P. setacea that grow on the hydroid Nemertesia antennina at a depth of 15 m in Torbay, Devon, are mature when about two months old; early sexual reproduction ensures continued colonization of N. antennina, which lives for only 4-5 months. Growth and reproduction of P. setacea occur throughout the year. The growth of the generally unbranched stolon is oriented toward the tip of the N. antennina hydrocaulus, which always bends downcurrent. The pinnate hydrocauli of P. setacea are held across the current in the most efficient position for feeding. The oldest tissue of P. setacea is continually resorbed towards the younger parts of the hydroid, leaving an empty perisarc, which eventually degenerates. This behaviour, together with the oriented growth of the stolon, means that the hydroids `move' distally along the N. antennina hydrocauli. On concrete substrata at ELWS at Felixstowe, Suffolk, P. setacea reproduces only from June to August. The young hydroids may briefly achieve an exponential growth rate, but this probably occurs infrequently because of mortality and interference by predators and competitors. The initial growth of the stolon is unbranched; subsequent branched growth produces a complex network of stolons, which covers the available substrata. The oldest tissue of these hydroids is also continually resorbed. In addition, in autumn the coenosarc of all hydrocauli is resorbed into the stolon and P. setacea overwinters as a dormant stolon. Rapid regrowth of large mature hydrocauli occurs in May and June from small sections of the stolon that are not destroyed during dormancy. These hydroids die after reproducing and therefore have a life-span of about one year. The modular construction of P. setacea, contrary to previous assumptions on hydroid biology, does not confer the potential for an infinite growth and life-span, but endows the potential for variable growth and form that enables P. setacea to survive, as opportunists, in these different habitats.