Use of metabolism to evaluate the sublethal toxicity of mercury on Farfantepaneus brasiliensis larvae (latreille 1817, crustacean).

ABSTRACT Penaeid shrimp are important resources for worldwide fisheries and aquaculture. In Brazil, Farfantepenaeus brasiliensis, is an important commercially exploited species and an ideal animal for studying the impairment caused by the effects of heavy metals that are often detected in coastal areas. The main purpose of this study is to detect the acute toxicity of mercury to F. brasiliensis larvae, and to investigate its effects on oxygen consumption and ammonium excretion, which have not been carried out in this species before. We examined the acute toxicity Hg to F. brasiliensis larvae revealed at 24, 48, 72 and 96-h of exposure and the medium lethal concentration (L[C.sub.50]) values obtained were of 0.13 mg [L.sup.-l]; 0.054 mg [L.sup.-l]; 0.047 mg [L.sup.-1] and 0.045 mg [L.sup.-1], respectively. Furthermore, we also found that exposure of shrimp to Hg caused an inhibition in oxygen consumption of 53.42% lower than that of the control. However, after separate exposure to Hg, elevations in ammonium excretion were obtained, which were 217.64%, higher than the control.

KEY WORDS: shrimp, Farfantepaneus brasiliensis, mercury, metabolism effect

INTRODUCTION

The metabolism of a shrimp includes 3 well-differentiated levels: standard metabolism, routine metabolism and active metabolism. The standard metabolism is the minimum energy required for the shrimp to survive, associated to rest state. The routine metabolism is the fraction of energy used by rest shrimp with movement of spontaneous swimming, or routine activity. Routine metabolism is the mean rate oxygen consumption measured when precautions are taken against the shrimp being influenced by outside stimuli (Fry 1971). The active metabolism already represents the metabolic rate in the maximum level of activity. Brett and Groves (1979) demonstrated that fast swimmer fishes can increase its metabolism up by 10 times the standard metabolism, whereas fishes of colder waters increase their metabolic rate by 2 or 3 times. In addition, the swimming performance (Beamish, 1978) was recorded as the highest position-maintaining velocity plus the fraction of the time interval of the velocity in which they became exhausted (Hymel et al. 2002, Wicks et al. 2002) (i.e., the physical capacity that facilitates them to swim against a certain water direction during a certain period of time (Howard 1975).

The metabolic rate of an organism is an useful and sensitive indication of its daily consumption of energy. Therefore, in aerobic animals, the quantification of the rate of oxygen consumption is directly associated to the amount of energy liberated from the oxidation of food substratum. Based on the amount of oxygen consumed by an animal for a certain period of time, it is possible to evaluate the energy spent during the same period to maintain its vital processes (Carvalho 1992).

Evaluation of metabolism was used, for example, to study toxicant effects caused by aromatic compounds (Lemaire et al. 1996), heavy metals (Wu & Chen 2004, Lorenzon et al. 2000, Lorenzon et al. 2004), detergents (Christiansen et al. 1998, Barbieri et al. 1998, Barbieri et al. 2000, Barbieri et al. 2002) and various toxins (Boudou & Ribeyre 1989).

Heavy metals have been widely recognized as highly toxic when dissolved and in ionic form (Mance 1987, Lorenzon et al. 2004). In Brazil, heavy metals enter the coastal seawater mainly through discharge of industrial effluents and disposal of sewage (Damato & Barbieri 2003, Barbieri et al. 2004). High concentrations of heavy metals have been reported in coastal waters (Eysink et al. 1988a), rivers entering into estuaries (Eysink et al. 1988b) and tissues of coastal marine organisms (Carvalho et al. 2000, Carvalho et al. 2001). Mercury (Hg) is one of the most common and persistent heavy metals in aquatic environments and known to be highly toxic to marine and estuarine crustaceans (Papathanassion 1983, Wu & Chen 2004). Hg effects on the respiratory rate of marine and estuarine organisms have not been extensively studied (Amand et al. 1999). This study investigates the acute toxicity effect of the Hg on routine metabolism and ammonium excretion of Farfantepenaeus brasileiensis, an eurihalinic and eurithermic shrimp, an important commercially exploited species in Brazil.

MATERIALS AND METHODS

Considering the high economic and ecologic importance of this species, and the problems related to pollution that estuarine regions are subjected. The acute toxicity of Hg in larvae of the shrimp (F. brasiliensis) exposed to different concentrations of this substance for a period of up to 96 h was evaluated. A total of 527 larvae of cultivated shrimp with 0.8 [+ or -] 0.3 g of average wet weight and 1.2 [+ or -] 0.5 cm of total length were used. Groups of 15 individuals each were put in 50-L tanks containing seawater at 36[per thousand] and 25[degrees]C. Five replicates of groups of 15 individuals were exposed to one of the following concentrations of HgS[O.sub.4]: control 0.01, 0.05, 0.10, 0.25, 050 and 1.00 mg [L.sup.-1]. Dead shrimp were taken off the tanks and counted at 24, 48, 72 and 96 h of exposure.

To calculate L[C.sub.50] values, the mortality of each treatment was considered and processed using a basic program from the Probit Analysis described by Finney (1971).

A total of 150 shrimp, with averages of 1.27 [+ or -] 0.4 g and 1.10 [+ or -] 0.36 cm, was used for the routine metabolism measure utilizing sealed respirometers. Ten shrimp with 3 replicates were subjected to the oxygen consumption measure in 1 of the 4 concentrations (control; 0.02, 0.04, 0.08 and 0.133 mg [L.sup.-1]) of Hg. We determined the pH and oxygen concentration of the test solution (Table 1).

Before the beginning of the experiments, the animals were maintained in the respirometer with a continuous water circulation during at least 90 min to attenuate the management stress. Then, the water supply was suspended and the respirometer was closed, so that the shrimp could consume the oxygen present in the known water volume for a period of 2 h. The respirometers were protected by an antepar to isolate the animals from possible moves in the laboratory. The difference between the oxygen concentrations determined at the beginning and at the end of the confinement was used to calculate the consumption during the period. To minimize the effect of the low oxygen concentration and the metabolites accumulation on the metabolism, the experiments duration was regulated so that the oxygen concentration by the end of experiments was larger than the 70% of its initial concentration. The dissolved oxygen was determined through the Winkler method (Table 1).

To obtain the Hg desired concentration, the necessary volume of the main (mother) solution (1.0 mg HgS[O.sub.4] [mL.sup.-1]) was calculated to each volume of respirometer and set (with the help of a micropipet) at the end of the acclimation. As soon as HgS[O.sub.4] was put, the entry orifice was immediately sealed. Additionally the seawater in the bottle was sampled at the beginning and end of the oxygen consumption analysis. Determination of ammonium-N within seawater was based on the phenolhypochlorite method (Solarzano 1969). The average of the shrimp oxygen specific consumption and ammonium excretion was tested, using analysis of variance (ANOVA). Before statistical tests, all data tested using Tukey test (P < 0.05).

RESULTS

Mortality

[LC.sub.50]

The acute toxicity of Hg in larvae of the shrimp exposed to different concentrations, for period of up to 96 h were 0.13 mg [L.sup.-1] at 24 h of exposure; 0.054 mg [L.sup.-1] at 48 h; 0.47 mg [L.sup.-1] for 72 h and 0.45 mg [L.sup.-1] at 96 h (Table 2).

The percentage mortality of F. brasiliensis exposed to Hg in each 24 h interval is shown in Table 3. No deaths of control animals were observed. The higher the concentration of metals the shrimp were exposed to, the higher mortality we observed. After Hg treatment, 100% death became apparent in the first 24 h at a concentration of 1.0 mg Hg [L.sup.-1]. Mortality rates of 100% were observed after a 48 h exposure at concentrations of 0.25 mg Hg [L.sup.-1].

Oxygen Consumption and Ammonium Excretion

Results presented in Figure 1 clearly reveal inhibition effects on oxygen consumption caused by Hg. Farfantepenaeus brasileiensis larvae that consumed different quantities of oxygen at different Hg concentrations. After exposure to 0.133 mg Hg [L.sup.-1] for 2 h, levels of oxygen consumed by treated Farfantepenaeus brasileiensis were obviously lower 53.42% than those of control individuals. However, after exposure for 2 h, the average levels of ammonium excreted by F. brasileiensis treated with 0.133 mgHg [L.sup.-1] were 217.64% higher compared with average amount of the control animals (Fig. 2).

[FIGURES 1-2 OMITTED]

The average was increased to the Hg concentration and there was a significant (P < 0.05, Tukey) difference between the obtained rates at all concentration studied (0.02, 0.04, 0.08 and 0.133) when compared with the control.

DISCUSSION

Surveys held with the mercury toxicity effect on the crab Eriocheir sinensis, showed that there was an increase in the toxic effect at low salinities for this species (Pequeux et al. 1996). The authors mention that the mercury interacts with an osmoregulator mechanism impeding the animal osmoregulator capacity, increasing, then, the toxicity of metal increasing in low salinities and decreasing in high salinities. For the gastropod Thiara tuberculata exposed to heavy metals (mercury and copper), with the increasing salinity, there was a toxicity decrease reflected by the oxygen decreasing consumption (Mule & Lomte 1994). In a review by Hall and Anderson (1994), the influence of salinity toxicity of the several kinds of chemical substances was analyzed and they found that the toxicity decreased as long as the salinity increased. Researches on [CL.sub.50] held with Cyprinodon variegatus in 96 h of exposure to the cadmium ([Cd.sup.2+]) in the Chesapeake bay in 3 salinities (15, 20 and 25), showed that the greater the salinity, the lesser the cadmium toxic effect on the fish. (Hall et al. 1995).

In this study, Hg showed great toxicity to F. brasiliensis. Acute toxicity of mercury has been studied in several crustacean species (Munhoz et al. 1986, Kraus & Weis 1988, Marino-Balsa et al. 2000). For example, 96 h [LC.sub.50] values of Hg for Mysidopsis bahia have been obtained as 3.5 [micro]g Hg [L.sup.-l] (Lussier et al. 1985). In addition Artemia are adversely affected by organic mercury at concentrations <0.1 [micro]M, the lowest level examined in this study.

Studies of the effect on the respiration of decapod crustaceans demonstrated that the oxygen consumption rates decrease was related to Hg concentration, exposure time and larval stage (Amand et al. 1999). When fiddler crab (Uca pugilator) larvae were exposed to 180 ppb Hg for 6 h, DeCoursey and Vernberg (1972) observed a decrease of 28% for the zoeae III stage and 62% for the zoeae V stage. McMahon (2001), in a review of the responses of aquatic crustaceans in low ambient dissolved oxygen, mentioned that many crustaceans possess an excellent regulatory ability in their oxygen consumption patterns and thus were called oxygen regulators. Our experiments also obviously demonstrate that oxygen consumed by F. brasiliensis showed no linear conformation to ambient oxygen levels regardless of whether the shrimp were exposed to a heavy metal (Fig. 1). Despite their regulatory capability, the oxygen consumption rate was indeed inhibited after F. brasiliensis was exposed to high concentrations of Hg (Fig. 1). Similar results were also found in different shrimp species (Amand et al. 1999, Chinni et al. 2000, 2002).

Respiratory impairment in crustaceans caused by exposure to heavy metals was also reviewed (Spicer & Weber 1991), and it was concluded that oxygen consumption generally decreases when crustaceans are acutely exposed to heavy metals. In addition, after exposure to a sublethal concentration (1.44 ppm) of lead (Pb) for 30 days, it was evident that Pb inhibits oxygen consumption in P. indicus; similar results have been obtained in other crustaceans studied (Chinni et al. 2000). Those authors assumed that cytologic damage should be related to the decrease in oxygen consumption because the gills are most likely to be the first target of waterborne heavy metals, including thickening of branchial epithelium and deep changes in hemolymph patterns in the gills with a concomitant increase in vacuolization and reduced hemolymph spaces causing perfusion stagnation. Cytologic and histologic damage caused by heavy metal exposure in P. japonicus was also reported (Soegianto et al. 1999a, 1999b). For example, an increased number of nephrocytes in gill filaments, a blackened appearance of the gills, necrosis of gill cells resulting in narrowed or obstructed hemolymphatic vessels, the appearance of a space between the cuticle and the epithelial cells, which contains black electron-dense material and even fragmentation of nuclei within gill cells could be observed when P. japonicus were exposed to different concentrations of heavy metals. Thus, the main pathologic effect on the respiratory system caused by Hg is the interference of the respiratory system, including cellular respiration (Spicer & Weber 1991, Koizumi et al. 1994).

Ammonium is one of the final products after catabolism, mainly of amino acids that might have an alimentary or muscular origin, depending on nutritional conditions (Mayzaud & Conover 1988). In addition to being used as energy substrates and components of body structures, amino acids can be more important than ions in the maintenance of osmotic pressure in prawns such as P. setiferus (McFarland & Lee 1963, Rosas et al. 1999). Normally, increases in ammonium excretion reflect an increase in catabolism of amino acids. However, when exposed to lethal concentrations of heavy metals, dysfunction of ammonium excretion control follows after gill damage. Chinni et al. (2000, 2002) found that ammonium excretion was inhibited in P. indicus postlarvae exposed to sublethal concentrations of lead. Although there is still no confirmed evidence, it is assumed that the decrease in ammonia-N excretion by P. indicus postlarvae in the presence of toxicants can be attributed to a reduction in the metabolic rate or to an interaction of lead with pathways for the production of ammonia-N. Differences with this study may be because of the metal used and their concentrations, shrimp species used and other abiotic factors such as salinity and temperature. However, much effort still needs to be devoted to determining the relationship between heavy metal exposure and ammonium excretion to verify these related questions.

CONCLUSION

In this study, Hg showed great toxicity to F. brasiliensis. Hg caused an inhibition in oxygen consumption down to 53.42% of the control. However, after separate exposure to Hg, elevations in ammonium excretion were obtained, up to 217.64%, of the control.

ACKNOWLEDGMENTS

The authors thank the Financiadora de Estudos e Projetos (FINEP/Convenio no 01.04.0030.00), for support.

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EDISON BARBIERI, (1) * ELISANGELA A. PASSOS (2) AND CARLOS A. B. GARCIA (2)

(1) Instituto de Pesca--APTA--SAA/SP. Caixa Postal 61, Cananeia, 11990-000, Sao Paulo Brazil;

(2) Departamento de Quimica da Universidade Federal de Sergipe, Av. Marechal Rondon s/n, Jardim Rosa Elze, 41100-000, Sao Cristovao. (SE) Brazil

* Corresponding author. E-mail: [e-mail]

TABLE 1.
pH and oxygen concentration of the solution in each concentration
of the HgS[O.sub.4] at the temperature of 25[degrees]C.

                                    Oxygen
Concentration                    Concentration
of HgS[O.sub.4]                  (mL[O.sub.2]
(mg [L.sup.-1])        pH         [L.sup.-1])

0.00                  8.20           6.22
0.01                  8.21           6.20
0.05                  8.20           6.25
0.10                  8.21           6.23
0.25                  8.21           6.20
0.50                  8.20           6.23
1.00                  8.22           6.22

TABLE 2.
Acute toxicity of Hg for Farfantepenaeus brasiliensis
larvae ([LC.sup.50]) with 95% confidence limits)
calculated by.

                                  Confidence
                  [LC.sub.50]      Limits,
                 HgS[O.sub.4] *     95% **

24h                  0.197        0.123-0.317
48h                  0.080        0.050-0.127
72h                  0.070        0.043-0.113
96h                  0.067        0.037-0.123

                  [LCS.sub.50]    Confidence
                       HG        Limits, 95% **

24h                  0.133        0.083-0.214
48h                  0.054        0.034-0.086
72h                  0.047        0.029-0.077
96h                  0.045        0.025-0.083 *

Concentration of HgS[0.sub.4] mg [L.sup.-1],

** average 5 replicates.

TABLE 3.
Cumulative percentage mortality (%) of F. brasiliensis exposed to
various mercury concentration (mg Hg [L.sup.-1]) for 96 h in five
replicates. It was used 75 individuals to calculate the percentage in
each concentration.

                                 Exposure Time (h)
Hg
Concentration         0 h            24 h           48 h

Control                0               0             0
0.01                   0               0            6.66
0.05                   0               0            26.6
0.10                   0             26.66           60
0.25                   0              60            100
0.50                   0              80            100
1.00                   0              100           100

                         Exposure Time (h)
Hg
Concentration         72 h           96 h

Control                0               0
0.01                   20            46.6
0.05                   40             56
0.10                   80             92
0.25                  100             100
0.50                  100             100
1.00                  100             100