Species-Specific Differences in the Net Assimilation of Zinc, Cadmium, Lead, Copper and Iron by the Terrestrial Isopods Oniscus asellus and Porcellio scaber

Abstract

(1) Specimens of the woodlice Oniscus asellus L. and Porcellio scaber Latreille (Crustacea: Isopoda) were collected from a contaminated deciduous woodland (Haw Wood) 3 km downwind of a primary zinc, lead and cadmium smelting works, and from a similar but uncontaminated site (Wetmoor Wood). They were maintained for 20 weeks in factorial combination on leaves of field maple (Acer campestre L.) collected from the litter layer at the two sites. (2) Twelve isopods were removed from each of the eight experimental tanks every 4 weeks and dissected into three ''tisue compartments'' (hepatopancreas, gut and rest) in which the concentrations of zinc, cadmium, lead, copper and iron were determined by flame and flameless atomic absorption spectrometry. (3) There were considerable differences in the extent to which the two species accumulated or lost zinc, cadmium and lead during the experiment. For example, zinc was retained by contaminated P. scaber fed on uncontaminated leaf litter but was lost rapidly from the hepatopancreas of O. asellus fed on the same diet. In contrast, cadmium and lead were accumulated to a much greater extent in O. asellus than in P. scaber. By the end of the experiment, mean concentrations of cadmium and lead in the hepatopancreas of O. asellus were at least five times higher than in the hepatopancreas of P. scaber in every case. (4) Copper was accumulated by the hepatopancreas of both species in all experimental situations. (5) Concentrations of iron increased in all the woodlice during the experiment but the metal was stored predominantly in the gut, not in the hepatopancreas. (6) The possible reasons for the differences in the net assimilation of zinc, cadmium and lead between the two species, and for the substantial increase in the concentration of copper in the hepatopancreas of isopods fed on leaf litter from their ''own'' site are discussed. (7) Comparisons between the concentrations of metals in the tissues of healthy and moribund isopods suggested that death of individual woodlice in the laboratory was due to zinc poisoning. Critical concentrations of zinc in the hepatopancreas (on a dry weight basis) were about 15,000 .mu.g g-1 in O. asellus and 25,000 .mu.g g-1 in P. scaber. Populations of the two species collected from sites less than 3 km from the smelting works at Avonmouth, occasionally contain moribund individuals in which these critical concentrations of zinc in the hepatopancreas are exceeded.