The evolutionary diversity of barnacles, with an updated classification of fossil and living forms

Abstract

We present a comprehensive revision and synthesis of the higher-level classification of the barnacles (Crustacea: Thecostraca) to the genus level and including both extant and fossils forms. We provide estimates of the number of species in each group. Our classification scheme has been updated based on insights from recent phylogenetic studies and attempts to adjust the higher-level classifications to represent evolutionary lineages better, while documenting the evolutionary diversity of the barnacles. Except where specifically noted, recognized taxa down to family are argued to be monophyletic from molecular analysis and/or morphological data. Our resulting classification divides the Thecostraca into the subclasses Facetotecta, Ascothoracida and Cirripedia. The whole class now contains 14 orders, 65 families and 367 genera. We estimate that barnacles consist of 2116 species. The taxonomy is accompanied by a discussion of major morphological events in barnacle evolution and justifications for the various rearrangements we propose.

Genomic insights into the sessile life and biofouling of barnacles (Crustacea: Cirripedia)

“…Barnacles (Thecostraca: Cirripedia) are sessile crustaceans ubiquitously present in marine environments from the intertidal to the deep-sea, and from the tropics to the polar regions; some of them even live on other marine organisms such as whales and turtles ( Chan et al., 2009 ; Buckeridge et al., 2018 , 2019 ; Kim et al., 2020 ). The subclass Cirripedia is divided into three infraclasses that differ in morphology and way of life: Thoracica includes acorn and stalked barnacles with phosphatic or calcified shells ( Chan and Høeg, 2015 ; Chan et al, 2021 ); Acrothoracica is characterized for not having shells but specifically burrowing into diverse calcareous substrates ( Chan et al., 2014 ; Nielsen et al., 2016 ); Rhizocephala includes endoparasitic barnacles of other crustaceans ( Anderson, 1994 ; Jung et al., 2019 ). Barnacles represent an excellent model system to study the evolution of body forms owing to their drastic changes from free-swimming planktonic naupliar and cyprid larval stages to sessile juveniles and adults with their forehead fixed to a solid substrate and their body placed upside down during the life cycle ( Qiu et al., 1997 ; Qiu and Qian, 1999 ).…”

A New Species of Waginella (Crustacea: Thecostraca: Ascothoracida) Parasitic on a Stalked Crinoid From Tasman Sea, With Notes on Morphology of Related Genera

“…parasites of non-crinoid echinoderms. The monophyly of these two orders is yet to be tested by molecular methods (Chan et al, 2021). The ascothoracidans are largely dioecious, with larger females and smaller, sometimes dwarf cypridiform males (Grygier and Fratt, 1984;Grygier, 1985Grygier, , 1987bGrygier, , 1991aKolbasov, 2007).…”

Flatfoot in Africa, the cirripedeChthamalusin the west Indian Ocean

“…Some of them, like Chthamalus stellatus Poli, 1791 , encompass a number of races or varieties that were later recognized as valid species, including new species of Chthamalus . This genus has the widest distribution in the family of Chthamalidae; currently, 27 nominal species of Chthamalus are recognized ( Chan et al, 2021 ) The morphological and ecological similarities within the genus may give rise to some taxonomic confusion. However, the use of molecular techniques has resolved some of the uncertainty and enabled the identification of a number of cryptic species within long-established nominal species.…”

“…Traditionally, the identification of barnacles is based on morphological features, although several taxa (from the entire class Thecostraca to subfamilies) are essentially only recognized by molecular analyses ( Chan et al, 2021 ). In the field, where only the morphology of the shell and opercular valves can be used for identification, the high variability in some species of Chthamalus , may give rise to confusion and lead to the misclassification of morphologically different species ( Southward, 1976 ).…”