and has often been raised in hatcheries and introduced into areas in which it is not native. These fish typically only reach a size of about and . Anatomy External anatomy Perch have a long and round body shape which allows for fast swimming in the water. True perch have "rough" or ctenoid scales. Perch have paired pectoral and pelvic fins, and two dorsal fins, the first one spiny and the second soft. These two fins can be separate or joined. The head consists of the skull (formed from loosely connected bones), eyes, mouth, operculum, gills, and a pair of nostrils (which has no connection to the oral cavity). They have small brush-like teeth across their jaws and on the roof of their mouth. The gills are located under the operculum on both sides of the head and are used to extract oxygen molecules from water and expel carbon dioxide; the gills have gill rakers inside the mouth. External anatomy can be used to determine the sex of perch in multiple ways. Perch have two posterior openings located on their abdomen, the anal and urogenital. In males, the shape of the urogenital opening is round and larger than the anal opening. In females, the urogenital opening is often a V- or U-shape which is a similar size to the anal opening. Also, males usually have a more brown-red colored urogenital opening compared to females. Internal anatomy The esophagus is a flexible tube that goes from the mouth to the stomach. The stomach is connected to the intestine via the pyloric sphincter. The intestines of perch consist of the small intestine and large intestine; the intestines have many pyloric caeca and a spiral value, the small intestine consists of a part called the duodenum. The spleen is located after the stomach and before the spiral value. The spleen is connected to the circulatory system, not part of the digestive tract. The liver is composed of three lobes: one small lobe (includes the gall bladder) and two large lobes. Perch have long and narrow kidneys that contain clusters of nephrons which empty into the mesonephric duct. They have a two-chambered heart consisting of four compartments: the sinus venous, one atrium, one ventricle, and conus. Perch have a swim bladder that helps control buoyancy or floating within the water, the swim bladder is only found in bony fish. In perch, the duct connecting