Thorson T B, Cowan C M, Watson D E
Science. 1967 Oct 20;158(3799):375-7. doi: 10.1126/science.158.3799.375.
All previously reported species of Chondrichthyes, from both marine and fresh water, have contained urea at concentrations ranging from about 300 to 1300 milligrams of urea nitrogen per 100 milliliters of fluid. Body fluids from two species of Potamotrygon, permanent residents of the Amazon basin, contained only 2 to 3 milligrams of urea nitrogen per 100 milliliters. Although they have abandoned the retention of urea exhibited by other chondrichthyans, the extent to which they have lost the mechanisms of retaining and tolerating urea in a hypertonic medium has not been determined.
此前报道的所有软骨鱼类，无论是海洋种还是淡水种，其体液中的尿素浓度范围约为每100毫升含300至1300毫克尿素氮。来自亚马逊河流域的两种淡水魟鱼的体液中，每100毫升仅含2至3毫克尿素氮。尽管它们已不再保留其他软骨鱼类所具有的尿素，但它们在高渗介质中失去保留和耐受尿素机制的程度尚未确定。
