Distribution and Ecology of Estuarine Ectoprocts: A Critical Review

While most gymnolaemates are restricted to waters of normal salinity, at least 3-6% are able to penetrate some distance into mixohaline water. Of this group, which includes 9 species of cyclostomes, 35 species of ctenostomes, 55 species of anascan and 21 species of ascophoran cheilostomes, the cyclostomes and the ascophorans are least tolerant of diluted salinities, the ctenostomes and the anascans are most tolerant. Like many other groups of benthic organisms, ectoprocts show a decrease in numbers of species with decreasing salinity. Only 5 species can penetrate into waters of less than 3‰. About 20 species can be considered truly brackish-water organisms, being most abundant in mixohaline waters. Apparently these species possess some means of active osmoregulation, probably at the tissue level. The distribution of brackish water ectoprocts depends not on salinity alone, but also on factors of temperature, substrate availability and the general stability of the environment. Research most necessary before distribution patterns can be explained concerns the salinity tolerance of larvae and adults, larval behavior, and physiology. Also needed are faunal studies, particularly in tropical estuarine localities.