Rathouisia leonina has a cylindrical and elongated body, with a resting length of less than 35 mm (1 in).
The body color usually ranges from light grayish-brown to dark gray, rarely reddish.
The bottom surface (sole) is light reddish brown.
The upper surface of the body has scattered black oblong spots.
The tentacles are short and black in color.

Rathouisia leonina is larger than its two congeners, Rathouisia pantherina and Rathouisia tigrina.
Adults weigh from 0.18 to 1.02 g. Their mucus is very sticky.
Rathouisia leonina has a cylindrical and elongated body, with a resting length of less than 35 mm (1 in). The body color usually ranges from light grayish-brown to dark gray, rarely reddish. The bottom surface (sole) is light reddish brown. The upper surface of the body has scattered black oblong spots. The tentacles are short and black in color.
Rathouisia leonina is larger than its two congeners, Rathouisia pantherina and Rathouisia tigrina. Adults weigh from 0.18 to 1.02 g. Their mucus is very sticky.
Rathouisia leonina