Groups of Neanthes arenaceodentata were fed different diets for four weeks then exposed to copper in seawater to determine if nutritional history would affect copper toxicity. Two experiments were conducted in which mortality was the endpoint. In the first experiment, the 28-day LC₅₀ concentrations were > 98 µg copper (Cu)/L for polychaetes fed a combination diet of cultured fresh Ectocarpus siliculosus and TetraMarin, 80 µg Cu/L for those fed field-collected dried Enteromorpha clathrata (fed rehydrated), and 38 µg Cu/L for those fed cultured fresh Enteromorpha clathrata. The greater sensitivity of worms fed the fresh E. clathrata may have resulted because the fresh E. clathrata accumulated copper while in the experimental system, therefore, worms fed this diet were probably also exposed to copper through the food. Metal analysis of the food revealed that the mean copper concentration of fresh E. clathrata was five to six times higher than the mean copper concentration of dried E. clathrata or TetraMarin after 48 h in any of the copper-dosed treatments. Mean copper concentration of the fresh E. siliculosus was three to five times higher than dried E. clathrata or TetraMarin after 48 h in the experimental system. However, it is uncertain whether worms fed the combination diet of E. siliculosus and TetraMarin were affected by the copper in the fresh alga, because much of the alga was used for tube construction. It is not known how much alga the worms actually consumed. Therefore, in the second experiment, live alga diets and combination diets were not used to avoid these problems.

In the second experiment, diet had an effect on the short-term results (4 to 10-day LC₅₀ values), but not the long-term results (28-day LC₅₀ values). Worms fed prawn flakes were significantly more resistant to copper than worms fed dried E. clathrata; the 10-day LC₅₀ values were 246 and 124 µg Cu/L, respectively. The 28-day LC₅₀ values for these two diets were not significantly different (83 and 86 µg Cu/L). The short-term LC₅₀ values in Experiment 2 (where diet-toxicant interaction was minimized) differed by less than a factor of 2, which is within the range of variability (1.18 to 3.31) in an interlaboratory comparison with this species.