Effects Of Water Level On Metamorphosis Duration,Morphological Phenotype And Immune Function In Two Amphibian Species

In the context of global climate change,the rate of water level desiccation severely affects the morphological phenotype and metamorphosis duration of post-metamorphosis amphibians.However,it is still unknown about the adaptive changes in aspects of their morphological phenotype and immune functions induced by water level desiccation.In this study,the Gosner 37 stages tadpoles of Bufo gargarizans and Pelophylax nigromaculatus from sympatric distribution in hydrostatic waters were selected,then were subjected to rapid,moderate and slow desiccation treatments,and with the constant water level group as the control group.The metamorphosis duration from Gosner stages 37 to 42 and 46,as well as the average metamorphosis survival rate was counted.The variations of body and internal organ size,responsiveness to phytohemagglutinin(PHA-P),percentages of any type of leukocyte and the ratio of neutrophils to lymphocytes(ratio of N/L)were measured in Gosner stage 46 toadlets and froglets.The results showed that:1.There were interspecific differences in both metamorphosis duration and average metamorphosis survival rate for Bufo gargarizans and Pelophylax nigromaculatus tadpoles.The metamorphosis duration from Gosner stage 37 to 42 or 46 for Bufo gargarizans displayed the highest value in the control group,and which was significantly greater in the control group than that of rapid,moderate or slow desiccation group,and the average metamorphosis survival rate for any group was more than 95%.The average survival rate of metamorphosis in Pelophylax nigromaculatus tadpoles ranged from 72.5% to 90.8%,with the highest value in the control group and the lowest value in the rapid desiccation group.2.In aspects of body and internal organ size,both toadlets and froglets showed different responsiveness to water level changes.There were no significant group differences in toadlets’ body and internal organ size.However,the body weight and body length of froglets in the control and slow desiccation groups were significantly higher than those in the moderate and rapid desiccation groups,and the ratio of body weight to body length,body width,wet weight of carcasses and wet weight index of lungs and fat body of froglets showed the minimum value in the rapid desiccation group.3.The maximal responsiveness to PHA-P in both toadlets and froglets occurred at 3 h after injection,but with different response patterns.In toadlets,it was significantly higher in the rapid and moderate desiccation groups than that in the slow desiccation group and the control group;In froglets,it was significantly higher in both the moderate and slow desiccation groups than that in the control group,and no significant difference were found between the rapid desiccation group and any other group.4.The morphological characteristics of different subtypes of leukocyte were similar in both toadlets and froglets,and no significant changes were observed.With accelerated water level desiccation,the percentage of neutrophils,eosinophils and the ratio of N/L increased,but the percentage of lymphocytes decreased in the blood of toadlets.In froglets,the percentage of neutrophils and the ratio of N/L increased,but the percentage of monocytes decreased.In summary,when exposed to the risk of shallower water levels,both amphibian species tadpoles accelerated their metamorphosis development processes,with smaller individuals,and taking decreased cell-mediated immune competence and elevated stress responses as their costs,which may be potentially detrimental to their future survival of toadlets and froglets.In addition,Pelophylax nigromaculatus tadpoles were more sensitive to water level changes than Bufo gargarizans tadpoles,and the morphological phenotypes and immune functions of the former were more susceptible to water level fluctuations,possibly corrected with specific survival environments and life history traits,but the mechanistic basis are waiting for further study.This study compared the variations of morphological phenotypes in two kinds of amphibian species after they were treated with water level changes,and explored the physiological cost of accelerated metamorphosis rate from water to terrestrial environment conversion process,which is of positive significance to promote the deep research of amphibian immunological ecology field,and can also provide a theoretical basis for the healthy breeding of frogs and toads.