Sea Pens (unknown species)

A sea pen encountered during a site survey between the Endurance Oregon Offshore 2-legged mooring EOM leg anchor and the low-voltage node LV01C. Photo Credit: NSF-OOI/UW/CSSF, Dive R1752, V14

Sea pens (order Pennatulacea) are colonial marine cnidarians that are widely distributed in tropical and temperate waters worldwide, and range from the intertidal to extreme depths. Sea pens, like sea whips (gorgonians) are considered to be octocorals.

As with other octocorals, sea pens have multiple, 8-tentacled polyps (like miniature sea anemones in appearance), but they are extremely specialized and modified in appearance. One polyp develops into a rigid stalk (the "rachis"), loses its tentacles, and forms a bulbous "root" or peduncle at its base that helps anchor it in the sediment. Water intake, feeding, and reproductive polyps branch out from this central stalk, and calcium carbonate spicules and a central rod strengthen the entire structure so it can stand upright.

The portion of the sea pen exposed above the sediment can grow up to 2 meters (6.6 ft) tall in some species, and they are often brightly colored. Sea pens are more commonly found in deep water, away from waves and turbulent currents that can uproot them. While they are generally sessile (staying in one place), sea pens can uproot and relocate if necessary. They are filter feeders, and orient themselves to the currents to ensure a regular flow of plankton and particulate matter. They are preyed on by nudibranchs and sea stars, some of which feed exclusively on sea pens, and they can force water out of their bodies to deflate and retreat into the peduncle if under attack.

Sea pens have been seen in soft sediment environments around OOI infrastructure at the Oregon Offshore, Slope Base, and Axial Base sites.