| Summary: | 博士 === 國立臺灣大學 === 動物學研究研究所 === 93 === White shrimp, Litopenaeus vannamei, a globally important cultured prawn species, is an ideal animal for studying the impairment caused by the effects of heavy metals that are often detected in coastal areas. In this present study, we detected the acute toxicity of cadmium (Cd) and zinc (Zn) to L. vannamei, and investigated their effects on gill functions, which have not been carried out in this species before, hoping to obtain some important significance. First of all, we examined the acute toxicity of cadmium (Cd) and zinc (Zn) to L. vannamei and obtained 24-, 48-, 72-, and 96-h medium lethal concentration (LC50) values of 2.58, 1.30, 1.14, and 1.07 mg Cd/L and 3.98, 2.14, 1.75, and 1.35 mg Zn/L, respectively. Furthermore, we also found that acute exposure of shrimp to Cd or Zn caused some alterations in gill functions, including inhibitions in oxygen consumption of 91.3% and 75.9% lower than that of the control, elevations in ammonium excretion, which were 153.7% and 144.1%, respectively, higher than the control, and decreases in osmotic pressure of shrimp blood. The distribution of Cd and Zn within gills was also determined. Furthermore, effects on gill structures of L. vannamei after acute exposure to Cd and Zn were studied. We demonstrated that acute exposure to high concentrations of Cd and Zn resulted in morphological alterations in gills of L. vannamei in this present study. Combining with these results, we further discussed aspects of the effects of heavy metals on respiratory function by examining the relations between gill structural changes and injuries, and gill dysfunction in the present article.
Using the results from acute toxicity experiments as foundations, we further studied the chronic toxicities of Cd and Zn on L. vannamei. We investigated the effects of Cd and Zn on the growth, food consumption, and biochemical parameters such as triacylglycerol (TAG), cholesterol, and total soluble protein (TSP) levels within the hepatopancreas, to determine the effects of Cd and Zn on the nutritional condition of L. vannamei. Results showed that Cd and Zn caused growth retardation in L. vannamei, which might be due to the decrease in their food consumption rates as well as changes in nutritional conditions, since alterations in levels of or unavailability of biochemical and nutritional materials within the hepatopancreas were observed. Histological injuries in hepatopancreas were also observed in L. vannamei after long-term exposure to Cd and Zn.
Metallothioneins (MTs) have been widely considered for their potential use as specific biomarkers to reflect the existence of heavy metal pollution, because their induction has been observed to be obviously elevated after heavy metal exposure in a large number of organisms studied. However, relatively fewer efforts have been made in MT-related studies of prawn species, such as L. vannamei. With the results from gel filtration chromatography, we demonstrated the existence of MTs or MT-like proteins in L. vannamei. We further studied the relationship between MT induction and metal accumulation after long-term exposure to the heavy metals Cd and Zn. From our results, it is very clear that the response of L. vannamei to Cd differs from that to Zn, and this should be considered when using MTs in field applications to monitor metal contamination. Also, induction of glutathione S-transferase (GST) in L. vannamei was observed after exposure to Cd and Zn.
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