Functional morphology of the balanomorph barnacles Tesseropora rosea (Krauss) and Tetraclitella purpurascens (Wood) (Tetraclitidae)

Tesseropora rosea of eastern Australia inhabits exposed, wave-washed rocks, is an extension feeder in fast currents, and has monometric growth, with orifice enlargement by erosion. It has a deep, mobile operculum. Tergal depressor muscles are large, scutal depressors moderately large. The opercular valves play an active part in respiratory pumping beat. Tergal depressors effect opercular withdrawal. Rostral scutal depressors assist in pumping beat. Lateral scutal depressors assist in opercular opening. Cim IV-VI of T. rosea unfurl rapidly in response to fast water currents and are captorial, either singly or in concert. The antenniform third maxillipeds (cirrus 111) turn back laterally in the extended position but fold over and hold down the captorial cirri in the curled position. Tetraclitella purpurascens of southern Australia inhabits crevices and overhangs on rocky shores, is an extension feeder in moderate currents, and has diametric growth, with orifice enlargement by circumferential growth of radii and parietes. It has a shallow operculum with only slight mobility. Tergal depressors are large, scutal depressors small. Valve action is mainly hinge-like and slow, with little apicobasal movement. The small scutal depressors assist the large tergal depressors in opercular withdrawal and have only a very minor role in pumping beat. Cirri IV-VI of T. purpurascens are held erect for long periods in moderate water currents. Captorial movements are similar to but slower than those of T. rosea. The third maxillipeds have short rami involved in forward food transfer. Setation correlates with function on the cirri of both species. The oral cone of T. rosea is broader and the mouthparts relatively larger than those of T. purpurascens, possibly mediating a more powerful triturating mechanism. A comparison of structure and function shows that T. purpurascens retains many primitive balanomorph features, whereas T. rosea has a more advanced functional configuration. In the evolution of tetraclitids, it seems likely that the tetraclitellines diverged early, before the austrobalanines gave rise to the tetraclitines. The four-plated wall may have evolved independently in tetraclitellines and austrobalanines.