Three new species of sea star parasite Dendrogaster (Crustacea: Thecostraca) from Japan.

Three new species of ascothoracidan crustaceans, Dendrogaster danni sp. nov., Dendrogaster tanabensis sp. nov., and Dendrogaster jinshomaruae sp. nov. are described from the sea around the Kii Peninsula on the Pacific coast of central Japan. They are found in the coelomic cavities of the sea stars, Neoferdina japonica Oguro & Misaki, 1986., Henricia sp., and Coronaster volsellatus (Sladen, 1889), respectively. Morphological examinations and DNA barcoding analyses of these new species are reported in this study. The emergence of Dendrogaster from their host sea stars is also noted. These findings represent the 11th to 13th species of Dendrogaster that infest Japanese sea stars.

Carcinoecium-Forming Sea Anemone Stylobates calcifer sp. nov. (Cnidaria, Actiniaria, Actiniidae) from the Japanese Deep-Sea Floor: A Taxonomical Description with Its Ecological Observations.

Here we describe Stylobates calcifer sp. nov. (Cnidaria, Actiniaria, Actiniidae), a new carcinoecium-forming sea anemone from the deep-sea floor of Japan. Stylobates produces a carcinoecium that thinly covers the snail shells inhabited by host hermit crabs Pagurodofleinia doederleini. The new species is distinct from other species by the shape of the marginal sphincter muscle, the distribution of cnidae, the direction of the oral disk, and host association. The species' novelty is supported by the data of its mitochondrial genes 12S, 16S, and COIII and nuclear genes 18S and 28S. Also, we conducted behavioral observation of this new species, focusing on the feeding behavior and interaction with the specific host hermit crab. Our observations suggest that this sea anemone potentially feeds on the suspended particulate organic matter from the water column or the food residuals of hermit crabs. When the host's shell changed, intensive manipulation for transference of S. calcifer sp. nov. was recorded. However, although the hermit crab detached and transferred the sea anemone to the new shell after shell change, the sea anemone did not exhibit active or cooperative participation. Our data suggest that the sea anemone may not produce a carcinoecium synchronously to its host's growth, contrary to the anecdotal assumption about carcinoecium-forming sea anemones. Conversely, the host hermit crab's growth may not depend entirely on the carcinoecium produced by the sea anemone. This study is perhaps the first observation of the behavioral interaction of the rarely studied carcinoecium-forming mutualism in the deep sea.

Rare endoparasitic Asteriomyzostomum (Annelida: Asteriomyzostomidae) from Japan, including three new species descriptions and their phylogenetic position within Myzostomida.

The rare myzostomid genus Asteriomyzostomum Jägersten, 1940 consists of two species, both parasitizing sea stars. The phylogenetic position of this genus among Myzostomida has not been previously shown using molecular data. In this study, three species of Asteriomyzostomum were collected from the Kumano Sea, Japan, and are described as A. hercules sp. nov., A. jinshou sp. nov., and A. monroeae sp. nov. Additional specimens of the genus Asteromyzostomum Wagin, 1954 were also collected from the Kumano Sea and briefly reported as Asteromyzostomum sp. A molecular phylogeny based on four gene markers (COI, 16S, 18S, H3) suggests that the three families Asteriomyzostomidae, Asteromyzostomidae, and Protomyzostomidae comprise a clade. The resulting topology of the tree indicates that a host change from Crinozoa (sea lilies and feather stars) to Asterozoa (asteroids and ophiuroids) occurred only once in the evolutionary history of Myzostomida.

Loss of the six3/6 controlling pathways might have resulted in pinhole-eye evolution in Nautilus.

Coleoid cephalopods have an elaborate camera eye whereas nautiloids have primitive pinhole eye without lens and cornea. The Nautilus pinhole eye provides a unique example to explore the module of lens formation and its evolutionary mechanism. Here, we conducted an RNA-seq study of developing eyes of Nautilus and pygmy squid. First, we found that evolutionary distances from the common ancestor to Nautilus or squid are almost the same. Although most upstream eye development controlling genes were expressed in both species, six3/6 that are required for lens formation in vertebrates was not expressed in Nautilus. Furthermore, many downstream target genes of six3/6 including crystallin genes and other lens protein related genes were not expressed in Nautilus. As six3/6 and its controlling pathways are widely conserved among molluscs other than Nautilus, the present data suggest that deregulation of the six3/6 pathway led to the pinhole eye evolution in Nautilus.

Genome structure analysis of molluscs revealed whole genome duplication and lineage specific repeat variation.

Comparative genome structure analysis allows us to identify novel genes, repetitive sequences and gene duplications. To explore lineage-specific genomic changes of the molluscs that is good model for development of nervous system in invertebrate, we conducted comparative genome structure analyses of three molluscs, pygmy squid, nautilus and scallops using partial genome shotgun sequencing. Most effective elements on the genome structural changes are repetitive elements (REs) causing expansion of genome size and whole genome duplication producing large amount of novel functional genes. Therefore, we investigated variation and proportion of REs and whole genome duplication. We, first, identified variations of REs in the three molluscan genomes by homology-based and de novo RE detection. Proportion of REs were 9.2%, 4.0%, and 3.8% in the pygmy squid, nautilus and scallop, respectively. We, then, estimated genome size of the species as 2.1, 4.2 and 1.8 Gb, respectively, with 2× coverage frequency and DNA sequencing theory. We also performed a gene duplication assay based on coding genes, and found that large-scale duplication events occurred after divergence from the limpet Lottia, an out-group of the three molluscan species. Comparison of all the results suggested that RE expansion did not relate to the increase in genome size of nautilus. Despite close relationships to nautilus, the squid has the largest portion of REs and smaller genome size than nautilus. We also identified lineage-specific RE and gene-family expansions, possibly relate to acquisition of the most complicated eye and brain systems in the three species.

Evolution of the cephalopod head complex by assembly of multiple molluscan body parts: Evidence from Nautilus embryonic development.

Cephalopod head parts are among the most complex occurring in all invertebrates. Hypotheses for the evolutionary process require a drastic body-plan transition in relation to the life-style changes from benthos to active nekton. Determining these transitions, however, has been elusive because of scarcity of fossil records of soft tissues and lack of some of the early developmental stages of the basal species. Here we report the first embryological evidence in the nautiloid cephalopod Nautilus pompilius for the morphological development of the head complex by a unique assembly of multiple archetypical molluscan body parts. Using a specialized aquarium system, we successfully obtained a series of developmental stages that enabled us to test previous controversial scenarios. Our results demonstrate that the embryonic organs exhibit body plans that are primarily bilateral and antero-posteriorly elongated at stereotyped positions. The distinct cephalic compartment, foot, brain cords, mantle, and shell resemble the body plans of monoplacophorans and basal gastropods. The numerous digital tentacles of Nautilus develop from simple serial and spatially-patterned bud-like anlagen along the anterior-posterior axis, indicating that origins of digital tentacles or arms of all other cephalopods develop not from the head but from the foot. In middle and late embryos, the primary body plans largely change to those of juveniles or adults, and finally form a "head" complex assembled by anlagen of the foot, cephalic hood, collar, hyponome (funnel), and the foot-derived epidermal covers. We suggest that extensions of the collar-funnel compartment and free epidermal folds derived from multiple topological foot regions may play an important role in forming the head complex, which is thought to be an important feature during the body plan transition.