Water relations of the endemic New Zealand frogs Leiopelma archeyi, L. Hamiltoni and L. Hochstetteri

Abstract

Aspects of habitat and water balance physiology were compared between the endemic and anatomically primitive New Zealand frogs Leiopelma archeyi, L. hamiltoni and L. hochstetteri. Field studies in an area of the Coromandel Range showed that the partially sympatric L. archeyi and L. hochstetteri had nearly disjunct distributions. L. archeyi occurred in terrestrial habitats on the forested ridgetops, whereas most L. hochstetteri were found in semiaquatic habitats along the edges of streams. L. archeyi also showed less preference for water than L. hochstetteri in the laboratory. Climatic conditions in the ridgetop habitat of L. archeyi were usually cool and humid, and resembled those previously reported for the allopatric and completely terrestrial L. hamiltoni. In laboratory studies, all three species dehydrated rapidly in dry conditions. Resistance to evaporative water loss (EWL) was similar to that for a free water surface. L. archeyi and L. hamiltoni rehydrated rapidly (in 1-2 h) when returned to shallow water after dehydration to 9-15% weight loss. Rapid rehydration was due to increased water uptake through ventral skin and to reduced bladder urine accumulation (cutaneous and antidiuretic water balance responses). L. hochstetteri rehydrated very slowly (rehydration incomplete within 16 h) and showed only an antidiuretic response. L. hochstetteri also rehydrated more slowly from moist soil when compared with L. archeyi. Treatment with arginine vasotocin (AVT, 7 x 10⁻¹¹ mol g⁻¹) produced water balance responses similar to those seen during rehydration. L. archeyi and L. hamiltoni placed in shallow water showed rapid water retention, reaching weight gains of 31% and 29% respectively 4-5 h after treatment. L. hochstetteri gained weight at a slow and linear rate (9% over 9 h). Unlike L. archeyi and L. hamiltoni, L. hochstetteri showed no increase in cutaneous water uptake, and water retention was attributed solely to antidiuresis. In vitro experiments confirmed that AVT (3 x 10⁻⁸ mol 1⁻¹) caused increased osmotic water flow (OWF) through ventral pelvic skin of L. archeyi and L. hamiltoni. However, pelvic skin from L. hochstetteri and ventral pectoral and dorsal skin from all three species showed no hydroosmotic response. The bladders of the three species showed small increases in OWF in response to AVT. In vivo, glomerular filtration rate fell following AVT treatment in all species, although the effect on L. hochstetteri was not statistically significant. Pelvic skin of L. archeyi showed no obvious structural specialisation for rapid water uptake, when examined using light and scanning electron microscopy. Pelvic skin was smooth, structurally similar to non-glandular areas of pectoral or dorsal skin, and less vascularised than pelvic skin of L. hochstetteri. Adults of all three species were ureotelic. In the terrestrial eggs and larvae of L. archeyi, intracapsular larvae were ammonotelic, but most larvae became ureotelic within 4 d after hatching. The eggs had no resistance to EWL, but absorbed water rapidly from moist surfaces. Male parental care (egg brooding, larval transport) led to reduced EWL from eggs and hatched larvae of L. archeyi. This study demonstrates that the presence of a cutaneous hydroosmotic response varies between adults of the three species in an inverse manner with habitat water availability. It also supports previous studies indicating a closer ecological similarity and evolutionary relationship between L. archeyi and L. hamiltoni, than between either and L. hochstetteri.