Ahnfeltia durvillaei and Gymnogongrus furcellatus, two intertidal red algae from central Chile, often grow sympatrically and their gametophytic phases are dominant in the field, but recruitment is low. A shortterm comparative analysis of the reproductive behavior of their gametophytes was carried out to determine quantitative levels of reproduction and their degree of functional similarity. Size, rather than age, was evaluated as predictor of fecundity. Both species showed the same qualitative patterns, although maintaining quantitative differences. Reproductive plants were recorded throughout the size range, and fecundity increased continuously and directly with size. A size-independent threshold in reproductive effort was found for each species, and the variance of effort values decreased inversely with size. Vertical, size-specific life tables revealed low dependence of survivorship with size, and reproductive values exhibited a maximum at the same size class for both species. This last pattern, and a negative relationship found between fecundity and survivorship, suggest the existence of costs or trade-offs operating at the phenotypic level, but they would not be supported in a selective context.
Keywords: Algal life history; Fecundity; Maximization; Size; Trade-off.