Why do some fish have projections around their mouths?
These projections, called barbels, are common in certain fish, typically those found in muddy waters. Barbels help to provide the fish with sensory information about its environment. The shape of these barbels can vary greatly - they are relatively short and inconspicuous in the case of carp such as koi, becoming much more highly developed in many catfish.

The shape and length of a catfish's barbels can be a very useful guide to its behaviour. Those with long, narrow barbels such as the tiger shovelnose (Pseudoplatystoma tigrinum) seen above are usually active predators, swimming fast and hunting other fish, often after dark.
On the other hand, those with barbels located on the underside of the mouth, such as corydoras catfish, are much more likely to spend their time rooting around on the substrate of their aquarium or pond, searching for worms and other food here, as bottom-feeders.