As its common names suggest, S. clava is club-shaped with an elongated oval body and a long peduncle for attaching to a substrate.

As described by some of its common names, S. clava has a tough, wrinkled or irregularly grooved skin and comes in two variations of color dependent on size.

S. clava is predominant in the littoral zone, preferring sheltered localities free of strong wave action and floating objects, making artificial surfaces in harbors and marinas exceptional habitat.

[2][3][6] Outside of its native range, S. clava has proven to be an increasingly successful invasive species due to physiological adaptations and environmental tolerances.

[1][7] The lack of a natural predator already gives S. clava an advantage over native tunicates, but their large size as well allows them to outcompete other filter-feeding species such as oysters or mussels for food and substrate space.

The earliest sighting of S. clava outside of its natural range was on the United States’ west coast in Californian coastal waters in the early-1900s.

The current European countries with established S. clava populations are England, Ireland, Belgium, Netherlands, Denmark, France, Portugal and Spain.

[2][1][4] S. clava is a solitary species, but with optimal conditions can reach high densities, up to 500 - 1,000 individuals, fouling man-made substrates resulting in boat and fishing gear difficulties.