FERTILIZATION IN NATURAL-POPULATIONS OF THE DIOECIOUS BROWN ALGA FUCUS-CERANOIDES AND THE IMPORTANCE OF THE POLYSPERMY BLOCK

The brown alga Fucus ceranoides L. was studied in several estuaries on the Isle of Man (Great Britain) in the summers of 1989 and 1990. The objectives were to determine the success of natural fertilization in a dioecious organism with external fertilization and to contribute to our understanding of processes such as polyspermy blocks and propagule settlement under natural conditions. Gamete release occurred on a semilunar cycle near full and new moons; at such times, settlement densities of 500 zygotes cm-2 d-1 were common. Gamete release was largely restricted to daytime high tides. Fertilization success was high: about 95 to 100% of all eggs released were fertilized, and 1 to 9% of these zygotes were polyspermic. The incidence of polyspermy increased towards the upper limit of F. ceranoides in the estuary of the river Neb; this may be significant in the context of the Na+-dependent block against polyspermy. Levels of polyspermy were not related to fertilization success (R2 = 0.06). Rates of pronuclear migration and karyogamy determined in the laboratory were used as an internal clock to estimate when eggs were fertilized in the field. Most eggs were fertilized 30 to 120 min after plants were immersed by the incoming tide, which corresponded to the period 60 min before to 30 min after high tide. The high success of fertilization suggests that polyspermy blocks are important in nature, and, in combination with the high settlement of zygotes, shows that population size in F. ceranoides is determined by post-settlement mortality, not by propagule availability.