Gender and size effects of metal bioaccumulation on the rock crab, Thalamita crenata, in Dapeng Bay, southwestern Taiwan

Abstract

Most benthic crustaceans with restricted mobility live on benthic chippings. However, they frequently come in contact with bed mud which is often the final destination of various pollutants and, therefore, contains the highest concentrations when compared with any other medium in an aqueous environment (Chapman, 1989; Chapman et al., 1998). It follows that, since heavy metal accumulation in benthic crustaceans generally reflects the status of the pollution of an environment (Dixit and Witcomb, 1983; Rainbow, 1990, 1996), concentrations observed can be an indicator of heavy metal pollution (e.g. Uthe et al., 1986, 1987; Chou and Uthe, 1993; Park and Presley, 1997; Sanders et al., 1998; Chou et al., 2000; MacFarlane et al., 2000). In the family of Portunidae, the rock crab Thalamita crenata is the largest among the 17 species of the genera Thalamita found in Taiwan. It is one of the most widelyeaten crabs in the bayou areas of the country (Huang and Yu, 1997). Their main habitat, at a depth of 1– 5 m, is tideland with reef and bedload. Typically, these benthic crustaceans can be traced to tideways that can be reached by seawater. Owing to their predominance in Dapeng Bay, and their wide distribution in the bayou areas of Taiwan, T. crenata has long been believed to be a good bioindicator of such environments. However, for biomonitoring purposes, two biological factors, namely body size and gender, need to be evaluated before any decision as to their indicator potential can be confirmed (Rainbow, 1996). In addition, establishing baseline data is essential before selecting an ideal bioindicator. With this in mind, we determined the metal concentrations in T. crenata prior to Dapeng Bay being designated as a national park, and in so doing, we investigated and determined the best method for future biomonitoring. Dapeng Bay, located at Donggang (Tungkung), Pingtong county, in southwestern Taiwan, has a water depth of 3–5 m and covers an area of 532 ha. It is a traditional agricultural and aquacultural area, with rice paddies and various ponds for the culture of grass shrimp, grouper and eel, among others. With a population density of approx. 1500/km (2.5 times greater than the national average), the Bay receives a substantial amount of local domestic as well as aquaculture wastewater. In addition, by 2001, oyster farming prevailed over the entire surface waters of the Bay. With only a single opening in the north, the water exchange is poor in the inner southern part of the Bay. By and large, salinity, pH, DO and BOD are being maintained at acceptable levels, namely 29–32, 7.6–8.6, 4–11 mg O2/L and 1.6–2 mg/L, respectively. The Zn, Cu, Cd, Pb, As and Se concentrations of the water have been determined as 4–20.1, 2–6, <0.1–0.7, <1–8, 1–4 and 1–2 lg/L, respectively. In addition, the Mn, Zn, Cr, Cu, Pb, Ni, Co, As, Se, Cd, Ag and Hg concentrations of the sediments are Edited by Bruce J. Richardson