Shrimp that have received carrageenan via immersion and diet exhibit immunocompetence in phagocytosis despite a post-plateau in immune parameters

• Carrageenan caused degranulation of shrimp haemocyte, and induced immunity in vitro.
• Immune parameters increased in shrimp received carrageenan via immersion and orally.
• Shrimp received carrageenan showed resistance against Vibrio alginolyticus and WSSV.
• Shrimp received a diet containing carrageenan up-regulated gene expressions.
• Shrimp received carrageenan had phagocytosis despite of declined immune parameters.

The effect of carrageenan on the immune response of white shrimp Litopenaeus vannamei, was studied in vitro and in vivo. Shrimp haemocytes receiving carrageenan at 1 mg ml−1 experienced change in cell size, reduction in cell viability, increase in PO activity, serine proteinase activity, and RB in vitro. Shrimp received carrageenan via immersion at 200, 400 and 600 mg L−1 after 3 h and orally at 0.5, 1.0 and 2.0 g kg−1 after 3 weeks showed higher proliferation of haematopoietic tissues (HPTs) together with increases in haemocyte count and other immune parameters. Shrimp that fed a diet containing carrageenan at 0.5 g kg−1 after 3 weeks significantly up-regulated gene expressions of several immune-related proteins. The immune parameters of shrimp that received carrageenan via immersion and orally increased to a plateau after 3 h and after 3 weeks, but decreased after 5 h and 6 weeks, respectively. Phagocytosis and clearance of Vibrio alginolyticus remained high in shrimp that had received carrageenan via immersion after 5 h and orally after 6 weeks, respectively. Resistances of shrimp against V. alginolyticus and white spot syndrome virus were higher over 24–144 h and 72–144 h, respectively in shrimp that received carrageenan at 600 mg L−1 via immersion after 3 and 5 h. It was concluded that carrageenan effectively triggers an innate immunity in vitro, and increases mitotic index of HPT, immune parameters, gene expressions and resistance against pathogens in vivo. Shrimp received carrageenan via immersion and orally exhibited immunocompetence in phagocytosis and clearance of V. alginolyticus, and resistance to pathogen despite the trend in immune parameters to recover to background values.