ABSTRACT Sipuncula is a group of unsegmented, coelomate, bilaterally symmetrical and wormlike marine deposit-feeding benthic macro-invertebrates. They are abundant along the intertidal habitats and around 150 species have been recorded worldwide. Sipunculans likely accumulate many anthropogenic contaminants by feeding on the contaminated sediments or benthos, and since they are edible marine species and have long been used in Southwest Pacific cuisine, these contaminants may transfer into the food chain and affect the public health of humans.Trace elements and PAHs are the most common contaminants that appear in the coastal areas worldwide. These contaminants lead to losses in oceanic yield and may hazardous effects on health when they enter the food chain. The previous studies of sipunculans along the coast of Taiwan were mostly focused on the taxonomy and distribution of the species living on the hard -ground habitat (rubble), one of the purposes of this study is aiming at the sipunculans on the seagrass beds and wetland soft-ground habitat for species diversity, population dynamic and ecological related research. Among 10 native sipunculans species, Sipunculus nudus is the dominant species living at the segrass bed and wetland in Taiwan. Siangshan wetland locates at the area of industrial pollution and S. nudus here is a species with aquatic economy, but is known limitedly of its population and environmental physiology. Therefore, the other purpose of this research is to investigate the S. nudus population in this wetland and analyze the trace elements: arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), and zinc (Zn) and the organic pollutants: PAHs, in order to understand the bioaccumulation of S. nudus in the above two types of soft-substrate regarding the pollutants. The main results of this study are as follows: (1) This study represents a pioneering investigation in Taiwan regarding the species diversity of sipunculans on the seagrass meadow soft-ground sediment, which describes the species and their morphological characteristics. A total of 42 specimens 10 sipunculan species belonging to 3 families were identified. (2) It is the first time that S. nudus is realized as the dominant species in these soft habitats. Because of its cultivation potential, its population dynamics and reproductive cycle in Siangshan wetland were studied. The population size of S. nudus in this area is 0.965¡Ó 0.46 ind.m-2 and the sex ratio is around 1:1; the reproductive period is from April to September in the year with the peak reproductive period from May to August; the body length of matured individual is over 11cm. (3) This is the first report in Taiwan on the trace elements in sipunculan and their surrounding sediments. The bioaccumulation characteristics of arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), and zinc (Zn) were analyzed and compared in two sipunculan species, S. nudus and Siphonosoma vastum, which were collected from seagrass beds and wetlands in Taiwan. Both sipunculans in the these soft habitats had a high Cu bioaccumulation mechanism. Multivariate analysis, principle component analysis, and partial least squares for discriminant analysis of trace element levels and bioaccumulation factors were used to distinguish the element distributions that corresponded to the two habitats (seagrass beds and wetlands). Different levels of certain trace elements in these two sipunculan species may result not only from the environmental factors of various habitats but also from the accumulation characteristics of various species. The As, Cd, Cr, Cu, Hg, and Zn concentrations were markedly lower in sipunculan than in other invertebrates from the adjacent polluted regions. (4) This is the first report on the accumulation situation of PAHs in different body tissues (tentacle, nephridium, intestine, and muscle) of S. nudus in Taiwan. It was found that the smaller individuals had higher amount of PAHs with higher percentage of low-ring PAHs and they were mostly concentrated in the nephridium. This study provides the bioaccumulation pattern of PAH in the tentacle, nephridium, intestine, and muscle of S. nudus. The logKow and logBAF parameters show that S. nudus may excrete or decompose high-ring PAHs via biotransformation. By comparing the t-PAH of S. nudus with that of the surrounding substrate and invertebrates along the coasts of other countries, the t-PAH content in the muscle of S. nudus is lower than the sediment and the invertebrates. As a result, I suggest removing the internal organs and utilizing the muscle portion as good to reduce the uptake and accumulation of PAHs from the sipunculans.