No abstract

The presence and status of harmful algal blooms (HABs) in Japan are reviewed, revealing a decrease in red tides; however, toxic blooms are found to be increasing in western Japan. Environmentally friendly control strategies against HABs are also compared with integrated agricultural pest management. Very high densities (105–108 CFU/g) of algicidal and growth-inhibiting bacteria were found in biofilm on seagrass and seaweed surfaces and in surrounding coastal seawater. The situation in freshwater ecosystems is similar to coastal seas for toxic cyanobacterium, Microcystis aeruginosa, and aquatic plants. These findings offer new insights into the ecology of influential bacteria and harmful algae, suggesting that protection and restoration of native seagrasses and seaweeds in coastal marine environments should be implemented to suppress HABs. Diatom blooms were successfully induced with bottom sediment perturbation to prevent the occurrence of harmful flagellates such as Chattonella spp. and Alexandrium catenella in the Seto Inland Sea; however, this method requires robust and reproducible verification. “Sato-Umi” is a helpful concept for HAB control in the sea and freshwater ecosystems when adequately managed by people (e.g., appropriate bottom perturbation; protection and restoration of seaweeds, seagrasses, and aquatic plants; application of polycultures of fish, seaweeds, etc.).

The gut contents of anguilliform leptocephali collected by research cruises in the western North Paciˆc were examined in order to investigate their food sources. Gut contents of 26 leptocephali (Anguillidae, Congridae and Serrivomeridae) examined were amorphous food material containing many small spherical particles. Alcian Blue (AB)-stained aggregates containing spherical particles approximately 2 10 mm in diameter were observed in 14 leptocephali from the 2015 survey, and Coomassie Brilliant Blue (CBB)-stained aggregates containing spherical particles about 2 mm in diameter were observed in 12 leptocephali from the 2016 survey. Red auto‰uorescence of phytoplankton was exhibited in several sized cells in amorphous food material. Furthermore, polysaccharide-containing transparent exopolymer particles (TEP), proteinaceous Coomassie stainable particles (CSP), and pico/ nano-planktonic particles about 2 mm in diameter were observed in environmental water samples. These results suggest that anguilliform leptocephali utilize phytoplankton-derived particulate organic matter (POM) as one of the food sources. Such feeding ecology makes it possible for anguilliform leptocephali to avoid food competition from predatory, zooplanktivorous and omnivorousˆshes that have higher trophic positions in the oligotrophic ocean. キーワードウナギ，凝集物，消化管内容物，餌料，多糖類，タンパク質，ピコナノプランクトン，レプトセ ファルス

Natural diets of leptocephalus larvae have been enigmatic. In this study, we collected DNA samples from the gut contents and body surface of leptocephali belonging to the five Anguilliform families (Anguillidae, Chlopsidae, Congridae, Muraenidae, and Serrivomeridae) from the northwest Pacific and performed next-generation 18S rDNA sequencing. Wide variety of eukaryotes was detected in both samples, from which eight eukaryotic groups (jellyfish, conoid parasite, tunicate, copepod, krill, segmented worm, fungi, and dinoflagellate) were selected on the basis of abundance. All groups except conoid parasites were common in both the samples. Cnidarian 18S rDNA reads were the most abundant in both the samples; however, the number of samples having cnidarian reads and the read counts were significantly higher in the body surface scraping samples than in the gut content samples, regardless of careful rinsing of the body surface. These results indicate that the cnidarian DNAs are most likely found because of cross contamination from the body surface and/or environment. 18S rDNA read counts of copepod and tunicate in the gut contents were greater than or comparable with those in the body surface scraping samples, which may correspond to the previous observations of fecal pellets and larvacean houses in the leptocephali gut. Thus, the present study supports previous implications that leptocephali utilize detritus materials, so called marine snow.