Sea pen in the context of "Filter-feeding"

Filter feeders are aquatic animals that acquire nutrients by feeding on organic matter, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a specialized filtering organ that sieves out and/or traps solids. Filter feeders can play an important role in condensing biomass and removing excess nutrients (such as nitrogen and phosphate) from the local waterbody, and are therefore considered water-cleaning ecosystem engineers. They are also important in bioaccumulation and, as a result, as indicator organisms.

Filter feeders can be sessile, planktonic, nektonic or even neustonic (in the case of the buoy barnacle) depending on the species and the niches they have evolved to occupy. Extant species that rely on such method of feeding encompass numerous phyla, including poriferans (sponges), cnidarians (jellyfish, sea pens and corals), arthropods (krill, mysids and barnacles), molluscs (bivalves, such as clams, scallops and oysters), echinoderms (sea lilies) and chordates (lancelets, sea squirts and salps, as well as many marine vertebrates such as most species of forage fish, American paddlefish, silver and bighead carps, baleen whales, manta ray and three species of sharks—the whale shark, basking shark and megamouth shark). Some water birds such as flamingos and certain duck species, though predominantly terrestrial, are also filter feeders when foraging.

Anthozoa is one of the three subphyla of Cnidaria, along with Medusozoa and Endocnidozoa. It includes sessile marine invertebrates and invertebrates of brackish water, such as sea anemones, stony corals, soft corals and sea pens. Almost all adult anthozoans are attached to the seabed, while their larvae can disperse as plankton. The basic unit of the adult is the polyp, an individual animal consisting of a cylindrical column topped by a disc with a central mouth surrounded by tentacles. Sea anemones are mostly solitary, but the majority of corals are colonial, being formed by the budding of new polyps from an original, founding individual. Colonies of stony corals are strengthened by mainly aragonite and other materials, and can take various massive, plate-like, bushy or leafy forms.

Members of Anthozoa possess cnidocytes, a feature shared among other cnidarians such as the jellyfish, box jellies and parasitic Myxozoa and Polypodiozoa. The two classes of Anthozoa are class Hexacorallia, with members that have six-fold symmetry such as stony corals, sea anemones, tube anemones and zoanthids, and class Octocorallia, with members that have eight-fold symmetry, such as soft corals, gorgonians (sea pens, sea fans and sea whips), and sea pansies. Some additional species are also included as incertae sedis until their exact taxonomic position can be ascertained.

Coelenterata is a rejected phylum encompassing the animal phyla Cnidaria (corals, true jellies, sea anemones, sea pens, and their relatives) and Ctenophora (comb jellies). The name comes from Ancient Greek κοῖλος (koîlos) 'hollow' and ἔντερον (énteron) 'intestine', referring to the hollow body cavity common to these two phyla. They have very simple tissue organization, with only two layers of cells (ectoderm and endoderm), along with a middle undifferentiated layer called the mesoglea, and radial symmetry. Coelenterata lack a specialized circulatory system, relying instead on diffusion across the tissue layers.

Octocorallia, along with Hexacorallia, is one of the two extant classes of Anthozoa. It comprises over 3,000 species of marine and brackish animals consisting of colonial polyps with 8-fold symmetry, commonly referred to informally as "soft corals". It was previously known by the now unaccepted scientific names Alcyonacea and Gorgonacea, both deprecated c. 2022, and by the also deprecated name of Alcyonaria, in earlier times.

Its only two orders are Malacalcyonacea and Scleralcyonacea, which include corals such as those under the common names of blue corals, sea pens, and gorgonians (sea fans and sea whips). These animals have an internal skeleton secreted by their mesoglea, and polyps with typically eight tentacles and eight mesenteries. As is the case with all cnidarians, their complex life cycle includes a motile, planktonic phase (a larva called planula), and a later characteristic sessile phase.

A holdfast is a root-like structure that anchors aquatic sessile organisms, such as seaweed, other sessile algae, stalked crinoids, benthic cnidarians, and sponges, to the substrate.

Holdfasts vary in shape and form depending on both the species and the substrate type. The holdfasts of organisms that live in muddy substrates often have complex tangles of root-like growths. These projections are called haptera and similar structures of the same name are found on lichens. The holdfasts of organisms that live in sandy substrates are bulb-like and very flexible, such as those of sea pens, thus permitting the organism to pull the entire body into the substrate when the holdfast is contracted. The holdfasts of organisms that live on smooth surfaces (such as the surface of a boulder) have flattened bases which adhere to the surface. The organism derives no nutrition from this intimate contact with the substrate, as the process of liberating nutrients from the substrate requires enzymatically eroding the substrate away, thereby increasing the risk of the organism falling off the substrate.