Chemical suppression of embryonic cane toads Rhinella marina by larval conspecifics

1. Mechanisms that evolved to suppress the development of potential competitors may offer novel methods for species-specific control of invasive organisms. The tadpoles of cane toads Rhinella marina compete for limited food resources in small ponds and older tadpoles eliminate competitors not only by eating newly laid eggs, but also by releasing a chemical that suppresses development of conspecific eggs.

2. We conducted laboratory trials to assess the magnitude and generality of this suppression effect and to identify the developmental stages capable of producing the suppressor cue and those that are vulnerable to its presence. 

3. In all clutches that we tested, the suppressor cue reduced larval growth, development and survival (>95% mortality vs. <50% in controls). The suppressor exerts a general effect on larval development, rather than causing malformations of specific organ systems.

4. Free-swimming tadpoles of all developmental stages produce the cue (but are not affected by it), whereas embryos do not produce the cue, but are vulnerable to it. Tadpoles produce the cue consistently rather than facultatively (e.g. in response to detecting eggs). 

5. A preliminary trial shows that the cue is effective in natural spawning ponds as well as in the laboratory.

6. Synthesis and applications. Our data have strong implications for current methods of toad control (culling tadpoles will remove suppression effects on the next generation, so it may be better to maintain live toad tadpoles in secure containers rather than removing them from the pond). Longer-term, understanding the chemical basis of this powerful mechanism for intraspecific competition may facilitate invader control. Our data have strong implications for current methods of toad control (culling tadpoles will remove suppression effects on the next generation, so it may be better to maintain live toad tadpoles in secure containers rather than removing them from the pond). Longer-term, understanding the chemical basis of this powerful mechanism for intraspecific competition may facilitate invader control.