Fish exhibit a remarkable diversity of feeding strategies, occupying nearly every ecological niche. The question of whether fish consume waste matter, specifically feces, often arises from observations in both natural waterways and home aquariums. Understanding these habits reveals important processes of nutrient recycling and food web complexity that sustain underwater ecosystems.
Defining the Behavior: Coprophagy Versus Detritivory
The consumption of waste falls into two distinct biological categories. Coprophagy is the scientific term for the deliberate consumption of feces, meaning a fish actively seeks out and ingests fresh fecal matter for immediate nutritional gain.
This is less common than detritivory, which is what most people observe. Detritivores consume detritus, a broader mix of decomposing organic matter, including decaying plants, dead organisms, and aged fecal material mixed with sediment. These fish target the biofilm, bacteria, and other microorganisms that colonize this matter, extracting nutrients from the microbial life rather than the waste material itself. While detritivory involves consuming material that contains feces, it differs fundamentally from the targeted consumption of fresh waste.
Specific Examples of True Coprophagic Fish
While detritivory is common, true coprophagy—where fish actively select fresh feces—is documented in resource-limited environments like coral reefs. Certain herbivorous species, such as Caribbean parrotfish and surgeonfish, deliberately feed on the fecal pellets of plankton-eating fish. This targeted behavior provides nutritional supplementation, as a diet consisting solely of algae is often low in protein and certain micronutrients.
The feces of planktivores are rich, containing higher levels of protein, carbohydrates, and micronutrients compared to the algae herbivores usually consume. In the Indo-Pacific, studies have documented at least 45 fish species engaging in coprophagy, consuming feces from various sources. This behavior immediately recycles valuable, partially-digested nutrients back into the food web. The Remora, or sucker fish, also includes feces in its diet when feeding on host animals like sharks or turtles, alongside parasites and dead skin.
Why Bottom Feeders Consume Organic Waste
The common assumption that popular bottom-dwelling fish, such as Plecostomus, Corydoras catfish, and loaches, are “poop eaters” misunderstands their detritivorous diet. These species process the organic matter that settles on the substrate, which is a mix of uneaten food, decaying plants, and aged feces. They are not primarily consuming the waste itself but rather the secondary food sources that flourish on it.
The surfaces of organic waste and sediment are quickly colonized by dense mats of bacteria, fungi, and microalgae, collectively known as biofilm. These microorganisms are highly nutritious, and detritivores essentially harvest this microbial community. The fish use specialized mouths, often with a downturned orientation and sensory barbels, to sift through bottom materials. They ingest sediment, “winnow” out the edible organic particles and associated microbes in their orobranchial chambers, and expel the inedible sediments.
This behavior serves an important ecological function in all aquatic systems, as these detritivores are responsible for a significant portion of nutrient cycling. By consuming and processing decaying organic material, they prevent its accumulation and the release of harmful compounds into the water column. Their activity helps manage water quality by converting complex organic compounds into forms readily available to other organisms, supporting the entire ecosystem.