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What are Sea Pens?

By Ken Black
Updated Mar 05, 2024
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Sea pens may be some of the most unique creatures in an ocean environment. They are marine cnidarians that live in clusters, resembling one individual creature that is very reminiscent of an old-fashioned quill pen. These creatures, known as polyps, work together to benefit the entire organism.

Sea pens come in a variety of colors and sizes. They can grow up to 2 feet (60 cm) in length and are often found in tropical waters, but can be found in cooler waters as well. Their color ranges from dark orange to yellow and white. Some even express qualities of bio-luminescence at night.

Found on the bottom of the ocean floor, their shape makes them very distinguishable. They can be found to a depth of up to 225 feet (68.5 m). They are present both in inshore waters and deeper offshore waters where the current is calmer. However, because they are suspension feeders, they do need some type of current in order to move food into their grasp.

Though sea pens are very easy to overlook and not much is known about them, they may be one of the most abundant type of cnidarians on the planet. In fact, one researcher estimates the mass of tissue in sea pens would equal or surpass that of all other cnidarians combined. Given this abundance, it is somewhat surprising more research has not been done on them.

Sea pens open up in their full splendor at approximately the time they begin feeding. They are suspension feeders, meaning they depend on drifting organic particulates and zooplankton in order to get their energy. Once done feeding the sea pens then retreat and contract. This process usually takes place several times a day.

Sea pens are especially vulnerable to some predators. Sea stars, especially feed heavily on sea pens. Sea pens can also fall prey to a number of diseases, including a parasitic form of green algae. However, if left without predation or disease, sea pens can live as long as 15 years.

Sea pens can also be kept in aquariums and tropical varieties can be very popular. However, they may not be recommended for amateurs as they take a considerable amount of space and expertise in their care. They must be fed a specialty food and need a base of sand of at least 1 foot (30 cm) in order to give them a good anchor.

Frequently Asked Questions

What exactly are sea pens?

Sea pens are marine invertebrates belonging to the order Pennatulacea. They are a type of soft coral that anchors themselves in soft-bottomed marine environments. Resembling antique quill pens, they consist of a central stalk, called a rachis, with numerous polyps that extend outwards, which are responsible for feeding and gas exchange.

How do sea pens reproduce?

Sea pens can reproduce both sexually and asexually. Sexually, they release sperm and eggs into the water column, leading to external fertilization. Asexually, they can reproduce through fragmentation, where a piece of the sea pen breaks off and develops into a new individual. This dual strategy enhances their resilience in various marine environments.

Where can sea pens be found?

Sea pens are distributed globally, predominantly found in the deeper parts of the ocean, from shallow tidal zones to depths of over 6,000 meters. They prefer soft substrates like sand or mud where they can anchor themselves. Their presence is a good indicator of a healthy benthic ecosystem.

What do sea pens eat?

Sea pens are filter feeders, meaning they consume plankton and other small particles suspended in the water. Their feathery polyps catch food as currents flow by. This diet allows them to play a role in the ocean's nutrient cycles, filtering the water and maintaining ecological balance.

Are sea pens important to the marine ecosystem?

Yes, sea pens are ecologically significant. They provide habitat and shelter for various marine species, including juvenile fish. As filter feeders, they help maintain water clarity and quality. Their presence indicates a stable and unpolluted environment, making them important indicators for marine conservation efforts.

Are sea pens at risk from human activities?

Sea pens are vulnerable to human activities such as bottom trawling, which can physically damage or dislodge them from their habitats. Pollution and climate change also pose threats by altering the delicate balance of their marine ecosystems. Conservation measures are crucial to protect these unique organisms and the roles they play underwater.

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Discussion Comments

By sunnySkys — On Feb 06, 2012

It's a little disappointing that so little research has been done about these sea pens. It seems like it would be easy to do, since sea pens are so abundant. Also, they belong to the same family of organisms as the jellyfish, which we do seem to know a lot about.

I suppose we have a lot more motivation in the case of jellyfish, because people actually get stung and hurt by them. I've never heard of anyone getting stung by a sea pen!

By SZapper — On Feb 06, 2012

@fify - I've actually seen a sea pen live and in person. There are some at my local aquarium and I have to say, they are pretty cool looking. I remember seeing one that was a very vibrant shade of orange and another that was a sunny shade of yellow.

I have to wonder why see pens evolved to be so brightly colored. It sounds like it would be much better for them to be a blue green color and blend in with the water. Since they mostly stay in one place and don't move, it seems like they don't really have a defense against predators. If they were camouflaged, I think it would probably up their chances of survival.

By strawCake — On Feb 05, 2012

What an interesting kind of sea marine life! I find it fascinating that many sea pens basically live together as one organism. That's taking teamwork to a whole new level, I think. I wonder how they coordinate their movements and other bodily functions when they're linked up like that?

Anyway, I can't imagine linking my survival to a group of people that way. However, it sounds like sea pens probably benefit from this arrangement much more than a group of people would. I'm sure they're much less susceptible to predators when they cluster together as a group.

By fify — On Feb 05, 2012

Has anyone seen pictures of sea pens? I saw some at school and they were amazing! I had never seen such sea creatures before.

The best part was that there were so many different kinds! My teacher brought in sea life pictures to class and there were several kinds of sea pen pictures too. The first one she showed us did look like the old-type feather pens a lot. The other ones were pretty different though.

There was one that was really cool. It was shaped like a sausage and had these little arms with ends in the shape of stars sticking out from everywhere. It was so different than the pen shaped one.

From the pictures, some of them seemed to have the texture of jelly-fish and most of them were whitish or tan in color. There was one that was a bright yellow though.

By anon40143 — On Aug 06, 2009

I'm doing some research on the sea pen. I found your information to be extremely valuable. Thank you.

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