Thecostraca (Cirripedes and Relatives)

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Thecostraca

(Cirripedes and relatives)

Phylum Arthropoda

Subphylum Crustacea

Class Maxillopoda

Subclass Thecostraca

Number of families 48

Thumbnail description
Highly modified, mainly free-living sessile and parasitic crustaceans, usually enclosed within a calcareous carapace or forming a chitinous saclike body

Evolution and systematics

The calcareous carapaced forms of thecostracans have provided the richest fossils. The oldest known thecostracan fossil is dated from the Middle Cambrian; traces of the parasitic forms (without carapaces) have been dated from Cretaceous. The great diversity of the species began to occur in the Upper Cretaceous and was essentially completed by the end of the Miocene.

Until 1834 barnacles were classified as mollusks because of their calcareous shells. Only in 1829, when thecostracan larval stages were first discovered, were their affinities with other crustaceans fully recognized.

Thecostracans are included among maxillopodan groups. The group Maxillopoda is not generally accepted as a natural group, and there are doubts over its monophyly and component groups. The thecostracan lineage is founded on morphological, ontogenetic, molecular, and fossil data. Some ascothoracicans show some similarities with other maxillopodans, and are now considered the primitive group of thecostracans. Cirripedes are a notable exception among maxillopodans because of their adaptations to a sessile way of life. The facetotectans are one of the biggest remaining mysteries of crustacean diversity; the latter group's affinity with the tantulocarids is still under investigation.

Two-thirds of thecostracan species are free-living or commensals, and one-third have different degrees of parasitism. There are about 1,400 species divided into three infraclasses: Cirripedia (barnacles), with three suborders (Thoracica, Acrothoracica, and Rhizocephala) and 41 families with free-living, commensal, and parasitic species; Ascothoracica, with six families, includes ecto- and endoparasites of cnidarians and echinoderms; Facetotecta is composed of one genus, Hansenocaris, which is microscopic, Y-shaped, and free-living.

Physical characteristics

As in some extant ascothoracicans, the body is primitively composed of a head containing some cephalic structures, a thorax with six segments and appendages, and a segmented abdomen. In all groups the head is reduced, and the abdomen has no limbs; the second antennae is absent in some groups. Telson and compound eyes are absent in adults. The mouth appendages have some reductions and modifications. Most adult cirripedes are modified for a life attached to an object, or as a parasite.

Cirripedes are unique among crustaceans because they are sessile. A number of barnacle peculiarities may be correlated with sessility. During the larval stage almost all cirripedes find

an object to attach to by using their first antennae (antennules), after which the preoral region becomes fixed to the object. Later, during metamorphosis, the body, mouth, eyes, and the adductor muscles separate from the antennules. All openings of the body are located on the side of the body that is opposite from the object they are attached to, that is, the free side of their body. Barnacles are known as animals that sit on their heads and kick food into their mouths.

A bivalved and chitinous carapace (mantle) encloses the body in ascothoracicans. In cirripedes, the mantle forms a sac. In the thoracicans, the mantle secretes calcareous plates, which form an outer wall or shell that is permanent. Thoracian species of barnacles such as goose barnacles are pedunculate (stalked), while sessile species are non-pedunculate. The pedunculate species have a fleshy peduncule that hangs along the head and entire body. Facetotectans, ascothoracicans, acrothoracicans, and rhizocephalans have no calcareous plates and are all chitinous.

The mantle cavity is a spacious chamber where the mouth, anus, and sexual organs are located and from which the larvae are freed into seawater.

Most thecostracans are recognized as crustaceans due to their paired, chitinous, and jointed thoracic appendages, the cirri, which can be uni- or biramous as in other crustaceans, and are sometimes heavily fringed with bristles. Cirri are mainly used during feeding and as a respiratory organ. Only rhizocephalans in their larval stages have appendages; adults have a ramified (branching off) structure that penetrates the tissues of the host. Rhizocephalans are the most highly modified of all thecostracans.

Facetotectans are less than 0.039 in (1 mm) long. Most parasites and simple forms are only a few millimeters in length. Stalked barnacles range from a few millimeters to more than 27.5 in (70 cm) in length. The majority of sessile species are a few inches (centimeters) in length and can reach 9 in (23 cm) in height and up to 3.1 in (8 cm) in diameter. Free-living barnacles are white, pink, red, purple, orange, violet, or brown.

Distribution

Thecostracans are exclusively marine and/or estuarine. Most species are intertidal or subtidal. Some thoracican species live in the high tide and others are found near abyssal hydrothermal vents. The group occurs worldwide, but barnacles are less conspicuous in tropical rocky shores. A number of species have commensal relationships with some pelagic animals and their distribution is limited only by the range of their host.

Habitat

Primitive ascothoracicans, facetotectans, and males of some species are free-swimming. Some ascothoracicans attach to their hosts using a prehensile first antenna, which has glands that secrete cement for the attachment. The cement is produced throughout their lifespan, and repairing partial detachment is possible. The attachment of most thecostracans is done by cyprid larvae after settlement. These animals can live on almost any hard object in the seawater. Most free-living sessile barnacles attach to rock. Common pedunculate and sessile barnacles attach to inanimate objects such as wood, floating logs, bottoms of ships, wooden pilings, and empty bottles. Commensals and parasites attach to living organisms, including pelagic animals such as corals, sea anemones, jellyfishes, mussels, crabs, shrimps, lobsters, copepods, other barnacles, echinoderms, tunicates, sea turtles, and the skin of whales and sharks. Thecostracans attach to their host or object by the cypnid larval stage.

Ascothoracicans bore on calcareous substrates such as mollusk shells, dead corals, or carapaces of sea urchins. They bore using chitinous teeth, as well as by excreting chemicals that lead to dissolution. As larvae, parasites, attach to some part of the host's body, making a perforation on the tegument.

Behavior

Most thecostracans can move about freely only as larvae. In some species, adults retain the ability to swim throughout their lives, attaching only temporarily for feeding.

Barnacles are very resistant to abiotic factors. Many species of sessile barnacles, common in rocky shores, live in the intertidal zone on the coast. During low tide, these animals are exposed to the air. They hermetically close the valves present in the carapace to avoid desiccation, high temperatures, and freshwater rain. They can form bands for miles along the coast, with high population densities of 1,000–2,500 individuals in 15.5 in² (100 cm²). Some species have a high growth rate in areas with high wave rates, because turbulance and strong currents promote the movement of plankton toward the coast line; plankton is a major food source for barnacles.

Cyprid larvae settle in dense numbers in areas where other living or dead barnacles occur. A protein present in the exoeskeleton of older attached individuals has been shown to attract larvae. This behavior ensures that individuals will be close enough for cross-fertilization and settlement to take place, as these animals are sessile.

Feeding ecology and diet

Most thoracicans are filter feeders. They feed actively by extending their long, feathery, birramous cirri out of the carapace, in a fan-like manner, to filter feed on suspended material from the surrounding water. The bristles of the cirri overlap to form an effective filtering net. The water is filtered and the food is passed to the mouthparts.

Food particles range from 0.0000787 to 0.039 in (2 µm to 1 mm) in size, and includes detritus, bacteria, algae, and zoo-plankton. Food is detritus for those species that are found within estuaries and bays.

Cirripedes can be predators. Stalked barnacles are capable of preying upon larger planktonic animals by coiling a single cirrus around the prey.

Ectoparasitic thecostracans send roots into the tissues of their pelagic hosts in order to feed. Some parasites have modified mouthparts that form suctorial cones for piercing tissues and sucking out the body fluids of their hosts. In rhizocephalans, the ramified body lacks an alimentary system, and nutrients are absorbed directly from the host's tissues.

Starfishes, snails, fishes, worms, and birds feed on barnacles.

Reproductive biology

Most species are hermaphrodites, but some are accompanied by additional males, and are called complementals. Ascothoracicans, acrothoracicans, and rhizocephalans have separate sexes. Males of these groups are called dwarves. Complemental and dwarf males are greatly reduced in size and do not feed frequently. They attach to females.

Free-living barnacles generally cross-fertilize, because a suitable substrate almost always contains a large number of adjacent individuals. The penis of the free-living barnacle can be extended out of the body and into the mantle cavity of another individual in order to deposit sperm.

In all cirripedes, the eggs develop within the ovisac, present in the mantle cavity. In most species, free-swimming nauplii larvae hatch from the eggs. Other naupliar instars occur before the transformation to cyprid larvae. The entire body is enclosed within a bivalved carapace; one pair of sessile compound eyes and six pairs of thoracic appendages are also present.

Conservation status

No species are listed by the IUCN.

Significance to humans

Barnacles often attach to the bottom of ships, where they grow to such a degree that they can reduce its speed by as much as 35%. Significant effort and money have been expended toward the development of special paints that will prevent barnacles from attaching.

The barnacle species Balanus nubilis is eaten by Native Americans in the United States. The rock barnacle, Balanus psittacus, can reach 9 in (23 cm) in height and 3.1 in (8 cm) in diameter, and is a popular local seafood in South America.

Species accounts

List of Species

Trypetesa lampas
Ascothorax ophioctenis
Root-like barnacle
Common goose barnacle
Rock barnacle

No common name

Trypetesa lampas

order

Apygophora

family

Trypetesidae

taxonomy

Alcippe lampas Hancock, 1849.

other common names

None known.

physical characteristics

Species does not have a calcareous carapace. Females reach 0.78 in (2 cm) in length; body is colorless or yellowish and covered by a large mantle. Cavity is exposed to water by the narrow fissure-like orifice. The preoral region has a wide and flat disc plate that larvae use to form attachments to a host's shell. Thorax is bent by segmentation. Three pairs of appendages within the mouth; one pair of reduced cirri near the mouth. Only three uniramous pairs of cirri located at the end of the thorax. Females have an incomplete gut so ceca are able to reach into various parts of the body; anus is absent. Dwarf males are 0.047 in (1.2 mm) long, bottle-shaped, legless, and attach to females. Antennae are the only appendages present and there are no internal organs other than reproductive ones.

distribution

Shores of the Pacific and Atlantic Oceans in the Northern Hemisphere.

habitat

Adult females spend their lives boring into the shells of living or dead gastropods and hermit crabs; they then live within the aperture or the columela. Adults bore with the non-mineralized spines present on the mantle, and by secreting chemicals.

behavior

When molting, the female does not shed the non-mineralized layers present on the attachment disc.

feeding ecology and diet

Females expand their mantle and bend their body away from the mantle slit so that water is forced in and out of the mantle cavity. Three posterior cirri then collect particles of food from the water. Cirri present in the mouth draw up against the posterior cirri to sweep particles into the mouth.

reproductive biology

Sexes are separate; males attach to females as cyprid larvae and undergo metamorphosis into dwarf males. Testes open into extensible penis to fertilize eggs. Nauplii hatch from eggs and after four naupliar stages, cyprid larvae with six pairs of appendages and compound eyes search for a suitable substrate in which to burrow.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Ascothorax ophioctenis

order

Dendrogastrida

family

Ascothoracidae

taxonomy

Ascothorax ophioctenis, Djakonov, 1914, Novaya Zemlya, Russia, at a depth of 328 ft (110 m).

other common names

None known.

physical characteristics

Females 1.1–1.5 in (3–4 cm) in length; males one-third to one-sixth female's size. Body consists of head, six thoracic segments, and five abdominal segments; last segment bears the caudal rami. The first antenna is long, thick, and subchelate; cement glands are absent. The body is often orange in color. Males have bivalved, oval, laterally compressed, and uncalcified carapaces surrounding the mantle cavity; the carapace is attached to the head, enclosing the head and thorax. The adductor muscle can close the carapace.

Females are heart-shaped with a bivalved carapace that is much larger than the trunk. First and sixth thoracic cirri are uniramous, the others are biramous. Thorax and abdomen are both short, and the thorax is sharply bent at the fourth segment.

distribution

Found from Greenland to Norway, and east to Novaya Zemlya and Franz Josef Land; including the Barents, Greenland, Norwegian, Kara, and Laptev Seas.

habitat

Parasitic; lives in the bursa present between two arms of its host, the serpent stars Ophiocten sericeum s and O. gracilis. Once it has attached and is fixed, the host's bursa becomes inflated so that the parasite's presence can be recognized from the outside.

behavior

Males attach to female's body using the subchelate first antennae. In females, one or both antennal chelae are extended and attach to bursa wall of the host. Slit present between carapace valves toward the bursal opening permits limbs to circulate a current of water that allows breathing to occur. Prevents its host from developing gonads.

feeding ecology and diet

Mouthparts of males and females are stylet-like structures enclosed in a conical labrum modified for piercing and sucking. Labrum is pressed to bursal wall where penetration of host occurs. Gutter-like first maxillae forms a tube through which cell debris and fluids are drawn from host.

reproductive biology

First abdominal segment bears male intromittent organ. Fertilization is external, taking place in the brood chamber of the female. Nauplii hatch from eggs and develop within the brood chamber where they metamorphosize into cyprid-like larvae that are capable of feeding and have a bivalved carapace, but no second antennae. During the larval stage—called the ascothoracid stage—young leave the brooding chamber present in females and begin to search for their own host.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Root-like barnacle

Sacculina carcini

order

Kentrogonida

family

Sacculinidae

taxonomy

Sacculina carcini Thompson, 1836.

other common names

Dutch: Krabbezakje.

physical characteristics

Body of adult females is made of tissue and is covered with a thin cuticle devoid of calcareous plates, traces of segmentation, distinct regions, appendages, or an alimentary tract. The body consists of a central nucleus, which attaches itself onto the mid-gut of the host from where it sends root-like appendages (the interna) into every part of the host's body. This nucleus connects to the exterior of the host with the external knob sac (the externa). Males are dwarves, microscopic cyprids that attach to females.

distribution

Occurs in European seas; predator of the green crab, Carcinus maenas and other crabs.

habitat

Uses its root-like extensions to attach to the host's visceral organs, nervous system, gonads, and appendages, but do not penetrate inside the host.

behavior

A host's gonads are castrated by this parasite; in adult males, the abdomen becomes feminized. Young crabs do not form gonads when infested.

feeding ecology and diet

Nourishment is accomplished by diffusing nutritive substances from host's blood. It may attack organs with enzymes it secretes. Males receive nourishment from the female's body.

reproductive biology

External sac has branched ovaries and a pair of male sperm receptacles. A male cyprid swims into the female's body. After attachment, male extrudes its whole cell content into the female. Male takes up residence and its function is to produce sperm. Fertilization and brooding occur in the externa; nauplii hatch out of the eggs, pass through four naupliar stages. One parasite can release many broods during one season, but the peak of reproduction is during summer; there is a rest period in the winter. The period of incubation is 12–35 days; after 10 days the parasite settles. Female cyprids attach to the base of their host's bristles. Females abandon their limbs, thorax, mantle, and carapace, and develop a curved stylet through which their cells pass into the host's body. After an internal phase, which can last up to 3 years, females produce an externa, attracting male cyprids.

conservation status

Not listed by the IUCN.

significance to humans

Suggested as a biological control agent against the invasive green crab, Carcinus maenas, which is negatively affecting coastal ecosystems worldwide.

Common goose barnacle

Lepas anatifera

order

Pedunculata

family

Lepadidae

taxonomy

Lepas anatifera Linnaeus, 1758.

other common names

English: Goose barnacle, gooseneck barnacle; French: Anatife; Portuguese: Anatifas, conchas marrecas, lepas.

physical characteristics

Average body length is 9.8 in (25 cm); largest specimen found was 29.5 in (75 cm). Color of body is dark brown and calcareous plates are whitish. Body divided into two regions: the peduncle (stalk), and the capitulum. The peduncle is fleshy, large, and long, and it attaches to the substrate using the first antennae. The body is compressed laterally, covered by two folds of mantle, where five thin calcareous plates are attached. The carina is a dorsal unpaired plate, which forms a central keel. Paired scuta are large, and are located at the anterior region of the body. Paired terga are short and are located at the posterior-most region of the body. Six pairs of thoracic, biramous cirri bordered with chaetae are visible through an aperture present in the mantle cavity. The adductor muscle closes the mantle cavity. In the mantle cavity, there is a short head, a thorax with six thoracic, biramous limbs, a mouth, and a long, setose penis.

distribution

Cosmopolitan in tropical and temperate seas.

habitat

Small colonies attach to floating objects such as logs and ships.

behavior

Cirri, stalk, and mantle are covered by sensory bristles (chaetae). Stimulation of these chaetae causes the withdrawal of the body and the mantle aperture to close.

feeding ecology and diet

Plankton is collected with movements by the thoracic cirri. From between plates, cirri are extended and spread out like a fan-shaped net; cirri are then withdrawn and the aperture is closed. Filtering of food from water is done rhythmically through combined movements of the cirri and the closure of the aperture; food is then transferred to mouthparts.

reproductive biology

Hermaphroditic, but without complemental males. The long, extensible penis is inserted into the mantle cavity of adjacent individuals. Eggs brood within the mantle cavity and nauplii hatch from eggs. After several molts, larvae reach the cyprid stage. The cyprid attaches to the substrate using its antennae and begins metamorphosis; during this stage it can grow almost 0.039 in (1 mm) per day, attaining a length of 3.9 in (10 cm) within 113 days.

conservation status

Not listed by the IUCN.

significance to humans

Scientific name means "goose carrier" because they resemble goose eggs. They attach to ships.

Rock barnacle

Semibalanus balanoides

order

Sessilia

family

Archaeobalanidae

taxonomy

Balanus balanoides Linnaeus, 1767.

other common names

English: Acorn shell, common barnacle, acorn barnacles, stalkless barnacle; French: Balane, gland-de-mer; Portuguese: Bálanos, bolotas-do-mar, caracas, glandes-do-mar.

physical characteristics

Reaches 0.19–0.59 in (5–15 mm) in diameter. Carapace is conical, with six gray or white fused calcareous plates forming a lateral wall. The dorsal anterior plate is the largest. The carine is posterior and two pairs of plates are located laterally. On top, two pairs of short calcareous and articulated plates, the terga and scuta (one pair on each side) form the operculum in a diamond-shaped arrangement; it encloses the mantle cavity and can be opened and closed by muscles. When rotated laterally, they form an aperture through which leads to the mantle cavity and the six pairs of birramous cirri bordered by chaetae, the first three being shorter than other three. Tissue inside opercular aperture is usually white or pinkish. Attaches to rock using the flattened and broad membranous attachment disc.

distribution

Found in intertidal rocky shores in North America, Europe, and the Arctic.

habitat

Common and abundant in rocky shores. It settles on a wide variety of solid objects including pilings, rocks, and shell crabs. Prefers low-tide areas occasionally splashed by water.

behavior

The calcareous carapace protects the animal, and the operculum hermetically seals the carapace; when the tide goes out, they close the carapace. Survives freezing weather during the winter in tide zones of the Arctic Ocean, as well as the daily dry 6–9 hours between high tides during summer. Eggs and nauplii in

the mantle cavity also survive. In very dense colonies, young do not find enough space to grow, so the wall plates become elongate, resembling stalks, elevating the feeding position as in pedunculate barnacles; on rocky coasts, they form a wide band that can stretch for miles. Turbulence and waves are of importance since the current brings plankton for feeding. Fewer individuals settle in areas without strong waves.

feeding ecology and diet

Feeds only during high tide when the carapace is covered by water. Food is small and collected by filtering; animal opens operculum and extends the last three long pairs of cirri, which are spread and curved outward resembling a fan-shaped net. It rhythmically extends and retracts the cirri and collects material, transferring it to mouthparts. Cirri can do 33–37 beats per minute to collect food. Preyed on extensively by the dog whelk, Nucella lapillus, and the shanny, Lipophrys pholis.

reproductive biology

Hermaphrodite; has an extensible penis that fertilizes neighboring individuals. Eggs are fertilized in the mantle cavity; gravid individuals each contain 6,000–13,000 nauplii in mantle cavity. After some stages as a nauplius, a cyprid larva begins to seek a place of attachment. The antennae and cement glands make the attachment onto objects. During metamorphosis animals can grow to a diameter of 0.23–0.27 in (6–7 mm) in 58 days. Lives for 3–8 years. In some localities, all young molt synchronously. Individuals in same place hatch nauplii at same time; a good number of larvae settle in same place, assuring cross-fertilization.