Sporeling coalescence in Gracilaria chilensis Bird, McLachlan et Oliveira produces genetically polymorphic, chimeric individuals. If this is common in red algae, it may have significant biological consequences. In this study, we evaluate the hypotheses that coalescence is widespread among the Rhodophyta and that specific and convergent morphological and ecological responses characterize this as a unique growth style among marine algae. A literature survey on coalescence was undertaken to assess the distribution of this condition in the Florideophycidae. Sixty-two (54.9%) of 113 species considered germinated to form a disk. Subsequent development in 37 of these species showed crust formation and coalescence during development with other crusts in 31 species (84%). Coalescing red algae were members of the orders Ahnfeltiales, Corallinales, Gigartinales, Gracilariales, Halymeniales, Palmariales, and Rhodymeniales. Ultrastructural studies in species of Ahnfeltiopsis, Chondrus, Gracilaria, Mazzaella, and Sarcothalia suggested a common pattern of early development. Newly released, naked spores may fuse into a single cell, as they do in Chondrus canaliculatus, or they may develop individual cell walls that later are surrounded by a thickened common wall. Ultrastructural studies demonstrated two kinds of immediate development after the first mitotic division: direct development by symmetric divisions resulting in discoid sporelings or an indirect asymmetric arrangement of divisions before a diskoid sporeling was formed. Germination in coalescing species is a linear function of the initial spore density, whereas in noncoalescing species maximum germination occurs at intermediate densities. In the field, coalescing species may recruit either from solitary or aggregated spores. However, survival is significantly higher for plantlets grown from a larger number of coalescing spores. Total number of erect axes formed by the coalesced mass is a logarithmic function of the initial number of spores. Thus, germlings grown from a larger number of coalescing spores exhibited a larger photosynthetic canopy than do plantlets grown from a few spores. Juveniles and mature clumps grown from a coalescing mass may exhibit size inequalities among erect axes, with the larger axes located toward the center of the clump. These larger axes mature first or, in some cases, are the only to produce spores. The widespread occurrence of coalescence in 1 roughly half the number of orders of the Florideophycidae, the similarity of the coalescence process, and the finding of various adaptive traits associated with coalescence characterizes this as a unique growth style, splitting the diversity of species now included in the Florideophycidae into two major groups: coalescing and noncoalescing Rhodophyta.

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.

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Coalescence in seaweeds is known to occur in the laboratory among young and older sporelings and in the field between neighboring conspecific clumps. However, because spores and germlings are difficult to study in the field, it is as yet unknown at which stage of population development coalescence is most important. Since many seaweeds disperse aggregated propagules, often with a sticky mucilagous envelope around the spores, aggregated recruitment and coalescence might be more important at early stages of population establishment than among fully grown, well established clumps. Using recruitment plates maintained during several experimental times in the field, we are evaluating the above idea with mid‐intertidal populations of Mazzaella laminarioides. During high fertility seasons, close to 45% of the spores settling within or at close (<1 m) distances of the bed exhibited aggregated recruitment, forming groups of 2 to 150+spores. The probability of aggregated recruitments is a function of dispersal distance and spore density. The number of sporelings produced is a function of spore density and coalescence. Highest after‐recruitment mortality (first 15 days) occurs among solitary recruits, followed by sporelings conformed by small number of spores (2–4). Approximately 50% of the spores recruited, isolated or in group, coalesce within these 15 days, gradually forming massive sporelings with increasingly larger basal areas. Thus, after recruitment, sporelings may disappear (die), survive or coalesce. These three alternatives are integrated in a new demographic model for coalescing seaweeds (supported by grant FONDECYT 1020855).