Salinity Tolerance of the Brackish-Water Echinoderm Ophiophragmus filograneus (Ophiuroidea)

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Abstract
O. filograneus is an infaunal brittlestar inhabiting shallow brackish waters of Florida (USA). Its tolerance to salinity changes was studied in the laboratory using burrowing time as an activity criterion. Complete acclimation of burrowing time occurred within 21 days of continuous acute exposure to salinities of 17-38.permill.. All test animals died within 2 days at 42.permill. and within 19 days at 10.permill.. Gradual reduction to 8.permill. from ambient salinity (25.permill.) during 43 days exposure gave results similar to those of the 1st experiment. A 3rd experiment gave evidence of acclimatization in a well established population in the Indian River lagoon, where salinity dropped to 9.permill. in Aug. 1978. Salinity tolerance of O. filograneus allows it to live in the organically rich sediments of Florida bays and lagoons, to which it is restricted, unlike the more stenohaline O. wurdemani from nearby marine, sandy habitats. This is the 1st documented case of an endemic brackish water echinoderm.