RAPID RHIZOID PRODUCTION IN HALIMEDA-DISCOIDEA DECAISNE (CHLOROPHYTA,CAULERPALES) FRAGMENTS - A MECHANISM FOR SURVIVAL AFTER SEPARATION FROM ADULT THALLI

Halimeda discoidea Decaisne is a common, coenocytic, calcified, reef m acrophyte that uses rhizoids to anchor itself either in sandy habitats or on rocks. In Hawaii, small fragments of this macroalga are often b roken off from adult thalli by physical disturbances and herbivores. V irtually all H. discoidea fragments rapidly produced new attachment rh izoids, regardless of the (1) source of fragmentation (fish grazing, s torm damage, razor blade), (2) location where fragments were held (fie ld, laboratory), (3) fragment size (0.5 to 4.0 segments), (4) location along the original plant axis, or (5) breakage orientation. Verticall y-cut segments produced rhizoids significantly earlier than horizontal ly-cut segments or node-cut fragments, even though the rate of rhizoid al production was constant among the three cutting orientations after rhizoid production began (mean +/- SE = 0.01 +/- 0.01 mg/day ash-free dry weight). Additionally, no differences existed either in the percen t of fragments producing rhizoids or the timing of rhizoid production within a cutting orientation. The selective advantage of quickly plugg ing wounds in large coenocytic organisms is clear. H. discoidea is str iking in its ability not only to plug wounds, but to rapidly produce r hizoids in this wound region. Fragments as small as 15 mm2 with damage to three of four edges were able to produce rhizoids within 3 days. T hus, through this novel means of vegetative fragmentation, grazers and wave action potentially increase plant numbers by clonally propagatin g nearby adults in reef habitats.