Image of elephant fish by fir0002 | flagstaffotos.com.au Canon 20D + Tamron 28-75mm f/2.8 - Own work, GFDL 1.2, https://commons.wikimedia.org/w/index.php?curid=864471
Seawater contains sulfate concentrations that are nearly 40 times those measured in plasma. Therefore, it is easy to see why fish would need to develop mechanisms to keep sulfate within a physiologically normal range. The kidneys of teleost fish have been known to excrete excess sulfate in the urine. However until now, it was not known whether the kidneys of cartilaginous fish do the same thing as their kidneys are rather complex. In a new study published in the American Journal of Physiology Regulatory, Integrative and Comparative Physiology, researchers characterized the cDNA of elephant fish (Callorhinchus milii) kidneys. Similar to mammals and teleost fish, the researchers determined that elephant fish also have transporters responsible for excreting sulfate in the kidneys to maintain a normal balance.
Source: