Cirral activity of barnacles

Cirral activity is more varied than Darwin realised, and the variation can be related to phylogeny, ecology and physiology. It is simplest in ibloids and scalpellids, which show a slow respiratory pumping beat and hold the cirri extended as a net to catch plankton. The neustonic lepadids are more specialised; their cirri are almost always extended, and rhythmic respiratory pumping involves partial curling and immediate re-extension of the cirri on each beat, with pauses in the extended condition. In small species such as Lepas pectinata the maxillipedes (smaller cirri) filter off fine particles during pumping. Verrucomorphs use the asymmetric cirral net to capture crawling prey, but the shallow water/ intertidal species Verruca stroemia also displays a pumping beat that allows microfiltration by the maxillipedes. Chthamaloids have a pattern of behaviour similar to that of scalpellids: a basic respiratory pumping beat and capture of small planktonic prey by the cirral fan extended in response to water currents. The cirral fan has a primitive cupped posture in Catomerus, Octomeris and Chamaesipho, but is more active and upright in Euraphia and Chthamalus. Some coronuloids follow the chthamaloid behaviour; others show vigorous pumping beat and more dynamic movement of the cirral fan for captorial feeding. Tetraclita can also display a strong pumping beat that allows microfiltration. Some primitive balanoids show respiratory pumping beat and captorial feeding by extension of the cirral net, but others, such as Semibalanus balanoides, also display rhythmic beating of the cirral fan (normal and fast beat). The true balanids characteristically perform rhythmic cirral movements (normal beat and fast beat), in which the cirral fan is vigorously swept through the water. The larger species emphasize strong pumping beat with microfiltration or captorial feeding by prolonged extension. Captorial feeding may involve single rami, single cirri or the whole fan in different ways in different species. The maxillipedes transfer food from the larger cirri to the mouthparts, but their setae can also provide a fine filter for micro-feeding, a mechanism separately evolved in various lepadomorphs and balanomorphs. Rates of rhythmic cirral beating decrease with size and age but increase with temperature. Modes of activity also change with temperature, with increased emphasis on pumping beat and fast beat at higher temperatures, when barnacles spend more time active. Boreal and warm-water species are adapted to different temperature regimes, and though each 136group is capable of limited acclimation, there is little evidence for genetic selection within the species. Differences in rate between high tide and low tide populations can be explained by temperature acclimation. Ventilation of the mantle cavity is sufficient for basic oxygen requirements, and is controlled by a receptor sensitive to carbon dioxide concentration that regulates the proportion of time spent ventilating. Ventilatory pumping beat appears to have been an evolutionary precursor of filter feeding and captorial normal beating. In the balanomorphs normal beat, fast beat and strong pumping beat may have evolved more than once.