Liver The liver carries out a wide range of duties and operations that affect numerous body organs and regions.In marine habitats, gills also help with ammonia excretion and salt balance regulation.The liver helps control the amount of various chemicals in the blood and transforms certain compounds into other forms that are subsequently eliminated in the urine, which helps to purify the blood of hazardous toxins.(ii) Heart A marine fish's heart is situated close to its gills and is rather basic, often consisting of two chambers: an atrium and a ventricle.In addition to producing new nutrients the body requires, this also entails breaking down certain chemicals and keeping them at regular levels.It is shielded by the gill coverings and contained in a pericardial sac.The kidneys of marine fish are essential for eliminating excess salt from the body and preserving water.Through the gill membranes, oxygen from the water diffuses into the blood, while carbon dioxide is expelled.