Sea Pen Facts: They Are Actually Animals and Can Move

Look like a plant, but the sea pen is actually a marine animal. Surprised!!! With their distinctive shape and vibrant colors, sea pens are a remarkable sight to behold in the sea world. Let’s explore some of the most interesting facts about these remarkable creatures with us now! 

Sea pen
Phylum: Cnidaria
Class: Anthozoa
Order: Pennatulacea

1. Sea pens are octocorals

The sea pen is a soft coral species that bears a resemblance to a chubby, antique quill pen. This is the reason why they are called so. However, not all sea pens conform to this appearance. Some species may resemble pinwheels, umbrellas, or clubs instead, lacking the feather-like structure.

Ocean pens are colonial marine animals, much like anemones or by-the-wind sailors. They are made up of thousands of individual polyps that operate together as a unified organism. The polyp is basically a sac-like body with a mouth and a ring of tentacles surrounding it. Each polyp comes equipped with 8 tentacles.

The Ptilosarcus gurneyi or orange sea pen or fleshy sea pen/Cre: in pic

There are about 300 sea pen species in 14 families. They have a wide variety of shapes, colors, and sizes. However, these species all have one thing in common – a central stem a.k.a the rachis. The rachis is a specialized polyp that drops all of its tentacles and transforms into a sea pen stem. It features a bulb at its base referred to as the peduncle. The main responsibilities of this polyp are to attach sea pens to the seafloor and provide support for the colony.

The remaining polyps attach themselves to the rachis to create branches. Each of them performs various functions including water intake, eating, and reproduction.

These species often have a size of about 46 centimeters (1.5 ft) in height. Some species can grow to be over a meter. For instance, the tall sea pen (Funiculina quadrangularis) can reach a height of 2 meters (6.6 feet).

2. Habitat

Sea pens can be found in almost all temperate and tropical waters around the world. They can live in both shallow and deep water. However, these species have a common preference for deeper habitats where they are less likely to be uprooted by turbulence.

The purple sea pen on the ocean floor

Certain species can be found at depths of up to 6,600 feet (2,000 meters) below the surface. An Umbellula species was discovered dwelling in freezing, pitch-black water near Antarctica.

3. Diet

Like all other corals, sea pen animals are filter feeders. They mainly feed on plankton, including invertebrate larvae, rotifers, detritus, ciliates, bacterioplankton, and possibly phytoplankton. In addition, they consume organic matter.

To catch food in the surrounding water, the creatures use their polyps autozooids. Their unique shape is an adaptation that allows them to increase their prey capture rate while also reducing the effects of turbulent water in their habitat.

We all know that these soft corals use their bulbous form to anchor themselves to soft bottoms such as sand or mud. However, this is not permanent. They can move easily if necessary. These species prefer to position themselves in areas with a steady current that can provide them with various types of food.

The invertebrate animals have the ability to significantly expand and contract their body. With nocturnal behavior, they frequently retract fully beneath the sandy surface to rest during the day, only to extend to their full height at night for feeding purposes.

These creatures face predation from nudibranchs and sea stars like the red star or the leather star. Their juveniles will be eaten by nudibranches while the adults are preyed upon by those sea stars.

4. They’re bioluminescent in the dark

When stimulated, these species can emit a blue-green color. The bioluminescence phenomenon is caused by the polyps and spreads across the sea pen in waves. This is a type of defense mechanism designed to frighten and deter predators.

A sea pen glow in the dark

However, not all sea pens can glow like this. Those with this ability are called phosphorescent sea pens and they are a favorite meal of Dover sole. To protect themselves from these fish, the creatures use their possesses narcotic and appetite-suppressing characteristics to discourage hungry sole fish.

Besides this, these creatures may protect themselves by expelling water, causing them to deflate and retract into their peduncle. Quite interesting, right?!

5. Life cycle

These species attain sexual maturity between the ages of 4 and 6. Their height and size can tell you about their age. Each colony is either male or female, with females outnumbering males. However, there are some instances of hermaphroditic colonies, where each individual possesses both male and female reproductive organs.

Most species of sea pens utilize broadcast spawning as their reproductive method. The time they reproduce is often linked to the moon cycle, food availability, and water temperature.

When engaging in broadcast spawning, male and female colonies discharge their eggs and sperm into the water column through their mouth. These marine species have high reproductive output, as a single colony can release anywhere from 2,000 to 200,000 eggs.

After fertilization, the planktonic larvae will float in the ocean for several days to a month, looking for a suitable site to settle. During this phase, they eat “yolk” that is given to them at birth.

Upon discovering the ideal substrate (like rubble, mud, or sand), the larva will establish itself and transform into the primary polyp. The polyp will then produce daughter polyps, leading to the growth of the sea pen.

The lifespan of sea pens is quite long, some of them can live up to 100 years.

7. They play important roles

Sea pen fields play a crucial role in providing habitats for numerous marine species like redfish. Invertebrates and fish often rely on these octocorals as a shelter, substrate, and source of food. These creatures also use sea pens as a refuge for their eggs, ensuring the survival of future generations.

These creatures also affect the flow of water currents at the sediment-water interface, leading to greater retention and sedimentation of nutrients and organic matter, as well as sediment bioturbation and oxygenation(1).

The fat sea pen

Their populations have reduced in places of Puget Sound where they were once abundant. Their decline is concerning and can have far-reaching impacts on the ecosystem. It is unclear why these species, as well as their predators, have disappeared. Problems such as pollution, climate change, overfishing, or habitat loss may be playing a role in this decline.



I'm so fascinated about animals and want to share with you these amazing, interesting, amusing and beautiful creatures. Hope you enjoy it!

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