Free amino acids and osmoregulation in the intertidal pulmonate Onchidium tumidium

The aim of this study is to elucidate the osmoregulatory capabilities of th e intertidal pulmonate Onchidium tumidium. Our results indicate that O. tum idium could tolerate hyperosmotic stress more effectively than hypoosmotic stress. In 10% seawater (SW), it was capable of maintaining its plasma hype rosmotic and hyperionic to the external medium. However, it rapidly gained weight in 10% SW, and no restoration of body weight was observed. In contra st, in 90% SW it was able to recover its body weight partially. In a hypero smotic medium, accumulation of intracellular free amino acids (FAA) was res ponsible for cell volume regulation. The accumulated FAA might originate fr om protein catabolism, which occurred in the internal organs (IO). In contr ast, the relatively smaller amination :deamination ratio of glutamate dehyd rogenase in the body wall (BW) and IO of specimens exposed to hypoosmotic s tress might lead to a higher rate of amino acid catabolism. This was suppor ted by the greater ammonia contents in the BW and IO of, and higher rate of ammonia excretion by, specimens exposed to 10% SW.