Sea pens are
marine cnidaria
Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...
ns belonging to the
superfamily Pennatuloidea,
which are
colony
A colony is a territory subject to a form of foreign rule, which rules the territory and its indigenous peoples separated from the foreign rulers, the colonizer, and their ''metropole'' (or "mother country"). This separated rule was often orga ...
-forming
benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...
filter feeders within the
order Scleralcyonacea.
There are 14
families within the order and 35 extant
genera
Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...
, and it is estimated as of 2011 that, of 450 described
species
A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
, around 200 are valid.
Sea pens have a
cosmopolitan distribution
In biogeography, a cosmopolitan distribution is the range of a taxon that extends across most or all of the surface of the Earth, in appropriate habitats; most cosmopolitan species are known to be highly adaptable to a range of climatic and en ...
, being found in tropical and temperate waters worldwide, from
intertidal shallow waters to
deep seas of more than .
The earliest accepted sea pen fossils are known from the
Cambrian
The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...
-aged
Burgess Shale (''
Thaumaptilon''). Similar fossils from the
Ediacaran
The Ediacaran ( ) is a geological period of the Neoproterozoic geologic era, Era that spans 96 million years from the end of the Cryogenian Period at 635 Million years ago, Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last ...
may show the dawn of sea pens. Precisely what these early fossils are, however, is not decided.
Taxonomy
The superfamily Pennatulacea consists of the following families:
*
Anthoptilidae
*
Balticinidae
*
Chunellidae
*
Echinoptilidae
*
Funiculinidae
*
Gyrophyllidae
*
Kophobelemnidae
*
Pennatulidae
*
Protoptilidae
*
Pseudumbelludlidae
*
Renillidae
*
Scleroptilidae
*
Stachyptilidae
*
Umbellulidae
*
Veretillidae
*
Virgulariidae
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.
As
octocorals, sea pens are colonial animals with multiple
polyps (which look somewhat like miniature
sea anemone
Sea anemones ( ) are a group of predation, predatory marine invertebrates constituting the order (biology), order Actiniaria. Because of their colourful appearance, they are named after the ''Anemone'', a terrestrial flowering plant. Sea anemone ...
s), 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.
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''.
Using their root-like peduncles to anchor themselves in sandy or muddy substrate, the exposed portion of sea pens may rise up to 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 , sea pens prefer deeper waters where turbulence is less likely to uproot them. Some species may inhabit depths of or more.
While generally sessile animals, sea pens are able to relocate and re-anchor themselves if need be.
They position themselves favourably in the path of currents, ensuring a steady flow of
plankton
Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...
, the sea pens' chief source of food. Their primary predators are
nudibranch
Nudibranchs () are a group of soft-bodied marine gastropod molluscs, belonging to the order Nudibranchia, that shed their shells after their larval stage. They are noted for their often extraordinary colours and striking forms, and they have b ...
s 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. 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.
Like other
anthozoa
Anthozoa is one of the three subphyla of Cnidaria, along with Medusozoa and Endocnidozoa. It includes Sessility (motility), sessile marine invertebrates and invertebrates of brackish water, such as sea anemones, Scleractinia, stony corals, soft c ...
ns, 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
A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...
. Analysis of rachis growth rings indicates sea pens may live for 100 years or more, if the rings are indeed annual in nature.
Some sea pens exhibit
glide reflection symmetry,
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.
Gallery
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)
References
External links
*
{{Taxonbar, from=Q1122081
Octocorallia
Bioluminescent cnidarians
Extant Cambrian first appearances
Anthozoan orders