Hu F et al. (2020), Effects of the brown algae Sargassum horneri an...

ECB-ART-49716

Sci Rep 2020 Jul 27;101:12495. doi: 10.1038/s41598-020-69435-8.

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Effects of the brown algae Sargassum horneri and Saccharina japonica on survival, growth and resistance of small sea urchins Strongylocentrotus intermedius.

Hu F , Yang M , Ding P , Zhang X , Chen Z , Ding J , Chi X , Luo J , Zhao C , Chang Y .

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Mass mortality of the long line culture of the sea urchin Strongylocentrotus intermedius in summer, which is greatly associated with their disease, energy storage and resistant abilities, is the most serious problem for the development of the aquaculture. Here, a feeding experiment was conducted for ~ 9 weeks to investigate the survival, growth and gonadal development of small S. intermedius (~ 3 cm) fed either brown algae Sargassum horneri or Saccharina japonica. Subsequently, we assessed their resistant abilities via observing the behaviors of righting, tube feet extension and Aristotle's lantern reflex at both moderately elevated and acutely changed water temperatures. Sea urchins fed S. horneri showed significantly fewer diseased individuals and slower gonadal development than those fed S. japonica. Consistently, significantly greater Aristotle's lantern reflex occurred in sea urchins fed S. horneri at moderately elevated temperatures. These findings suggest that S. horneri has direct application potential as food for the long line culture of S. intermedius in summer because of the advantage in health, energy storage (avoid the energy loss caused by gonadal development at small body sizes) and resistance abilities. In comparison, sea urchins fed S. japonica outperformed those fed S. horneri for all experimental behaviors under the acutely changed water temperatures. These findings clearly suggest that S. intermedius fed S. japonica is more suitable for the areas with cold water mass in summer, because it can effectively avoid or reduce the negative impacts of acute changes of water temperature on sea urchins. The present study provides valuable information into the management of the long line culture of S. intermedius in summer.

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	Figure 1. Dried food consumption (g indâ1Â dayâ1) of Strongylocentrotus intermedius fed different diets for six consecutive days (meanâÂ±âSD, Nâ=â5). Sargassum horneri and Saccharina japonica refer to the experimental and control groups, respectively. The asterisks *** mean Pâ<â0.001.
	Figure 2. Histology of ovaries (A,C) and testes (B,D) of Strongylocentrotus intermedius (Nâ=â6) fed different brown algae afterâ~â9Â weeks. Sargassum horneri and Saccharina japonica refer to the experimental and control groups, respectively. In the gonads of sea urchins in the experimental group, the primary oocytes only attached to the follicular wall of ovaries (A) and sperms only occurred in the follicular wall of testes (B). Regarding the gonads of sea urchins in the control group over the same period, the oocytes were detached from the wall and gradually replaced nutritive phagocytes in the follicular cavity (C). Consistently, the basophilic clusters of sperm with a length ofâ~â2Â Î¼m occurred in both the follicular wall and follicular cavity of testes (D). NP means nutritive phagocytes.
	Figure 3. Righting response time (meanâÂ±âSD, Nâ=â15; A,D), tube feet extension rating (meanâÂ±âSD, Nâ=â15; B,E) and Aristotle's lantern reflex (meanâÂ±âSD, Nâ=â7 for experimental group and Nâ=â10 for control group at moderately elevated temperatures; (C) meanâÂ±âSD, Nâ=â10 for both groups; (F) in either treatment in respond to moderately elevated or acutely changed water temperatures. Sargassum horneri and Saccharina japonica refer to the experimental and control groups, respectively. Moderately elevated water temperatures refers to seawater temperature rose from 23.5 to 26.5Â Â°C at a rate of 0.5Â Â°C per day and maintained for 1 week. To simulate the changes of water temperature in Haiyang island near Dalian (39Â° 03â² N, 123Â° 09â² E) where water temperature frequently fluctuates from 22 to 16Â Â°C instantly by the cold water mass, sea urchins were transferred quickly from 23.5 to 15Â Â°C, maintaining at 15Â Â°C for an hour and subsequently quickly returned to 23.5Â Â°C for another hour to finish one cycle of acutely changed temperatures. After four cycles, these behaviors were observed. The asterisks ,* and *** mean Pâ<â0.05 and Pâ<â0.01 mean Pâ<â0.001.
	Figure 4. The conceptual diagrams show the black-mouth disease (A), spotting disease (B) and without disease performance (C) of sea urchin as well as the devices for righting behavior (D) and Aristotle's lantern reflex (E).
	Figure 5. Daily records of the water temperature in Heishijiao sea area, Dalian (~â2Â m water depth, 38Â° 51â² N, 121Â° 33â² E) in the summers of 2017 and 2018.

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