Volume 21/Number 7/July 1990 Alarine Pollution Bulletin, Volume21, No. 7, pp. 321-324, 1990. Printed in Great Britain. 0025-326X/90 $3,00+0.00 O 1990 PergamonPress pie Viewpoint is a column which allows authors to express their own opinions about current events. The Significance of Trace Metal Concentrations in Marine Invertebrates A Need for Laboratory Investigation of Accumulation Strategies PHILIP S. RAINBOW, DAVID J. H. PHILLIPS and MICHAEL H. DEPLEDGE Philip Rainbow is a Reader in Marine Biology at Queen Mary and Westfield College, University of London.' David Phillips is Divisional Director, Environmental Services, Mott MacDonald, and has extensive experi- ence of biomonitoring in Australia, Scandinavia, Hong Kong and USA. Michael Depledge holds the Chair of Ecotoxicology at Odense University, Denmark, with particular interest in the role of physiological monitoring in toxicity studies. Much of the literature on trace metal concentrations in marine invertebrates contains lists of data, with authors not in a position to comment on the significance of results. The reader again and again is left with the ques- tion 'so what?'. This viewpoint makes a plea for labora- tory experiments as a necessary complement to field data to gain information on the metal accumulation strategies available to marine organisms. An under- standing of the accumulation strategy adopted by a particular invertebrate will then place the experimenter in a position to conclude whether a measured trace metal concentration in an organism is high, low or of little significance. Whether essential or not, all trace metals are poten- tially toxic at a threshold bioavailability. Factors affect- ing toxicity include those influencing metal uptake and those affecting the organism's ability to handle and detoxify accumulated trace metals. ' Marine invertebrates are bathed in a solution of trace metals dissolved at concentrations ranging from nano- grams per litre in the open ocean (e.g. Ag, Hg) to micro- grams per litre in coastal seas (e.g Cu, Zn). Despite the relatively low concentrations of trace metals in the surrounding medium, marine organisms take up and accumulate them in soft tissues to concentrations orders of magnitude above ambient environmental levels. In many cases, the bioavailable forms of dissolved trace metals appear to be the free metal ions, although how these ions cross cellular barriers is unclear. Typically, concentrations of the free metal ion (resulting from com- plexation equilibria) are proportional to the total dissolved concentrations of the trace metals, much of which in seawater may be present in the form of in- organically or organically chelated complexes--e.g. CdC12", CuCo3°, AgCI2-, etc. One model of the uptake of trace metals from solution is via passive diffusion along a concentration gradient, involving the binding of the free metal ion to a carrier in a facilitated diffusion process. The concentration gradient is maintained because the internal concentration of diffusable metal species is maintained at a level below that of the external concen- tration. This may result from the rapid binding of such metal species in the organism to non-diffusable intra- cellular ligands of high molecular weight prior to trans- fer to blood proteins, in turn to be removed from the site of metal uptake in the circulation. Trace metals are also accumulated from other sources including food, in this case by the absorption of bioavailable forms of the metal after digestion in the alimentary tract. A variable pro- portion of any trace metal taken up by a marine inverte- brate will remain passively adsorbed onto the outside of the animal and not be available to metabolic processes. Thus, trace metals are taken up by marine inverte- brates with the potential for accumulation to high body contents and concentrations (content per unit dry weight). Ma@ factors affect the uptake of trace metals, including extrinsic physico-chemical factors controlling the metal bioavailability: e.g. dissolved metal concentra- tion, temperature, salinity, presence or absence of chelating agents, presence and absence of other metals; and intra- and inter-specifically variable intrinsic factors such as surface impermeability, nutritional state, stage of moult cycle, throughput of water by osmotic flux, etc., many of which are in turn affected by physico-chemical factors. The bo~ly content of a trace metal in any organism 321