Leathesia difformis

Leathesia difformis, the sea potato, is a species of littoral brown algae in the class Phaeophyceae and the order Ectocarpales which is commonly attached to other seaweeds and sometimes rocks. When young, the organism is solid but as it matures it become hollow and somewhat convoluted and has the appearance of a small leathery brown bag about the same size as a tennis ball. The texture is rubbery and the outer surface smooth.