Salicornia ramosissima Biomass as a Partial Replacement of Wheat Meal in Diets for Juvenile European Seabass (Dicentrarchus labrax)

The green tips of Salicornia ramosissima are used for human consumption, while, in a production scenario, the rest of the plant is considered a residue. This study evaluated the potential of incorporating salicornia by-products in diets for juvenile European seabass, partially replacing wheat meal, aspiring to contribute to their valorization. A standard diet and three experimental diets including salicornia in 2.5%, 5% and 10% inclusion levels were tested in triplicate. After 62 days of feeding, no significant differences between treatments were observed in fish growth performances, feeding efficiency and economic conversation ratio. Nutrient digestibility of the experimental diets was unaffected by the inclusion of salicornia when compared to a standard diet. Additionally, salicornia had significant modulatory effects on the fish muscle biochemical profiles, namely by significantly decreasing lactic acid and increasing succinic acid levels, which can potentially signal health-promoting effects for the fish. Increases in DHA levels in fish fed a diet containing 10% salicornia were also shown. Therefore, the results suggest that salicornia by-products are a viable alternative to partially replace wheat meal in diets for juvenile European seabass, contributing to the valorization of a residue and the implementation of a circular economy paradigm in halophyte farming and aquaculture.

Thermal tolerance limits and physiological traits as indicators of Hediste diversicolor's acclimation capacity to global and local change drivers

Global projections predict significant increases in ocean temperature and changes in ocean chemistry, including salinity variations by 2100. This has led to a substantial interest in the study of thermal ecophysiology, as temperature is a major factor shaping marine ectotherm communities. However, responses to temperature may be influenced by other factors such as salinity, highlighting the relevance of multiple stressor studies. In the present work, we experimentally evaluated the thermal tolerance of the marine ragworm Hediste diversicolor under predicted global change scenarios. Organisms were subjected to an experimental trial under control (24 °C), and two temperature treatment scenarios (ocean warming +3 °C - (27 °C) and heat wave +6 °C - (30 °C)), combined with salinity variations (20 and 30) in a full factorial design for 29 days. Environmental data from the field were collected during 2019 and 2020. At day 30 post exposure, upper thermal limits (Critical Thermal Maximum - CTMax), thermal safety margins (TSM) and acclimation capacity were measured. Higher acclimation temperatures led to higher thermal tolerance limits, confirming that H. diversicolor features some physiological plasticity, acclimation capacity and a positive thermal safety margin. This margin was greater considering in situ temperature data from 2019 than maximum temperatures for 2020 (CTMax > maximum habitat temperature-MHT). Moreover, smaller organisms displayed higher upper thermal limits suggesting that thermal tolerance is size dependent. Ragworms subjected to higher salinity also showed a higher CTMax than those acclimated to lower salinity. However, temperature and salinity showed an additive effect on CTMax, as no significant interaction was detected. We conclude that H. diversicolor can easily acclimate to increased water temperature, independently of salinity variations. Given the key role of ragworms in food webs in estuaries and coastal lagoons, substrate bioturbation and aquaculture, this information is relevant to support conservation actions, optimize culture protocols and identify thermal resistant strains.

Salinity shapes the stress responses and energy reserves of marine polychaetes exposed to warming: From molecular to functional phenotypes

Estuarine systems are critical transition zones influenced by sea, land and freshwater. An array of human activities impacts these areas leading to multiple-stressor interactions. Temperature and salinity are among the most relevant drivers in estuaries, shaping species growth, reproduction and distribution. However, few studies provide an overview of cellular rewiring processes under multiple-stressor environments. Here, we tested how salinity could shape the response of ragworms Hediste diversicolor, an important bioindicator and commercial species, to elevated temperature. We exposed polychaetes to three temperatures for a month, simulating control, ocean warming and heatwave conditions (24, 27 and 30 °C, respectively) combined with two salinities (20 and 30). We quantified whole-organism performance (wet weight gain and survival), along with cellular stress response (CSR) and energy reserves of worms after 14 and 28 days of exposure. Significant three-way interactions between temperature, salinity and exposure time show the non-linearity of molecular responses. Worms at a salinity of 20 were more sensitive to warming than worms exposed to a salinity of 30. The combination of high temperature and low salinity can act synergistically to induce oxidative stress and macromolecular damage in worm tissues. This finding was supported by an induction of the CSR, with a concomitant decrease of energy reserves, pointing towards a metabolic compensation strategy. However, under a higher salinity (30), the need for a CSR upon thermal challenge was reduced and energy content increased with temperature, which suggests that environmental conditions were within the optimum range. Heatwaves striking low-salinity areas of estuaries can therefore negatively impact the cellular physiology of H. diversicolor, with greater metabolic costs. However, extreme stress levels were not reached as worms incremented wet weight and survival was high under all conditions tested. Our findings are important for the optimization of ragworm aquaculture and adaptive conservation strategies of estuarine systems.

Unravelling the fatty acid profiles of different polychaete species cultured under integrated multi-trophic aquaculture (IMTA)

Polychaetes can be successfully employed to recover otherwise wasted nutrients present in particulate organic matter (POM) of aquaculture effluents. The present study describes the fatty acid (FA) profile of four different polychaete species cultured in sand filters supplied with effluent water from a marine fish farm. The FA profile of cultured and wild Hediste diversicolor was compared and revealed a ≈ 24.2% dissimilarity, with cultured biomass displaying a higher content in two essential n-3 highly unsaturated FA (HUFA) (EPA [20:5 n-3] and DHA [22:6 n-3]—eicosapentaenoic and docosahexaenoic acid, respectively). The comparison of the FA profile of cultured H. diversicolor with that of other polychaete species whose larvae successfully settled on the sand filters (Diopatra neapolitana, Sabella cf. pavonina and Terebella lapidaria) revealed that their FA profile, which is here described for the first time, displayed high levels of EPA and DHA (≈ 1.5–4.8 and 1.0–1.1 µg mg⁻¹ DW, respectively). The highest concentration of total FA per biomass of polychaete was recorded in H. diversicolor and T. lapidaria, with both species being the ones whose FA profiles revealed a lowest level of dissimilarity and more closely resembled that of the aquafeed used in the fish farm. In the present work it was demonstrated that it is possible to produce polychaetes biomass with high nutritional value through an eco-design concept such as integrated multi-trophic aquaculture (IMTA). Indeed, this framework promotes a cleaner production and, in this specific case, allowed to recover essential fatty acids that are commonly wasted in aquaculture effluents.

Calcium homeostasis and stable fatty acid composition underpin heatwave tolerance of the keystone polychaete Hediste diversicolor

Extreme weather events, such as heatwaves, are becoming increasingly frequent, long-lasting and severe as global climate change continues, shaping marine biodiversity patterns worldwide. Increased risk of overheating and mortality across major taxa have been recurrently observed, jeopardizing the sustainability of ecosystem services. Molecular responses of species, which scale up to physiological and population responses, are determinant processes that modulate species sensitivity or tolerance to extreme weather events. Here, by integrating proteomic, fatty acid profiling and physiological approaches, we show that the tolerance of the intertidal ragworm Hediste diversicolor, a keystone species in estuarine ecosystems and an emergent blue bio-resource, to long-lasting heatwaves (24  vs 30 °C for 30 days) is shaped by calcium homeostasis, immune function and stability of fatty acid profiles. These features potentially enabled H. diversicolor to increase its thermal tolerance limit by 0.81 °C under the heatwave scenario and maintain survival. No growth trade-offs were detected, as wet weight remained stable across conditions. Biological variation of physiological parameters was lower when compared to molecular measures. Proteins showed an overall elevated coefficient of variation, although decreasing molecular variance under the heatwave scenario was observed for both proteins and fatty acids. This finding is consistent with the phenomenon of physiological canalization in extreme environments and contradicts the theory that novel conditions increase trait variation. Our results show that keystone highly valued marine polychaetes are tolerant to heatwaves, confirming the potential of H. diversicolor as a blue bio-resource and opening new avenues for sustainable marine aquaculture development.

Recovering wasted nutrients from shrimp farming through the combined culture of polychaetes and halophytes

The bioremediation and biomass production of organic extractive organisms (polychaetes Arenicola marina, Hediste diversicolor and halophyte Salicornia ramosissima) was assessed in an integrated multi-trophic aquaculture (IMTA) framework. Culture trials were performed outdoors using the nutient rich effluent from a shrimp farm employing recirculated aquaculture systems. Similar bioremediation efficiencies were obtained in cultures using a single polyculture tank (1 T) or two trophic levels separated tanks (2 T; ≈ 0.3 and 0.6 m² operational area, respectively), with a reduction of 74-87% for particulate organic matter (POM), 56-64% for dissolved inorganic nitrogen (DIN) and 60-65% for dissolved inorganic phosphorus (DIP). Hediste diversicolor adapted well to culture conditions, reaching densities up to 5.000 ind. m^-2 (≈ 78-98 g m^-2). Arenicola marina failed to cope with water temperature that exceeded the species thermal limits, displaying a survival < 10% (20 °C often pointed as the maximum thermal threshold for this species). Productivity of S. ramosissima with 1 T was about twice that obtained with 2 T (≈ 150-170 and ≈ 60-90 g FW m^-2 edible aboveground biomass, respectively). The yellowish coloration of cultured plants was likely due to the chemical oxidation and rapid sand filtration pre-treatment applied to the brackish groundwater used in the aquaculture facility, that removed iron (and probably other essential elements). Overall, 1 T design combining H. diversicolor and S. ramosissima displayed the best bioremediation performance and biomass production, while also allowing reducing in half the operational area required to implement this IMTA framework.