BULLETIN OF MARINE SCIENCE, 33(3): 645-655, 1983 CIRRAL ACTIVITY AND FEEDING IN THE CORONULOID BARNACLES TESSEROPORA ROSEA (KRAUSS) AND TETRACLITELLA PURPURASCENS (WOOD) (TETRACLITIDAE) D, T. Anderson and J. Buckle ABSTRACT Adult T. rosea and T. purpurascens are extension-feeding planktivores, dependent on water currents to evoke extension of the eirral fan. T. rosea displays testing activity in still and slowly moving water and respiratory pumping beat in a moderate water current, but requires a fast current for cirral extension and feeding. T. purpurascens displays pumping beat in still and slowly moving water, and performs cirral extension in a moderate water current. Fast flow causes distortion of the cirral fan in T. purpurascens and often provokes cirral withdrawal. Juveniles of T. purpurascens show the same cirral responses as adults. Juveniles of T. rosea display pumping beat and normal beat in still, slowly moving and moderate water flow, but withdraw the cirri in response to fast flow. Transitional behavior is shown by T. rosea individuals of about 2 mm aperture length. The cirral responses to water flow in the two species are related to habitat. T. rosea inhabits wave washed rocks subject to fast water flow. T. purpurascensoccupies more sheltered habitats in which water flow is reduced. Juvenile T. rosea appear to feed when immersed and to survive fast flow by a protective withdrawal of the delicate cirri. The observation of cirral beating in a juvenile coronuloid adds to the number of cases of convergent evolution ofcirral beating among thoracican barnacles. It also raises the possibility that adult balanoids with cirral beating are a product of neotenic evolution. Two species of tetraclitid barnacle feature prominently on the rock platforms of the open coast of New South Wales, dominating the rock faces between the mean high water level of the neap tides and a level about 0.5 m above mean low water neaps (Dakin et al., 1948; Dakin, 1953). One species, Tesseropora rosea (Krauss), occurs mostly on areas of rock exposed to direct sunlight and relatively strong wave action. The other species, Tetraclitella purpurascens (Wood), is found mainly in crevices and caves and under ledges, where there is considerable shade and reduced wave action. Denley and Underwood (1979) showed that each species settles in the area of the other, but does not survive. T. purpurascens is unable to withstand the physical stresses of high temperature and desiccation in sunny habitats. T. rosea survives in shaded areas provided that the water flow resulting from strong wave action is not reduced by the topography of the substratum, but is unable to survive in locations with a reduced water flow. The little that is known of feeding mechanisms in tetraclitid barnacles (Mori, 1958; 1961; Crisp and Southward, 1961; Southward and Crisp, 1965; Anderson, 1981) indicates that they are extension-feeding planktivores, dependent on an external water flow to evoke cirral extension. It is thus possible that the failure of T. rosea to survive in areas of reduced water flow is related to the pattern of cirral activity and feeding in this species. In T. purpurascens, in contrast, a pattern of cirral activity and feeding appropriate to the circumstances of reduced water flow might be expected. In the present investigation, we set out to determine experimentally the responses of the two species to external water currents of different velocities, to establish as far as possible the diet of the two species and to relate these observations to the distribution of the species on the shore. We included an examination of the cirral activities of juveniles as well as adults of 645 646 BULLETIN OF MARINE SCIENCE, VOL. 33, NO.3, 1983 the two species, since Denley and Underwood (1979) had emphasized the heavy mortality of newly settled juveniles within 2 months of experimental transfer to typical habitats of the other species. METHODS Adults and juveniles of both species attached to pieces of rock were collected from a numbcr of rock platforms in the vicinity of Sydney, N.S.W. (Harbord, Fairlight, Cape Banks). Juveniles were defined as individuals with an aperture length of less than 2 mm. The animals were held at 21·C in the laboratory and were used experimentally within 2 days of collection. Water currents were directed across the aperture using a small, immersible bilge pump controlled by a variable rheostat. The pump generated a jet of water at three rates of flow: slow (0.3-0.6 m ·sec-I), moderate (0.8-1.2 m ·sec-I) and fast (1.4-1.8 m·sec-I). These flow rates fall within the range of measured rates of water flow on rocky shores (Riedl, 1971; Koehl, 1977). Responses were observed visually and filmed using the methods of Anderson (1978). Films were subjected to frame by frame analysis. Mean times for rhythmic events were determined from 20 sequential repetitions of the event in each of 4 animals. Milk diluted with seawater was used to display water currents generated by the animals during cirral activity, following the method of Crisp and Southward (1956). The diet of adults of the two species were investigated by gut content analysis. 50 adults of each species were collected on the outgoing tide at Cape Banks, N.S. W., in March and August 1981. The animals were fixed in 7% formalin in seawater (v/v) at the time of collection. Stomach contents were later dissected out and analyzed, as a pooled sample for each species, for particle size distribution and qualitative composition. RESULTS Cirral activity in the present study was found to fall mainly within the definitions of testing, pumping beat, normal beat and extension described for balanomorphs by Crisp and Southward (1961) and Anderson (1981). Adults of T. rosea T. rosea, a typical tetraclitinid, has moderately long first and second maxi IIipeds, elongate antenniform third maxillipeds and three pairs of long posterior cirri (Pope, 1945). In still or slow flowing water, adult T. rosea exhibit only testing activity. The operculum is raised slightly at the carinal end and opened partially. Sometimes the body can be seen moving back and forth within the mantle cavity. Occasionally a single antenniform ramus ofa third maxilliped is protruded through the aperture. Testing activity begins almost immediately following immersion and usually precedes any other type ofcirral activity. Adult T. rosea kept out of water for some time also display testing activity in air. When the water flow across the animal is increased to moderate, the animal commences pumping beat. In this regular, rhythmic activity, the aperture is opened more widely and the cirri are protruded and withdrawn, remaining curled. The scuta 1 ends of the opercular valves are also raised and lowered in conjunction with the cirral protrusion and withdrawal of each beat. Rates of pumping beat ranged from 7 to 9 per 10 sec, with cirral protrusion taking longer (range of means 0.39-0.45 sec) than withdrawal (range of means 0.17-0.22 sec) on each beat. The interval between withdrawal and the next protrusion was 0.60-0.78 sec. Milk traces revealed that pumping beat in T. rosea is accompanied in the usual way by a flow of water through the mantle cavity (Fig. 1), drawn in at the rostral end on each upstroke (cirral protrusion and scutallift) and ejected as a jet at the carinal end on the following downstroke (cirral withdrawal and scutal depression). The length of the exhalant jet varies. In the example shown in Figure 1, an adult of 7-mm shell height and 5-mm aperture length, pumping at 9 beats per 10 sec, ejected a jet 14 mm above the orifice on each stroke. Jets up to 20 mm long were ANDERSON AND BUCKLE: ClRRAL ACTIVITY IN TETRACLlTlDS 647 1 2 '33' 2 lOmm 3 -~ jifJ /'445 ~ Moderate ~Xler~ > low Figure 1. Production of a through-mantic current by T. rosea during pumping beat at 9 per 10 sec in response to a moderate external water flow. (a)-(c) represent three frames of a cinefilm at spaced intervals of 0.22 sec. The milk-laden plumes of water generated by three beats are shown. The times indicated for each plume represent the intervals following the cirral withdrawal that ejected each plume. In (a) the circal withdrawal of the third beat is about to occur. In (c) the first plume is dissipating, the second is drifting and the third is almost discharged. observed in other individuals. The successive jets are carried away on the water current flowing externally across the animal. The exhalant jet, and thus the water flow through the mantle cavity per beat, had a volume of about 0.16 ml in the above example. Water was thus flowing through the mantle cavity at a rate of about 0.5 I per h. When the external water flow is fast (> 1.4 m· sec-I), adult T. rosea react by prolonged cirral extension. The opercular valves are raised and opened and the long cirri are extended as an upright fan at the carinal end of the aperture. The long rami of the third maxillipeds flank the cirral fan. The first and second max- illipeds of each side emerge at the rostral end of the aperture and turn back against the opercular plates. In the extended position, the cirral fan may rotate up to 90° in either direction towards the impinging current. Cirral extension in response to fast water flow is a dynamic process in T. rosea. l At rates of flow approaching 1.8 m'sec , the cirri are held extended for periods of up to 4.5 sec (Fig.