Delineating metal accumulation pathways for marine invertebrates

Delineating the routes of metal uptake in marine invertebrates is important for understanding metal bioaccumulation and toxicity and for setting appropriate water and sediment quality criteria. Trace element biogeochemical cycling can also be affected if the rates of metal uptake and regeneration by marine animals are dependent on the routes of metal accumulation. In this paper we review recent studies on the pathways of metal accumulation in marine invertebrates. Both food and water can dominate metal accumulation, depending on the species, metal and food sources. Trace elements which exist in seawater primarily in anionic forms (e.g. As and Se) are mainly accumulated from food. For metals that tend to associate with protein, uptake from water can be an important source. Kinetic modeling has recently been used to quantitatively separate the pathways of metal uptake in a few marine invertebrates. This approach requires measurements of several physiological parameters, including metal assimilation efficiencies (AE) from ingested food, metal uptake rates from the dissolved phase, and metal efflux rates (physiological turnover rates) in animals. For suspension feeders such as mussels and copepods, uptake from the dissolved phase and food ingestion can be equally important to metal accumulation. Metal AE and partition coefficients for suspended particles, which are dependent on many environmental conditions, can critically affect the exposure pathways of metals. For marine surface deposit feeding polychaetes such as Nereis succinea, nearly all metals are obtained from ingestion of sediments, largely because of their high ingestion rates and low uptake from solution. The bioavailability of metals from food and the trophic transfer of metals must be considered in establishing water and sediment quality.