We determined the complete mitochondrial genome sequences of an asteroid Linckia laevigata belonging to the order Valvatida. The complete mitogenome of L. laevigata was 16,371 bp in length and consisted of 13 protein-coding genes (PCGs), two rRNA, and 22 tRNA. The orders of PCGs and rRNAs were identical to those of the recorded mitogenomes of asteroids. Phylogenetic analyses placed L. laevigata as the sister group to the species of the other Paxillosida.
Our recent morphological studies on the echinoderm collection of the National Museum of Nature and Science, Tokyo (NMST), indicated that the goniasterid starfishes Ceramaster japonicus (Sladen, 1889) and Ceramaster patagonicus (Sladen, 1889) are distributed in the Pacific Oceanside of Japan. In this study, we studied the NMST samples of C. japonicus and C. patagonicus from Japan, by using two mitochondrial DNA genes, the COI and 12S-rRNA, as markers to test relationships between these species in Japan. C. patagonicus sequences from GenBank were mined and included in the analyses. Results of phylogenetic and haplotype network analyses of both genes (final sequence lengths: COI = 317 bp, 12S = 477 bp) suggested that "Ceramaster patagonicus" and "Ceramaster japonicus" from Japanese waters are almost certainly synonymous without any population structure inside Japan.
A new commensal ostracod Asterositus ohtsukai n. g., n. sp. (Podocopida: Cytheroidea: Paradoxostomatidae) is described. This new taxon occurs on the ambulacral grooves and wreath of pedicellariae around superomarginal spines of the sea star Sclerasterias euplecta (Fisher) (Asteroidea: Forcipulatida: Asteriidae). Although the family Paradoxostomatidae Brady & Norman, 1889 contains four ectoparasitic or commensal genera, the association with asteroideans has never hitherto been reported. Morphological observations suggested that A. ohtsukai n. g., n. sp. is closely associated with the host sea star as it typically has distal hooks on the antenna and fifth to seventh limbs, and specialised mandibula and maxillula structures that are considered adaptations to a commensal life-style. In addition, this study discussed the feeding habits of A. ohtsukai n. g., n. sp. based on comparison with previous works. The extremely reduced palp and endites of maxillula of A. ohtsukai n. g., n. sp. imply that they have unique feeding habits that are not based on the feeding function of maxillula. From morphological comparisons with siphonostomatoid copepods, we suggest that A. ohtsukai n. g., n. sp. may feed on the body of host sea star by injuring them with styliform needle-like mandibular coxa and sucking the tissues, body fluid, or mucus with the suctorial disc on the oral cone.
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