Development of an Algal Diet for Rearing Juvenile Freshwater Mussels (Unionidae)
Feeding selectivity by the rainbow mussel (Villosa iris) was examined for three age groups; 2-3 days old, 50-53 days old, and 3-6 years old. The mussels were fed an algal diet consisting of Scenedesmus quadricauda (22.3 - 44.5 μm), Nannochloropsis oculata (2.8 – 8.1 μm), and Selenastrum capricornutum (3.6 – 8.5 μm) in equal cell densities. The change in relative abundance of each algal species within feeding chambers over a 5 hr feeding trial was used to discern selectivity. At the conclusion of the feeding trials, the gut contents of mussels were analyzed for preferential ingestion. The mussels selected for N. oculata and S. capricornutum over S. quadricauda (p < 0.05). This may be an indication of particle size-dependent selection. Feeding trials also suggest that selectivity by the rainbow mussel does not change with age. Gut content analyses showed a preferential ingestion of algae, in the sequence N. oculata, S. capricornutum, then S. quadricauda.
The suitability of two algal diets, S. quadricauda and N. oculata, for rearing captive juveniles of V. iris in 145-L recirculating culture systems was compared. Juveniles were fed their assigned diet at a density rate of approximately 30,000 cells/ml for 42 days, and sampled weekly for percent survival and shell length. Regardless of diet, juvenile survival decreased rapidly after 21 days, and growth did not exceed approximately 450 μm. High mortality rates and slow growth of juveniles was likely due to inadequate diets. Juveniles that were fed S. quadricauda lacked chlorophyll coloration in their guts, indicating that the juveniles did not ingest this species of algae. Colonies of S. quadricauda were likely too large for the juveniles to ingest. The gut content of juveniles fed N. oculata showed chlorophyll coloration, indicating that the juveniles ingested this species, but N. oculata may have been difficult for the juveniles to assimilate. Under the culture conditions provided, survival and growth did not compare favorably to those of other studies with V. iris.
Newly metamorphosed juveniles of V. iris were reared in 145-L recirculating culture systems containing sediment (< 600 μm) of two depths, 5 mm and 15 mm. Mussels were fed a bi-algal diet of Nannochloropsis oculata and Neochloris oleoabundans. Survival differed significantly between treatments (p=0.04), and was higher for juveniles reared in 5 mm of sediment over a 40-day period. Growth was not significantly different between treatments. After 40 days, juveniles achieved a mean length of approximately 578 μm in both treatments. Survival and growth of juveniles compared favorably to those of other culture studies using juveniles of V. iris. A shallow layer of sediment is recommended for the culture of juvenile mussels.