The Bullhead (also known as Miller's Thumb or Tommy Logge), is the only freshwater cottid found in the Czech Republic. The Cottidae (sculpins) are mostly marine fish, with around 300 species worldwide. A small species, the bullhead rarely exceeds 15 cm in length and a weight of 28 g. It is easily identified by its large head (which can account for 25% of body length), with eyes on the top and a dorso-ventrally flattened tapering body adapted to life on the bottom of flowing waters.

A further adaptation to a benthic habit is the lack of a swim bladder, conferring negative buoyancy. The mottled skin, which varies in shade according to background, offers good camouflage among stones and leaf litter. The mouth has an extremely wide gape, and villiform teeth are present on both jaws, as well as on the front of the vomer bone.

Bullheads predominantly occur in stony streams and rivers where the flow is moderate and the water is cool and oxygen-rich. These range from high-altitude streams to the rivers of uplands and lowlands. However, they may also be found in lakes, where they not only occur in stony margins, but also in the profundal zone in depths of >20 m.

Bullheads are retiring fish, actively hiding from light under stones or any other available objects. This may be linked to their vulnerability to predation. For this reason, shade and cover are important components of their preferred habitat.

Bullhead is a significant element of fauna of Czech cleanest streams and cannot live in even little polluted water.