Effects of low salinity pressure on biological tissue and immunity enzymes activities of Sipunculus nudus

In order to understand the response of the biological and physiological indicators of Sipunculus nudus to low salinity, we evaluated the effects of low salinity pressure on the survival rate, osmotic pressure, biological tissue and immunity enzymes activities of S. nudus which were cultured for 96 h in water with abrupt salinity drop of 0 (S30), 5 (S25), 10 (S20), 15 (S15), 20 (S10) and 25 (S5). The results show that: 1) S. nudus could tolerate the salinity drop of 10, but would die when the salinity dropped over 15. 2) The coelomic fluid of S. nudus increased with the decrease of water salinity significantly, and there was a significant positive correlation between the body mass and coelomic fluid volumn (R²>0.96). 3) The osmotic pressure and body protein contents decreased with the decrease of water salinity (P<0.05). 4) In the low salinity treatments, the connective tissue of longitudinal and circumferential muscle fibers of the body wall were thin, and the staining of columnar cells was lighter. For the tentacle, its adhesion ability was weaker as the epithelial cells became larger. 5) SOD and CAT activities in the groups of S30 and S25 were relatively stable, while the body SOD and coelomic fluid CAT in the groups of S10 and S15 first decreased and then increased; however, the CAT of coelomic fluid in the groups of S10 and S15 first increased and then decreased. SOD and CAT activities in the coelomic fluid had strong varation. In summary, low salinity pressure can affect the survival rate, osmotic pressure, biological tissue and immunity enzymes activities of S. nudus significantly.