The Ultrastructure of Retinacula in the Rhizocephala (Crustacea: Cirripedia) and their Systematic Significance

Abstract Scanning electron microscopy is used to examine the mantle surfaces of the reproductive body (externa) in a range of rhizocephalan barnacles. The cuticular structures called retinacula are described in detail and classified into various morphological types. The retinacula can be single or grouped in repeated patterns, or even united by a common base into a variety of complex structures. The surface of the individual retinaculum can be smooth or ornamented in various ways. The different types of retinacula intergrade to some extent and may all be derived from a single type, but they exhibit taxonomically informative differences. They are comparatively easy to study on traditional alcohol specimens and seem to offer the potential of an important suite of taxonomic characters which can be used also by non-specialists. It is recommended that all future species descriptions in the Rhizocephala use SEM to illustrate retinaculum morphology.