Bud formation and metamorphosis inCassiopea andromeda (Cnidaria: Scyphozoa): A developmental and ultrastructural study

Asexual reproduction by formation of swimming buds which metamorphose directly into polyps plays a most important role in the propagation ofCassiopea andromeda (Cnidaria: Scyphozoa). (C. andromeda polyps, originally supplied by the Löbbecke Museum and Aquarium Düsseldorf, FRG, were cultured in our laboratories.) We have defined five budding stages and investigated epithelial recruitment and dynamics during bud formation using intracellular vital stains. The region of cell recruitment was found to encircle the budding site asymmetrically. The aboral side contributing considerably less to the developing bud than the oral and lateral sides. Furthermore, it was found that the epithelial flow involved in bud formation is part of a permanent apico-basal displacement of ectodermal cells. Light and electronmicroscopic investigations revealed that no drastic changes occur at the cellular level, neither in the ectoderm nor in the endoderm which both participate in bud formation. Scanning and transmissionelectron microscopic investigations of the swimming bud revealed that the ectoderm is composed of three, and the endoderm of two, cell types. Nerve elements have been detected near the mesoglea between both ecto- and endodermal cells. Amoebocytes are regularly found either at the basis of epidermal cells or within the mesoglea, whereas symbionts are located in the endoderm. Buds induced to metamorphose by a bacterial-inducing factor were used to investigate morphological and ultrastructural changes occurring during metamorphosis and scyphistoma morphogenesis. Metamorphosis starts with the settling of a bud, followed by the formation of the pedal disk in which desmocytes, as typical cnidarian adhesive structures, are differentiated. Metamorphosis is completed with the formation of the mouth and tentacles. Whereas metamorphosis of cnidarian planulae implies considerable changes at the cellular level, tissue remodeling processes prevail in bud metamorphosis ofC. andromeda.