The dynamics of multiple mouth formation in Fungia granulosa: Possible patterning mechanisms

We studied patterning mechanisms acting on the formation of new mouths during regeneration of wedge-shaped fragments of the solitary coral Fungia granulosa. Mouth formation dynamics was studied in fragments taken from different parts of the adult polyp. The corals were fragmented into central and peripheral fragments in relation to each polyp's centrally positioned mouth. The study revealed that the maintenance of the original body plan and growth axis during the process of regeneration, appears to depend on the existence of the polyp's mouth. When damage to the coral polyp includes only partial damage to the original mouth, the regeneration process is characterized by maintenance of the growth axis and body plan. By contrast, complete removal of the original mouth results in the formation of multiple mouths and is followed by a drastic change in body plan and growth axis. Some of these mouths develop later into new polyps, which eventually detach from the original coral. During the first 2-3 mo of regeneration, 'preliminary' mouths developed and regressed until stabilization was attained. The appearance of new mouths in fragments farther from the original mouth (peripheral fragments) preceded by a week the appearance of new mouths in fragments that were more proximal to the original mouth (central fragments). Additionally, more mouths formed in peripheral than in central fragments. We interpret the results in light of the developmental model suggested by Meinhardt for Hydra. The formation of new mouths in regenerating fragments of Fungia is explained in terms of a source density gradient field, auto-activation, and lateral inhibition exerted by the original central mouth. These effectors determine the position and timing of mouth(s) development.