Great Ram'S-Horn

Planorbarius corneus is the largest European species of ramshorn snail (family Planorbidae), with a shell typically measuring 35 mm (1.4 in) in diameter and 12 mm (0.47 in) in width when fully-grown. The shell is a reddish-brown or olive color, though albino variants may have lighter colors. The shell is somewhat reflective, with visible growth lines, and juvenile snails possess hairs along the shell's surface. There are between 5–5.5 whorls, or revolutions of the shell. Each whorl is round, with no observable keel. Each successive whorl grows in size, leaving the center of the shell (the umbilicus) sunken. Sutures between whorls are deep. The opening of the shell, called the aperture, is kidney-shaped. The animal's skin can be brown or black in coloration, but albino variants may appear red or white.

Planorbarius corneus is distributed from western Europe, through central Europe and into the Caucasus, north into Siberia and south into the Middle East. Additionally, as the species is commonly sold in the aquarium trade, it may be found outside of its main range in ponds where they have been introduced. The species is very common in central Europe and the Balkans, where it has been recorded at densities of up to 40 individuals per square meter. It is not as common in southwestern Europe.

Planorbarius corneus may be found in freshwater ponds, rivers, and canals, where it clings to vegetation. The species seemingly prefers water which is slow moving, highly vegetated, and contains lots of dissolved minerals. The water in which it inhabits may sometimes be poorly oxygenated. It is thought that, similar to Peregriana peregra, this species stays close to shallow water, as the emergent plants growing there provide access to the surface for oxygen. The snail has two methods of respiration. In well-oxygenated waters, it is able to breathe through simple diffusion of oxygen through its skin. However, in poorly oxygenated waters, it must extend its siphon above the surface of the water to fill its lungs. Additionally, its blood contains the protein hemoglobin, acting as an oxygen transporter in low-oxygen environments.

Like most freshwater pulmonates, Planorbarius corneus probably follows a yearly reproduction cycle. In a 1963 study in southern England, eggs were thought to hatch in May, with the reproductive period ending before June. However, the snails may be able to live for longer than one year, with reports of them living as long as 6 years in captivity.