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Seawater and sediment samples from various sites in or near Halifax Harbour were cultured for Paramoeba, as were tissues of the natural sea urchin host Strongylocentrotus droebachiensis sampled from Hahfax Harbour. An solate of Paramoeba from a Halifax Harbour water sample was not virulent on injection into healthy sea urchins. However, some sea urchins in cages near the outfall pipe of Dalhousie University's seawater system were infected during the annual peak of the seawater temperature cycle. Results indicate that (1) there is no evidence of a free-living endemic population of Paramoeba invadens in or near Halifax Harbour, (2) S. droebachiensis, the natural host for P. invadens, do not harbour a reservoir population of this orgamsm at temperatures sub-optimal for paramoebiasis, and (3) Paramoeba which are morphologically ~ndistinct from laboratory stocks of P. ~nvadens were recovered near the outfall pipe. These organisms may have been released via the outfall pipe into Halifax Harbour and subsequently lost virulence. A pathogenic marine amoeba, Paramoeba invadens Jones, 1985, was found to be the causative agent of sea urchin Strongylocentrotus droebachiensis mass mortalities along the Atlantic coast of Nova Scotia from 1980 to 1983 (Miller & Colodey 1983, Scheibling & Stephensor, 1984, Jones & Scheibling 1985, Jones et al. 1985). Most species of the genus Paramoeba are freeliving and ubiquitous in the marine environment, and are commonly found in the surface microlayer, water column and bottom sediments in a range that extends from the North Atlantic to the Gulf of Mexico (Page 1970, 1973, 1976, 1983, Davis et al. 1978, Bovee & Sawyer 1979, Sawyer 1980). Only 2 species in this genus are known to be parasitic (Page 1983, Jones 1985). Paramoeba perniciosa appears to be an obligate parasite (Sprague et al. 1969, Page 1983) in the blue crab Callinectes sapidus. P. invadens is known only Addressee for correspondence; present address. Department of Microbiology, Sir Charles Tupper Medical Building, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4H7 O Inter-Research/Printed in F. R. Germany from the tissues of diseased sea urchins and their surrounding seawater (Jones & Scheibling 1985), but it can be cultured on bacteria and is therefore thought to be a free-living opportunistic parasite (Jones 1985, Jones & Scheibling 1985, Jones et al. 1985, Jellett & Scheibling 1988b). The only known species of freeliving Paramoeba morphologically similar to P. invadens is P. aestuanna (Page 1983). Recent observations by Rogerson (1988 and pers. comm.) may bring into question their position as different taxonomic species. The origin (endemic or exotic) of the infective population of Paramoeba invadens involved in the sea urchin mass mortalities along the Nova Scotia coast has remained unresolved (Jones & Scheibling 1985,

Non-occurrence of free-living Paramoeba invadens in water and sediments of Halifax Harbour, Nova Scotia, Canada