Sea Pens: Learn about these interesting marine species.

In our tradition to offer the best information about all marine species, we have this article about sea pens for you. Thus, don´t lose this time to learn about all the wonders of the sealife.

Sea pens are invertebrate creatures belonging to the Pennatulacea order, class Anthozoa (phylum Cnidaria), whose name comes from their resemblance to bird feathers. They inhabit shallow waters from the polar seas to the tropics.

Main Characteristics of the Sea Pens

Although the name of the group is due to its supposed resemblance to the old writing pens, only the sea pens belonging to the suborder Subselliflorae comply with this feature fully. Those that belong to the much larger suborder called Sessiliflorae, lack of feathery structures and grow in radiant forms. The last suborder includes what is commonly known as sea pansy.

However, sea pens are not animals, but consist of many separate animals called polyps, which use their tentacles to trap food. Some marine pens are more than 5 feet (1.5 m) in height, although most of them are smaller than this. The sea pens are anchored to the seabed.

sea pens : Close up of polyps from a sea pen. Pennatulacea resemble sea anemones but much smaller. Their stem has a lower part, called peduncle, which anchors the colony in mud or sand, and an upper part, the rachis, which contains polyps (hollow stems with a mouth and tentacles on the free end) or branches having several polyps. The central stem is known as the primary or axial polyp. Those that are derived from it are secondary polyps. Colonies of some genera, such as Chunella, have few polyps, while others have up to 35,000, which indicates that these are very diversified species.  The mouth and tentacles of the primary polyp are degenerated, and consist of little more than a fleshy stem reinforced by a central corneal rod.

Sea pens can deflate or expand when they expel or drink water through the hollow channels of interconnecting polyps. They feed on small organisms captured by the tentacles at the end of each polyp. Most sea pens light up, or bright, when touched or stimulated in some way which is known as bioluminescence.

Like other anthozoans, marine feathers reproduce by releasing sperm and eggs in the water column which may occur seasonally or during the year. Fertilized eggs develop into larvae called planulae, which move freely for about a week before settling on the substrate. Adult sea pens usually provide shelter to other animals such as cubomedusae. As for their life expectancy, we can indicate that it is 100 years or even more in some cases.

They live in marine lakes at depths between 20 and 2,000 meters, found in the North Atlantic and the Mediterranean but also in Japan and New Zealand. They are usually sessile animals, so they can relocate and re-anchor if necessary. They usually position themselves in the path of currents, to ensure a constant flow of plankton, which is their main source of food.

Their main predators are nudibranchs and starfish, some of which feed exclusively on them. (we invite you to read our article starfish to learn more about these interesting creatures)

Some marine pens exhibit reflective slip symmetry, which is rare among non-extinct animals. Occasionally they are commercialized to be used in aquariums, although they are difficult to care because they need a very deep substrate and have special feeding requirements.

Here they are…

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