Development of digestive enzymes and in vitro digestibility of different species of phytoplankton for culture of early juveniles of the freshwater pearl mussel, Hyriopsis (Hyriopsis) bialatus Simpson, 1900

Abstract

Summary In vitro culture of the glochidia of the freshwater pearl mussel, Hyriopsis (Hyriopsis) bialatus Simpson, 1900, in M199 medium supplemented with common carp plasma resulted in 95 ± 2% survival, of which 97 ± 2% were transformed to juveniles. Transformation occurred within 10 days. After transformation, early juvenile mussels were reared and fed for 15 days with a mixture of four species of phytoplankton (Chlorella sp.2, Kirchneriella incurvata, Navicula sp. and Coccomyxa sp.). Digestive enzyme activities, not detected in glochidia, were induced after feeding in transformed juveniles. Analysis of juvenile crude enzyme extracts after 15 days of feeding revealed the presence of amylase, proteinases, lipase and cellulase with specific activities 7.29 ± 0.12 U mg protein−1, 0.003 ± 0.0001 U mg protein−1, 1.85 ± 0.24 U mg protein−1 and 0.03 ± 0.01 U mg protein−1, respectively, which corresponded to 39%, 25%, 42% and 6%, respectively, of those found in the gastrointestinal tract of adult mussels. In vitro digestibility of the four species of phytoplankton using juvenile crude enzyme extract resulted in digestion of carbohydrate, protein and lipid content of Chlorella sp.2, K. incurvata, Navicula sp. and Coccomyxa sp., respectively, at the following levels: carbohydrate, 414 ± 46, 234 ± 38, 98 ± 19 and 188 ± 31 μg of reducing sugar per mg phytoplankton; protein, 75 ± 13, 164 ± 19, 63 ± 7 and 120 ± 11 μg of cleaved peptides per mg phytoplankton; lipid, 369 ± 24, 18 ± 3, 56 ± 7 and 80 ± 11 μg of fatty acid per mg phytoplankton. Among different nutrient digestibilities, that of protein is apparently the key factor determining food quality, followed by carbohydrate as the secondary factor in herbivorous species like mussels. The data indicate that a combination of Chlorella sp.2 and K. incurvata is a suitable food formula for juvenile culture.