APICAL ELONGATION OF FUCOID ALGAE EXPOSED TO LOW SALINITIES

Apical elongation of five intertidal fucoids (Pelvetia canaliculata (L .) Dec. et Thur., Ascophyllum nodosum (L.) Le Jol., Fucus spiralis L., Fucus vesiculosus L. and Fucus serratus L.) was measured during expos ure to different salinities. The response could be separated into two distinct phases, an initial (0-3 h) very rapid elongation due to osmot ic effects, and after about 24 h an approximately steady state where e longation rates probably represented normal growth rates. Elongation r ates during the initial phase were inversely correlated to salinity. I n the steady state phase, all species demonstrated a very wide salinit y tolerance for elongation rate. For P. canaliculata, the maximum elon gation rate was observed at the lowest salinity applied, 2.6 parts per thousand. Cumulative elongation including both phases showed a maximu m at salinities less than 20 parts per thousand, and exposure to a var iable salinity regime (34 parts per thousand for 23 h, 5 parts per tho usand for 1 h) for 5 days showed stable elongation rates after 2 days of exposure, 2-3 times higher than at 34 parts per thousand. This indi cates that exposure to low or variable salinities increases total elon gation, and it is suggested that this may enhance competition for ligh t and thus be of ecological significance.