A study of holding conditions, feed ration, and algal foods for the captive care of freshwater mussels
The use of glass racks and suspended pocket nets for holding freshwater mussels collected from the Ohio River and relocated to lined-ponds was studied over 3 years. Survival of mussels in ponds was 73 % after 1 y, 44 % after 2 y, and 5 % after 3 y. The glycogen levels of mussels in ponds for ly were significantly greater than that of mussels in ponds after 2.5 y and 3 y, indicating a chronic decline in body condition in mussels. Despite the presence of a diverse and dense assemblage of algae and organic detritus in the ponds, the stomachs examined at 3 y were empty and the bodies were emaciated.
In the laboratory, I determined the amount of algae cleared by the rainbow mussel, Villosa iris (Lea, 1829) fed different algal rations, and estimated the algae concentration needed to maintain mussels in captivity. Filtration rate in the first feeding hour was highest in ration B (1.0 mg dry wt L⁻¹) and lowest in ration C (3.4 mg dry wt L⁻¹). After 1 h, filtration rates declined in ration B but increased in rations C and A. V.iris likely achieved gut satiation in the first hour using maximum filtration (712.5 mL h⁻¹g⁻¹) and then decreased filtration (259 mL h⁻¹g⁻¹) thereby regulating ingestion rate during the following 2 h. I estimate, therefore, that K V. iris daily maintenance requirement for carbon is 8.2 mg C (1.2 x 10⁹ cells of N. oleoabundans) or ca. 2.4% of dry body weight. Assimilation efficiencies (AE) and carbon budgets also were established for the rainbow mussel, Villosa iris (Lea 1829), using radio-labeled cultures of Neochloris oleoabundans (Chantanachat and Bold 1962) at three cell concentrations. Approximately 70% of the ingested carbon was assimilated (assimilation efficiency) by V. iris fed 5 x 10⁵ cells mL⁻¹) (3.4 mg dry weight L⁻¹). At 5 x 10⁴ cells' mL⁻¹) (0.34 mg dry weight L⁻¹), AE was 47.5 %. At 5 X 10³ cells mL⁻¹) (0.034 mg dry weight L⁻¹), AE was 40%. V. iris had the greatest amount of energy available for growth, reproduction, and body condition in captivity at 3.4 mg dry weight L⁻¹).
The gross composition (protein, carbohydrate (CHO), and lipid) of four algae (Bracteacoccus grandis, Neochloris oleoabundans, and Scenedesmus quadricauda, and Phaeodactylum tricomutum) was examined at four different phases of growth. The CHO content (% algal dry wt) increased with growth phase (age of the algal culture) with the exception of B. grandis. N. oleoabundans and P. tricomutum contained the greatest CHO content (33.07 ± 6.89% and 39.37 ± 3.07%, respectively) at late stationary phase. The total lipid content increased with growth phase for N. oleoabundans and P. tricomutum. Lipid content of B. grandis decreased with age, and S. quadricauda showed no difference in lipid content (% algal dry wt) between growth phase. N. oleoabundans' lipid content (31.85 ± 9.4%) was greater than all other species. Generally, there was no effect of phase on the sterol content, with the exception of the sterol content of S. quadricauda increased with growth phase. The mean sterol content of the four algae ranged 1.0 % ± 0.4 to 1.8 ± 1.8 of the total lipid dry wt; maximum sterol % of lipid was 5 % for Scenedesmus and 4.4% for B. grandis. There was no effect of growth phase or species on the protein content (% of algal dry wt). The protein content ranged 60.6 ± 17.1 to 70.3 ± 9.5 % of algal dry wt.