps) | doi = 10.1017/S0025315400023687 | journal = [[Journal of the Marine Biological Association of the United Kingdom]] | volume = 37 | issue = 2 | pages = 287–297 | year = 2009 | url = http://plymsea.ac.uk/1927/1/Observations_on_the_gymnosomatous_pteropod_Clione_limacina_%28Phipps%29.pdf | access-date = 2019-07-12 | archive-date = 2017-09-22 | archive-url = https://web.archive.org/web/20170922013055/http://plymsea.ac.uk/1927/1/Observations_on_the_gymnosomatous_pteropod_Clione_limacina_%28Phipps%29.pdf | url-status = live }}</ref> to propel their "perfectly streamlined"<ref name=Morton1958/> bodies through the upper 20 m of the water column. Although usually slow-moving, beating their wings once or twice per second, they are capable of bursts of speed when they need to pursue their prey, calling a separate suite of muscles into action to obtain the higher beat frequency.<ref name=Seibel2007/>

==Taxonomy==
In the new [[Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)|taxonomy of Bouchet & Rocroi]] (2005), the [[clade]] Gymnosomata is arranged as follows :
In the new [[Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)|taxonomy of Bouchet & Rocroi]] (2005), the [[clade]] Gymnosomata is arranged as follows :


Edit summary
moved cladogram into "Taxonomy" section; merged hermaphrodite paragraph into development section
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