Nowadays, aquaculture has undergone significant transformations, increasingly adopting more sustainable practices. Low-trophic organisms, particularly seaweeds, are now recognized as strategic components for regenerative aquaculture. Among them, Ulva spp. has emerged as one of the most promising seaweeds for integrated multi-trophic aquaculture (IMTA) systems due to its high nutrient uptake capacity, fast growth rates in tank-based production, and wide spectrum of industrial applications.

Beyond regeneration, aquaculture can also play a restorative role in the context of environmental crisis and biodiversity loss. In this study, we explored the potential of Ulva sp. as a biofilter to enhance water quality in the hatchery of the fan mussel Pinna nobilis, a critically endangered Mediterranean species. Semi-closed experimental systems were designed to evaluate whether Ulva sp. could efficiently remove dissolved nitrogen (N) and phosphorus (P) remaining compatible with P. nobilis rearing.

During entire experimental period the ammonia values were under safe limits for mussels in hatchery, up to 0.22 mg.L ^-1. Nutrient concentrations were reduced in 35% and 32% daily for N and P, respectively. The daily growth rate (DGR) of Ulva sp. averaged 2.75 ± 0.79 %·day⁻¹ in IMTA and 3.55 ± 0.79 %·day⁻¹ in the control, without significant differences (T = –1.24; p = 0.28).

Despite relatively low growth performance, Ulva sp. proved efficient in nutrient removal, maintaining water quality within suitable ranges for P. nobilis hatchery conditions. Importantly, the IMTA approach was safe for P. nobilis, with no evidence of negative interactions.

These findings highlight the dual role of Ulva sp. in regenerative and restorative aquaculture, supporting its use as a biofilter in conservation-oriented systems and as a tool to integrate sustainable production with biodiversity restoration, amplifying the ecological and practical applications of Ulva sp. in Mediterranean aquaculture.