SCANNING ELECTRON-MICROSCOPY OF MOUTH APPENDAGES IN 6 SPECIES OF BARNACLES (CRUSTACEA, CIRRIPEDIA, THORACICA)

The morphology and setation of mouth appendages (trophi) are investigated with scanning electron microscopy in the balanomorph barnacles Semibalanus balanoides, Balanus balanus and B. nubilus and in the pedunculate barnacles Pollicipes polymerus, P. cornucopia and Lepas anatifera. It is difficult to uphold a clearcut distinction between denticles and setae and several types of setae also intergrade with each other. The trophi of Pollicipes polymerus and P. cornucopia have the most 'generalized' morphology. The palps of Pollicipes and L. anatifera are simpler than those in balanomorphs and carry a single type of seta. In L. anatifera the mandibles and maxillules are adapted to a semi-predatory feeding habit by carrying large, pointed teeth, but this species lacks the foliate-serrate setae which populate the palps in the other species studied. Compared with Pollicipes and Lepas, the three balanomorphs have palps with a complex setation. The trophi in S. balanoides differ from the two species of Balanus in numerous features of setation and denticulation, notably in having the palps populated by plumodenticulate setae homologous to purely denticulated types in Balanus. It is suggested that SEM studies of barnacle trophi will provide characters of use in estimating phylogeny.