Rapid rhizoid production in Halimeda discoidea decaisne (Chlorophyta, Caulerpales) fragments: a mechanism for survival after separation from adult thalli

Abstract

Halimeda discoidea Decaisne is a common, coenocytic, calcified, reef macrophyte that uses rhizoids to anchor itself either in sandy habitats or on rocks. In Hawaii, small fragments of this macroalga are often broken off from adult thalli by physical disturbances and herbivores. Virtually all H. discoidea fragments rapidly produced new attachment rhizoids, regardless of the (1) source of fragmentation (fish grazing, storm damage, razor blade), (2) location where fragments were held (field, laboratory), (3) fragment size (0.5 to 4.0 segments), (4) location along the original plant axis, or (5) breakage orientation. Vertically-cut segments produced rhizoids significantly earlier than horizontally-cut segments or node-cut fragments, even though the rate of rhizoidal 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 percent of fragments producing rhizoids or the timing of rhizoid production within a cutting orientation. The selective advantage of quickly plugging wounds in large coenocytic organisms is clear. H. discoidea is striking in its ability not only to plug wounds, but to rapidly produce rhizoids in this wound region. Fragments as small as 15 mm 2 with damage to three of four edges were able to produce rhizoids within 3 days. Thus, through this novel means of vegetative fragmentation, grazers and wave action potentially increase plant numbers by clonally propagating nearby adults in reef habitats.