Takenori Sasaki

Novel Chemoautotrophic Endosymbiosis between a Member of the Epsilonproteobacteria and the Hydrothermal-Vent Gastropod Alviniconcha aff. hessleri (Gastropoda: Provannidae) from the Indian Ocean

ABSTRACT The hydrothermal-vent gastropod Alviniconcha aff. hessleri from the Kairei hydrothermal field on the Central Indian Ridge houses bacterium-like cells internally in its greatly enlarged gill. A single 16S rRNA gene sequence was obtained from the DNA extract of the gill, and phylogenetic analysis placed the source organism within a lineage of the epsilon subdivision of the Proteobacteria. Fluorescence in situ hybridization analysis with an oligonucleotide probe targeting the specific epsilonproteobacterial subgroup showed the bacterium densely colonizing the gill filaments. Carbon isotopic homogeneity among the gastropod tissue parts, regardless of the abundance of the endosymbiont cells, suggests that the carbon isotopic composition of the endosymbiont biomass is approximately the same as that of the gastropod. Compound-specific carbon isotopic analysis revealed that fatty acids from the gastropod tissues are all 13C enriched relative to the gastropod biomass and that the monounsaturated C16 fatty acid that originates from the endosymbiont is as 13C enriched relative to the gastropod biomass as that of the epsilonproteobacterial cultures grown under chemoautotrophic conditions. This fractionation pattern is most likely due to chemoautotrophy based on the reductive tricarboxylic-acid (rTCA) cycle and subsequent fatty acid biosynthesis from 13C-enriched acetyl coenzyme A. Enzymatic characterization revealed evident activity of several key enzymes of the rTCA cycle, as well as the absence of ribulose-1,5-bisphosphate carboxylase/oxygenase activity in the gill tissue. The results from anatomic, molecular phylogenetic, bulk and compound-specific carbon isotopic, and enzymatic analyses all support the inference that a novel nutritional strategy relying on chemoautotrophy in the epsilonproteobacterial endosymbiont is utilized by the hydrothermal-vent gastropod from the Indian Ocean. The discrepancies between the data of the present study and those of previous ones for Alviniconcha gastropods from the Pacific Ocean imply that at least two lineages of chemoautotrophic bacteria, phylogenetically distinct at the subdivision level, occur as the primary endosymbiont in one host animal type.

Host-Symbiont Relationships in Hydrothermal Vent Gastropods of the Genus Alviniconcha from the Southwest Pacific

ABSTRACT Hydrothermal vent gastropods of the genus Alviniconcha are unique among metazoans in their ability to derive their nutrition from chemoautotrophic γ- and ε-proteobacterial endosymbionts. Although host-symbiont relationships in Alviniconcha gastropods from the Central Indian Ridge in the Indian Ocean and the Mariana Trough in the Western Pacific have been studied extensively, host-symbiont relationships in Alviniconcha gastropods from the Southwest Pacific remain largely unknown. Phylogenetic analysis using mitochondrial cytochrome c oxidase subunit I gene sequences of host gastropods from the Manus, North Fiji, and Lau Back-Arc Basins in the Southwest Pacific has revealed a new host lineage in a Alviniconcha gastropod from the Lau Basin and the occurrence of the host lineage Alviniconcha sp. type 2 in the Manus Basin. Based on 16S rRNA gene sequences of bacterial endosymbionts, two γ-proteobacterial lineages and one ε-proteobacterial lineage were identified in the present study. The carbon isotopic compositions of the biomass and fatty acids of the gastropod tissues suggest that the γ- and ε-proteobacterial endosymbionts mediate the Calvin-Benson cycle and the reductive tricarboxylic acid cycle, respectively, for their chemoautotrophic growth. Coupling of the host and symbiont lineages from the three Southwest Pacific basins revealed that each of the Alviniconcha lineages harbors different bacterial endosymbionts belonging to either the γ- or ε-Proteobacteria. The host specificity exhibited in symbiont selection provides support for the recognition of each of the host lineages as a distinct species. The results from the present study also suggest the possibility that Alviniconcha sp. types 1 and 2 separately inhabit hydrothermal vent sites approximately 120 m apart in the North Fiji Basin and 500 m apart in the Manus Basin.

Gastropods from Recent Hot Vents and Cold Seeps: Systematics, Diversity and Life Strategies

Since the first discovery of hydrothermal vents at the Galapagos Spreading Center in 1977, gastropods have gained considerable attention as a major constituent of the chemosynthesis-based biological communities, especially the colonies of large species like Alviniconcha, Ifremeria and Lepetodrilus, or morphologically strange ones like the scaly-foot snail. Various types of symbiotic relations to bacteria have broadened the interest in them. During more than 30 years numerous vent and seep biotopes have been found mainly in temperate seas, but recently hydrothermal vents off Jan Mayen in the North Atlantic at 80°N and cold seeps off Norway between 67°N and 70°N (Haakon Mosby Mud Volcano) with chemosynthesis-based fauna (see Waren and Bouchet 2001; Desbruyeres et al. 2006: 516–517 for map). Biology of these vent/seep organisms is still in an early state of exploration; much of what is known is summarized in the monographs by Van Dover (2000) and Desbruyeres et al. (2006). Vent/seep taxa have also been a major target in research on higher phylogeny and systematics of gastropods. Several new families were established in the 1980s, based on seemingly “great differences” in morphology.

The shell structure of the Recent Patellogastropoda (Mollusca: Gastropoda)

ABSTRACT The shell microstructure of 44 species belonging to 19 genera and 5 families of Patellogastropoda was observed by scanning electron microscopy on the basis of material mainly from the Northwest Pacific. As a result, 17 microstructures of prismatic, crossed, and lamellar structures were recognized. The comparison among species revealed 20 shell structure groups which are defined by microstructures and shell layer arrangement. The relations between taxa and shell structural composition indicate that the Recent patellogastropods generally have distinctive and stable shell structures at the genus level. This high level of consistency provides a firm basis for the application of shell structural characters to identify fossil patellogastropods. However, the evolutionary process of microstructures and homology across different shell layers are mostly ambiguous in the absence of robust phylogeny and undoubted positional criteria for comparison. More studies from phylogenetic, ontogenetic and mineralogical viewpoints should be undertaken to discuss the process of shell structure diversification in patellogastropods.

Central Nervous System of Chaetoderma japonicum (Caudofoveata, Aplacophora): Implications for Diversified Ganglionic Plans in Early Molluscan Evolution

The organization of the central nervous system of an “aplacophoran” mollusc, Chaetoderma japonicum, is described as a means to understand a primitive condition in highly diversified molluscan animals. This histological and immunocytochemical study revealed that C. japonicum still retains a conservative molluscan tetra-neural plan similar to those of neomenioids, polyplacophorans, and tryblidiids. However, the ventral and lateral nerve cords of C. japonicum are obviously ganglionated to various degrees, and the cerebral cord-like ganglia display a lobular structure. The putative chemosensory networks are developed, being composed of sensory cells of the oral shield, eight precerebral ganglia, and eight neuropil compartments that form distinct masses of neurites. In the cerebral cord-like ganglia, three anterior, posterior, and dorsal lobes are distinguished with well-fasciculated tracts in their neuropils. Most neuronal somata are uniform in size, and no small globuli-like cell clusters are found; however, localized serotonin-like immunoreactivity and acetylated tubulin-containing tracts suggest the presence of functional subdivisions. These complicated morphological features may be adaptive structures related to the specialized foraminiferan food in muddy bottoms. Based on a comparative scheme in basal molluscan groups, we characterize an independent evolutionary process for the unique characters of the central nervous systems of chaetoderms.

Color Polymorphism and Historical Biogeography in the Japanese Patellogastropod Limpet Cellana nigrolineata (Reeve) (Patellogastropoda: Nacellidae)

Cellana nigrolineata is amongst the most common and largest patellogastropod limpets in Japan, and has two color morphs. Analyses of anatomical and morphological characters, shell structure, and mitochondrial COI data (658bp) of these color morphs suggested that they represent intraspecific genetic variation. Molecular analysis demonstrated that the species can be subdivided into three genetically distinct groups: (Clade 1) Honshu, Shikoku to Eastern Kyushu, (Clade 2) Western Kyushu and (Clade 3) Southern Kyushu. Clade 1 and Clade 2 + 3 are distributed on the coastlines adjacent to two warm-water currents, the Kuroshio and Tsushima Currents, respectively. The southern population (Clade 3) is currently isolated by inhospitable sandy shores. The subdivision of these groups likely dates from the glacial period of Plio-Pleistocene time.

Pre-Bomb Marine Reservoir Ages in the Western Pacific

In this study, molluscan shells housed at the University Museum, the University of Tokyo, provided a new set of region-specific correction values ( Δ R) for the western Pacific, in particular for the central part of the main islands in the Japanese Archipelago and the southwest islands of Japan. The values of 40 total samples were calculated from 11 regions. North of the main islands and in the Ryukyu Islands, the mean Δ R values showed comparatively small values, 5-40 14C yr; in the central part of the main islands, these values were 60-90 14C yr.

An In-silico Genomic Survey to Annotate Genes Coding for Early Development-relevant Signaling Molecules in the Pearl Oyster, Pinctada fucata

The pearl oyster Pinctada fucata has great potential as a model system for lophotrochozoan developmental biology research. Pinctada fucata is an important commercial resource, and a significant body of primary research on this species has emphasized its basic aquaculture biology such as larval biology and growth, aquaculture, pearl formation and quality improvement, shell formation, and biomineralization. Recently, a draft genome sequence of this species was published, and many experimental resources are currently being developed, such as bioinformatics tools, embryo and larva manipulation methods, gene knockdown technique, etc. In this paper, we report the results from our genomic survey pertaining to gene families that encode developmental signaling ligands (Fgf, Hedgehog, PDGF/VEGF, TGFβ, and Wnt families). We found most of the representative genes of major signaling pathways involved in axial patterning, as well as copies of the signaling molecule paralogs. Phylogenetic character mapping was used to infer a possible evolutionary scenario of the signaling molecules in the protostomes, and to reconstruct possible copy numbers of signaling molecule-coding genes for the ancestral protostome. Our reconstruction suggests that P. fucata retains the ancestral protostome gene complement, providing further justifications for the use of this taxon as a model organism for developmental genomics research.

New Molluscan Larval Form: Brooding and Development in a Hydrothermal Vent Gastropod, Ifremeria nautilei (Provannidae)

Despite extreme differences between some shallow and deep-sea habitats, the developmental modes and larval forms of deep-sea animals are typically similar to those of their shallow-water relatives. Here we report one of the first documented exceptions to this general rule. The hydrothermal vent snail Ifremeria nautilei displays two novel life-history traits: (1) an unusual uniformly ciliated larva that we here name Warén’s larva, and (2) internal brood protection in a modified metapodial pedal gland. Warén’s larva emerges from the internal brood pouch as a fully ciliated lecithotrophic larva with a unique external cuticle. The larvae swim with their posterior end forward and metamorphose into typical veliger larvae after 15 days at room temperature. Warén’s larva is the only known example of a free-swimming pre-veliger larval stage in the higher gastropods and is the first new gastropod larval form to be described in more than 100 years.

Thermal dependency of shell growth, microstructure, and stable isotopes in laboratory-reared Scapharca broughtonii (Mollusca: Bivalvia)

We experimentally examined the growth, microstructure, and chemistry of shells of the bloody clam, Scapharca broughtonii (Mollusca: Bivalvia), reared at five temperatures (13, 17, 21, 25, and 29°C) with a constant pCO2 condition (∼450 μatm). In this species, the exterior side of the shell is characterized by a composite prismatic structure; on the interior side, it has a crossed lamellar structure on the interior surface. We previously found a negative correlation between temperature and the relative thickness of the composite prismatic structure in field-collected specimens. In the reared specimens, the relationship curve between temperature and the growth increment of the composite prismatic structure was humped shaped, with a maximum at 17°C, which was compatible with the results obtained in the field-collected specimens. In contrast, the thickness of the crossed lamellar structure was constant over the temperature range tested. These results suggest that the composite prismatic structure principally accounts for the thermal dependency of shell growth, and this inference was supported by the finding that shell growth rates were significantly correlated with the thickness of the composite prismatic structure. We also found a negative relationship between the rearing temperature and δ18O of the shell margin, in close quantitative agreement with previous reports. The findings presented here will contribute to the improved age determination of fossil and recent clams based on seasonal microstructural records.