Are toxic effects of alien species affected by their prey? Evaluation by bioassay with captive-bred toad embryos and a vulnerable predator

Toads of the Family Bufonidae possess neurotoxins (bufadienolides) which are generally considered to be synthesized de novo. Thus, invasive toad species pose a threat to native predators via toxic effects. However, the influence of diet on toad toxicity is poorly understood. We evaluated the effect of diet on toxicity of embryos of the invasive toad Bufo japonicus formosus in Japan, using native tadpoles (Rana pirica) as a bioassay. Specifically, we compared the toxicity of embryos spawned by captive toads reared entirely on non-toxic prey versus embryos spawned by wild toads. Embryos of captive-reared toads and wild toads were comparable in terms of toxic effects: all tadpoles that consumed a toad embryo died, irrespective of origin of embryos. Embryo toxicity appears to be via parental (likely maternal) provisioning. Our study indicates that adult B. j. formosus produce toxins (likely bufadienolides) in amounts sufficient for embryos to be fatal to native predator tadpoles irrespective of whether the adult diet contains toxins. Thus, the toxic effects of invasive B. j. formosus on R. pirica, and possibly on other sensitive native predator species, are likely to be widespread and occur regardless of variation in prey biota in invaded areas in Japan.