Seawater is known to contain a significant concentration of boric acid, which can be toxic and deadly to living systems. As such, fish living in marine habitats need to be able to excrete boric acid in order to maintain a healthy boron balance. Tokyo Tech researchers have now identified the gene and mechanism of boric acid transport in seawater fish and contrasted it to freshwater species.
Marine fishes live in highly saline environments with ionic concentrations that are vastly different from their blood plasma. Seawater contains a variety of toxic ion species that can build up in the body if the fish does not excrete them. One example of this is boric acid, which—in small amounts—is a vital micronutrient for animals but can prove toxic in excess. Hence, marine fish must develop physiologic means to excrete boric acid. However, how they do this is, as yet, unknown. Now, an international team led by researchers from Tokyo Institute of Technology (Tokyo Tech) has unveiled and demonstrated the molecular mechanisms underlying boric acid secretion in marine pufferfish.
Associate Professor Akira Kato of Tokyo Tech is the principal author of the study, which was published in the Journal of Biological Chemistry. He tells us more about it. "We compared euryhaline pufferfish (which are pufferfish that can survive in varying levels of salinity) accustomed to saltwater, brackish water, and freshwater. On comparing fish from these three habitats, we found that the urine of a seawater pufferfish (Takifugu pufferfish) contained 300 times more boric acid than pufferfish blood, and 60 times more boric acid than seawater." The urine of freshwater fish contained almost 1000 times less boric acid than that of seawater pufferfish. These findings established that Takifugu pufferfish living in seawater excrete boric acid in their urine. Just like in humans, the process of excretion via urine in pufferfish is mediated by the kidneys.