Left High and Dry: Froth Nesting Allows Eggs of the Anuran Amphibian to Complete Embryogenesis in the Absence of Free-Standing Water
For amphibians that oviposit in temporary aquatic systems, there is a high risk of desiccation-induced offspring mortality when water evaporates prior to the completion of embryo or tadpole development. Such a strong selective pressure has led to the evolution of a variety of traits in reproducing females and their offspring to improve the odds of reproductive success when free-standing water is temporarily available. Herein, we describe an adaptive function of froth nesting in the Sandpaper Frog, Lechriodus fletcheri, as a possible maternal strategy for protecting embryos from the immediate risk of desiccating in highly ephemeral pools that frequently dry prior to hatching. Field observations revealed that embryos located near the core of nests remained alive and continued to develop for several days after becoming stranded out of water due to declining water levels, with embryo viability maintained long enough for additional rainfall to recharge pools and support hatching of tadpoles into water in some cases. In laboratory trials, the proportion of embryos surviving in nests exposed to desiccating conditions was positively correlated with nest volume, while the rate of water loss relative to nest mass declined, both of which are likely a function of reduced surface area relative to volume in larger nests. We suggest that the encapsulation of embryos within an aerated mucus shields them from desiccation by trapping moisture around their external surfaces. As embryos of L. fletcheri complete development rapidly, the ability for the froth nest to protect against desiccation for several days may allow embryogenesis to be completed largely out of water despite the larval phase not being terrestrial. These results suggest froth nesting has played an important role in facilitating this species' use of ephemeral habitats that most other amphibians with aquatic reproductive modes are incapable of exploiting.
Froth nests of L. fletcheri stranded on land after their pools dried. The left photograph shows live embryos in moist leaf litter just prior to hatching, within 24 hours of water levels receding. The right photograph shows two nests at the base of a completely dried pool more than 48 hours after free-standing water had evaporated. The top, exposed surface of these two nests had dried with residing embryos in this layer desiccated, while deeper mucous layers remained moist with embryos still alive.
Physical properties of froth nests of L. fletcheri prior to desiccation. Relationships shown include (A) initial nest volume versus initial surface area, (B) initial nest volume versus initial surface area to volume ratio, (C) initial nest volume versus initial weight, (D) initial nest volume versus total egg number, and (E) initial nest volume versus the proportion of eggs present on the periphery compared to the core of nests.
Differences in desiccation rate of embryos of L. fletcheri in two froth nests after 48 hours out of water. Note the desiccated eggs at the surface of both nests, and the nearly fully developed embryos in the core and around the base of nests (marked by arrows).
The effects of nest size of L. fletcheri on desiccation after 48 hours in the absence of free-standing water. Relationships shown include (A) initial nest weight versus nest weight change, (B) initial nest weight versus the proportion of nest weight change, and (C) initial nest volume versus the proportion of eggs that desiccated.
Contributor Notes
Associate Editor: D. S. Siegel.