REDUCING RELIANCE ON LIVE MICROALGAE IN BIVALVE HATCHERIES: POWDERED DIET SUPPORT GROWTH OF JUVENILE Ruditapes decussatus
Introduction
Aquaculture production of bivalves relies heavily on the continuous supply of high-quality live microalgae1, particularly during early juvenile stages characterized by high feeding rates and elevated nutritional demands. However, large-scale microalgae production remains technically demanding, costly, and subject to variability in both nutritional quality and availability, posing significant operational constraints for hatcheries2. These limitations have driven growing interest in alternative or complementary feeding strategies capable of reducing dependence on live feeds without compromising biological performance. Among these, powdered or inert diets have emerged as promising options, offering advantages in terms of storage, handling, and consistency3. Despite these benefits, their effectiveness as partial or total substitutes for live microalgae remains insufficiently understood, particularly during juvenile development. This knowledge gap is especially relevant for high-value species such as Ruditapes decussatus, in which early-stage nutrition strongly influences growth performance and overall physiological condition4. Therefore, this study aimed to evaluate the effects of partial and total replacement of live microalgae with a formulated powdered diet (PearlDiet) on the growth and physiological condition of juvenile R. decussatus.
Materials and Methods
Juvenile R. decussatus (~1 mm initial shell length) were reared for 28 days in 8 L closed systems using ultrafiltered seawater. Clams were distributed as pooled samples (~0.5 g wet weight, WW) across five dietary treatments in triplicate: 1) live microalgae (LM) Dose 1; 2) LM, Dose 2; 3) 50% LM + 50% PearlDiet (PD); 4) 25% LM + 75% PD; and 5) 100% PD. LM consisted of Tisochrysis lutea, Chaetoceros calcitrans, and Skeletonema costatum (1:1:1, v/v), while PD comprised a microalgal blend fortified with docosahexaenoic acid (DHA) and supplemented with carbohydrates, probiotics, and yeast. Diets were administered daily in two feeding events at fixed rations: LM at 10% (Dose 1) and 5% (Dose 2) and PD at 15% (g dry weight g-1 WW of juveniles). Water was renewed every two days and continuous aeration was provided. Environmental conditions were monitored daily. Sampling was performed weekly, with biomass standardized at each time point to maintain comparable system density. Growth was assessed based on changes in WW and shell morphometrics (shell length, SL; shell height, SH; shell area, SA), while physiological condition was evaluated through organic matter content (OMC).
Results
At the end of the experiment, there were no significant differences in WW, SL, or SA among juveniles fed all LM-containing diets, except for those fed LM Dose 1, which showed the lowest growth (WW, SL, and SA) among these treatments. The growth of juveniles fed 25% LM + 75% PD (WW: 2.09 ± 0.13 g, SL: 3.51 ± 0.82 mm, SA: 7.17 ± 2.9 mm2) did not differ significantly from that of juveniles fed LM Dose 2 (WW: 2.26 ± 0.16 g, SL: 3.68 ± 0.94 mm, SA: 7.33 ± 3.11 mm2). Juveniles fed 100% PD showed the lowest WW, SL, SA, and OMC values, which were significantly lower than those observed in most dietary treatments. No significant differences in OMC were observed between juveniles fed 25% LM + 75% PD and those fed LM Dose 2 or any other LM-containing diets. Juveniles fed 100% PD showed significantly lower OMC values (10.55 ± 0.25%) compared to all other treatments.
Figure 1. Growth performance and physiological condition of juvenile Ruditapes decussatus over a 28-day feeding trial under different dietary treatments. (A) pooled wet weight (g); (B) organic matter content (OMC, %). Values are presented as mean ± SD (n = 3). Different lowercase letters indicate significant differences among treatments at day 28 (one-way ANOVA, Tukey post-hoc test, p < 0.05). LM: live microalgae; PD: PearlDiet
Discussion
The present study demonstrates that a substantial reduction in live microalgae supply is possible without compromising the growth and physiological condition of juvenile R. decussatus. Comparable growth and OMC between the 25% LM + 75% PD and LM treatments, particularly relative to LM Dose 2 (the best-performing LM treatment), indicate that a substantial proportion of live microalgae can be replaced by a formulated powdered diet while maintaining juvenile clam performance. This finding is especially relevant, as live microalgae constitute the natural diet of bivalves and their production remains a major technical and economic constraint for bivalve seed production1. Similar approaches have been reported in bivalve spat, where non-live or concentrated microalgal diets have been shown to support growth when used as complementary feeds rather than as complete substitutes3. The lower performance observed in the 100% PD treatment suggests that further improvements in formulation are needed to fully match the nutritional and functional properties of live microalgae. However, the strong performance observed at high inclusion levels of inert diet highlights the potential of partial replacement strategies to improve hatchery efficiency. Overall, these findings support the use of PearlDiet as a complementary feed to reduce reliance on live microalgae, thereby enhancing production sustainability and reducing operational costs.
Acknowledgment
Supported by Project SHELLBLOOM-17410 (ALGARVE 2030, COMPETE2030-FEDER-01193100, Portugal 2030) and FCT doctoral scholarship 2022.09708.BDANA
References
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4) Albentosa, M., et al. (1996). Evaluation of live microalgal diets for the seed culture of Ruditapes decussatus using physiological and biochemical parameters. Aquaculture 148(1), 11–23.