Stalked Sea Squirt

S. clava is a solitary tunicate. Including both the club-shaped body and peduncle, larger specimens of S. clava can have a maximum length of around 130 mm (5.1 in) while smaller specimens only reach 30 mm (1.2 in) in length. Smaller specimens tend to lack a distinct peduncle. 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. Larger specimens have a light brown body and a darker brown peduncle while smaller specimen are yellow-brown.

S. clava is a marine invertebrate animal. Adults are entirely sessile, growing attached to hard subtidal substrata as deep as 25 meters (82 ft). They can be found on virtually any hard surface such as rocks, buoys, pilings and shells of mussels. 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 habitats. It is a hardy species that can live in a wide range of temperatures from -2 °C to 27 °C and can tolerate high salinity waters (26% - 28% salinity) that would be lethal to other tunicate species.

Like most tunicates, S. clava is hermaphroditic and produces short-lived pelagic lecithotrophic larvae. They reproduce externally via broadcast spawning, and the reproductive period is highly dependent on sea surface temperatures reaching a critical temperature threshold, between 16 °C and 20°C. The reproductive period can vary from 4 to 10 months depending on location. Along the Californian coast in the United States, the reproduction period occurs for 4 months from June to September, while in Denmark and England, the reproduction period is also 4 months but occurs from July to October.