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
Sargassum species grow on rocks and dead coral, forming dense seaweed beds in their growth seasons. Sargassum beds play a range of ecological roles in coastal waters, but their sustainability is threatened by pollution and reclamation within ASEAN countries with rapid economic growth. Before conservation initiatives can be implemented, it is necessary to establish their present distribution. These beds can be mapped using the noncommercial satellite, ALOS, which can provide multiband images using high spatial resolution optical sensors (sensitive to 10 m-2 plots), AVNIR-2. These images are of sufficient quality for examining coastal ecosystems and, in this study, for mapping the Sargassum beds in the waters off the coast of Sattahip, Chon Buri Province, Thailand. This coastal zone is a natural marine park reserve and protected from human activities such as fishery and reclamation. Biomass data were obtained in January, February, March, and December 2009 from quadrat sampling. The biomass of Sargassum aquifolium (Turner) C.Agardh at a bottom depth of 1 m and 1.5–2 m was determined to be 7.73 and 92.75 g dw m-2, while that of Sargassum oligocystum Montagne was found to be 44.05 and 87.97 g dw m-2, respectively. Ground truth data were obtained in February and October 2012 from serial images taken by manta tow. Supervised classification is a procedure for identifying spectrally similar areas on an image by identifying “training” sites of known targets and then extrapolating those spectral signatures to other areas of unknown targets. By applying this methodology, the Sargassum beds off Sattahip can be detected with an accuracy of about 70%. It is estimated that the error is caused by mixel effects of the bottom substrates in individual pixels, each of which covers an area of 10×10 m. Our results indicate that the images captured by ALOS AVNIR-2 are informative and useful for mapping the Sargassum beds in Southeast Asia.
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