A radiole is a heavily ciliated feather-like tentacle

Tentacle

A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...

 found in highly organized clusters on the crowns of Canalipalpata

Canalipalpata

Canalipalpata, also known as bristle-footed annelids or fan-head worms, is an order of polychaete worms, with 31 families in it including the Sabellida and the Alvinellidae, a family of deep-sea worms associated with hydrothermal vents.The Canalipalpata have no teeth or jaws. Most are filter feeders...

. Canalipalpata is an order of sessile

Sessility (limnology)

In limnology, sessility is that quality of an organism which rests unsupported directly on a base, either attached or unattached to a substrate. It is a characteristic of vegetation which is anchored to the benthic environment. There are two families of sessile rotifers: Flosculariidae and...

 marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 polychaete worms consisting of 31 families (including the Sabellidae

Sabellidae

Sabellidae are sedentary marine polychaete tube worms where the head is mostly concealed by feathery branchiae. They build tubes out of parchment, sand, and bits of shell...

, Serpulidae

Serpulidae

Serpulidae is a family of sessile, tube-building annelid worms in the class Polychaeta. The members of this family differ from the sabellid tube worms in that they have a specialized operculum that blocks the entrance of their tubes when they withdraw into the tubes. In addition, serpulids secrete...

, Terebellidae

Terebellidae

Terebellidae is a family of polychaete worms. They are surface deposit feeders, catching falling particles with numerous elongate prostomial tentacles splayed out on the sea floor. These tentacles, which are the most or only normally visible portion of the animal and are reminiscent of spaghetti,...

, and Alvinellidae

Alvinellidae

The Alvinellidae are a family of small, deep-sea polychaete worms endemic to hydrothermal vents in the Pacific Ocean. Belonging to the order Terebellida, the family contains two genera, Alvinella and Paralvinella; the former genus contains two valid species and the latter eight...

, a family of deep-sea worms associated with hydrothermal vents). These benthic

Benthos

Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...

 annelid tube worms employ radioles primarily for alimentation. While their primary role is to function as an organ for filter feeding

Filter feeder

Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...

, radioles also serve as respiratory

Aquatic respiration

Aquatic respiration is the process whereby an aquatic animal obtains oxygen from water.-Fish:In most fish respiration takes place through gills. Lungfish, however, possess one or two lungs...

 organs. Because of their role in gas exchange, radioles are often referred to as "gill

Gill

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

s".

Anatomical location of radioles

Canalipalpata have a head located at the anterior end of the body. The head is formed by the fusion of a funnel-shaped, symmetrical peristomium with the prostomium

Prostomium

Prostomium is the first body segment in annelid worms. It is in front of the mouth, being usually a small shelf- or lip-like extension over the dorsal side of the mouth. It sometimes bears antennae and eyes. It often functions like a kind of overlip when the animal is feeding...

. The prostomium bears a specialized mouth

Mouth

The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....

 appendage

Appendage

In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment...

 which is referred to as a branchial crown. The crown functions as both a sieve

Sieve

A sieve, or sifter, separates wanted elements from unwanted material using a woven screen such as a mesh or net. However, in cooking, especially with flour, a sifter is used to aerate the substance, among other things. A strainer is a type of sieve typically used to separate a solid from a liquid...

 and a gill. The animal can extend the crown from its calcareous

Calcareous

Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...

 tube for feeding and gas exchange, and rapidly retract it when disturbed or threatened.

The crown consists of two bundles (one right and one left) of featherlike tentacles known as branchiae, or radioles. Each of these bundles consists of a single row of radioles attached to a branchial stalk and curved into a semicircle. These two semicircles form the funnel-shaped branchial crown. The mouth is located at the apex

Apex

Apex may refer to:- Biology :* Apex , the tip of the spire of the shell of a gastropod* Apex is the Apical meristem or its remnant on a flower...

 of the funnel, between the two branchial stalks.

An adult worm typically has about forty radioles in its crown. The radioles themselves are covered by tiny hair-like branches called cilia

Cilium

A cilium is an organelle found in eukaryotic cells. Cilia are slender protuberances that project from the much larger cell body....

. This arrangement gives the crown the appearance of a small fan or feather duster (for which the animals are often referred to as fanworms, or feather duster worms). When extended, these heavily ciliated radioles trap particles of organic matter

Organic matter

Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...

 and transport them towards the mouth.

Use of radioles in filter feeding

The ventral surface of each radiole is covered by cilia that rhythmically move in such a way as to create a current

Current (fluid)

A current in a fluid is the magnitude and direction of flow within that fluid. An air current presents the same properties for a gaseous medium.Kinds of fluid currents include.* Boundary current* Current , a current in a river or stream...

 in the surrounding water column. This current carries plankton

Plankton

Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

ic particles from the underside of the crown upwards through the net of radioles to the dorsal surface.

The dorsal or upper side of each radiole has a ciliated longitudinal radiolar food groove running down its center, extending along its longitudinal axis from the tip to the center of the crown. Planktonic food particles are swept into these grooves, where they become trapped in a coating of mucus

Mucus

In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...

. At this point, the animal subjects the particles to an examination and selection process, whereby any particles that are determined to be unsuitable due to size or chemical composition are rejected by the animal and discarded back into the water column. Once the selection process is complete, the cilia transport the particles towards the mouth of the animal, from where they enter the digestive tract.

Use of radioles in respiration

While they are primarily feeding structures, the radioles also serve as respiratory organs. It is because of this role in gas exchange that the structures are commonly referred to as "gills".

Pigmentation of radioles

The radioles of different species of canalipalpata vary widely in color. Those of the serpulid

Serpulidae

Serpulidae is a family of sessile, tube-building annelid worms in the class Polychaeta. The members of this family differ from the sabellid tube worms in that they have a specialized operculum that blocks the entrance of their tubes when they withdraw into the tubes. In addition, serpulids secrete...

 tubeworms are typically red, pink, or orange in color, with white transverse bands. Astaxanthin

Astaxanthin

Astaxanthin is a carotenoid. It belongs to a larger class of phytochemicals known as terpenes. It is classified as a xanthophyll, which means "yellow leaves". Like many carotenoids, it is a colorful, lipid-soluble pigment. Astaxanthin is found in microalgae, yeast, salmon, trout, krill, shrimp,...

, a carotenoid

Carotenoid

Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...

 pigment, is responsible for the bright red color of the crown of Serpula vermicularis

Serpula vermicularis

Serpula vermicularis, known by common names including the calcareous tubeworm, fan worm, plume worm or red tube worm, is a species of segmented marine polychaete worm in the family Serpulidae. It is the type species of the genus Serpula and was first described by Linnaeus in 1767...

.

Growth and regeneration of radioles

Juvenile

Juvenile (organism)

A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in terms of their colour...

 and other worms of small size have small crowns and radioles and therefore prefer to capture and eat very small particles, like bacterioplankton

Bacterioplankton

Bacterioplankton refers to the bacterial component of the plankton that drifts in the water column. The name comes from the Ancient Greek word , meaning "wanderer" or "drifter" , and , a Latin neologism coined in the 19th century by Christian Gottfried Ehrenberg...

 and single-celled phytoplankton

Phytoplankton

Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...

 and zooplankton

Zooplankton

Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...

. As a worm matures and grows in size, so does its crown. The larger crown allows the animal to feed on larger multicellular plankton. The preferred food size depends on the maximum size achieved by the adult worm.

Canalipalpata worms often lose one or more radioles, or even the entire crown as a result of predation

Predation

In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...

 by other animals or other types of trauma. Some species of canalipalpata even appear to have the ability to control the loss of their tentacular crowns, in much the same manner as when a lizard loses its tail. In certain circumstances, sacrifice of the crown may permit escape or confer some other benefit to the animal. Separation of the crown occurs at a preestablished zone of abscission

Abscission

Abscission is a term used in several areas of biology. In plant sciences it most commonly refers to the process by which a plant drops one or more of its parts, such as a leaf, fruit, flower or seed...

, located at the base of the crown.

These animals have the ability to regenerate

Regeneration (biology)

In biology, regeneration is the process of renewal, restoration, and growth that makes genomes, cells, organs, organisms, and ecosystems resilient to natural fluctuations or events that cause disturbance or damage. Every species is capable of regeneration, from bacteria to humans. At its most...

 new radioles to replace those that have been amputated

Amputation

Amputation is the removal of a body extremity by trauma, prolonged constriction, or surgery. As a surgical measure, it is used to control pain or a disease process in the affected limb, such as malignancy or gangrene. In some cases, it is carried out on individuals as a preventative surgery for...

, or even the entire crown if necessary. Any would-be predators that pass by after a worm has lost its crown will get the impression that the worm has died; this protects the animal from further attack. The crown typically reappears after about two weeks. When it does reappear, it is initially smaller in size, but it eventually grows back to its former size and color.

Specialized radioles

In addition to having ordinary radioles, some canalipalpata possess one or more highly modified radioles located on the dorsal part of the head. This specialized structure is called an operculum

Operculum (bryozoa)

In the bryozoan order Cheilostomata, the operculum is a calcareous or chitinous lid-like structure that protects the opening through which the polypide protrudes....

. The operculum is a cone-shaped cartilaginous

Cartilage

Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...

 structure located at the distal end of a long cartilaginous stalk. When threatened or disturbed, the animal withdraws rapidly into its protective calcareous tube and employs the operculum as a plug to occlude the entrance to the tube. The operculum, which is usually similar in color to the other radioles, secretes a mucus which seems to possess antibiotic

Antibiotic

An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

 properties. It is not unusual for the animal to have two crowns, and hence two opercula.

Serpulids and sabellids are two families

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 of the Sabellida

Sabellida

Sabellida is a suborder of annelid worms in the class Polychaeta. They are filter feeders with no buccal organ. The prostomium is fused with the peristomium and bears a ring of feathery feeding tentacles...

 suborder

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 of canalipalpata tubeworms that are similar in nearly every respect, but they can be readily distinguished by the fact that while both have radioles, the sabellids (such as Sabella pavonina

Peacock worm

The peacock worm is a marine polychaete worm belonging to the family Sabellidae. It occurs along the coasts of Western Europe and the Mediterranean. It is found in shallow, tidal waters with a bed of mud, sand or gravel. It is sometimes found on rocks or shipwrecks.It is 10-25 centimetres in...

) lack an operculum.