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Fleshy Sea Pen (Ptilosarcus gurneyi)
[Fleshy Sea Pen Phylum: Cnidaria] [Class: Octocorallia] [Order: Scleralcyonacea] [Family: Pennatulidae]
The general characteristics of the orange sea pen, frequently called the fleshy sea pen, is what makes it one of the most recognisable of all colonial cnidarians.
This segment contains interesting facts about Ptilosarcus gurneyi, including where they thrive best, what they eat, and how a soft coral that looks like a writing quill reproduces.
Orange Sea Pens Distribution and Habitats
The cosmopolitan distribution of the sea pen species means they are widespread - even existing in some polar seas.
But the geographic range of Ptilosarcus gurneyi is concentrated along the Northeastern Pacific coastline.
In fact, the orange sea pen is mostly found in the western shores of North America, from Alaska down to southern California.
They're particularly prevalent in the Gulf of Alaska, and very common in British Columbia, Canada.
Even though orange sea pens can inhabit some shallow environments (close to the low tide line), they can also tolerate life in much deeper water between fourteen (14) and fifty (50) metres deep (165 feet).
In more detail...
Sea pens use their basal peduncle to anchor themselves in soft sediment at the ocean floor, usually sand or mud.
For Ptilosarcus gurneyi, this often takes place in environments with 'low energy' substrates, such as subtidal zones (sublittoral) and sheltered bays.
Pro Tip: The phylum of sea pen is Cnidarian. Thus, it's also grouped together with some common species of coral, jellyfish, tube-dwelling sea anemones, and sea whip gorgonians.
Fleshy Sea Pen Characteristics
These colony-forming benthic filter feeders are in fact marine invertebrates (cnidarians). The order Pennatulacea contains fourteen (14) families and scientists have identified and validated more than two hundred (200) species of sea pens.
It may appear to be a single animal. However, each sea pen is a colony of soft-bodied coral polyps (tiny animals) with varying roles and functions that are essential for its survival.
Here's the thing:
The general appearance of the orange sea pen (Ptilosarcus gurneyi) is one that resembles an old-fashioned quill pen. The primary colouration of the quill and feathers is bright orange and yellow with a slight reddish tint.
Even though adult fleshy sea pens often grow to fifty (50) centimetres tall (20 inches), the feather boa sea pen (Anthoptilum grandiflorum) is a much taller species.
Some of the specialised soft fleshy polyps are able to take in water and inflate the colony.
Doing so means sea pens are able to drift to new locations if and when it becomes necessary.
Likewise, expelling water quickly results in a deflated colony and allows the organism to retreat into its bulbous foot (or mud) if it feels threatened.
Some species of sea pens can also produce a bioluminescent bright-greenish glow if they are disturbed or they perceive a danger.
Interesting Fact: Almost all sea pens have some dark orange, yellow, brown, or red colouration. Despite being mostly sessile in nature, some Pennatulacea can inflate and deflate certain body parts to help them "walk" (or move slowly) over the seafloor.
What Do Orange Sea Pens Eat?
They are suspension feeders, and the name of the feeding polyps is autozooids. You may see them filter-feeding as they extend and wave their tentacles to try and catch drifting plankton in the water column.
But, because these filter feeding stationary benthic invertebrates need ocean currents to deliver a food source, the typical diet for Pennatulacea will also include:
- Detritus (tiny food particles and organic debris)
- Phytoplankton (microscopic algae)
- Protists (single-celled organisms)
- Zooplankton (tiny animals such as copepods)
Following that, a different kind of polyp (siphonozooids) use a complex water circulation system to distribute their food throughout the colony.
How Do Fleshy Sea Pens Reproduce?
In actual fact, sea pens (of the order Pennatulacea) are able to increase their population by sexual reproduction or without the fusion of gametes (asexually).
Sexual Reproduction
Like many marine creatures, sea pens have two sexes (gonochoric) and spawn through external fertilisation. Thus, the female releases her eggs and the males release sperm into the water.
It's quite common for environmental events, such as increased water temperature or lunar cycles, to trigger these synchronised spawning events.
However, the timing of plankton cycles - and subsequent seasonality of reproduction - in California differs to that in British Columbia and Alaska.
Then...
After fertilisation occurs, the eggs will develop into free-swimming planula larvae and drift with ocean currents for up to several weeks.
Once the larvae settle on a suitable substrate, they can metamorphose into a primary polyp and start to form the colony.
The primary polyp develops into the rachis (central stalk) and new polyps start budding in "branches" from it.
Asexual Reproduction
A juvenile sea pen might also develop by budding new polyps from the main body. In simple terms, the colony is able to expand without the need for further fertilisation.
Orange Sea Pen Predators and Threats
In fact, sea pens can have a lifespan of more than one hundred (100) years. Nonetheless, shorter life cycles are more common due to disease, pollution in the ocean, and habitat erosion.
So, what animal eats the orange sea pen in the wild?
The main enemies are some sea slugs (such as the striped nudibranch) and sea stars, especially the leather star (Dermasterias imbricata), vermillion sea star (Mediaster aequalis), and common sunstar (Crossaster papposus) - sometimes called the rosy sun star.
In fact, sea stars (frequently called starfish) attack sea pen coral by everting (turning outward) their own stomach onto the tissues of a sea pen animal.
In case you were wondering...
Our fish aquarium guide for beginners explains why some aquarists use sea pens in reef aquariums. But, they will only survive in deep substrates and they have special dietary requirements.
As a consequence of that, worldwide populations of some sea pen species are decreasing in certain areas of the world.
Important: The IUCN Red List of Threatened Species is a comprehensive source of information about the global conservation status of animals, fungi, and plants. Currently, the IUCN lists many sea pen species as being "Near Threatened" (NT), such as the wild star sea pen (Kophobelemnon stelliferum).