Abstract

Although spores, sporelings and juveniles of many ecologically important and competitively dominant seaweed species inhabiting intertidal and shallow subtidal habitats may coalesce, it is unknown whether coalescence among holdfasts of well-established clumps occurs naturally in the field. Using a combination of field and ultrastructural studies, we evaluate if inter- and intraspecific coalescence occurs between clumps of Mazzaella laminarioides and Nothogenia fastigiata and, if so, whether such a process modifies clump shapes and sizes. Interspecific encounters always resulted in cell compression and destruction of the contacting tissues and in the eventual mortality of 1 of the partners (in this case N. fastigiata). In contrast, intraspecific encounters in both species always resulted in coalescence. In these field populations, coalescence seems to be frequent and dynamic and is probably modulated by other community structuring mechanisms such as grazing, abiotic extremes, and mussel recruitment. This field demonstration of coalescence suggests that many intertidal and subtidal populations of coalescing Rhodophyta might be chimeric.