Cell selection technique for establishment of low salinity tolerance strain in Pyropia tenuipedalis

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Pyropia tenuipedalis is characterized by reddish thalli and direct budding from the shell substratum. The distribution of this species is limited to coastal areas in the Seto Inland Sea,Ise Bay and Tokyo Bay in Japan. In Yamaguchi prefecture, this species has been locally used for direct human consumption. Moreover, alanine and glutamic acid contents of P. tenuipedalis are three times and two times higher, respectively than those of P. yezoensis “nori”. Yamaguchi Prefectural Fisheries Research Center began development of the mariculture technique of P. tenuipedalis in 2002, and succeeded in its commercialization in 2007. However, production has continued to decrease since 2012. More recently, intense disappearance of thalli is observed during January to February. One of the reasons for the production decrease is thought to be low salinity due to recent heavy rain fall supposedly associated with global climate change. The aim of this study was to develop low salinity tolerance strains of P. tenuipedalis through a cell selection technique. Punched out disks of thallus were cultured in a 10% seawater medium diluted with distilled water with modified 1/2SWM-III medium for 1 to 4 months. A few cells survived in this medium and were subsequently cultured in a 100% seawater medium with 1/2SWMIII. Surviving cells divided and regrew to thallus, and stock culture strains were established with self-fertilization. A field culture trial of the established low salinity tolerance strain and the conventional strain was carried out in Koto River estuary from December 2018 to February 2019. The low salinity tolerance strain showed better growth than the conventional strain, suggesting the efficacy of the cell selection technique for breeding. However, some of the thalli growing on the culture plates were suddenly shortened or disappeared in early January. Time-lapse observation with an underwater camera revealed that the disappearance of the thalli was due to predation by blackhead seabream, Acanthopagrus schlegelii. Immediate establishment of not only strains resistant to changing environments, but also effective measures for countering predation are necessary to increase production of P. tenuipedalis.

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