CUES FOR ZOOSPORE RELEASE BY MARINE OOMYCOTES IN NATURALLY DECAYING SUBMERGED LEAVES

Two common potential environmental impacts upon leaf-decomposers in th e marine intertidal zone (mild drying, dilution of salinity/lowering o f temperature due to rainfall) were simulated and tested as enhancers of zoospore release by species of Halophytophthora. Senescent leaves o f mangroves and oak were used as bait in flow-through chambers to quan titate zoospore passage from disks of naturally decaying leaves. Older -decaying leaves of red and black mangrove had low rates of zoospore r elease by H. vesicula, the predominant oomycote encountered (< 15% of bait samples occupied in 24 h), and environmental cues had no detectab le effect. Younger-decaying leaves of red mangrove had considerably hi gher rates of release, and rates were stimulated 2-fold by dilution (t o 50% of bait samples positive). Rates for oak leaves were doubled by both dilution and drying (to 86-92% positive). We speculate that: i) h alophytophthoras complete their occupation of submerged leaves (from a ttachment of zoospore cysts to release of zoospores from sporangia) ea rly in the decomposition of leaves, before substantial entry into the leaves of labyrinthulids (predators); ii) environmental cuing results in high concentrations of halophycophthoran zoospores in shallow sheet s or pools of low-tidal waters, enhancing probability of rapid contact with new-fallen leaves.