The idea that all fish waste exits through a single opening is a common simplification that does not apply to the vast majority of species. Bony fish (Osteichthyes), which make up over 95% of all fish, utilize distinct openings for the expulsion of solid and liquid waste. Understanding this process requires examining the unique anatomy and specialized physiological processes fish use to manage waste in water.
Anatomy of Waste Elimination
The physical separation of waste is managed by two primary external openings located near the tail on the underside of the fish. The digestive tract concludes at the anus, or vent, which is dedicated solely to the expulsion of solid, undigested food matter. This opening is the final exit point for the digestive system.
Slightly behind the anus is the urogenital pore or orifice. This structure is the exit point for liquid waste produced by the kidneys and for reproductive materials like eggs or sperm. In some species, the urinary and genital ducts may have their own separate pores. These openings are typically situated near the base of the anal fin, differentiating them from the digestive exit.
How Fish Eliminate Liquid Waste
The way a fish eliminates liquid waste is highly specialized compared to terrestrial animals. The primary nitrogenous waste product from protein metabolism is ammonia, which is highly toxic but extremely water-soluble. Up to 90% of this ammonia is released directly into the surrounding water via diffusion across the extensive surface area of the gills. This process is so efficient that for most fish, the gills are the main organ for nitrogenous waste excretion, effectively acting as the major “pee” mechanism.
The kidneys do produce a small amount of true urine, but its main function is not to remove nitrogenous waste. Instead, the kidneys play a major part in osmoregulation, which is the maintenance of the fish’s internal water and salt balance. Freshwater fish constantly absorb excess water through their skin and gills due to osmosis, so their kidneys produce a large volume of highly dilute urine to expel this water.
Conversely, marine fish tend to lose water to their salty surroundings, so they constantly drink seawater. Their kidneys produce very minimal, concentrated urine, while specialized cells in their gills actively excrete the excess salt they ingest. The gills handle both nitrogen and salt elimination, while the urogenital pore only expels a small amount of water and minimal waste.
How Fish Eliminate Solid Waste
Solid waste elimination begins with the ingestion and mechanical breakdown of food by the mouth and pharynx. The food then moves down the esophagus and into the stomach, where initial digestion by acids and enzymes takes place. The food then passes into the intestine, which is the primary site for nutrient absorption.
The length and complexity of the intestine vary significantly based on the fish’s diet. Carnivorous fish, which eat protein-rich, easily digestible food, tend to have a shorter, relatively straight digestive tract. Herbivorous fish, whose plant-based diet is more difficult to break down, possess much longer, coiled intestines to maximize the time available for nutrient extraction.
Once the digestive process is complete, the remaining undigested material (feces) is compacted and expelled from the body. This solid waste exits exclusively through the anus. The appearance of the waste depends heavily on the fiber and moisture content of the fish’s natural diet.
When Fish Use a Single Opening
While bony fish follow the two-hole rule, a significant group of species does align with the single-opening idea. This exception is primarily found among the cartilaginous fish, which belong to the class Chondrichthyes. This group includes sharks, skates, and rays, which all possess a structure called a cloaca.
The cloaca is a posterior opening that serves as the single exit for the intestinal, urinary, and reproductive tracts. All solid waste, liquid waste, and reproductive products are channeled into this common cavity before being expelled externally. Primitive jawless fish, such as lampreys, also possess a cloaca, which is considered an ancestral feature in vertebrate evolution.