Sea pen

{{short description|Order of colonial marine cnidarians}} {{Automatic taxobox | image = Haeckel Pennatulida.jpg | image_caption = "Pennatulida" from Ernst Haeckel's ''Kunstformen der Natur'', 1904 | fossil_range = {{fossil range|Cambrian|Recent}} | taxon = Pennatuloidea | authority = Ehrenberg, 1834 | subdivision_ranks = Families | subdivision = see text }}

'''Sea pens''' are marine cnidarians belonging to the superfamily '''Pennatuloidea'''. They are fleshy colonial organisms anchored to the seabed, with tiny polyps embedded in a soft matrix. They have a cosmopolitan distribution, being found in tropical and temperate waters worldwide, from intertidal shallow waters to deep seas of more than {{cvt|6100|m}}.<ref name="williams"/>

The earliest accepted sea pen fossils are known from the Cambrian-aged Burgess Shale (''Thaumaptilon''). Similar fossils from the Ediacaran may show the dawn of sea pens.<ref>{{cite web |first=Gary C. |last=Williams |title=Aspects of the Evolutionary Biology of Pennatulacean Octocorals |url=https://research.calacademy.org/research/izg/EvolutionaryBiology.htm |publisher=Department of Invertebrate Zoology and Geology |agency=California Academy of Sciences |access-date=2023-01-27}}</ref> Precisely what these early fossils are, however, is not decided.<ref>{{Cite journal | last1 = Antcliffe | first1 = J.B. | last2 = Brasier | first2 = M.D. | year = 2008 | title = Charnia at 50: Developmental Models for Ediacaran Fronds | journal = Palaeontology | volume = 51 | issue = 1 | pages = 11–26 | doi = 10.1111/j.1475-4983.2007.00738.x| s2cid = 83486435 | doi-access = free | bibcode = 2008Palgy..51...11A }}</ref>

== Taxonomy == ''Sea pens'' are marine cnidarians belonging to the superfamily ''Pennatuloidea'',<ref name=WoRMS>{{cite taxon|WoRMS|title=Pennatuloidea Ehrenberg, 1834 |id=1609360 |db=octocorallia|access-date=7 September 2025}} </ref> which are colony-forming benthic filter feeders within the order Scleralcyonacea.<ref name=WoRMS /><ref name="williams">{{cite journal |last=Williams |first=Gary C. |date=2011-07-29  |title=The Global Diversity of Sea Pens (Cnidaria: Octocorallia: Pennatulacea) |journal=PLoS ONE |volume=6 |issue=7 |article-number=e22747 |bibcode=2011PLoSO...622747W |doi=10.1371/journal.pone.0022747 |pmc=3146507 |pmid=21829500|doi-access=free }}</ref> The order comprises 16 families and 44 extant genera, with around 235 accepted species.<ref name=WoRMS-tree>{{cite taxon|WoRMS|title=Pennatuloidea treeview |db=octocorallia |url=https://www.marinespecies.org/aphia.php?p=browser&accepted=1&id[]=1609355&id[]=1609360&id[]=128480&id[]=1508602&id[]=196247&id[]=1613205&id[]=128485&id[]=182877&id[]=128487&id[]=128488&id[]=1758274&id[]=1565686&id[]=266954&id[]=266953&id[]=196246&id[]=128481&id[]=128484&id[]=128483#focus|access-date=7 September 2025 }}</ref> The superfamily Pennatulacea consists of the following families:<ref name=WoRMS/>

*Anthoptilidae {{small|Kölliker, 1880}} *Balticinidae {{small|Balss, 1910}} *Chunellidae {{small|Kükenthal, 1902}} *Echinoptilidae {{small|Hubrecht, 1885}} *Funiculinidae {{small|Gray, 1870}} *Gyrophyllidae {{small|López-González,Drewery & Williams, 2022}} *Kophobelemnidae {{small|Gray, 1860}} *Pennatulidae {{small|Ehrenberg, 1834}} *Protoptilidae {{small|Kölliker, 1872}} *Pseudumbellulidae {{small|López-González in López-González & Drewery, 2022}} *Renillidae {{small|Gray, 1870}} *Scleroptilidae {{small|Jungersen, 1904}} *Stachyptilidae {{small|Kölliker, 1880}} *Umbellulidae {{small|Lindahl, 1874 (1840)}} *Veretillidae {{small|Herklots, 1858}} *Virgulariidae {{small|Verrill, 1868}}

==Biology== Due to their wide geographic distribution and long evolutionary history, genetic variation within the different species of sea pen is quite large. Throughout evolution, most sea pens have kept their original mitochondrial gene order, but a certain clade of sea pens shown unique rearrangements through ancestral state reconstruction. There are many populations of sea pens found in mainly Indian waters. It is their polyps that are affected genetically, as they have dispersed within the different waters and islands, and how they use their polyps (tentacles) to protect themselves and other species.<ref>{{cite journal |last1=De Clippele |first1=L. H. |last2=Buhl-Mortensen |first2=P. |last3=Buhl-Mortensen |first3=L.|author-link3=Lene Buhl-Mortensen |title=Fauna associated with cold water gorgonians and sea pens |journal=Continental Shelf Research |date=15 August 2015 |volume=105 |pages=67–78 |doi=10.1016/j.csr.2015.06.007 }}</ref>

As octocorals, sea pens are colonial animals with multiple polyps (which look somewhat like miniature sea anemones), each with eight tentacles. Unlike other octocorals, however, a sea pen's polyps are specialized to specific functions: a single polyp develops into a rigid, erect stalk (the ''rachis'') and loses its tentacles, forming a bulbous "root" or ''peduncle'' at its base.<ref name="IZ">{{cite book |last=Barnes |first=Robert D. |year=1982 |title=Invertebrate Zoology |pages=168–169 |publisher=Holt-Saunders International |isbn=0-03-056747-5 |place=Philadelphia, PA}}</ref> The other polyps branch out from this central stalk, forming water intake structures (''siphonozooids''), feeding structures (''autozooids'') with nematocysts, and reproductive structures. The entire colony is fortified by calcium carbonate in the form of ''spicules'' and a central ''axial rod''.{{Citation needed|date=May 2025}}

Using their root-like peduncles to anchor themselves in sandy or muddy substrate, the exposed portion of sea pens may rise up to {{convert|2|m|ft}} in some species, such as the tall sea pen (''Funiculina quadrangularis''). Sea pens are sometimes brightly coloured; the orange sea pen (''Ptilosarcus gurneyi'') is a notable example. Rarely found above depths of {{convert|10|m|ft}}, sea pens prefer deeper waters where turbulence is less likely to uproot them. Some species may inhabit depths of {{convert|2000|m|ft}} or more.{{Citation needed|date=May 2025}}

While generally sessile animals, sea pens are able to relocate and re-anchor themselves if need be.<ref name="IZ" /> They position themselves favourably in the path of currents, ensuring a steady flow of plankton, the sea pens' chief source of food. Their primary predators are nudibranchs and sea stars, some of which feed exclusively on sea pens. The sea pens' ability to be clumped together and spatially unpredictable hinders sea stars' predation abilities.<ref>{{cite journal |last=Birkeland |first=Charles |date=February 1974 |title=Interactions between a Sea Pen and seven of its predators |journal=Ecological Monographs |volume=44 |issue=2 |pages=211–232 |doi=10.2307/1942312 |jstor=1942312 |bibcode=1974EcoM...44..211B }}</ref> When touched, some sea pens emit a bright greenish light; this is known as bioluminescence. They may also force water out of their bodies for defence, rapidly deflating and retreating into their peduncle.{{Citation needed|date=May 2025}}

Like other anthozoans, sea pens reproduce by coordinating a release of sperm and eggs into the water column; this may occur seasonally or throughout the year. Fertilized eggs develop into larvae called ''planulae'' which drift freely for about a week before settling on the substrate. Mature sea pens provide shelter for other animals, such as juvenile fish. Analysis of rachis growth rings indicates sea pens may live for 100 years or more, if the rings are indeed annual in nature.{{Citation needed|date=May 2025}}

Some sea pens exhibit glide reflection symmetry,<ref>{{cite web |last=Zubi |first=Teresa |date=2016-01-02 |title=Octocorals (Stoloniferans, soft corals, sea fans, gorgonians, sea pens) - Starfish Photos - Achtstrahlige Korallen (Röhrenkorallen, Weichkorallen, Hornkoralllen, Seefedern, Fächerkorallen) |website=starfish.ch |url=http://www.starfish.ch/c-invertebrates/octocorallia.html#Pennatulacea |access-date=2016-09-08}}</ref> rare among extant animals.

==Aquarium trade== Sea pens are sometimes sold in the aquarium trade. However, they are generally hard to care for because they need a very deep substrate and have special food requirements.{{Citation needed|date=May 2025}}

== Gallery == <gallery style="text-align:center;" mode="packed"> Image:Actinoptilum molle at Windmill BeachPA111765.JPG|''Actinoptilum molle'' (Echinoptilidae) Image:Sea Pansy - Flickr - Andrea Westmoreland (1).jpg|''Renilla sp.'' (Renillidae) Image:Pennatula phosphorea.jpg|''Pennatula phosphorea'' (Pennatulidae) Image:Veretillum sp. (Sea pen) at night.jpg|''Veretillum sp.'' (Veretillidae) Image:Virgularia sp. (Purple sea pen).jpg|''Virgularia sp.'' (Virgulariidae) </gallery>

==References== {{reflist|25em}}

==External links== {{Commons category|Pennatulacea}} {{Wikispecies|Pennatulacea}} *{{cite web |url=http://www.starfish.ch/c-invertebrates/octocorallia.html#Pennatulacea |title=Photos of Sea Pens |website=starfish.ch}}

Category:Pennatulacea Category:Octocorallia Category:Bioluminescent cnidarians Category:Extant Cambrian first appearances Category:Anthozoa orders