Abstract:Litopenaeus vannamei was fed with basal diets respectively supplemented with 0%, 1%, 2% and 4% salt and cultured in low salinity water for 40 d. The study was conducted to investigate the histology and morphology of the shrimp reared in low salinity using light microscope and electron microscope, and also compared with the seawater shrimp. Light microscope observation showed porosity of muscle fibre in shrimps of the control and 1% treatment group, some were gathered cord, some were bent, and the gap were wide between some perimysium. But as the addition of salt increased, muscle fiber characteristics were improved, especially for the 2% and 4% treatments. Muscle fibres of shrimps in these treatments were better distributed than the other group. The ultrastructure of the shrimp under electron microscope showed irregular array of the myofilament, and incomplete in the control and 1% treatment. The distribution of myofilament in 2% treatment was relatively homogeneous, but the thin filaments had small diameter and large quantity. Some irregular filaments were arranged in clumps. But the 4% treatment was much better, and not different from seawater shrimp; the filaments all distributed uniformly and in the form of hexagonal lattice array. On longitudinal section, the sarcomere in the 1% and 2% treatments were untidy, some were twisted and overlapping; while those in the 4% treatment were well-shaped, clearly distinguishable, and the length of sarcomere were significantly greater than the other treatments (P<0.05). As the addition of dietary salt increased, myofibril diameter, solid thick filament diameter, hollow thick filament diameter, and A and H band width showed no significant difference（P>0.05）.
