Studies on Amyloodinium ocellatum (Dinoflagellata) in Mississippi Sound: Natural and Experimental Hosts

Four species of parasitic dinoflagellates have been found to occur naturally on the gills and fins of Mississippi Sound fishes: Amyloodinium ocellatum (Brown 1931) Brown and Hovasse 1946, Oodinium cyprinodontum Lawler 1967, and two undescribed species. Sixteen of 43 species of fishes examined had natural gill infections of A. ocellatum. Seventy-one of 79 species of fishes exposed to A. ocellatum dinospores were susceptible, and succumbed, to the dinoflagellate. Eight did not die even though exposed to numerous dinospores. The most common signs in an infested fish were spasmodic gasping and uncoordinated movements. Trophonts of A. ocellatum were found on the gills, skin, fins, eyes, pseudobranchs, membranes of the branchial cavity and around the teeth; and in the lateral line pits, nasal passages, esophagus, and intestine of experimentally infected fishes. The dinoflagellate causes extensive mortalities of fishes held under closed-system mariculture conditions. INTRODUCTION Parasitic dinoflagellates were first reported from vertebrates by Brown (1931), who described Oodinium (= Amyloodinium) ocellatum from marine aquarium fishes. Additional references to this species are those by Brown (1934), Nigrelli (1936, 1939, 1940), Brown and Hovasse (1946), Chatton (1952), Porter (1952), Dempster (1955, 1956, 1972), Laird (1956), Simkatis (1958), Hdjgaard (1 962), Paccaud (1 962), Chlupaty (1962), Buxton (1962), Graaf (1962), Valenti (1968), Kingsford (1975), Becker (1977), and Lawler (1977a, 1977b, 1979). Oodinium limneticum, a species from freshwater fishes, was described by Jacobs (1946); additional references are those by Kozloff (1948, 1949), Patterson (1949), and Lewis (1963). Another freshwater species, 0. pillularis Schiiperclaus, 1954, has received considerable attention in Europe (Weiser 1949; Schiiperclaus 195 1 ; Hirschmann and Partsch 1953; Reichenbach-Klinke 1954, 1955, 1956, 1961; Schubert 1959; Geus, 1960a, 1960b, 1960c , 1 9 6 0 d ; Reichenbach-Klinke and Elkan 1965; and Lucky 1970. An estuarine species, 0. cyprinodontum, was described from Virginia cyprinodontids by Lawler (1967). It has been reported by Dillon (1966), Lawler (1967, 1968a, 1968b), Lom and Lawler (1971, 1973), and Williams (1972). Of these species, only 0. pillularis has not been reported from North America. Unidentified parasitic dinoflagellates from North American fishes have been reported by Overstreet (1968), Lom and Lawler (1971, 1973), and Papema and Zwerner (1976). Oodinium cyprinodontum has been reported from the Gulf of Mexico (Williams 1972); Kingsford (1975) illustrated a trophont from a mangrove snapper (Lutjanus griseus) with an A. ocellatum infection, presumably from the eastern Gulf of Mexico. 'This study was conducted in part in cooperation with the U.S. Department of Commerce, NOAA, National Marine Fisheries Service, under PL 18-309, Roject No. 2-262-R. Manuscript received August 1 , 1979; accepted October 10,1979. Amyloodinium ocellatum (Brown 193 1) causes extensive mortalities of fishes in confined areas (Brown 1931, 1934; Nigrelli 1936, 1939, 1940; H$jgaard 1962; Lawler 1977a, 1977b), and has been reported from many species of marine teleosts (Brown 1934; Nigrelli 1936, 1940; Brown and Hovasse 1946; Dempster 1955, 1956; Chlupaty 1962; Graaf 1962; Paccaud 1962; Straughan 1970; Lawler 1979). Oodinium cyprinodontum Lawler 1967 has not yet been found to cause fish deaths in aquaria (Lawler, unpublished). In October 1971, I started trying to control A. ocellatum on Micropogonias undulatus which were being held for experiments on lymphocystis (Cook 1972). Studies were initiated to ascertain the following: (1) identity of the species of parasitic dinoflagellates naturally present on fishes of Mississippi Sound; (2) the natural hosts of parasitic dinoflagellates in Mississippi waters; (3) those fishes susceptible to A. ocellatum in aquaria; and (4) the methods for controlling dinospores and trophonts of A. ocellatum in aquaria. The present report includes my findings thus far on the first three items. The results of efforts to control the parasites will be presented in a future report. MATERIALS AND METHODS Fishes were collected mainly in Mississippi Sound, from Biloxi Bay to Horn Island, by means of trawls, traps, dip nets, and hook and line. Salinity was determined with a refractometer. Fishes were held alive in water from their place of capture and examined within a day. The fishes were bled by gutting them or cutting their tail to draw the blood away from the as, thus facilitating examination of the excised gills under reflected light with a dissecting microscope. The gdls of fishes collected were examined for natural infections in autoclaved seawater to avoid introduction of fortuitous dinoflagellates. Infected filaments were removed with irridectory scissors and the live trophonts were examined with a compound microscope for the presence of a stigma, stomopode, and other morphological characters. 403