ABSTRACT-Rhode Rlver (USA) populatlons of the red-tlde d~noflagellates Gyrnnodinium sanguineum Hlrasaka, 1922, Cyi-odinium uncatenum Hulburt, 1957, and Scnppsiella trochoidea (Steln) Loeblich 111, 1976, were commonly infected by thelr parasltlc relative Amoebophrya cei-atil Cachon, 1964, d u n n g the summer of 1992. Mean ~nfection levels were relatively low, wlth data for vertically Integrated samples averaging 1.0, 1.9, and 6 5 % for G. sangujneum, G. uncatenum, and S , trocho~dea, respectively However, epldemlc outbreaks of A. ceratii (20 to 8 0 % hosts parasitized) occurred in G. uncatenum a n d S. trochoidea on several occasions, wlth peak levels of parasitism associated wlth decreases ~n host abundance. Estimates for paraslte Induced mortality indlcate that A, ceratil 1s capable of removlng a significant fraction of dinoflagellate blomass, with epldemics In the upper estuary cropplng u p to 5 4 % of the dominant bloom-forming species, G uncatenum, dally. However, epldemics were usually geographically restncted and of short duration, with dally losses for the 3 host species d u e to parasitism averaging 1 to 3 % over the summer. Thus, A ceratli appears capable of exerting a controlling Influence on bloonl-form~ng dinoflagellates of the Rhode River only when conditions a r e suitable for production of epidemlc infections. Interestingly, epidemics falled to occur in multlple d~noflagellate taxa sunultaneously, even when alternate host specles were present at hlgh densities. This observation, along with laboratory experiments demonstrating that parasites Isolated from G sanguineum wcre unable to infect G. uncatenum, S. trocholdea, and Ceratium furca, suggests that the dinoflagellate taxon A cerati] may represent a cluster of relatively host-speclfic specles.

In order to experimentally investigate feeding by mixotrophic dinoflagellates, we developed protocols for the use of live protistan prey as markers of ingestion. CMFDA (5-chloromethylfluorescein diacetate), a vital green fluorescent stain, was used to label cultures of photosynthetic nanoflagelIates, a diatom, and an oligotrichous ciliate. Cryptophytes were not readily stained with CMFDA, but phycoerythrin-containing members of this phylum havo a distinct yellow-orange fluorescence and thus can be used unstained to demonstrate ingestion. M'ith these complen~entary techniques, we qualitatively demonstrated feeding by the dinoflagellates Ceratium furca, G)/~nnodinium sanguineum, Gj,rodinium estuariale, Prorocentrum n~~n i m u m (= mariae-lebouriae) and Peridinium brevipesin natural dsscmblages from Chesapeake Bay, USA. LVe also used CMFDA-stained Isochrysis galbana (Prymncsiophyta) and unstained Cryptomonas sp (Cryptophyta) in laboratory and field studles, respectively, to examine prevalence of feeding by C estuariale as a function of prey dens~ty However, determination of in situ grazing rates for m~xotrophic dlnoflagellates proved difficult, as only a small percentage of cells contained labeled food vacuoles follolving short incubations (5 4 h) with stained prey added at tracer concentrations. The use of CMFDA-stained cells and phvcoerythrincontaining prey as markers of ingestion should also be applicable to species-specific feeding studies with other phagotrophic protists and micro-metazoa. The protocols prescmtcd here have advantaqas over the use of fluorescent microspheres or fluorescently labeled heat-killed algae (FLA) for investigating grazing or predation because many micrograzers do not readily ingest, or discriminate agalnst. inert particles.

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A 55-year-old man presenting with a 4-week history of abdominal swelling was found to have hepatomegaly. An abdominal ultrasound scan was performed using a 3.5 MHz transducer and a longitudinal section through the right upper quadrant shown below (Fig. 1).