Influence of nutrients on ploidy‐specific performance in an invasive, haplodiplontic red macroalga

Worldwide, macroalgae have invaded near‐shore marine ecosystems. However, their haplodiplontic life cycles have complicated efforts to predict patterns of growth and spread, particularly since most theoretical predictions are derived from diplontic taxa (i.e., animals). To complete one revolution of the life cycle, two separate ploidy stages, often including separate haploid sexes, must pass through development and reproduction. In the case of the invasive, red macroalga Agarophyton vermiculophyllum, during the invasion of soft‐sediment estuaries throughout the Northern Hemisphere, diploid tetrasporophytes came to dominate all free‐floating populations and haploid gametophytes were consistently lost. The ecological hypothesis of nutrient limitation might contribute to an explanation of this pattern of tetrasporophytic dominance in free‐floating populations. Under this hypothesis, gametophytes should outperform tetrasporophytes under nutrient limited conditions, but tetrasporophytes should be better able to exploit optimal or even abundant nutrient conditions, such as in eutrophic estuaries. We sampled tetrasporophytes, male gametophytes, and female gametophytes from two sites each located on either side of the Delmarva Peninsula that separates the Chesapeake Bay from the Atlantic Ocean. We subjected apices excised from multiple thalli from each life cycle stage to a nutrient‐enriched and a nutrient‐poor seawater treatment and assessed growth and survival. While nutrient addition increased growth rates, there was no significant difference among ploidies or sexes. Gametophytes did, however, suffer higher mortality than tetrasporophytes. We discuss how nutrient‐dependent differences in growth and survival may contribute to observed patterns of tetrasporophytic dominance in soft‐sediment A. vermiculophyllum populations.