Effects Of Low Concentration Cadmium Stress On Physiological Characteristics Of Benthic Macroinvertebrate

Cadmium（Cd）is a heavy metal with high biological toxicity and the biomagnification of which in aquatic organisms has been extensively studied.The water quality has been improved since recent twenty years due to the programs for water pollution control and treatment.However,the physiological characteristics of benthic macroinvertebrates under low-concentration Cd stress remain unclear.In this study,Asia clam（Corbicula fluminea）and field snail（Bellamya aeruginosa）were selected as experimental objectives under low concentration Cd stress(solution gradient:0.05 mg L^-1,0.10 mg L^-1,and 0.20 mg L^-1)as the exposure solution.The indoor aquatic toxicological culture experiment was designed with exposure（14 d）and depuration（7 d）phases.A total of 19 variables were measured to explore the toxic effects and mechanisms on bivalves and gastropods(the water quality situation,abnormal physiological phenomenon,FR:filtration rate,OR:oxygen consumption rate,NR:ammonia nitrogen excretion rate,O/N:OR/ON×1000,the movement speed,the tissue damage,ACh E:acetylcholinesterase,ATPase,Ca²⁺/Mg²⁺-ATPase,T-ATPase,SOD:superoxide dismutase,CAT:catalase,GR:glutathione reductase,GST:glutathione S-transferase,MDA:malondialdehyde,Cd concentration,and mortality).The main results of this study are as follows:The digestive system and reproductive system of benthos were affected seriously by Cd.On the second day of the experiment（D2）,there were 12,21,and 29 preterm individuals in 0.05 mg L^-1,0.10 mg L^-1 group,and 0.20 mg L^-1 group,respectively.The Cd directly affected the digestive system of the C.fluminea and B.aeruginosa,resulting in the discharge of the incompletely digested spirulina powder,which led to turbidity of the water.Cd also posed a serious threat to the reproductive system of B.aeruginosa when it damaged the digestive system.When pregnant individuals failed to protect the offspring,the offspring might be excreted with excretion behavior.The low-concentration Cd significantly inhibited the behavioral characteristics（FR,OR,NR,and O/N）of C.fluminea and the locomotor capacity of B.aeruginosa.On D14,the FR,OR,NR,and O/N of the 0.20 mg L^-1 group were inhibited by 83%,83%,38%,and72%,respectively.In response to the threat of Cd,C.fluminea would reduce the opening amplitude and frequency of the valves,which resulted the decrease of FR,OR,and NR.When the valve-closing behavior could not completely resist the threat of Cd,the body would produce excessive reactive oxygen species（ROS）.In order to resist oxidative damage,C.fluminea required more energy consumption and more dependent on protein metabolism.On D21,the improvement of water quality would slightly improve the FR and O/N of C.fluminea.Cd could affect the nervous system and caused oxidative damage to the body,so locomotor capacity of B.aeruginosa in 0.05 mg L^-1 group was weakened by93%.The tissue slice of C.fluminea showed that Cd causes tissue lesions such as cilia degeneration and muscle vacuolization in the feet and cilia degeneration,cellular swelling,contractions,and adhesions in haemolymphatic vessels in the gills of C.fluminea.The degree of tissue damage increased with the increased of Cd concentration.The appearance of tissue damage confirmed that Cd could cause oxidative damage to the benthic body.The Cd led the increase the enzyme activity in benthic macroinvertebrate.The activities of ACh E,ATPase,antioxidant enzymes,and MDA content of C.fluminea and B.aeruginosa（gills and digestive glands）were enhanced in treatments.Only the activities of ACh E,SOD,and CAT in treatment group on D14 were significantly different from those in control group（p<0.05）.This showed that low-concentration Cd could induce excessive production of ROS in the body on D7,and the body could increase the activities of various enzymes in order to eliminate oxidative stress.On D14,the ROS were almost eliminated in the SOD and CAT stages,and the activities of the other enzymes gradually returned to normal state.The activities of Ca²⁺/Mg²⁺-ATPase and GST increased during the depuration phase.The concentration of Cd in C.fluminea increased gradually during exposure phase.The bioaccumulation of Cd in C.fluminea was as follows:0.20 mg L^-1>0.05 mg L^-1>0.10 mg L^-1,and only 0.10 mg L^-1 group was still bio-enriching at the depuration phase.The reverse concentration enrichment may be due to the fact that the decreased of the filtering ability of C.fluminea at 0.10 mg L^-1 Cd was greater than 0.05 mg L^-1 Cd,which increased the risk of Cd ingestions in the 0.05 mg L^-1 group.