Thecostraca

Gruvel, 1905

Barnacles and Allies

Order Guides

Lepadiformes(stalked barnacles)
Sessilia(acorn barnacles)

Thecostraca is a class of marine crustaceans encompassing over 2,200 described , with barnacles (subclass Cirripedia) comprising the vast majority. The group includes three major lineages: suspension-feeding barnacles, parasitic ascothoracidans that infect cnidarians and echinoderms, and the enigmatic Facetotecta, known only from planktonic larvae. Members undergo distinctive larval development, typically featuring a nauplius stage followed by a cypris stage that facilitates settlement. Thecostraca exhibits remarkable diversity, ranging from free-living suspension feeders to with highly derived morphologies.

Lepas by (c) Takaaki Hattori, some rights reserved (CC BY), uploaded by Takaaki Hattori. Used under a CC-BY license.

Thecostraca by (c)
Gustav Paulay, Florida Museum of Natural History, some rights reserved (CC BY). Used under a CC-BY license.

Conchoderma auritum by (c) Cricket Raspet, some rights reserved (CC BY), uploaded by Cricket Raspet. Used under a CC-BY license.

Pronunciation

How to pronounce Thecostraca: /θiːkoʊˈstrækə/

These audio files are automatically generated. While they are not always 100% accurate, they are a good starting point.

Identification

Thecostraca can be distinguished from other crustacean classes by the presence of cypris larvae—a specialized non-feeding settlement stage with a bivalved and . forms vary dramatically: Cirripedia possess calcareous shell plates (in Thoracica) or are shell-less burrowers (Acrothoracica) or highly reduced (Rhizocephala); Ascothoracida have enlarged, often ornamented carapaces with thoracic horns; Facetotecta are known only from y-larvae with a distinctive faceted cephalic shield. The nauplius larvae, when present, bear a frontal filament complex and paired lattice organs—structures unique to the group.

Images

Habitat

Exclusively marine; distribution spans from intertidal zones to deep-sea environments exceeding 200 m depth. Cirripedia occupy diverse substrates including rocky shores, ship hulls, whale skin, and hydrothermal vents. Ascothoracida are or of anthozoans (scleractinian and zoantharian corals, antipatharians, alcyonaceans) and echinoderms (asteroids). Facetotecta occur as planktonic larvae in all world oceans; unknown but suspected parasitic.

Distribution

Global marine distribution. Facetotecta larvae documented from plankton worldwide including Okinawa (Japan), Taiwan, and Atlantic localities. Ascothoracida exhibit Tethyan reliction patterns with disjunct Western Pacific/Eastern Atlantic distributions in gorgonian-infecting . Cirripedia are in marine environments. Specific records include: Minho River estuary (Iberian Peninsula) for barnacles; Emperor Seamount Chain for deep-water ascothoracidans; Kii Peninsula (Japan) for sea star .

Diet

Highly variable across subclasses. Thoracican barnacles are suspension feeders on plankton and organic particles. Rhizocephalan barnacles are parasitic, with nutrient uptake from decapod crustacean . Ascothoracida exhibit mixed strategies: Petrarca feeds directly on coral host tissue; Baccalaureus appears to steal food from host gastric cavities rather than feed on host tissue; Dendrogaster inhabit sea star coelomic cavities with feeding mode inferred parasitic. Facetotecta larvae are lecithotrophic (non-feeding); diet unknown.

Life Cycle

Characterized by distinctive metamorphic development. Typically includes: (1) Nauplius stage—often planktonic, either lecithotrophic or planktotrophic, with 5-6 instars; (2) Cypris stage—obligatory, lecithotrophic, with bivalved , , and antennules for substrate exploration and settlement. In Facetotecta, from final nauplius to cypris involves abrupt development of free carapace and thoracopods, with a unique internal 'ghost' structure (support sling) visible in . Rhizocephala exhibit extreme metamorphosis with kentrogon or akentrogon invasion stages and internal reproductive development.

Behavior

Cypris larvae exhibit substrate-specific settlement , using antennulary biomechanics to explore surfaces and identify suitable attachment sites. Thoracican barnacles cement permanently to substrate after settlement. Rhizocephalan females undergo invasion via kentrogonid or akentrogonid mechanisms, with Clistosaccus paguri exhibiting a novel type. Ascothoracida display host-specificity: Gorgonolaureus infect octocorals of suborder Halaxonia ( Plexauridae) but never calcaxonian families; Synagoga species associate with antipatharians; Dendrogaster species inhabit specific asteroid hosts.

Ecological Role

Thoracican barnacles serve as engineers, creating complexity on rocky shores and hard substrates; they are important biofouling organisms affecting maritime industries. Rhizocephala function as castrators of decapod , manipulating host and . Ascothoracida may influence coral and echinoderm through parasitic , though impacts remain poorly quantified. As suspension feeders, barnacles contribute to benthic-pelagic coupling and nutrient cycling in coastal ecosystems.

Human Relevance

Barnacles are significant biofouling agents on ships, increasing drag and fuel consumption, necessitating antifouling measures. Some barnacle are harvested for food in Iberian Peninsula and other regions. Thecostraca have been extensively studied in evolutionary since Darwin's monograph on Cirripedia; they provide model systems for studying evolution, , and the transition to . Facetotecta remain a major unresolved mystery in crustacean biology due to unknown form.