Introduction and methods
Sea urchin aquaculture, particularly Paracentrotus lividus, is gaining increasing interest due to the high market value of its gonads and the need to reduce fishing pressure on wild populations. At the same time, the invasive seaweed Rugulopteryx okamurae has become an ecological and economic concern along the Portuguese coast, affecting biodiversity, fisheries and tourism (Boesiger, 2024). However, its high biomass availability may represent an opportunity for valorization within a circular blue economy framework. This study evaluated the use of R. okamurae as a feed source for P. lividus, either as a dried macroalgal diet or incorporated into formulated feeds. Four diets were tested: dried Ulva sp., dried R. okamurae, a formulated diet containing 20% Ulva sp., and a formulated diet containing 10% Ulva sp. + 10% R. okamurae. Ulva biomass was produced at IPMA facilities and solar-dried in greenhouse trays, while R. okamurae was harvested and dried by Easy Harvest Lda. Formulated diets were produced by Sparos I&D using a previously validated formulation for P. lividus (Ara��jo et al., 2023). The trial lasted 139 days and was conducted in open-circuit systems under natural environmental conditions, monitored on a daily basis. Diet effects were assessed in juveniles through survival, feed conversion ratio and growth performance, and in pre-commercial sea urchins through gonadosomatic index (GSI) and gonad proximate composition. Sea urchins were fed every two days, with uneaten feed recorded before each feeding. Weight and test diameter were measured monthly. Differences among treatments were analyzed using one-way ANOVA or Kruskal–Wallis tests, followed by post-hoc comparisons.
Results and discussion
The results of this study demonstrate that the invasive macroalga R. okamurae can be incorporated into the diet of sea urchins (P. lividus), although its effectiveness is highly dependent on the mode of inclusion. Sea urchins fed exclusively with dried R. okamurae exhibited poor zootechnical performance, including weight loss, minimal test growth and a markedly reduced gonadosomatic index (GSI), indicating limited nutritional suitability as a sole feed source (see table1). These findings are consistent with the presence of secondary metabolites, such as diterpenes, which are known to reduce palatability and digestibility (Casal-Porras, 2021). Nevertheless, continuous ingestion and high survival rates (>94%) suggest the absence of acute toxic effects, highlighting the trophic plasticity of the species and its capacity to adapt to novel food sources. In contrast, the inclusion of R. okamurae in a formulated diet resulted in significantly improved performance, yielding the highest somatic growth and GSI values among treatments, surpassing both macroalgal and control feed diets. This indicates that processing and dietary balancing can mitigate the limitations associated with raw biomass, enhancing nutrient availability and feed efficiency. Proximate analysis of the gonads revealed that dietary treatments influenced their biochemical composition, reflecting differences in nutrient assimilation and allocation. In particular, formulated diets promoted a more favourable nutritional profile, consistent with enhanced gonadal development and improved production performance. Overall, the results support the potential valorization of R. okamurae as a functional ingredient in aquafeeds, contributing to circular economy strategies by transforming an abundant invasive species into a valuable resource. However, further research is required to optimize inclusion levels, assess long-term effects and evaluate the sensory attributes and market acceptance of the produced gonads, which remain critical for commercial application.
Table 1: Paracentrotus lividus production parameter: Total Diameter Gain (TDG), Total Specific Growth Rate for Diameter (TSGRD), Feed Conversion Rate (FCR) and gonads proximal analysis
Diet
TDG (%)
TSGRD (% day-1)
FCR sea urchins production parameters
Dry Ulva spp.
9.01 ± 6.33 ab
0.06 ± 0.04 ab
11.17 ± 1.87 a
Dry R. okamurae
0.97 ± 1.08 b
0.01 ± 0.01 b n.a.
Ulva-based Feed
13.27 ± 2.72 a
0.09 ± 0.02 a
6.5 ± 0.96 a
Mix Feed
11 ± 2.88 a
0.07 ± 0.02 a
7.58 ± 0.98 a
Diet
Protein (% DM)
Fat (% DM)
Energy (MJ/kg DM)
Sea urchins gonads proximal analysis
Dry Ulva
60.10 ± 4.38
10.11 ± 1.28
20.19 ± 0.53
Dry R. okamurae
55.54
10.13
20.77
Mix Feed
54.77 ± 5.77
13.29 ± 0.95
21.38 ± 0.68
Ulva-based Feed
55.32 ± 1.29
13.44 ± 0.70
21.53 ± 0.20
Acknowledgment
This research was funded by Project AQUAFISH 0.0—Improving acceptance and social awareness in the consumption of sustainable marine food products developed under the zero waste philosophy (co-financed by the Interreg Atlantic Area Programme through the European Regional Development Fund).
References
Ara��jo, J., Loureiro, P., Candeias-Mendes, A., Gamboa, A., Bandarra, N., Cardoso, C., Soares, F., Dias, J., & Pous��o-Ferreira, P. (2023). The effect of a formulated feed on the body growth and gonads quality of purple sea urchin (Paracentrotus lividus) aquaculture produced. J Aquac Mar Biol. 2023;12(1):11���18.
Boesiger, L. M. (2024) Ecological impact of the invasive algal species Rugulopteryx okamurae in the Atlantic, Southwest Portugal. Master's thesis. Universidade do Algarve, Faculdade de Ci��ncias e Tecnologia. Available at: http://hdl.handle.net/10400.1/27556
Casal-Porras, I., Zub��a, E., Brun, F.G. (2021) Dilkamural: A novel chemical weapon involved in the invasive capacity of the alga Rugulopteryx okamurae in the Strait of Gibraltar, Estuarine, Coastal and Shelf Science, Volume 257, 107398, ISSN 0272-7714