Sea pens are colonial organisms found across the world’s oceans, ranging from shallow coastal areas to abyssal depths exceeding 6,000 meters. Despite their feather-like appearance, these organisms are animals belonging to the phylum Cnidaria, making them relatives of corals and sea anemones. A sea pen colony anchors itself into soft substrates like sand or mud using a muscular base called a peduncle. Understanding how these colonies obtain nutrition reveals a highly specialized method of feeding in the marine environment.
The Passive Feeding Strategy of Sea Pens
Sea pens are obligate suspension feeders, relying entirely on the surrounding water to deliver food particles. They do not actively move or hunt, instead positioning their colonial structure to intercept whatever ocean currents carry past them. This passive strategy is linked to their sessile lifestyle, as they remain anchored in place. The colony’s central stalk, known as the rachis, extends upward into the water column to maximize exposure to the flow.
This exposure ensures a constant stream of food flows across the thousands of feeding structures on the rachis. The sea pen acts as a living sieve, relying on hydrodynamic forces to bring particles into contact with its capture surfaces. Species often position themselves in locations with a steady, moderate current that optimizes the delivery of food without causing physical stress.
Primary Food Sources in Deep-Sea Habitats
The diet of the sea pen is composed exclusively of microscopic particles suspended in the water column, including plankton and various forms of organic detritus. Plankton components are often phytoplankton (microscopic plants) and zooplankton (tiny marine animals). Smaller life forms such as copepods and the larval stages of invertebrates are regularly consumed.
Particulate organic matter (POM) forms a significant portion of their diet, including non-living fragments of decaying organisms and waste products. In deep-sea environments, this material often arrives as “marine snow,” a continuous shower of organic debris falling from surface waters. Since the sea pen cannot choose its prey, feeding success depends on the local concentration and quality of these transported particles. The availability of these food sources can fluctuate seasonally, impacting the colony’s growth rate and health.
Specialized Polyps and Capture Mechanisms
The sea pen is a colony of many individual, specialized polyps. Feeding is performed by specialized individuals called autozooids, which are small, eight-tentacled polyps emerging from the main stalk. These autozooids are equipped with tentacles featuring microscopic stinging cells known as nematocysts. While these cells are known for defense in other cnidarians, sea pens primarily use them to ensnare and secure minute food particles from the passing current.
When a plankton organism or detrital particle contacts the autozooid’s extended tentacles, the nematocysts fire to immobilize and hold the prey. The tentacles then manipulate the captured item, moving it toward the polyp’s central mouth for ingestion. The colony also contains another type of polyp, the siphonozooid, which does not feed.
Siphonozooids are smaller polyps that manage water for the colony. They control the influx and expulsion of water through a complex internal canal system. By pumping water in, siphonozooids inflate the colony, making the rachis rigid and ensuring the feeding autozooids are fully extended for maximum particle capture. When disturbed, they rapidly expel water, allowing the colony to deflate and retract into the substrate for protection.
How Environment Determines Dietary Success
The amount of food a sea pen receives is influenced by the physical conditions of its habitat. Water current speed is a determining factor for feeding success. If the current is too slow, fewer food particles are delivered to the polyps, leading to reduced intake. Conversely, if the current becomes too swift, the force can deform the colony or make it difficult for the autozooids to capture particles.
The depth and composition of the seafloor also play a role in their diet. Sea pens require a soft substrate to securely anchor their peduncle and remain upright in the flow. The depth influences the availability of marine snow and other sinking organic matter, which can be the dominant food source in deeper locations.