The Japanese Common Toad, Bufo japonicus formosus, Contains Toxin in the Egg Stage

Abstract

Bufonid toads generally possess cardiotoxic steroids called bufadienolides as defensive chemicals. Although knowledge of the life stages at which the toad species possess the poison is important for our understanding of diversity of toxicity among bufonid toads, this knowledge is limited. In the present study, we revealed that the Japanese common toad, Bufo japonicus formosus, possesses toxins at the unfertilized egg stage by conducting a bioassay experiment. Recent studies documented that hatchlings of B. j. formosus have lethal toxic effects on native frog tadpoles (Rana pirica) in the invasive area of the toad (Hokkaido). In our bioassay experiment using R. pirica tadpole as a predator, no tadpoles died when they did not consume any prey item during two-days experimental period. However, approximately 90% of R. pirica tadpoles immediately died when they consumed an unfertilized egg of B. j. formosus. These results suggest that the toxin at the early life stages of B. j. formosus is, at least partly, provided from female parent.

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

“…However, we consider it unlikely that the individual captive female toad was uniquely capable of biosynthesizing toxin from non-toxic prey. So far, we have confirmed that wild B, j. formosus embryos collected from various geographic regions exhibit significant toxicity to R. pirica tadpoles without exception (Kazila and Kishida 2019;Oyake et al 2020;Okamiya et al 2021). Furthermore, the toxicity of embryos of the captive-reared toad to R. pirica in our experiment was comparable to the toxicity of wild toad embryos.…”

“…This indicates that B. bufo tadpoles synthesize their toxins de novo, as confirmed by histological studies (Delfino et al 1995). Because unfertilized eggs and fertilized embryos of B. j. formosus contain toxin (Okamiya et al 2021; present study), the toxin present in eggs and embryos of this species is probably due to maternal provisioning. In this context, our study provides insight into the synthesis of toxicity in B. j. formosus, as we showed that embryos spawned by a captive-reared female toad exhibit high toxicity.…”

Native frogs (Rana pirica) do not respond adaptively to alien toads (Bufo japonicus formosus) 100 years after introduction

“…Hakodate-A (see above for location) to test with frog tadpoles. We believe these toad populations to be representative of toad toxicity because previous studies testing multiple B. j. formosus clutches from various populations on Hokkaido have consistently shown highly toxic effects on native frog tadpoles (i.e., 100% mortality: Kazila & Kishida, 2019;Okamiya et al, 2021;Oyake et al, 2020).…”

“…Recent studies have documented that B. j. formosus embryos and hatchlings have lethal toxic effects on native frog tadpoles (Rana pirica) on Hokkaido (Kazila & Kishida, 2019;Okamiya et al, 2021;Oyake et al, 2020). In invaded areas, both native and alien anurans often share breeding sites, and in such natural ponds, R. pirica tadpoles frequently experience mass mortality shortly after hatching of toad eggs (Kishida and Yamaguchi, unpublished data).…”

Utilizing environmental DNA for wide-range distributions of reproductive area of an invasive terrestrial toad in Ishikari river basin in Japan

“…268 km). This species has a neurotoxin (bufotoxin) in their body through their life cycle, and it has been reported that their embryos and hatchlings has lethal toxic effects on larvae of native amphibian species in Hokkaido (i.e., Hokkaido brown frog Rana pirica, Hokkaido salamander Hynobius retardatus) (Kazila & Kishida 2019;Oyake et al 2020;Okamiya et al 2021). The toxic effects (Kazila & Kishida 2019) suggest the potential threats of B. j. formosus as a substantial cause of declines of the native predators in invaded regions.…”