Thecostraca () is a class of marine invertebrates containing over 2,200 described species. Many species have planktonic larvae which become sessile or parasitic as adults.
The most prevalent subgroup are the barnacles (subclass Cirripedia), constituting a little over 2,100 known species.
The subgroup Facetotecta contains a single genus, Hansenocaris, known only from the tiny planktonic nauplii called "y-larvae". These larvae have no known adult form, though it is suspected that they are parasites, and their affinity is uncertain. Some researchers believe that they may be larval tantulocaridans. No larval tantulocaridans are currently known.
The group Ascothoracida contains about 110 species, all parasites of coelenterates and echinoderms.
The nauplius larvae (sometimes absent) can be both lecithotrophic (non-feeding) and planktotrophic (feeding), and is followed by a larval stage called the cyprid, which is always lecithotrophic. The cypridoid larvae are referred to as the y-cyprid in the Facetotecta, the a-cyprid in the Ascothoracida, and the c-cyprid, or just cyprid, in the Cirripedia.
Classification This article follows Chan et al. (2021) and the World Register of Marine Species in placing Thecostraca as a class of Crustacea and in the following classification of thecostracans down to the level of orders. Previously, Thecostraca was considered a subclass of Maxillopoda. Significant changes in the organization of Cirripedia's orders, families, and genera were introduced in 2021 by Chan et al. and accepted by the World Register of Marine Species.
Class Thecostraca Gruvel, 1905 Subclass Ascothoracida Lacaze-Duthiers, 1880 Order Laurida Grygier, 1987 Order Dendrogastrida Grygier, 1987 Subclass Facetotecta Grygier, 1985 Subclass Cirripedia Burmeister, 1834 Infraclass Acrothoracica Gruvel, 1905 Order Lithoglyptida Kolbasov, Newman & Hoeg, 2009 Order Cryptophialida Kolbasov, Newman & Hoeg, 2009 Infraclass Rhizocephala Müller, 1862 Infraclass Thoracica Darwin, 1854 Superorder Phosphatothoracica Gale, 2019 (paraphyletic) Order Eolepadomorpha Chan et al., 2021 Order Iblomorpha Buckeridge & Newman, 2006 Superorder Thoracicalcarea Gale, 2015 Order Calanticomorpha Chan et al., 2021 Order Brachylepadomorpha Withers, 1923 Order Archaeolepadomorpha Chan et al., 2021 Order Verrucomorpha Pilsbry, 1916 Order Scalpellomorpha Buckeridge & Newman, 2006 Order Pollicipedomorpha Chan et al., 2021 Order Balanomorpha Pilsbry, 1916
Phylogeny The following cladogram depicts the internal relationships of the Thecostraca as of 2021.
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