Performance of non-motile male gametes in the sea: analysis of paternity and fertilization success in a natural population of a red seaweed,Gracilaria gracilis

In haploid–diploid red seaweeds, the dispersal of male gametes is presumed limited due to their lack of flagella. It has been suggested that this group suffers from sperm limitation and, consequently, that fertilization is relatively inefficient. Fertilization in most floridean rhodophytes results in the formation a cystocarp, a swelling on the haploid female thallus housing the diploid zygote and its thousands of diploid daughter spores. To study the performance of non-motile male gametes in the sea, we evaluated both female and male fertilization success in a natural population of the red marine alga Gracilaria gracilis. Female fertilization success, estimated by cystocarp yield per unit female thallus, was evaluated with respect to the availability of male gametes. Male fertilization success, estimated by the individual contribution of different males to zygotes, was assessed by paternity analyses on 350 cystocarps produced in one reproductive season using two microsatellite loci. The results show that cystocarp yield is not sperm limited and that the large variation in male fertilization success cannot be solely explained by the distance travelled by the male gamete to find a mate. Taken together, the results suggest that, not only is fertilization efficient, but that male–male competition and/or female choice may play a role in shaping population mating patterns.