Sea Pens

InvertebrateCosmopolitan
Written By Alexander Julius Jensen

Pennatulacea spp.

A sea pen may look like an underwater quill, but it's actually a colony of individual polyps — one polyp becomes the stalk of the sea pen and the bulb that attaches it to the sea floor, while the rest form feathery branches. Some sea pens have only a few polyps, while others have as many as 35,000.

As you survey the sea floor on your dive, you're taken aback at what looks like an old-fashioned writing instrument sticking out of the sand. Its feathery body is some 40 cm (16 in) long and it undulates in the slight ocean current. You gently poke the oddity and it responds with a faint luminescent glow. This plump, colourful, glowing quill is a sea pen.

Strange Stationary

The sea pen seems like a strange creature — and it is in many ways — but it's not much stranger than the corals that form reefs and vibrantly colour our coasts. Both corals and pens seem like pretty dubious animals; they are far from lively or animated. But animals they are, and they belong to the same group known as Cnidaria; a phylum including jellyfish, sea anemones, corals, and sea pens. Over 12,000 Cnidarian species have been described, and of these, sea pens make up some 300.

Composing the order Pennatulacea, sea pens are a surprisingly diverse group. They live the world over; from freezing polar oceans and abyssal depths of 6000 metres (19,685 ft) to warm tropical shallows and sheltered bays. They come in an array of colours, including yellow, orange, red, pink, purple, and white. Many can bioluminesce in flashes of greenish-blue light when excited. Their feathery stalks can be no taller than 20 cm (8 in), as big as a typical writing quill, or grow to flowing lengths of over 2 metres (6.6 ft), resembling the pale tentacles of an eldritch monster reaching from beneath the sand, rather than any kind of pen. Some are naked of "feathers" and resemble erect sticks or columns poking from the sand. Others are squat and wide like mushrooms or umbrellas. A few just look like amorphous blobby worms.

Colonial Animals

But one thing all sea pens have in common — with one another and with corals and sea anemones — is their colonial structure. A sea pen isn't one animal, but a collection of several, individual tiny polyps. However, unlike coral polyps, those of sea pens are each specialised for a specific job. One polyp becomes the base and "stem" of the sea pen; known as the 'primary polyp', it creates a muscular foot called a 'peduncle', which anchors the pen in sand or mud, and a central stalk supported by an internal bone-like structure. Along the stalk (called the 'rachis') grow the other types of polyps. There are the 'oozoids' which form leaf-like structures and give the pen its feathery look. Specialised 'siphonozooids', usually resembling little bumps or holes, pump water into and throughout the colony body to keep it rigid. 'Autozooids' are tiny eight-armed polyps that emerge from the "body" at high tide to sting and capture floating plankton, before retracting again at low tide. Together, these individual mini-creatures live as a single animal, with some sea pens composed of just a few polyps while others are mass colonies of as many as 35,000.

Friends & Allies

Naturally cooperative by their biology, sea pens are also friendly with completely disparate organisms. Some sea pens shelter a type of symbiotic algae called 'zooxanthellae' inside their bodies, which can photosynthesis and provide the sea pen with additional nutrients. The copious filamentous "feathers" of many sea pens make excellent homes for minuscule creatures. Pairs of pale, painted porcelain crabs hide amongst the tentacles of the common sea pen, while tiny transparent shrimp find camouflage on the bodies of flowery sea pens. Relationships with some other creatures are less amicable. Sea stars and nudibranchs (sea slugs) are notorious predators, with a few species specializing exclusively in sea pens. Sea pens aren't completely helpless, however. Being a Cnidarian, the sea pens polyps can deploy stinging tentacles to defend themselves. Sea pens are also able to detach from the sea floor and re-anchor in a better location, or, if the danger is fast approaching, a sea pen can expel the water from its body, deflate, and retreat into its peduncle (its "foot") beneath the sand.

Humans, unfortunately, are also often enemies of the sea pen. While the changing climate adversely affects the entire ocean ecosystem with acidification and increased temperatures, practices like bottom trawling and dredging directly tear sea pens from their anchored positions. Coastal-living sea pens are sometimes trampled by careless swimmers. And sea pens harvested for aquariums usually don't survive for long — requiring a deep substrate to anchor into and a specialized diet, most sea pens starve soon after being caught.

How does an animal composed of many smaller animals go about creating more of itself? Many sea pens, embracing their alien-esque identity, can produce asexually by budding off polyps that form new sea pens. Others take the more familiar sexual route. The tentacled 'autozooid' polyps that capture food can also release gametes — eggs and sperm — into the water column. When these come into contact, they fertilise and become free-swimming larvae (called 'ciliated planula'), which eventually settle on the sea floor, metamorphose into sessile polyps, and eventually form a new sea pen. Most will live for around 15 years, however, if the growth rings observed in some sea pens are annual, then certain sea pens can live to be 100 years old!

Where Does It Live?

⛰️ A diverse range of habiats; from freezing polar oceans and abyssal depths of 6000 metres (19,685 ft) to warm tropical shallows and sheltered bays.

📍 Most of the world’s oceans.

  • Size // Tiny to Huge

    Length // Less than 20 cm (8 inches) to 2 meters (6.5 feet)

    Weight // N/A

  • Activity: N/A

    Lifestyle: N/A

    Lifespan: N/A

    Diet: Omnivore

    Favorite Food: Plankton 𓇼

  • Phylum: Cnidaria

    Class: Octocorallia

    Order: Pennatulacea

  • There are around 300 different species of sea pens in the order Pennatulacea. They are most closely related to jellyfish, sea anemones, and corals — in the phylum Cnidaria.

    While many sea pens do look like feather quills, the group is varied; there are those with long stalk-like bodies, squat umbrella-like forms, featherless "poles", and some that just look like big slugs. Many sea pens are also bioluminescent.

    A sea pen's stalk can be no taller than 20 cm (8 in) or can grow to flowing lengths of over 2 metres (6.6 ft).

    Along the pen's stalk grow several specialised polyps. There are the 'oozoids' which form leaf-like structures and give the pen its feathery look. Specialised 'siphonozooids', usually resembling little bumps or holes, pump water into and throughout the colony body to keep it rigid. While 'autozooids' are tiny eight-armed polyps that emerge from the "body" at high tide to sting and capture floating plankton, before retracting again at low tide.

    Feathery sea pens often give shelter to small ocean crustaceans such as painted porcelain crabs and tiny transparent shrimp.

    Sea pens are able to detach from the sea floor and re-anchor in a better location, or, if the danger is fast approaching, a sea pen can expel the water from its body, deflate, and retreat into its peduncle (its "foot") beneath the sand.

    Sea pens live the world over; from freezing polar oceans and abyssal depths of 6000 metres (19,685 ft) to warm tropical shallows and sheltered bays.
 
Alexander Julius Jensen
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