Complete cycle of receptacle development in parasitic barnacle, Parasacculina pilosella (Van Kampen and Boschma, 1925) (Rhizocephala: Polyascidae)

The complete cycle of receptacle development in the parasitic barnacle Parasacculina pilosella (Van Kampen and Boschma, 1925) (Rhizocephala: Polyascidae) was studied by light and electron microscopy. This rhizocephalan has two globular receptacles located outside the visceral sac. Two variants of development of the receptacles from the same externa were observed: (1) with both receptacles fertile and (2) with one of them (usually larger one) fertile and the other sterile. The presence of the second fertile receptacle probably extends the breeding season in P. pilosella. The following stages in the development of the fertile receptacle were identified: trichogon-free, trichogon-containing, early, middle, late, and degenerating. The sterile receptacle that has not received a trichogon, continued to develop and, after reaching the middle stage, atrophied. The cell types composing the receptacle wall (accessory, large female, polygonal, small-sized, and sheath cells) and the meshwork formed by the intercellular matrix were examined in detail. The accessory cells that separated spermatogenic and large female cells were probably derived from the trichogon which implanted cells of both germ and somatic lines. Globular and tubular receptacles had different fine organizations. Globular receptacles, as well as tubular ones, lacked morphological structures separating female and male organisms. Further studies, involving a larger number of other rhizocephalan species, are needed to clarify whether the receptacle ultrastructure can be useful in resolving taxonomic issues.