Remarkable convergent evolution in specialized parasitic Thecostraca (Crustacea)

<p>Abstract</p> <p>Background</p> <p>The Thecostraca are arguably the most morphologically and biologically variable group within the Crustacea, including both suspension feeders (Cirripedia: Thoracica and Acrothoracica) and parasitic forms (Cirripedia: Rhizocephala, Ascothoracida and Facetotecta). Similarities between the metamorphosis found in the Facetotecta and Rhizocephala suggests a common evolutionary origin, but until now no comprehensive study has looked at the basic evolution of these thecostracan groups.</p> <p>Results</p> <p>To this end, we collected DNA sequences from three nuclear genes [18S rRNA (2,305), 28S rRNA (2,402), Histone H3 (328)] and 41 larval characters in seven facetotectans, five ascothoracidans, three acrothoracicans, 25 rhizocephalans and 39 thoracicans (ingroup) and 12 Malacostraca and 10 Copepoda (outgroup). Maximum parsimony, maximum likelihood and Bayesian analyses showed the Facetotecta, Ascothoracida and Cirripedia each as monophyletic. The better resolved and highly supported DNA maximum likelihood and morphological-DNA Bayesian analysis trees depicted the main phylogenetic relationships within the Thecostraca as (Facetotecta, (Ascothoracida, (Acrothoracica, (Rhizocephala, Thoracica)))).</p> <p>Conclusion</p> <p>Our analyses indicate a convergent evolution of the very similar and highly reduced slug-shaped stages found during metamorphosis of both the Rhizocephala and the Facetotecta. This provides a remarkable case of convergent evolution and implies that the advanced endoparasitic mode of life known from the Rhizocephala and strongly indicated for the Facetotecta had no common origin. Future analyses are needed to determine whether the most recent common ancestor of the Thecostraca was free-living or some primitive form of ectoparasite.</p

Synchronism of naupliar development of Sacculina carcini Thompson, 1836 (Pancrustacea, Rhizocephala) revealed by precise monitoring

International audienceSacculina carcini is member of a highly-specialized group of parasitic cirripeds (Rhizocephala) that use crabs (Carcinus maenas) as hosts to carry out the reproductive phase of their life cycle. We describe the naupliar development of S. carcini Thompson, 1836 from a very precise monitoring of three different broods from three specimens. Nauplii were sampled every 4 h, from the release of the larvae until the cypris stage. Larval development, from naupliar instar 1 to the cypris stage, lasts 108 h at 18 °C. A rigorous sampling allowed us to describe an additional intermediate naupliar instar, not described previously. Naupliar instars are renumbered from 1 to 5. Nauplius 1 (N1) larvae hatch in the interna; N2 are released from the interna and last between 12 and 16 h; N3 appear between 12 and 16 h after release; N4 appear between 28 and 32 h; and N5 appear between 44 and 48 h. The cypris stage appears between 108 and 112 h. The redescribed morphologies allowed us to identify new characters. Antennular setation discriminates naupliar instars 3, 4 and 5. Telson and furca morphologies discriminate all naupliar instars. Furthermore, we demonstrate that the speed of larval development is similar within a single brood and between broods from different specimens, suggesting synchronization of larval development. From precise monitoring of broods every 4 h, we demonstrate that the life cycle of S. carcini includes five instars of naupliar larvae instead of four. The morphological characters of the larvae discriminate these naupliar instars and allow the identification of S. carcini from other Rhizocephala species. S. carcini larvae develop synchronously. Consequently, they might be an informative model to study larval development in crustaceans