Introduction
The growing demand for high-quality seafood – driven by population growth and increasing consumer awareness of the health benefits of fish consumption – is propelling aquaculture expansion, which in turn increase the need for aquafeed and sustainable raw materials. Black soldier fly larvae oil (Hermetia illucens, BSFLO), rich in lauric acid (C12:0; 30-70%), a medium-chain saturated fatty acid (SFA), is gaining attention for its potential functional properties. However, water temperature can significantly affect the digestion and absorption of dietary lipids in fish. This is particularly relevant for lipid sources rich in SFA, which generally have higher melting points than their unsaturated counterparts of the same chain length. As a result, temperature-dependent changes in the physicochemical properties of dietary lipids may influence their bioavailability and utilization by fish in cold water conditions. Therefore, this study aimed to explore the impact of the dietary BSFLO inclusion on macronutrient and fatty acid digestibility, as well as intestinal morphology, of European seabass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata) – two key species in Mediterranean aquaculture – reared at 15 °C and 23 °C, to simulate average seasonal conditions (winter and summer) in the Mediterranean.
Material and methods
A control diet (CTRL) was formulated with 6% fish oil and 5% poultry oil (PO). Three other experimental diets were formulated to include 1.7%, 3.4% and 5.2% of BSFLO (Entrogreen® - Ingredient Odissey S.A), at the expense of PO (BSFLO1.7, BSFLO3.4 and BSFLO5.2, respectively). All experimental diets contained 1% Cr₂O₃ as an inert marker. These diets were tested in triplicate groups of seabass and seabream, maintained under two temperature conditions (15 °C and 23 °C) in a recirculating aquaculture system (RAS) with sedimentation columns for faeces collection. Fish were fed to apparent satiation three times daily and faeces were collected daily and stored at -80 ºC. After freeze-drying, faecal chromium, crude protein, total lipids, crude energy, and fatty acids levels were determined, and their apparent digestibility coefficients (ADC) were calculated. In addition, at the end of digestibility trials, a histomorphometric evaluation of the fish anterior intestine was performed.
Results
Dietary inclusion of BSFLO did not impact the anterior intestinal histomorphology of either seabass or seabream. However, water temperature significantly influenced European seabass cross-sectional-area and -perimeter, as well as the thickness of the muscularis, submucosa, and lamina propria, villus length and the total number of acid goblet cells, which were significantly reduced at the lower temperature. These intestinal changes were associated with a slight reduction in protein digestibility from 91% at 23 ºC to 90% at 15ºC. Total lipid ADC values remained similar between temperatures, but the digestibility of SFA decreased at 15 °C (93% vs. 98%), resulting in lower energy ADC values (81% at 15 ºC vs. 83% at 23ºC). BSFLO dietary inclusion had no impact on seabass macronutrient, fatty acid, or energy digestibility, despite the higher SFA content in BSFLO diets. In contrast, in seabream, the digestibility of SFA was significantly higher in fish fed BSFLO3.4 and BSFLO5.2 diets than those fed the CTRL diet (95% vs. 97%), regardless of water temperature. Macronutrients, fatty acids and energy digestibility remained similar at 15ºC and 23ºC. Likewise, temperature had a minimal effect on the intestinal structure, with the only changes observed being a reduction in thickness of the muscularis and a lower number of acid goblet cells at 15ºC.
Discussion and conclusion
The results of this study highlight species-specific responses to dietary BSFLO and water temperature in European seabass and gilthead seabream. While BSFLO inclusion had no significant effects on intestinal histomorphology or nutrient digestibility in either species, water temperature played a more critical role in seabass, where rearing lower temperatures (15 °C) led to structural intestinal changes and a slight reduction in protein and SFA digestibility. In contrast, seabream exhibited a similar digestive function at 15ºC and 23ºC and even furthered from improved SFA digestibility with BSFLO dietary inclusion. These findings suggest that BSFLO is a viable lipid source for seabream and can be effectively used in aquafeeds without compromising nutrient absorption. However, long-term pilot studies are needed to validate these results in seabream and assess the physiological consequences observed in seabass, namely the potential impact on growth performance and fish health. In addition, complementing digestibility and histological evaluations with other types of analyses, such as digestive enzyme activity or gene expression profiling, would help to better understand the mechanisms underlying these species-specific responses.
Acknowledgments
This work has been supported by InsectERA Mobilising Agenda (PRR - InsectERA - The ERA of the insect industry), funded by Portugal and European Union - NextGenerationEU, with the identifier C644917393-00000032 and Daniel Acebes is supported by Fundação para a Ciência e Tecnologia (FCT), I.P., through a Ph.D. grant (2024.02126.BDANA). Strategic Projects UIDB/05748/2020 and UIDP/05748/2020 funded by FCT