The phenology of sexual reproduction by green algae (Bryopsidales) on Caribbean coral reefs

Recent field observations of highly organized, species- and sex-specific patterns of synchronous gamete release by tropical green algae (Bryopsidales) invite a variety of future studies into the ecology and life histories of an important component of tropical reef communities. This paper details sexual reproduction by 22 algal species within five common genera (Caulerpa, Halimeda, Penicillus, Rhipocephalus, and Udotea), including field observations on the spatial occurrence, timing, and color changes associated with fertility, data on gamete size and behavior, and descriptions of coincident changes in local species abundances. Ecologically ephemeral episodes of sexual reproduction involved macroscopic changes that reliably indicated developmental state and sexual identity. The time from onset of fertility to gamete release was 36 h (Halimeda), 48 h (Caulerpa, Penicillus, and Rhipocephalus), or 96 h (Udotea). All species produced flagellated, negatively buoyant, anisogamous gametes, Microgametes of all species were similar in size; however, considerable species-specific differences were seen in the size of macrogametes. In Caulerpa, Halimeda, and Udotea flabellum (Ellis and Solander) Lamouroux, the volumetric ratio of macrogametes to microgametes ranged from 2:1 to 45: 1, whereas more extreme levels of anisogamy (10(4):1) were observed for Penicillus spp., Rhipocephalus phoenix (Ellis and Solander) Kuetzing, and other Udotea spp, The macrogametes of Caulerpa and Halimeda showed strong positive phototaxis. Although only a subset (generally about 5%) of the thalli representing each species released gametes on a given morning, most species underwent bouts of sexual reproduction on numerous occasions during the seasonal peak of reproductive activity (March-May), As might be expected for holocarpic species, dramatic declines in local algal abundance coincided with these periods. The density of sand-dwelling genera such as Penicillus fell by 80-90% during this 3-month period in 1997. Similar declines in the cover of sprawling species such as Caulerpa racemosa ((Forsskal) J. Agardh) exposed large (35-50 m(2)) sections of previously overgrown reef substrate.