A study of the ovigerous fraena of barnacles

Abstract

A mainly microscopical study has been made on the paired ovigerous fraena of the stalked barnacle, Conchoderma auritum . An ovigerous fraenum is situated close to the adductor muscle in each sagittal half of the capitulum. Ovigerous fraena are simple extensions of the inner mantle occurring in sexually mature individuals. However, the distal region of an ovigerous fraenum is specialized in that many gland cells producing a proteinaceous secretion are present and numerous cuticular extensions, termed ovigerous setae, form a zone close to the margin on the inner face. Each seta is the product of the apical cytoplasmic extension of a single hypodermal cell and, as a cuticular projection, has the two distinct zones observed in the cuticle over the remaining parts of the ovigerous fraenum and inner mantle. There is an outer thin zone termed the cuticulin layer and a much more substantial inner lamellate layer. The slightly enlarged apical region of an ovigerous seta is characterized by back-pointing spines lying in a depression. The secretion, when released from the gland cells, passes out from raised papillae among the ovigerous setae and forms a sheet covering the ovigerous setae; many of the ovigerous setae have their apical region embedded in the secreted sheet. This anchored sheet is the adhesive that forms the permanent attachment between an ovigerous fraenum and the ovisac surrounding an egg mass. The presence or absence of ovigerous fraena was determined by dissection and scanning electron microscopy for a large number of thoracican barnacles. Ovigerous fraena were found in most, but not all lepadomorphs, and in a primitive balanomorph, Catophragmus ( Catomerus ) polymerus . C . polymerus was also found to possess branchiae (gills). Branchiae, which are restricted to balanomorphs, were previously thought to be modified ovigerous fraena; the occurrence of both ovigerous fraena and branchiae in C . polymerus demonstrates the independent origin of these two inner mantle structures.