Thecostraca

(Cirripedes and relatives)

Phylum Arthropoda Subphylum Crustacea Class Maxillopoda Subclass Thecostraca Number of families 48

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

Photo: Species of the genus Synagoga, found on the species Antipathella wollastoni at a depth of 131 ft (40 m) in waters off of the Azores. (Photo by Peter Wirtz. Reproduced by permission.)

Clase Thecostraca

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, Hanseno-caris, 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

Common goose barnacles (Lepas anatifera), with legs extended for feeding. (Photo by A. Flowers & L. Newman. Reproduced by permission.)

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. Thora-cian 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 thecostra-cans 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, jelly-fishes, 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 mol-lusk 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 ex-oeskeleton 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.

Northern rock barnacles (Balanus balanoides). (Photo by Animals Animals ©E. R. Degginger. Reproduced by permission.)

Food particles range from 0.0000787 to 0.039 in (2 pm to 1 mm) in size, and includes detritus, bacteria, algae, and zooplankton. 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 rhizo-cephalans, 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.

Anatomy Goose Barnacle

1. Root-like barnacle (Sacculina carcini); 2. Rock barnacle (Semibalanus balanoides); 3. Common goose barnacle (Lepas anatifera); 4. Ascotho-rax ophioctenis; 5. Trypetesa lampas. (Illustration by Jonathan Higgins)

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