Convergent biological processes in coalescing rhodophyta

Sporeling coalescence in Gracilaria chilensis Bird, McLachlan et Oliveira p roduces 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 Rhodop hyta and that specific and convergent morphological and ecological response s characterize this as a unique growth style among marine algae. A literatu re survey on coalescence was undertaken to assess the distribution of this condition in the Florideophycidae. Sixty-two (54.9%) of 113 species conside red germinated to form a disk. Subsequent development in 37 of these specie s showed crust formation and coalescence during development with other crus ts in 31 species (84%). Coalescing red algae were members of the orders Ahn feltiales, Corallinales, Gigartinales, Gigartinales, Halymeniales, Palmaria les, and Rhodymeniales. Ultrastructural studies in species of Ahnfeltiopsis , Chondrus, Gracilaria, Mazzaella, and Sarcothalia suggested a common patte rn of early development. Newly released, naked spores may fuse into a singl e cell, as they do in Chondrus canaliculatus, or they may develop individua l cell walls that later are surrounded by a thickened common wall. Ultrastr uctural studies demonstrated two kinds of immediate development after the f irst mitotic division: direct development by symmetric divisions resulting in discoid sporelings or an indirect asymmetric arrangement of divisions be fore a diskoid sporeling was formed. Germination in coalescing species is a linear function of the initial spore density, whereas in noncoalescing spe cies maximum germination occurs at intermediate densities. In the field, co alescing species may recruit either from solitary or aggregated spores. How ever, survival is significantly higher for plantlets grown from a larger nu mber of coalescing spores. Total number of erect axes formed by the coalesc ed mass is a logarithmic function of the initial number of spores. Thus, ge rmlings 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 inequalitie s among erect axes, with the larger axes located toward the center of the c lump. These larger axes mature first or, in some cases, are the only to pro duce spores. The widespread occurrence of coalescence in roughly half the n umber of orders of the Florideophycidae, the similarity of the coalescence process, and the finding of various adaptive traits associated with coalesc ence characterizes this as a unique growth style, splitting the diversity o f species now included in the Florideophycidae into two major groups: coale scing and noncoalescing Rhodophyta.