REPRODUCTIVE ECOLOGY OF FUCUS-DISTICHUS (PHAEOPHYCEAE) - AN INTERTIDAL ALGA WITH SUCCESSFUL EXTERNAL FERTILIZATION

Patterns of natural gamete release and fertilization success are repor ted for the open-coast macrophyte Fucus distichus L. (formerly Fucus d istichus ssp. distichus), which is restricted to tide pools in the upp er intertidal zone. The release and settlement of eggs and zygotes of F. distichus occurred during daytime low tide periods (DLT), largely o n days when the low tide fell between 10:00 and 14:00 h EST; very few gametes were released when high tides fell during the same time interv al. Gamete release during nighttime low tides was very low. During the DLT series, gametes were released in all pools and this was better co rrelated with the time of day than with time of low tide per se. The t iming of fertilization showed considerable variation both within and b etween pools during a single DLT. No relationship between either pool temperature or osmolality and gamete release was evident, and only sma ll episodes of gamete release coincided with the times of full or new moons. The restriction of gamete release to periods of very low water motion, when tide pools are isolated by the low tide, prevents gamete dilution by water exchange and turbulent flows, and results in externa l fertilization success between 78 and 100% with low levels of polyspe rmy (1 to 5%). Dispersal of zygotes among tide pools may be facilitate d by the low winter temperatures during reproduction, which retard adh esive production and zygote attachment. Our results demonstrate that i ntertidal organisms, living in habitats characterized by periodic, tur bulent flow regimes, may achieve high levels of fertilization success by releasing gametes under optimal hydrodynamic conditions for sperm-e gg encounters.