Sea potato algae

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.These brain-like brown seaweeds can be found growing on rock or as epiphytes on other seaweeds in the mid to low intertidal. Individuals can grow to ~15 cm in diameter. Young individuals can resemble other globular forms of algae. To determine if it is Leathesia, squeeze and flatten a piece between your fingers. Leathesia will break apart into filaments whereas other globular forms will flatten but remain intact. (wiki)
These sea potatoes, were spotted in the rock pools in the rocky shores of Houghton bay, Wellington NZ.