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  • Marine snow (TEP is the correct term?) and feeding of eel larvae

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    • ページ アイコン
      Description of TEP for acidic polysaccharides ページ

      Transparent Exopolymer Particle (TEP): Transparent extracellular polymeric particles are made of acidic polysaccharides. As explained below, algae are thought to produce, clothe, and shed TEP.


    • Eel larvae feeding on picoplankton attached to TEP 


      　The photosynthetic bacteria cyanobacteria, Prochlorococcus and Synechococcus, are dominant inhabitants of the oligotrophic subtropical ocean surface layer. They are the smallest primary producers, ranging in size from 0.2 to 2 μm. Plankton of this size are also called picoplankton. For Synechococcus, it has been found that it alone releases TEP and makes aggregate (Cruz and Neuer, Frontier in Microbiology, 2019).



      　Eels are thought to spawn in the subtropical Pacific Ocean and their larvae migrate just below the chlorophyll maximal layer in the oligotrophic subtropical waters. In that layer, there is a large amount of phytoplankton-derived TEP produced in the surface layer suspended in the water.


      ウナギ仔魚：Eel Larvae

      　When we examined the digestive tract of eel larvae, we found many pico phytoplankton-like fluorescent particles in addition to TEP-like gel material. (Pico phytoplankton, a photosynthetic bacteria, has photosynthetic pigments that emit a different wavelength of light, fluorescence, when exposed to light of a certain wavelength.)

      　Pico phytoplankton is a major primary producer in subtropical waters. Their cell membranes are composed of proteins, and although they are small, they are rich in nutrients. However, because their cell size is so small (about 1 to 2 μm), conventional research has thought that only protozoa prey on picophytoplankton.

      　Picophytoplankton attaches to TEP floating in the ocean surface layer, and it is present in high density even just below the surface. When eel larvae were collected in the subtropical Pacific Ocean and examined in the digestive tract, many gelatinous substances thought to be derived from TEP and fluorescent particles thought to be picophytoplankton were found (Tomoda et al., 2015).

      "Eel Larvae Ingest Marine Snow Origin Matters," Tomoda et al. Journal of the Japanese Society of Fisheries Science, 81, 715-721 (2015).

      　Tomoda et al, (2015) suggest that this is an important food resource for oligotrophic eel larvae. They also state that it provides important hints on how to raise eels during the larval stage, which is a problem in the complete aquaculture of eels.

      　Tomoda et al., (2015) is a publication in the Journal of the Fisheries Society of Japan, so they are focusing on it from the perspective of fisheries science, but from the perspective of oceanography, which is my specialty, it is very interesting because it provides a new route on the mass transport mechanism in oligotrophic waters.

    • URL アイコン
      Link to the course "Life History of Eels" URL



    • URL アイコン
      Download "Eel Larvae Ingest Marine Snow Origin Matters," Tomoda et al. (2015) URL

      TEP floating in the ocean is attached to other organic particles (e.g., zooplankton feces) and is also home to a number of microorganisms. It is considered a valuable food resource for microorganisms. TEP-like gelatinous material is often found in the digestive tract of eel larvae.


    • URL アイコン
      Download "Observations on the Digestive Tract Contents of Eel Leptocephalus Sampled in the Western North Pacific Ocean," Tomoda et al., Journal of the Japanese Society of Fisheries Science, (2018). URL

      Eel larvae are found at high densities just below the chlorophyll maximam layer. In that layer, there is a large amount of phytoplankton-derived TEP produced in the surface layer. In addition to TEP-like gel material, many picophytoplankton-like fluorescent particles were found in the digestive tract of eel larvae. Picophytoplankton is a major primary producer in subtropical waters. Their cell membranes are rich in nutrients, for example, they are composed of proteins. However, because the cell size is very small, about 1 to 2 μm, previous studies have thought that only protozoa prey on picophytoplankton. This study suggests that picophytoplankton attach to TEP floating in the ocean surface layer and exist at high density even just below the surface layer, where they become food for eel larvae.

      I thought this research is very interesting from the viewpoint of oceanic material cycles.



    • URL アイコン
      Download "Trophic Dynamics of Pico and Nanoplankton in an Oligotrophic Sea Area" by SATO Mitsuhide URL

      　We touched on the topic of picophytoplankton feeding by eel larvae. Speaking of "picophytoplankton" , Dr. SATO Mitsuhide of the University of Tokyo. Here is his paper in Japanese. He wrote about the feeding of picophytoplankton in detail.


    • ページ アイコン
      Discovering marine snow ページ

      　It was Dr. Inoue of Hokkaido University's School of Fisheries Science who discovered and named marine snow. I would like to introduce a little of the background of this discovery.


      　　Kuroshio, 1941


    • Marine snow

      　Deep in the ocean, there is a layer with large amounts of particles floating in it. In coastal areas with high biological productivity, the closer one gets to the seafloor, the more particles there are. When viewed in the light, these particles look like snow, so we call them "marine snow. Sometimes the marine snow settles down all at once.

      　The video below is an underwater view taken during a multiple core observation (※) in the northern Bering Sea on the Oshoro Maru voyage. The depth of the seafloor is 45 m in the shallow shelf area. The white specks are particulate organic matter called marine snow. Even if you sample a bottle of bottom water filled with this kind of marine snow and put it in a beaker on board, it looks completely transparent. It is a strange thing. Here are two reasons (imagined by the author, OOKI) why marine snow is not visible in a beaker.

      １：The vibration caused by the closing of the lid of the water sampler used in the observation, and the flow of water out of the sampler's cock causes the particles to shatter instantly and become invisible.

      ２：Marine snow is a particle that reflects light when shone on in total darkness, so you can't see it in bright light.

      （The hypotheses (1 and 2) have never been tested (as far as I know). Can someone please confirm this for me?)



      　The echinoderm "sand dollar" is said to be found around Hokkaido, but there are also many in the northern Bering Sea. Not all of the Bering Sea land shelf is covered with "sand dollar". Some areas are covered with "sand dollar", some with "brittle star", some with "seemingly few organisms," and so on (perhaps) in patches.

      ※　Multiple core observation: An observation in which sediments are sampled in multiple cores.



    • URL アイコン
      Link to Arctic ocean summary course URL

      Some of the waters of the Pacific Ocean travel northward through the Bering Sea shelf area into the Chukchi Sea (Arctic Ocean).

      Get to know the Arctic Ocean.

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    Thank you for your hard work.