Trophic accumulation and depuration of mercury by blue crabs (Callinectes sapidus) and pink shrimp (Penaeus duorarum)

Abstract

Mercury concentrations in blue crabs (Callinectes sapidus) collected from an area of mercury-contaminated sediments in Lavaca Bay, TX, USA, are more than an order of magnitude greater than concentrations in penaeid shrimp from the same area. Laboratory feeding experiments using mercury-contaminated fish as food showed that both blue crabs and pink shrimp (Penaeus duorarum) could accumulate mercury concentrations comparable to those in their food in 28 days. Calculated mercury assimilation efficiencies averaged 76% for blue crabs and 72% for pink shrimp. Significant depuration of mercury by blue crabs was not observed during a subsequent 28-day period, but pink shrimp lost mercury at a rate of about 0.012 day⁻¹. Model calculations predict biomagnification factors of mercury of about two to three at steady state for both species. The large difference in observed concentrations of mercury in field-collected blue crabs and penaeid shrimp does not result from differences in efficiency of mercury assimilation from their food or from differences in excretion rates. It is more likely the result of differences in residence times in the contaminated area and of differences in feeding habits.

Introduction

Mercury came to the forefront of environmental concerns in the 1960s and 1970s with cases of mercury poisoning such as occurred in Minamata Bay, Japan (Harada, 1966, Irukayama, 1967), and in Iraq (Bakir et al., 1973). Today there are approximately 34 states in the USA with fish consumption advisories for mercury [R. Hoffman (EPA), Washington, DC, personal communication]. However, only seven states have estuarine or marine areas closed to finfish and shellfish consumption due to levels of mercury greater than the Food and Drug Administration action level of 1.0 μg g⁻¹ wet weight as methyl mercury. One of these states is Texas, in which a chlor-alkali plant operated from 1966 to 1979 at Point Comfort, discharging mercury-contaminated wastewater from 1966 to 1970 into Lavaca Bay and into lagoons on a dredge material island in the bay west of the plant. As a result of monitoring by the Texas Department of Health for the past 29 years, a portion of Lavaca Bay (termed the “Closed Area”, Fig. 1) has been closed to crab and fish harvesting for consumption since 1988.

Higher trophic level aquatic organisms accumulate mercury mostly from the food they consume (Wiener & Spry, 1996). In Lavaca Bay, most food organisms are thought to derive their mercury in turn from sediments which have retained high concentrations of mercury since plant closure (Evans & Engel, 1994). Total mercury concentrations in blue crabs and in some species of fish have remained elevated as well, especially in and near the Closed Area. Since 1980, maximum measured concentrations of total mercury in muscle tissue have been 4.46 μg g⁻¹ wet weight for blue crabs, 6.62 μg g⁻¹ for black drum, 4.55 μg g⁻¹ for red drum, 2.92 μg g⁻¹ for sheepshead, and 1.68 μg g⁻¹ for southern flounder (Texas Department of Health, 1993). In contrast, total mercury concentrations in muscle tissue of harvestable-sized penaeid shrimp have not been measured above 0.28 μg g⁻¹ wet weight (Bowman, 1988). Historically, most mercury concentrations in such shrimp have been little different from the average of 0.07 μg g⁻¹ wet weight for the state of Texas (Texas Department of Health, 1993). As a result, the area has remained open to commercial harvesting of shrimp.

Lower concentrations of mercury in shrimp compared to blue crabs may be due to either exposure to lower mercury concentrations or less efficient accumulation and retention of ingested mercury. Penaeid shrimp, like blue crabs, are opportunistic omnivores. They are less predatory than blue crabs, however, depending more on plants and detritus for food (Britton & Morton, 1989, McTigue & Zimmerman, 1991). Because mercury (or at least methyl mercury) is generally biomagnified along food chains (Suedel, Boraaczek, Peddicord, Clifford & Dillon, 1994), organisms feeding low on the food web would be expected to have lower mercury concentrations.

Exposure to lower mercury concentrations could also result if shrimp spend only limited time feeding in the most contaminated part of the bay (Closed Area). Palmer (1992) and Palmer and Presley (1993) transplanted both brown shrimp and blue crabs into the Closed Area of Lavaca Bay from distant areas with lower mercury exposures. With emigration from the Closed Area prevented by caging, it was possible to test if these organisms could acquire elevated concentrations of mercury. Juvenile brown shrimp more than tripled their total mercury concentrations in 36 days of exposure, from 0.05 μg g⁻¹ wet weight (assuming a wet to dry wt. ratio of 7) to 0.17 μg g⁻¹ wet weight. Resident adult shrimp in the Closed Area had lower total mercury concentrations (0.03 μg g⁻¹ wet wt.: Woodward-Clyde Consultants, 1993; 0.03–0.13 μg g⁻¹ wet wt.: Presley, Palmer & Boothe, 1993), suggesting that resident shrimp had spent less than 36 days in the Closed Area (Palmer & Presley, 1993). Surprisingly, transplanted juvenile blue crabs showed no significant statistical increase in total mercury concentrations. Although natural variability may have obscured real uptake of mercury by crabs (Palmer, 1992), the turnover time of mercury in blue crabs may be substantially longer than in shrimp. Given longer exposure times, substantial mercury uptake might have been observed in crabs.

We have examined the potential accumulation and retention of mercury by pink shrimp and blue crabs in laboratory experiments using food with known concentrations of naturally accumulated mercury. Our studies demonstrate the ability of these animals to accumulate mercury from their food. If both species have similar uptake patterns when presented with mercury-laden food, then the observation of lower mercury concentrations in field-collected shrimp from the Closed Area would indicate differences in mercury exposure; shrimp either feed on food of lower mercury concentration than do crabs, or they do not remain in the area of exposure long enough for significant accumulation to occur.