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# Aquatic Ecosystem **Chapter 1** Multicellular organisms, such as fish, eliminate excess water that enters the body through the skin, mouth, and gills by the kidneys, in the form of dilute urine. In fish, the kidneys are located in the abdominal cavity on either side of the spine. **Fish in Salt...

# Aquatic Ecosystem **Chapter 1** Multicellular organisms, such as fish, eliminate excess water that enters the body through the skin, mouth, and gills by the kidneys, in the form of dilute urine. In fish, the kidneys are located in the abdominal cavity on either side of the spine. **Fish in Saltwater** While fish that live in saltwater need to swallow large amounts of sea water to compensate for the osmotic loss of water from their body, and then they excrete excess salts through their kidneys and specialized cells in their gills. **Sharks and Salinity** As a physiological adaptation to the high salinity of the ocean and sea water, sharks maintain the balance of water and salts within their bodies by controlling the level of urea in their blood. Urea is a nitrogenous compound that is excreted in the urine of many animals to get rid of it. Sharks keep a high concentration of urea in their blood, which increases their osmotic pressure, bringing it close to the osmotic pressure of the surrounding water. This helps minimize the loss of water from their body to the surrounding environment of high salinity. **Diagram of Fish Internal Structure** *(A diagram of a fish's internal organs is present. Labels include: spinal cord, gallbladder, kidney, brain, gills, heart, liver, esophagus, pelvic fin, pancreas, stomach, reproductive organ, intestine, bladder, anal fin, lateral line, and swim bladder. Also, there are labels for different fins, including spiny dorsal fin, soft dorsal fin, and caudal fin.)*