Sea squirt. Stalked ascidian, or tunicate, (class Ascidiacea) growing on a reef. Sea squirts filter plankton from the water current which circulates through them. They are sedentary but have free-swimming larvae. Ascidian sea squirts are characterized by a tough outer layer of the polysaccharide tunicin, unlike other tunicates which are less rigid. Photographed in the waters off Komodo National Park, Komodo, Indonesia.