Phylogenetic position, complete larval development and larval sexual dimorphism in a rhizocephalan barnacle, Lernaeodiscus rybakovi sp. nov. (Cirripedia: Rhizocephala: Peltogastridae), parasitizing the crab Pachycheles stevensii Stimpson, 1858 (Decapoda: Anomura: Porcellanidae)

Molecular and morphological methods are used to describe Lernaeodiscus rybakovi, a new rhizocephalan species parasitizing the porcellanid crab Pachycheles stevensii Stimpson, 1858, collected in Russian waters of the Sea of Japan. Molecular analysis of three species, including the new one, confirms the monophyly of the genus Lernaeodicus Muller, 1862 and its recent transfer to the family Peltogastridae. The main morphological features of the new species are also common with characters of the other species of the genus Lernaeodiscus Muller, 1862. Externa of L. rybakovi differs from the well studied Lernaeodiscus porcellanae from the E. Pacific by molecular markers, color and the absence of pronounced marginal lobes. Retinacula on the internal cuticle of L. rybakovi are found in the genus Lernaeodiscus for the first time. Some of the female hosts with adult externae were unusual in also carrying their own eggs on the pleopods, a rare situation among rhizocephalans. The complete larval development in the genus Lernaeodiscus is described here for the first time and includes five naupliar and one cypris instar. The main morphological features of Lernaeodiscus nauplii (the presence of flotation collar, morphology of the frontolateral horns and furcal spines, and the arrangement of dorsal setae on the shield head) are common with those of other known peltogastrid larvae, but also resemble nauplii of Peltogasterella in the presence of naupliar eyes and thin structure of the flotation collar. The sizes of male and female cyprids of L. rybakovi overlap slightly. In summer months, larval sex ratio is male-biased. We briefly review important larval characters in the Rhizocephala. (c) 2020 Elsevier GmbH. All rights reserved.