LOW-TIDE EXPOSURE OF SPONGES IN A CARIBBEAN MANGROVE COMMUNITY

Sponges on subtidal red-mangrove prop roots may become exposed to air many times per year during very low tides. Full exposure is stressful and potentially fatal, particularly if occurring in full sun. Large ro ot sponges show distinct species zonation between mean low water and - 0.5 m. Haliclona implexiformis and Lissodendoryx isodictyalis are near the top while Scopalina ruetzleri are near the lower end of the range . Temporary experimental desiccation resulted in 100% recovery of all three species after they had been exposed to either sun or shade for u p to 2 h. Scopalina is the least resistant and lost over 90% tissue wi thin 3 days after the 4-h and 6-h experiments; the remaining cell mass succumbed to infestation by microbes. Haliclona and Lissodendoryx rec overed from as much as 6 h in full sun but lost 85% and 80% of the ori ginal tissue volume, respectively. In Lissodendoryx, clusters of larva e developed in the regenerating fragments. Water loss tolerated by the three species is estimated as 66% of wet weight in Haliclona, 54% in Lissodendoryx and 38% in Scopalina. Salinity of interstitial seawater (pore water) extracted from exposed sponges rose from ambient 3.5% to 4.3-4.8% after 1 h, to 5.1-5.9% after 6 h. Most endobionts died or lef t their host during this last phase. Natural vertical zonation in thes e sponges reflects their resistance to tidal exposure.