Larval anurans with synchronous and asynchronous development periods: contrasting responses to water reduction and predator presence

Abstract

Summary The larvae of pond breeding frogs experience high rates of mortality, predominantly through predation and pond desiccation. We investigated how the tadpoles of two common species of frog respond to these hazards using factorial experiments conducted in a laboratory. The eastern brown froglet Crinia signifera Girard spawns in small, highly ephemeral ponds, while the spotted marsh frog Limnodynastes tasmaniensis Günther breeds in more permanent water bodies. Prior to our experiments it was known that C. signifera siblings develop quickly and reach metamorphosis comparatively synchronously ( c . 25–35 days), while L. tasmaniensis grow and develop more slowly and time to metamorphosis is highly asynchronous ( c . 60–300 days). It was anticipated that these species would have different strategies for coping with declining water and predators. A separate experiment was conducted for each species. The factors were Hydroperiod (either constant or gradually declining water volume) and Predator (either the presence or absence of a restrained mosquitofish Gambusia holbrookii Girard). A random block design was used with six replicates for each possible combination of the factors ( N = 24). Each replicate consisted of a 13‐l tub of water containing 20 tadpoles. Newly emerged frogs were transferred to one of 24 vivaria. In the arrangement of the vivaria, we maintained the block design of the experiment. Crinia signifera tadpoles responded to declining water by accelerating development and so reduced time to metamorphosis. However, the resulting metamorphs were smaller and survival rate in the terrestrial stage was significantly lower. This is a clear demonstration of the benefit and cost of phenotypic plasticity in metamorphic traits. Limnodynastes tasmaniensis tadpoles experiencing declining water were also smaller at metamorphosis, but there was no evidence that they had reduced the time to metamorphosis compared with those in constant water. Moreover the survival rates of the resulting frogs did not differ. Consequently no adaptive plasticity can be inferred. The data suggest that L. tasmaniensis has a bet‐hedging strategy in which some siblings develop more quickly at the expense of size, while others take much longer but are bigger at metamorphosis as a result. Tadpoles of both species showed clear behavioural responses to the presence of the predator. They tended to occupy the non‐predator halves of the tubs in replicates where a mosquitofish was present. Neither species altered development rate in the presence of the predator and there was no detectable impact on survivorship in the terrestrial stage.