DISTRIBUTIONAL SUCCESS OF THE MARINE SEAWEED FUCUS-VESICULOSUS L IN THE BRACKISH BALTIC SEA CORRELATES WITH OSMOTIC CAPABILITIES OF BALTIC GAMETES

To understand the unique success of the marine seaweed Fucus vesiculos us L. (Phaeophyceae) in the brackish Baltic Sea, the performance of ga metes from Baltic [4.1-6.5 parts per thousand S (Salinity)] and marine populations was studied. Sperm from Baltic F. vesiculosus swam with a path velocity of c. 30-110 mu m/s and could fertilize eggs in waters of salinities from 4 to 33 parts per thousand S. In their natural wate r, Baltic sperm were not negatively phototactic, unlike marine sperm i n seawater; this should decrease the sperm:egg concentration at the se afloor and reduce the likelihood of polyspermy. Marine (Iceland, Swede n) sperm in seawater had a path velocity of c. 80-100 mu m/s, but perf ormed poorly and could not fertilize eggs in natural or artificial Bal tic water less than or equal to 6 parts per thousand S; therefore, Bal tic populations have adapted or acclimated to their brackish habitat. Baltic populations appear better adapted to their natural low saliniti es because, even after culturing Baltic and marine individuals in wate r from both the Baltic (6.5 parts per thousand S) and the marine Skage rrak (21 parts per thousand S), Baltic sperm were in both cases still able to swim and fertilize eggs at lower salinities (4 parts per thous and S) than marine sperm; fertilization never occurred between marine gametes at 4-6 parts per thousand S. However, F. vesiculosus acclimate s to some salinities, since sperm from Baltic and marine males that ha d been cultured at 21 parts per thousand S swam better (higher velocit y, proportion that were motile and/or linearity) in marine salinities (21-33 parts per thousand S) than when they were cultured at 6.5 parts per thousand S. The effects of salinity on sperm motility and fertili zation were osmolar rather than due to specific ionic requirements, ov er the tested range. The osmolalities (< c. 100 mmol/kg) at which fert ilization success of Baltic gametes decreases nearly to zero correspon d to the osmolality of Baltic water at the northernmost limit of distr ibution of F. vesiculosus in the Baltic Sea. Therefore, the present ra nge of F. vesiculosus in the Baltic appears to correspond to the osmot ic tolerance of the gametes. Very small natural or anthropogenic incre ases in ambient osmolality would be likely to cause a substantial expa nsion of this species into the inner Baltic.