This video was taken in Monterey Bay Aquarium (USA)
Introduction: Sea pens are colonial invertebrate marine cnidarians which belong to the order “Pennatulacea”. They are named because of their upright feathered appearance that resemblance the old fashioned quill pens. It is believed they occur in shallow and deep waters in tropical and temperate waters worldwide. They live possibly as long as 15 years.
Habitat: Sea pens are found in sheltered inshore of calm current waters, or in deeper offshore waters at depths which could range from 12 m to deeper water of 100 m or possibly at greater depths. They often prefer deep water where turbulence is less likely to uproot them. Their preferred substrate is the sandy one that enables anchoring themselves in sand or mud beds on water beds.
Description: Sea pen is a colonial animal with a central rigid stem (developed from a polyp) that bears polyps (hollow stalks with a mouth and eight tentacles like leaves on either side of the stem which acts as the foot that anchors the colony in the sand or the mud, supports the whole colony and retracts the whole colony into the ground at low tide. This central stem is usually stiffened by an internal structure made of calcium. Although the Sea Pen is anchored into the ground, it is possible for them to detach the anchor from the sand or mud and move to a new spot whereas they get more protection or more food.
Sea pens come in a variety of colors and sizes. In regard to size, they can grow up to 60 cm in length. Their color ranges from dark orange to yellow and white.
Feeding habits: Sea pens are suspension feeders, meaning they need some water current for drifting their food into their grasp. Their food is mainly small organisms especially zooplankton which are captured by the tentacles at the end of each polyp. Once a feeding session is done, the sea pens retreat and contract.
Reproduction: The sexes are separate with each colony of polyps are either male or female. In some sea pens while the eggs and sperm are released from the polyps in other sea pen species whereas external fertilization occurs. A female colony can release more than 200,000 eggs. After about a week of hatching, the free-swimming larva (planula) settle to the sea floor and metamorphoses to form the axial polyp of a new colony.
Vulnerability to predators: Sea pens are preyed upon by some predators especially most sea stars. Their predators include also some snails and a number of nudibranchs.
Human uses: These animals are sometimes taken for the live aquarium trade. However, they require special care which is there with less experienced keepers.
References: BBC, Wikipedia, WiseGEEK, National Geographic, Wild Fact Sheets, Encyclopaedia Britannica, Scuba Diver Life, the Columbia Encyclopedia.