Evolutionary biological study on deep-sea hydrothermal barnacles as 'the living fossil'

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(1) A few years ago the most primitive living barnacles (so-called'living fossils') belonging to three major taxa within subclass Cirripedia were found as part of the extraordinary fauna associated with abyssal hydrothermal vents. They are Neolepas zevina Newman, 1979 (order Scalpellomorpha), Neoverruca brachylepadoformis Newman & Hessler, 1989 (order Verrucomorpha), and Eochionelasmus ohtai Yamaguchi, 1990 (order Balanomorpha) (Newman 1979, Newman & Hessler 1989, Yamaguchi & Newman 1990). The research reported here concerns additional forms of primitive barnacles from such environments. Neobrachylepas relica Newman and Yamaguchi, 1995, the first known living representative of the third order of sessile barnacles, Brachylepadomorpha, was found at a vent in the Lau Basin, southwest Pacific. A new species of Eochionelasmus was found off Easter Island in the east Pacific (Yamaguchi & Newman in press). Three geographically separated populations of Eochionelasmus ohtai from North Fiji, Lau … More and Manus Basins were compared in term of the arrangement of their shells' basal imbricating plates. The shell arrangement of the Manus population is different with that of the other two ; therefore, two subspecies of Eochionelasmus ohtai were recognized and described (Yamaguchi &Newman submitted).(2) Two vent barnacles, Eochionelasmus and Neoverruca, were recovered from vents in the Manus and Mariana Back-Arc Basins, respectively. The phylogenetic relationship between them was examined using parts of the nucleotide sequences of the 16S mitochondrial ribosomal RNA gene. The non-vent barnacle Capitulum mitella was representing the more pimitive scalpellomorphan cirripedes. The results implied that the Verrucomorpha (Neoverruca) were derived first from the Scalpellomorpha, and the Balanomorpha (Eochionelasmus) were derived from the Verrucomorpha. The relationship coincides with a phylogeny that had previously been inferred from the fossil record, adult morphology, ontogeny of larval development, and nervous system.(3) Yamaguchi & Newman (1990) considered Eochionelasmus to be more primitive than the generalized balanomorphan families Chthamalidae and Tetraclitidae, and to be the most primitive living balanomorphan barnacle. The inference has now been supported by a phylogenetic analysis using mitochondrial gene sequences (16S ribosomal RNA and COI) from 5 species of Chthamalidae and 7 species of Tetraclitidae. Also, the phylogenetic relationships among three species of Tetraclita (Tetraclitidae) has been described by Hasegawa (1996). Less