Effects of four microalgae diets on growth, nutritional composition, pearl producing function, shell nacre color and trace element content of Hyriopsis cumingii

China accounts for more than 95% of global freshwater pearl production, of which 80% is Hyriopsis cumingii. In high-density H. cumingii culture, large amounts of organic manure (duck manure, bird manure, soybean milk) and chemical fertilizers are used to produce sufficient quantities of feed. This method results in a large amount of feces, pseudo feces, and residual feed being discharged, causing eutrophication and negative effects on the benthic environment. Water pollution can result in frequent disease outbreaks and disease control drugs can further pollute the aquatic environment. Development of intelligent culture facilities, including high-quality nutritious diets for H. cumingii, is an urgent need. To dertimine the optimal microalgae diet for massive mussel culture, this experiment was conducted to investigate the effects of different microalgae diets (Cyclotella sp., Chlorella pyrenoidosa, Scenedesmus dimorphus, Monosporidium contortum, and mixed four microalgae) on the growth, pearl producing function, nutrition composition and shell nacre color of H. cumingii, using a single factor experiment method. The results showed that the growth of the mussel was significantly different with different microalgae diets. The mussels fed Cyclotella sp. and S. dimorphus showed weight growth (WG) of 9.94% ± 0.51% and 9.42% ± 0.89%, respectively, which were significantly higher than those of the other three groups; mussels fed M. contortum had the lowest WG 5.23% ± 0.30%. Similar results were obtained for shell length growth (SLG), with mussels fed Cyclotella sp. and S. dimorphus showing the highest values 1.99% ± 0.31% and 2.07% ± 0.53%, respectively. The group fed Cyclotella sp. showed the highest shell height growth (SHG) 6.55% ± 0.62%. The highest shell width growth (SWG) was also observed in the group fed Cyclotella sp. (5.31% ± 0.71%) while the group fed M. contortum group showed the lowest SWG (3.33% ± 0.91%); there was a significant difference between these two groups. Pearl weight growth (PWG) also differed significantly among the different treatment groups. The highest PWG was observed in mussels fed Cyclotella sp. (16.23% ± 1.23%). There were no significant differences among the groups fed C. pyrenoidosa (11.40% ± 2.09%), S. dimorphus (13.40% ± 2.00%), and mixed microalgae (11.80% ± 1.40%); the lowest PWG was observed in the group fed M. contortum (6.35% ± 1.90%). In summary, Cyclotella sp. and S. dimorphus were the optimal species for H. cumingii growth. Feeding different microalgae did not have a significant effect on the shape of the pearls (probability of producing round and near-round pearls), but did have a significant effect on the probability of producing round and oval pearls. The probability of a pearl shape being round was the highest in both the Cyclotella sp. and S. dimorphus groups (26.67% ± 4.71%) and was significantly higher than that in the M. contortum group. However, the S. dimorphus groups was significantly more likely to have oval round shape (16.67% ± 4.71%) than the other four groups. Different microalgae had significant effects on shell nacre color. The L* values for the Cyclotella sp. (64.89±1.97) and C. pyrenoidosa (65.18±2.24) groups were lower and significantly lower than the other three groups. And with the highest L* value in the mixed algae group (68.94±1.71), which was significantly higher than the other four groups. The mixed algae group had the highest dE* value (69.08±1.68), which was significantly higher than the other four groups. The smallest dE* value was found in the Cyclotella sp. group (65.23±1.89), which was not significantly different from the C. pyrenoidosa group (65.47±2.19). There was no difference between the S. dimorphus group (67.32±2.88) and the M. contortum group (67.52±2.46), but significantly higher than the Cyclotella sp. and C. pyrenoidosa groups. The largest C* value was found in the C. pyrenoidosa group (6.03±0.98), which was slightly higher than that in the Cyclotella sp. group (5.44±1.36) and significantly higher than those in the S. dimorphu, M. contortum and mixed groups. The trace element content of the mantle was significantly affected by diet. The highest Zn content was observed in mussels fed M. contortum (170.33 ±3.86) mg/kg, followed by S. dimorphus, and Cyclotella sp. (120.33 ± 2.05) and (122.33 ± 4.99) mg/kg. The mixed algae group had the lowest Fe content (169.33± 22.13) mg/kg, and the others had no significant difference. The content of Mg and Mn had same trend, M. contortum > S. dimorphus > Cyclotella sp.= C. pyrenoidosa > mixed algae. The Ca content of the foot was the highest (12 637.30 ± 624.39) mg/kg for group fed Cyclotella sp., followed by the group fed M. contortum (8 019.00 ± 513.72) mg/kg. The Ca content of the foot for these two groups was significantly higher than those of the other groups. The highest Ca content in the mantle was observed in the mussels fed S. dimorphus (25 049.30 ± 1 320.36) mg/kg, followed by the group fed M. contortum (24 903.70 ± 359.06) mg/kg. The groups fed Cyclotella sp., C. pyrenoidosa, and mixed algae showed significantly lower Ca content in the mantle. In conclusion, Cyclotella sp. may be the best diet choice for intensive H. cumingii, and followed by S. dimorphus.