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Home › Ocean Creatures › Marine Biology › Invertebrates › Sea Pen

Facts about Sea Pens (Pennatulacea)

[Sea Pen Phylum: Cnidaria] [Class: Anthozoa] [Subclass: Octocorallia] [Order: Pennatulacea]

They're often mistaken for plants. But, this species of colonial invertebrate animals get their common names from the resemblance to old-fashioned feather quill pens (once used in writing).

This section contains interesting facts about sea pen (Pennatulacea) soft coral, such as where they live, what they eat, and how they reproduce.

Sea Pens Geographical Range and Habitats

Having a somewhat cosmopolitan distribution means you can find sea pen species worldwide - even in the polar seas.

Yet, they tend to thrive best in most tropical waters, especially around the coasts of Australia, the Caribbean, and the Indo-Pacific regions.

Sea pens also inhabit most of the temperate zones, such as the Pacific shores of North America, North Atlantic Ocean, and the Mediterranean.

In all of these global habitats, the colonial marine cnidarians called sea pens (Pennatulacea) prefer to anchor themselves in the soft sediments and substrate (e.g. mud, sand, or silt) of the ocean floor.

You will find many species living in shallow waters (less than 10 metres) and others that can survive in deep-sea environments - deeper than 6,000 meters (twenty thousand feet).

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 whips (gorgonians).

Sea Pen Characteristics and Behaviour

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.

To the untrained eye, it may appear to be a single animal. Yet, each sea pen is a colony of soft-bodied coral polyps (tiny animals) with varying roles and functions that are essential for its survival.

For example:

Some polyps use nematocysts (stinging cells) to catch planktonic material when they feed. Others are responsible for reproduction, and the main role of some polyps is to force water in and out of canals to help ventilate the rest of the colony.

Once the primary polyp loses its tentacles, it develops into a strong rod-like stalk (axial polyp). Unlike most soft coral species, it can anchor itself into the muddy or sandy seabed using the large bulbous muscle at the base (a root-like structure called the basal peduncle).

There are many funny scientific names for fish and aquatic animals. But, only the suborder Subselliflorae species of sea pen actually resembles a quill pen.

In fact, members of the larger suborder Sessiliflorae (e.g. flat kidney-shaped sea pansies) do not have any of the customary feathery structures.

Here's the thing:

Some of the specialised soft fleshy polyps are able to take in water and inflate the colony. Doing so means it can drift to a new location.

Likewise, expelling water quickly results in a deflated colony and allows the organism to retreat into its bulbous foot or the 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.

One of the smallest sea pens is Echinoptilum sp measuring less than eight (8) centimetres. Then again, the feather boa sea pen (Anthoptilum grandiflorum) can grow more than two (2) metres in length.

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 Sea Pens Eat?

In sea pens, the name of the feeding polyps is autozooids. If you can get close, you will see them filter feeding by extending and waving 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)
• 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.

Sea Pen Species List

How Do 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.

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.

Threats and Predators

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, ocean pollution, and habitat erosion.

So, what eats sea pens in the wild? In fact, their common enemies are sea slugs, sea stars - especially the leather star (Dermasterias imbricata) and the red cushion sea star (Oreaster reticulatus).

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).

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