Effects of dietary supplementation of Gracilaria sp. extracts on fillet quality, oxidative stress, and immune responses in European seabass (Dicentrarchus labrax) Maria J. Peixoto 1,2 & Leonardo Magnoni 1,3 & José F. M. Gonçalves 1,2 & Robert H. Twijnstra 4 & Anake Kijjoa 2 & Rui Pereira 5 & Arjan P. Palstra 6 & Rodrigo O. A. Ozório 1 Received: 5 February 2018 /Revised and accepted: 15 May 2018 # Springer Science+Business Media B.V., part of Springer Nature 2018 Abstract The current study evaluated the effects of two different fractions derived from the methanolic extraction of the red seaweed Gracilaria sp. supplemented in European seabass (Dicentrarchus labrax) diets. With that purpose, three experimental diets were prepared: a basal diet (control), a control supplemented with the methanolic extract (GE, 0.5% diet) and a control supplemented with the insoluble residue of the GE extraction (GR, 4.5%). Seabass with an average initial weight of 16.5 ± 0.6 g were fed the experimental diets for 42 days, and the following parameters were evaluated: growth indicators, digestive enzyme activities, immune and oxidative stress responses, fillet pH and color (L*, a*, and b* values), and skin color. The dietary supplementation of GE or GR had no effect on growth performance, digestive enzyme activities, fillet pH, and color. Skin color was significantly lighter (L*) in fish-fed GE (83.9 ± 1.9) and GR (84.3 ± 2.3) diets when compared with the control group (81.9 ± 3.8). The dietary treatments did not affect the oxidative stress biomarkers. Alternative complement pathway (ACH50) was significantly higher in fish-fed GE diet (168.2 ± 13.4 EU mL −1 ) than in the control diet (113.1 ± 31.4 EU mL −1 ). No dietary effect was observed on peroxidase and lysozyme activities. The current study indicates that dietary supplementation of Gracilaria sp. methanolic extracts may have little influence on the innate immune system and skin color in seabass. Keywords Dicentrarchus labrax . Gracilaria sp. extract . Rhodophyta . Fillet quality . Dietary seaweed supplementation . Fish welfare Introduction Seaweeds are an abundant source of bioactive compounds, such as polysaccharides and pigments, which are extremely valuable for the food and pharmaceutical industries (Cardoso et al. 2014; Wells et al. 2017). The potential applicability of the resulting seaweed by-products generated by these industries has evoked considerable interest. Previous studies using fish spe- cies relevant for aquaculture have shown that seaweeds could be a valuable source of nutrients for aquafeeds (Valente et al. 2006; Soler-Vila et al. 2009; Ramalho Ribeiro et al. 2015). In fact, dietary seaweed supplementation has been associated with improvements in fish growth performance (El-Tawil 2010), im- mune response (Ganeshamurthy et al. 2014), and antioxidant capacity (Peixoto et al. 2016a), together with changes in flesh color (Soler-Vila et al. 2009). Conversely, Déléris et al. (2016) reported that the presence of anti-nutritional factors in seaweeds causes digestive enzymes inhibition in vitro. This suggests pos- sible negative effects on the fish gastrointestinal tract, which * Rodrigo O. A. Ozório rodrigo.ozorio@ciimar.up.pt 1 Present address: CIIMAR – Centro Interdisciplinar de Investigação Marinha e Ambiental, Universidade do Porto, Terminal de Cruzeiros do Porto de Leixões, Avenida General Norton de Matos, 4450-208 Matosinhos, Portugal 2 ICBAS – Instituto de Ciências Biomédicas de Abel Salazar, Universidade do Porto, Rua de Jorge Viterbo Ferreira 228, 4050-313 Porto, Portugal 3 IIB-INTECH, Av. Intendente Marino Km. 8.2, 7310 Chascomus, Argentina 4 Wageningen Marine Research (WMR), Wageningen University and Research, Korringaweg 5, 4401 NT Yerseke, The Netherlands 5 ALGAPLUS, Lda - Travessa Alexandre da Conceição S/N, 3830-196 Ilhavo, Portugal 6 Animal Breeding and Genomics Centre, Wageningen Livestock Research (WLR), Wageningen University and Research, De Elst 1, 6708 WD Wageningen, The Netherlands Journal of Applied Phycology https://doi.org/10.1007/s10811-018-1519-x