Heavy‐metal and microbial depuration of the clam Ruditapes decussatus and its effect on bivalve behavior and physiology

Abstract

The bivalve Ruditapes decussatus was evaluated as a possible biomonitor of heavy-metal contamination. Concentrations of copper (Cu), cobalt (Co), iron (Fe), nickel (Ni), and manganese (Mn) were measured in R. decussatus. Water and sediment samples were collected at two stations of Timsah Lake in Ismailia, Egypt, from October to November 2002, using atomic absorption spectrophotometry (AAS). Results from the heavy-metal and microbial analyses indicated that site II was less contaminated than site I. The bivalve showed accumulation of metals, with a bioaccumulation factor (BAF) greater than 4. The bioaccumulation of metals varied strongly according to the sampling site. After 48 h of depuration, Fe, Ni, Co, Cu, and Mn were reduced significantly, to 46.8% and 47.7%, 19.9% and 20.3%, 27.3% and 27.9%, 35.9% and 36.6%, and 18.2% and 26.6%, compared with the initial concentrations, in clam tissue at the two stations. In bivalves from site II the counts of total bacteria, fecal coliforms, and bacterial pathogens were reduced by more than 90%, whereas phage counts were only reduced by 56% after 4 days of depuration. The depuration of bivalves collected from the heavily polluted site (I) was not effective, as coliforms were reduced only by 85% after 4 days, whereas counts of pathogens and pathogenic indicators such as Vibrio, fecal Streptococcus, and coliphage decreased to less than 50% of the initial concentration. The time necessary to decrease contamination to 10%, 50%, and 90% for clams at both stations was consistently shorter for heavy metals than for microorganisms. Investigation of the effects of heavy-metal and microbiological depuration on valve movement and physiological rates (oxygen consumption and ammonia excretion) was carried out on R. decussatus to test the utility of physiological stress indices in assessing the health of depurated animals. Clams in the experimental tanks exhibited various states of activity, which were rated by identifying and scoring (0-4) the different parameters, including shell gap, siphon extension, and foot protrusion. Moreover, an increase in ammonia excretion was usually associated with an increase in respiration rate. The oxygen-to-nitrogen ratio provided a sensitive indicator of bivalve health. It can be concluded that shellfish monitoring and depuration data depended on the initial concentration of the pollutants. There were differences in the physiological responses of clams from the two sites during the periods of pre- and postdepuration of the contaminants. There was a significant correlation between reduction of metal concentration in clam tissue and enhancement of valve movement, as well as activity and increasing respiration rate.

The effect of depuration on heavy metals, petroleum hydrocarbons, and microbial contamination levels in Paphia undulata (Bivalvia: Veneridae)

“…This contradiction may be due to the degree of pollution, time of exposure and the molluscan species. Many studies reported the ability of bivalves to accumulate bacteria (Humphrey and Martin, 1993;El-Shenawy, 2004). At the same time, they can purge themselves of bacterial contaminants within 48 h, or they may be held up to 72 h in some cases (Dore and Lees, 1995).…”

“…On the other hand, depuration was found to be effective against Salmonella typhimurium after 72 h and Vibrio parahaemolyticus after 36 h in Mytilus galloprovincialis (Barile et al, 2009). El-Shenawy (2004) correlated the depuration of bacteria to the degree of pollution, for example Vibrio was reduced by less than 50% when collected from a heavily polluted site. In Paphia undulata the counts of pathogenic bacteria were reduced by 83% after three days of depuration.…”

“…To reduce or minimize the risk, the source of the shellfish should be investigated and better quality would be attained by appropriate treatment following the harvest. The best and the easiest strategy that has been developed for bivalve risk management is utilizing their capacity to eliminate pathogenic microorganisms and toxic substances when bivalves are kept in clean disinfected seawater tanks (Wong et al, 1997;Sobsey and Jaykus, 1999;El-Shenawy, 2004). The above method (depuration) helps bivalves to expel and separate contaminants from their gills and intestinal tract over a period of time and prevents their recontamination.…”

“…The above method (depuration) helps bivalves to expel and separate contaminants from their gills and intestinal tract over a period of time and prevents their recontamination. Although Arnold (1991) restricted the role of this type of depuration to remove bacterial contamination, others emphasized and encouraged it for reducing the toxicity of heavy metals (Wilson, 1980;Hung et al, 2001;El-Shenawy, 2004;Katayon et al, 2004) and petroleum hydrocarbons (Lee et al, 1972;Stegman and Teal, 1973;Linden et al, 1979;Clement et al, 1980;Rainio et al, 1986;Rantamaki, 1997). Many factors influence the degree of depuration as follows: the system design, initial water quality, oxygenation and flow rates, salinity, temperature, shellfish-to-water ratios, removal and settlement of faecal material, types of pollutants in seawater and the period of purification (Lee and Younger, 2002;Manfra and Accorneo, 2005).…”

“…A few studies used clams containing naturally high concentrations of heavy metals and followed their depuration at a relatively clean field (Okazaki and Panietz, 1981;El-Shenawy, 2004). No information was reported for Paphia undulata depuration or the impact of their depuration on survival.…”