ABSTRACT
Pioneering X-ray microanalytical work at Menai Bridge on zinc and copper in barnacles indicated the potential role of insoluble granules in the detoxification of accumulated toxic heavy metals in marine organisms. Barnacles accumulate heavy metals, with greater body concentrations in regions of high heavy metal availability. Zinc is accumulated by all barnacles as granules of zinc phosphate and in contrast to the decapod crustaceans, excretion plays little part in regulating the zinc content of the barnacle body. The number of granules and the body zinc concentration increase over the lifetime of barnacles, to levels as high as 153,000 ppm dry wt (Balanus improvisus; Thames Estuary, UK). The granules may also contain other heavy metals (Fe, Mn, Pb) and may represent a generalised metal detoxification system. The formation of these zinc phosphate granules takes place in the stratum perintestinale and prosomal parenchyma; barnacles lack a hepatopancreas, which is a central site of heavy metal detoxification in malacostracan crustaceans. Copper is excreted by barnacles to a greater degree than zinc, but the bodies of barnacles from an area of very high copper availability accumulated atypically high Cu concentrations (3750 ppm dw) and contained organic copper-rich deposits. Granules containing iron are lost from the midgut of barnacles. Iron and manganese are the major trace metal components of the calcareous shell plates, and the manganese concentration reflects ambient manganese availability. Barnacles may be useful indicators of high heavy metal availability if precautions are taken in the collection and analysis of data.
