Extraordinary mound building forms of Avrainvillea (Bryopsidales, Chlorophyta): their experimental taxonomy, comparative functional morphology and ecological strategies

The discovery of astounding mound-building forms ofAvrainvillea (to 30 m diam.) catalyzed this study. These colonial (possibly clonal) mounds dominate the standing stocks and productivity of protected, shallow, eutrophic interiors of Belizean mangrove islands. A comrnongarden reciprocal-transplant experiment showed that the mound formers (A. longicaulis f. laxa and A. asarifolia f. olivacea fiom Twin Cays), which we initially hypothesized to be undescribed species, readily acquired the morphological features consistent with the taxa characteristic of open-water habitats (A. longicaulis f. longicaulis and A. asarifolia f. asarifolia fiom Curlew Cay), thereby falsifjring the hypothesis that the mound formers are distinct species. In support of the coloniality hypothesis, the Twin Cays f. laxa and f. olivacea morphs were uniquely adapted to produce flabellar stipes that serve as shallow subterranean rhizomes which spread laterally to overgrow rich organic peat bottoms. The massive columnar rhizoidal holdfasts found in the Curlew Cay f. longicaulis and f. asarifolia morphs were adaptive for both anchoring and obtaining pore-water nutrients, but proved to be superfluous under placid enriched water-column nutrient conditions and were incapable of surviving the deeper anoxic conditions of the composting peat deposits. The large colonial mangrove morphs (i.e., f. laxa and f. olivacea) were not physically resistant to even the moderate current levels (3.6k0.5 cm per sec) encountered in the back-reef lagoon habitats ofthe deeply anchored morphs (i.e., f. longicaulis and f. asarifolia). However, smaller 2-3 blade clumps, with their stipes deeply buried, survived and grew. Consistent with the perennation hypothesis, only the experimentally amputated Curlew Cay morphs (both f. longicaulis and f. asarifolia) showed significantly more proliferations (1 00 %) than either the amputated Twin Cays morphs (both f. laxa and f. olivacea) or the uncut Curlew and Twin Cays control plants. The stipes and blades of the open-water morphs (f. longicaulis and f. asarifolia) serve as expendable assimilators with a major function of building a massive perennatinglstorage organ, the columnar holdfast, which comprises the bulk of the plant. Physical disturbances (such as storms and herbivory), as well as physiological stresses (such as epiphyte loading), can cause disproportionate losses of the relatively delicate expendable assimilators which are replaced subsequently by perennation fiom the long-lived subterranean holdfast during more favorable conditions.