Sea pens (order Pennatulacea), with around 300 species, can be found as deep as 20,000 feet in almost all oceans. Generally found in large fields, this order is one of few to have species in the frigid waters surrounding Antarctica. Having varying appearances, sea pens can range in color from orange to yellow to white, with some capable of emitting a bright greenish light when stimulated.
These octocorals, related to sea whips and sea fans, have a unique form – their central stalk, known as the primary polyp, is a modified polyp that has lost its tentacles and developed a water-filled bulb at its base to anchor the animal. Secondary polyps branch from the primary polyp and have specialized functions including capturing food, reproducing, and ventilating the colony by controlling water flow. This trait is unique among corals, which commonly have a collection of individual polyps living together, rather than dividing up tasks. There is huge variety in sea pens, with individuals ranging from just a few secondary polyps up to 35,000!
While this general body-type prevails among all sea pens, there are two suborders that sea pens can further be divided into. Subselliflorae, the smaller suborder of the two, consists of the individuals that look like old-fashioned quill pens. The other, larger order, Sessiliflorae, has a wider range of morphologies. Some animals lie flat against the sea floor, looking like a pond lily or flounder with polyps living on the side that faces up. Others look like clubs, with all the polyps collected at the end of a stalk.
Similar to sea fans, sea pens grow in soft sediment such as sandy or muddy bottoms, instead of the hard substrate other species of deep sea corals require. They are anchored by their bulb, which can extend a significant distance into the substrate. Red crabs can often be found near sea pens on these deep sandy bottoms.
Sea pens are less common in very shallow water than other corals partly due to their method of staying attached to the sea floor, which does not stand up well to high amounts of water turbidity, such as waves.
One unique and interesting ability of sea pens is their ability to contract and expand into the sand. When they are in the sand, which they do when they feel threatened, the only visible part of them is the small top of the animal that sits right on the seafloor. This is not a skill other orders of coral have due to their rigid structures and the type of substrate they live on.
The genus Renilla, known as the sea pansy and in the suborder Sessiliflorae, lies flat to the substrate, instead of growing vertically. These animals can be found off Florida and so far are only known to occur to about 250 feet, but as more of the deep ocean is explored deeper relatives may be discovered.
Sources: https://www.montereybayaquarium.org/animal-guide/invertebrates/sea-pen; http://www.advancedaquarist.com/2003/10/inverts;https://www.britannica.com/animal/sea-pansy; https://www.britannica.com/animal/sea-pen; https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search…
Hannah Leis is GRN’s Fisheries Associate