Kotaro Tsuchiya

Bioconcentration and biomagnification of polybrominated diphenyl ethers (PBDEs) through lower-trophic-level coastal marine food web.

Bivalves, crabs, fishes, seawater, and sediment collected from the inner part of Tokyo Bay, Japan, were measured for 20 polybrominated diphenyl ether (PBDE) and 5 polychlorinated biphenyl (PCB) congeners. To determine the trophic levels of the organisms, carbon and nitrogen stable isotope ratios (delta(13)C and delta(15)N) were also measured. Bioconcentration factors of PBDE and PCB congeners increased as the octanol-water partition coefficient (K(ow)) rose to log K(ow)=7, above which they decreased again. Biomagnification of PCBs and several PBDE congeners (BDE47, 99, 100, 153 and 154) up the trophic ladder was confirmed by a positive correlation between their concentrations and delta(15)N. Other PBDE congeners showed a negative or no correlation, suggesting their biotransformation through metabolism. The more hydrophobic congeners of both PBDEs (Br=2-6) and PCBs (Cl=6-9) were biomagnified more. It thus appears that PBDEs are less biomagnified than PCBs.

Embryonic and paralarval development of the central nervous system of the loliginid squid Sepioteuthis lessoniana.

The embryonic development of the central nervous system (CNS) in the oval squid Sepioteuthis lessoniana is described. It has three distinct phases: (1) The ganglionic accumulation phase: Ganglionic cell clusters develop by ingression, migration, and accumulation of neuroblasts. (2) The lobe differentiation phase: Ganglia differentiate into lobes. The phase is identified by the beginning of an axogenesis. During this phase, neuropils are first formed in the suboesophageal mass, then in the basal lobe system, and finally in the inferior frontal lobes and the superior frontal‐vertical lobe systems. (3) The neuropil increment phase: After the shape of the lobes reached its typical form, neuropil growth occurs, specifically in the vertical lobe. The paralarval central nervous system (CNS) is characterized by neuronal gigantism of the giant fibers and some suboesophageal commissures and connectives. The neuropil formation in the CNS of S. lessoniana occurs somewhat earlier than in Octopus vulgaris, although the principal developmental plan is quite conservative among the other coleoids investigated. Some phylogenetic aspects are discussed based on the similarities in the morphologic organization of their brains. J. Comp. Neurol. 437:449–475, 2001.

Early Ontogeny of the Japanese Common Squid Todarodes pacificus (Cephalopoda, Ommastrephidae) with Special Reference to its Characteristic Morphology and Ecological Significance

Abstract Early ontogeny of the Japanese Common Squid Todarodes pacificus was described for artificially inseminated and collected specimens to present new criteria for developmental stages in relation to its ecological adaptation. For the purpose, details for formation of the following organs and tissues were observed with special attention: cilia on the integument, mouth part, shell sac and stellate ganglia, visceral mass, funnel-collar complex, statocysts, eye parts, and ventral photosensitive vesicles. At the embryonic stage (i.e., pre-hatching), various types of epidermal cilia that seem to work as the embryonic rotation were detected. At the early postembryonic stage (i.e., post-hatching), the epidermal lines were characteristically arranged at the scattered condition on arms, tentacles, head, and funnel. Novel strong muscle fibers were distinct in the base of tentacles and funnel retractor muscles at the early postembryonic stage, which is clearly related to the head withdrawal behavior of the paralarvae. The lip cilia and toothed beak developed at the early postembryonic stage, but disappeared later; these apparatus were considered to be related with a change of unique feeding mode in the paralarval life. Based on such morphological features, four distinct stages, namely, paralarval stage 1, 2, 3, and juvenile stage are proposed. The present observations are discussed in relation to survival strategy at early life of T. pacificus and they are compared with those in other cephalopods.

Development of the Brain in the Oegopsid Squid, Todarodes pacificus: An Atlas Up to the Hatching Stage

Abstract An atlas of the developing brain up to hatching stage is established using conventional histo-logical methods in the oegopsid squid, Todarodes pacificus. The brain originates from placodal thickenings in the ectoderm at the end of epiboly. The neuroblasts composing the placodes ingress in a group and accumulate into ganglia under the proximal surface of the surface epithelium. Four pairs of the ganglia, pedal, palliovisceral, cerebral, and optic, form the brain primordium. These ganglia come into contact with one another, and eventually accumulate into a ring-like cluster (circumesophageal cluster) encircling the oral ingrowth and the inner yolk around the surface of the head. The circumesophageal cluster regionally differentiates into brain lobe anlagen through formation of neuropiles and nerve tracts. The neuropiles form a ladder-like structure with two longitudinal columns situated in the ventrolateral parts of the circumesophageal mass and some axonal tracts bridging the left and right columns (ladder-like framework). The brain is quite premature at the time of hatching, especially in the supraesophageal part. Though most brain nerves are already present, many brain lobes and commissures found in the adult brain are not yet differentiated. The present results show that the morphological processes of the brain formation are essentially common among the coleoid cephalopods. The similarity of the embryonic brain in Todarodes to the adult brain in Nautilus suggests that the coleoid brain has evolved on the basic plan as seen in the nautiloid brain.

Oocyte development of Metapenaeopsis dalei (Penaeidae, Decapoda, Crustacea)

Summary Oocyte development in the small penaeid shrimp Metapenaeopsis dalei was studied histologically. Seven stages were distinguished, namely (1) oocytes 10μm in diameter are strongly basophilic and spherical, (2) weakly basophilic ooplasm appears around the germinal vesicle, (3) follicle cells appear, (4) lipid globules appear in the ooplasm, (5) yolk granules start to appear in the ooplasm, (6) the germinal vesicle shrinks and migrates marginally, (7) the follicle cells disappear. This is the first report of a fully matured penaeid oocyte without cortical crypts which release materials forming a jelly coat when the eggs are exposed to seawater. Since ovulated oocytes occur in the same section of the ovary as earlier stages, M. dalei is concluded to be a multiple spawner which has short spawning intervals.

Biomagnification and debromination of polybrominated diphenyl ethers in a coastal ecosystem in Tokyo Bay

By field sampling and laboratory experiments we compared the mechanisms by which polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) are biomagnified. We measured PBDEs and PCBs, together with stable carbon and nitrogen isotopes as an index of trophic level, in low-trophic-level organisms collected from a coastal area in Tokyo Bay. PBDEs were biomagnified to a lesser degree than PCBs. The more hydrophobic congeners of each were biomagnified more. However, the depletion of BDE congeners BDE99 and BDE153 from fish was suggested. To study congener-specific biotransformation of halogenated compounds, we conducted an in vitro experiment using hepatic microsomes of two species of fish and five BDE congeners (BDE47, 99, 100, 153, and 154) and five CB congeners with the same substitution positions as the PBDEs. BDE99 and 153 were partially debrominated, but BDE47 and 154 were not debrominated. This congener-specific debromination is consistent with the field results. Both in vitro and field results suggested selective debromination at the meta position. The CB congeners were not transformed in vitro. This result is also consistent with the field results, that PCBs were more biomagnified than PBDEs. We conclude that metabolizability is an important factor in the biomagnification of chemicals, but other factors must be responsible for the lower biomagnification of PBDEs in natural ecosystems.

Development of the Brain in the Oegopsid Squid, Todarodes pacificus: An Atlas from Hatchling to Juvenile

Abstract Post-hatching development of the brain in the oegopsid squid, Todarodes pacificus was described using conventional histological and Cajals silver impregnation methods. The oegopsid squids spend a specific paralarval period before attaining adult-like juveniles. In the just-hatched paralarvae, the brain lobes (lower and intermediate motor centers) are differentiating only in the ventral part of the brain (subesophageal mass, SBM), and development of the dorsal part of the brain (supraesophageal mass, SPM) shows a heterochronic delay. In the SPM, an arched bundle of axonal tracts (transverse arch, TA) crosses the region over the oral ingrowth. In the early paralarval period, the basal lobes and precommissural lobe (higher motor centers) begin to develop along the TA. A little later, a pair of longitudinal axonal tracts (supraesophageal ladder, SPRL) elongates anteriorly from the TA, and accessory lobes (centers for memory and learning) and superior buccal lobes begin to differentiate along the SPRL. In the mid paralarval period, the lobes of the olfactory center and the peduncle lobe develop well in each optic tract region. In the late paralarvae, all brain lobes become identifiable and the brain shows substantially the same organization as that in the adults. The dorsal-most region of the SPM largely increases in volume with striking growth of the accessory lobes. The SBM elongates in anterior and posterior directions and the rostral end (anterior SBM) separates from the middle SBM. The optic lobes become very large with neuropils arranged in layers. In the juveniles, the neuropils increase in relative volume to the perikaryal layers, and neuronal somata enlarge markedly in some lobes. The retarded development of higher motor centers during paralarval development suggests that the early paralarvae of T. pacificus are not active predators but suspension feeders.

Allozyme variation of littleneck clam Ruditapes philippinarum and genetic mixture analysis of foreign clams in Ariake Sea and Shiranui Sea off Kyushu Island, Japan

Allozyme variation of the littleneck clam Ruditapes philippinarum was evaluated in four samples from Nameishi and Matsuo in the Ariake Sea, Ryugatake and Ushibuka in the Shiranui Sea off Kyushu Island, Japan, and in one sample from Jinzhou, China, in the Bohai Sea. A Ruditapes bruguieri sample imported from the Korean Bay off Nampo, North Korea was also studied. Among the R. philippinarum samples, heterozygosity varied from 0.265 to 0.301 and Fis estimates indicated significant homozygosity excess in 15 of 40 loci analyzed. Deviations from Hardy-Weinberg equilibrium were significant in all samples (P<0.05). Pairwise FST estimates indicate that genetic differences between the Chinese and Japanese samples were very low, but significantly different from zero. Mixture proportions with 95% confidence intervals of Chinese R. philippinarum in Nameishi and Matsuo were estimated at 0.4098 [0.2512, 0.5705] and 0.4899 [0.3262, 0.6540], respectively. However, genetic invasion of stocked Chinese R. philippinarum into wild populations in the Ariake Sea remains uncertain due to the low precision of the estimates caused by the high similarity of allele frequencies between Jinzhou and the Ariake Sea.

The complete mitochondrial genomes of deep-sea squid (Bathyteuthis abyssicola), bob-tail squid (Semirossia patagonica) and four giant cuttlefish (Sepia apama, S. latimanus, S. lycidas and S. pharaonis), and their application to the phylogenetic analysis of Decapodiformes

We determined the complete mitochondrial (mt) genomes of the deep-sea squid (Bathyteuthis abyssicola; supperfamily Bathyteuthoidea), the bob-tail squid (Semirossia patagonica; order Sepiolida) and four giant cuttlefish (Sepia apama, S. latimanus, S. lycidas and S. pharaonis; order Sepiida). The unique structures of the mt genomes of Bathyteuthis and Semirossia provide new information about the evolution of decapodiform mt genomes. We show that the mt genome of B. abyssicola, like those of other oegopsids studied so far, has two long duplicated regions that include seven genes (COX1-3, ATP6 and ATP8, tRNA(Asn), and either ND2 or ND3) and that one of the duplicated COX3 genes has lost its function. The mt genome of S. patagonica is unlike any other decapodiforms and, like Nautilus, its ATP6 and ATP8 genes are not adjacent to each other. The four giant cuttlefish have identical mt gene order to other cuttlefish determined to date. Molecular phylogenetic analyses using maximum likelihood and Bayesian methods suggest that traditional order Sepioidea (Sepiolida+Sepiida) is paraphyletic and Sepia (cuttlefish) has the sister-relationship with all other decapodiforms. Taking both the phylogenetic analyses and the mt gene order analyses into account, it is likely that the octopus-type mt genome is an ancestral state and that it had maintained from at least the Cephalopoda ancestor to the common ancestor of Oegopsida, Myopsida and Sepiolida.

Early development of young brooded in the enteron of the beadlet sea anemone Actinia equina (Anthozoa: Actiniaria) from Japan

Summary Histological observations were made to reveal the origin of young brooded in the enteron of adult Actinia equina from Japan. A total of 295 specimens were collected from four rocky intertidal areas of Sagami Bay and eastern Suruga Bay, Pacific coast of Japan, during the period from February 1994 to January 1995. In the enteron of adults, regardless of sex, 910 young individuals were found. Among them, blastulae with a structure resembling a syncytial blastoderm were found. Development of blastulae to young with tentacles was inferred. Early embryos were observed in both sexes all year round, although the spawning season of A. equina from Japan was restricted to early summer. This observation suggests that early embryos observed during the non-spawning season are produced asexually. Somatic embryogenesis is suggested as the mode of asexual reproduction of A. equina from Japan because of the existence of blastulae in the enteron of anemones in both sexes.