Is the distribution of Pelobates syriacus related to its limited osmoregulatory capacity?

1. 1. The osmoregulatory capacity of the spadefoot toad Pelobates syriacus, from Israel, was investigated in salt (NaCl) and urea acclimation conditions. 2. 2. The spadefoot cannot be acclimated to a NaCl solution more concentrated than 450 mOsm/l, but can be acclimated to 500 mOsm of urea. 3. 3. Plasma osmolarity is always hyperosmotic to the external solution, approaching isosmoticity at the high external osmolarities. 4. 4. Urine is normally hyposmotic, becoming isosmotic to the plasma at the hyperosmotic acclimation. 5. 5. In tap water, osmotic water uptake is rather low (0.75 ± 1.65 g/hr × 100 cm^-2; Mean ± SD). 6. 6. The hydrosmotic effect of neurohypophyseal hormones (oxytocin and vasopressin) is very poor, and greatly delayed, compared with Bufo viridis. 7. 7. Evaporative water loss in still air is 0.223 ± 0.051 g/hr × 100 cm² (Mean ± SD), comparable to that of Bufo viridis. 8. 8. The results are discussed with respect to the low genetic heterogeneity of Pelobates as compared to B. viridis, and in relation to its habitat in a narrow subterranean niche.