Feeding by common heterotrophic dinoflagellates and a ciliate on the red-tide ciliate Mesodinium rubrum
- Author: Lee Kyung Ha, Jeong Hae Jin, Yoon Eun Young, Jang Se Hyeon, Kim Hyung Seop, Yih Wonho
- Organization: Lee Kyung Ha; Jeong Hae Jin; Yoon Eun Young; Jang Se Hyeon; Kim Hyung Seop; Yih Wonho
- Publish: ALGAE Volume 29, Issue2, p153~163, 15 June 2014
-
ABSTRACT
Mesodinium rubrum is a cosmopolitan ciliate that often causes red tides. Predation by heterotrophic protists is a critical factor that affects the population dynamics of red tide species. However, there have been few studies on protistan predators feeding onM. rubrum. To investigate heterotrophic protists grazing onM. rubrum , we tested whether the hererotrophic dinoflagellatesGyrodiniellum shiwhaense, Gyrodinium dominans, Gyrodinium spirale, Luciella masanensis, Oblea rotunda, Oxyrrhis marina, Pfiesteria piscicida, Polykrikos kofoidii, Protoperidinium bipes , andStoeckeria algicida , and the ciliateStrombidium sp. preyed onM. rubrum. G. dominans, L. masanensis, O. rotunda, P. kofoidii , andStrombidium sp. preyed onM. rubrum . However, onlyG. dominans had a positive growth feeding onM. rubrum . The growth and ingestion rates ofG. dominans onM. rubrum increased rapidly with increasing mean prey concentration <321 ng C mL-1, but became saturated or slowly at higher concentrations. The maximum growth rate ofG. dominans onM. rubrum was 0.48 d-1, while the maximum ingestion rate was 0.55 ng C predator-1 d-1. The grazing coefficients byG. dominans on populations ofM. rubrum were up to 0.236 h-1. Thus,G. dominans may sometimes have a considerable grazing impact on populations ofM. rubrum .
-
KEYWORD
ciliate , growth , harmful algal bloom , ingestion , predation
-
Mesodinium rubrum is a globally distributed ciliate (Lindholm 1985, Crawford 1989, Williams 1996, Gibson et al. 1997) that sometimes causes red tides in coastal waters (Johnson et al. 2004, Yih et al. 2004, Hansen and Fenchel 2006, Hansen et al. 2013, Johnson et al. 2013, Kang et al. 2013).M. rubrum is capable of both photosynthesis and prey ingestion (Gustafson et al. 2000, Yih et al. 2004, 2013). In addition, this species is an important prey for some dinoflagellate predators (i.e.,Amylax triacantha, Alexandrium pseudogonyaulax, Dinophysis spp.,Neoceratium furca, andOxyphysis oxytoxoides ) and an effective grazer of cryptophytes (Yih et al. 2004, Park et al. 2006, 2011, 2013b , Blossom et al. 2012, Hansen et al. 2013, Johnson et al. 2013).The predation of
M. rubrum by heterotrophic protists is one of the critical factors that affect the population dynamics of red tide species. Heterotrophic protists play an important role in marine food webs, as they connect phototrophic plankton to higher trophic levels (Stoecker and Capuzzo 1990, Sherr and Sherr 2002, Myung et al. 2011, Garzio and Steinberg 2013). However, there have been few studies on the feeding patterns of common heterotrophic protists that frequently co-occur withM. rubrum .O. oxytoxoides is the only heterotrophic dinoflagellate that is known to feed onM. rubrum (Park et al. 2011). However, the growth and ingestion rates and / or the impact of heterotrophic protist grazing onM. rubrum have not been reported.Gyrodiniellum shiwhaense, Gyrodinium dominans, Gyrodinium spirale, Luciella masanensis, Oblea rotunda, Oxyrrhis marina, Pfiesteria piscicida, Polykrikos kofoidii, Protoperidinium bipes , andStoeckeria algicida , and naked ciliates having sizes of 30-50 μm have been reported to be present in many waters (Strom and Buskey 1993, Jeong et al. 2004, 2005, 2006, 2007, 2011a , 2011b , Kim and Jeong 2004, Yoo et al. 2010, 2013a , Seuthe et al. 2011, Kang et al. 2013). Furthermore, they often co-occur withM. rubrum (Hansen et al. 1995, Bouley and Kimmerer 2006, Kang et al. 2013). Thus it is worthwhile to explore interactions betweenM. rubrum and these heterotrophic protists.The results of the present study would provide a basis for understanding the interactions between
M. rubrum and heterotrophic protists.> Preparation of experimental organisms
M. rubrum (MR-MAL01) was isolated from water samples collected from Gomso Bay, Korea (35˚40′ N, 126˚40′ E) in May 2001 at a water temperature and salinity of 18℃ and 31.5, respectively. A clonal culture ofM. rubrum was established as in Yih et al. (2004). The culture was maintained withTeleaulax sp. (previously described as a cryptophyte) in 500-mL bottles on a shelf at 20℃ under an illumination of 20 µE m-2 s-1 of cool white fluorescent light on a 14 h : 10 h light-dark cycle (Yih et al. 2004).For the isolation and culture of the heterotrophic dinoflagellates
G. shiwhaense, G. dominans, G. spirale, L. masanensis, O. rotunda, O. marina, P. piscicida, P. kofoidii, P. bipes, S. algicida , and the naked ciliateStrombidium sp. plankton samples were collected from the waters of coastal area in Korea in 2001-2013, and a clonal culture of each species was established by two serial single-cell isolations (Table 1).The carbon contents for
M. rubrum (0.43 ng C cell-1, n= 40), the heterotrophic dinoflagellates, and the ciliates were estimated from cell volume according to Menden-Deuer and Lessard (2000). The cell volume of the preserved predators after each feeding experiment was conducted was estimated using the methods of Kim and Jeong (2004) forG. dominans andG. spirale , the protocol of Jeong et al. (2008) forO. marina , and the methods of Jeong et al. (2001) forP. kofoidii . The cell volume ofO. rotunda was calculated with an assumption that its geometry is an ellipsoid.Experiment 1 was designed to test whether
G. shiwhaense, G. dominans, G. spirale, L. masanensis, O. rotunda, O. marina, P. piscicida, P. kofoidii, P. bipes , andS. algicida , and the naked ciliateStrombidium sp. were able to feed onM. rubrum (Table 1).Approximately 10,000
M. rubrum cells were added to each of the two 42-mL polycarbonate (PC) bottles containing each of the heterotrophic dinoflagellates (2,000-10,000 cells) and the ciliates (10-80 cells) (finalM. rubrum prey concentration = ca. 1,000-5,000 cells mL-1). One control bottle (without prey) was set up for each experiment. The bottles were placed on a plankton wheel rotating at 0.9 rpm and incubated at 20℃ under an illumination of 20 μE m-2 s-1 on a 14 h : 10 h light-dark cycle.Five milliliter aliquots were removed from each bottle after 1, 2, 6, and 24 h incubation and then transferred into 6-well plate. Approximately 200 cells in the plate chamber were observed under a dissecting microscope at a magnification of 10-63× (SZX10; Olympus, Tokyo, Japan) to determine whether the predators were able to feed on
M. rubrum. Predator cells containing prey cells were transferred onto glass slides and then their photographs were taken at a magnification of 400-1,000× with a camera mounted on an inverted microscope (Zeiss-Axiovert 200M; Carl Zeiss Ltd., Gottingen, Germany).> Prey concentration effects on growth and ingestion rates
Experiment 2 was designed to measure the growth and ingestion rates of
G. dominans as a function ofM. rubrum concentration.Dense cultures of
G. dominans growing on the algal prey listed in Table 1 were transferred to 500-mL PC bottles containing filtered seawater. The bottles were filled to capacity with freshly filtered seawater, capped, and placed on plankton wheels rotating at 0.9 rpm and incubated at 20℃ under an illumination of 20 μE m-2 s-1 on a 14 h : 10 h light-dark cycle. To monitor the conditions and interaction between the predator and prey species, the cultures were periodically removed from the rotating wheels, examined through the surface of the capped bottles using a dissecting microscope, and then returned to the rotating wheels. At timepoints at which prey cells were no longer present in ambient water, they were still observed inside the protoplasm of the predators. We therefore decided to starve the predators for 1 day in order to minimize possible residual growth resulting from the ingestion of prey during batch culture. After this incubation period, cell concentrations ofG. dominans were determined in three 1-mL aliquots from each bottle using a light microscope, and the cultures were then used to conduct experiments.For each experiment, the initial concentrations of
G. dominans andM. rubrum were established using an autopipette to deliver predetermined volumes of known cell concentrations to the bottles. Triplicate 42-mL PC experiment bottles (mixtures of predator and prey) and triplicate control bottles (prey only) were set up at each predator-prey combination. Triplicate control bottles containing onlyG. dominans were also established at one predator concentration. To obtain similar water conditions, the water of predator cultures was filtered through a 0.7-μm GF/F filter and then added to the prey control bottles in the same amount as the predator culture for each predator-prey combination. All bottles were then filled to capacity with freshly filtered seawater and capped. To determine the actual predator and prey densities at the beginning of the experiment, a 5-mL aliquot was removed from each bottle, fixed with 5% Lugol’s solution, and examined using a light microscope to enumerate the cells in three 1-mL Sedgwick-Rafter chambers (SRCs). The bottles were refilled to capacity with freshly filtered seawater, capped, and placed on rotating wheels under the conditions described above. Dilution of the cultures associated with refilling the bottles was considered when calculating growth and ingestion rates. A 10-mL aliquot was taken from each bottle after 48-h incubation and fixed with 5% Lugol’s solution, and the abundance ofG. dominans and prey were determined by counting all or >300 cells in three 1-mL SRCs. Before taking the subsamples, the conditions ofG. dominans and their prey were assessed using a dissecting microscope as described above.The specific growth rate of G. dominans, μ (d-1), was calculated as:
, where P0 and Pt = the concentration of
G. dominans at 0 d and 2 d, respectively.Data for
G. dominans growth rates were fitted to a Michaelis-Menten equation:, where μmax = the maximum growth rate (d-1); x = prey concentration (cells mL-1 or ng C mL-1), x’ = threshold prey concentration (the prey concentration where μ = 0), KGR = the prey concentration sustaining 1/2 μmax. Data were iteratively fitted to the model using DeltaGraph (Delta Point).
Ingestion and clearance rates were calculated using the equations of Frost (1972) and Heinbokel (1978). The incubation time for calculating ingestion and clearance rates was the same as that for estimating the growth rate. Ingestion rate data for
G. dominans were also fitted to a Michaelis-Menten equation:, where Imax = the maximum ingestion rate (cells predator-1 d-1 or ng C predator-1 d-1); x = prey concentration (cells mL-1 or ng C mL-1), and KIR = the prey concentration sustaining 1/2 Imax.
Additionally, the growth and ingestion rates of
L. masanensis, O. rotunda , andStrombidium sp. onM. rubrum prey at a single prey concentration at which both growth and ingestion rates ofG. dominans onM. rubrum were saturated were measured as described above.> Cell volume of
Gyrodinium dominans After the 2-d incubation, the cell length and maximum width of
G. dominans preserved in 5% acid Lugol’s solution (n = 20-30 for each prey concentration) were measured using an image analysis system on images collected with an inverted microscope (AxioVision 4.5; Carl Zeiss Ltd.). The shape ofG. dominans was estimated to 2 cones joined at the cell equator (= maximum width of the cell). The carbon content was estimated from cell volume according to Menden-Deuer and Lessard (2000).We estimated grazing coefficients attributable to small heterotrophic
Gyrodinium spp. (25-35 μm in cell length) onMesodinium by combining field data on abundances of smallGyrodinium spp. and prey with ingestion rates of the predators on the prey obtained in the present study. We assumed that the ingestion rates of the other small heterotrophicGyrodinium spp. onM. rubrum are the same as that ofG. dominans . The data on the abundances ofM. rubrum and co-occurring small heterotrophicGyrodinium spp. used in this estimation were obtained from water samples collected in 2004-2005 from Masan Bay and in 2008-2009 from Shiwha Bay.The grazing coefficients (g, h-1) were calculated as:
, where CR is the clearance rate (mL predator-1 h-1) of a predator on
M. rubrum at a given prey concentration and GC is the predator concentration (cells mL-1). CR’s were calculated as:, where IR (h) is the ingestion rate (cells eaten predator-1 h-1) of the predator on the prey and x is the prey concentration (cells mL-1). CR’s were corrected using Q10 = 2.8 (Hansen et al. 1997) because in situ water temperatures and the temperature used in the laboratory for this experiment (20℃) were sometimes different.
Among the predators tested in the present study,
G. dominans, L. masanensis, O. rotunda, P. kofoidii , andStrombidium sp. preyed onM. rubrum (Table 1, Fig. 1). However,G. shiwhaense, G. spirale, O. marina, P. piscicida, P. bipes , andS. algicida did not attempt to attack, even when it encounteredM. rubrum .The specific growth rates of
G. dominans onM. rubrum increased rapidly with increasing mean prey concentration up to ca. 321 ng C mL-1 (746 cells mL-1), but slowly at higher concentrations (Fig. 2). When the data were fitted to Eq. (2), the maximum specific growth rate (μmax) ofG. dominans onM. rubrum was 0.48 d-1. The feeding threshold prey concentration for the growth ofG. dominans (i.e., no growth) was 23.3 ng C mL-1 (54 cells mL-1).The ingestion rates of
G. dominans onM. rubrum increased rapidly with increasing mean prey concentration up to ca. 321 ng C mL-1 (746 cells mL-1), but became saturated at higher concentrations (Fig. 3). When the data were fitted to Eq. (3), the maximum ingestion rate (Imax) ofG. dominans onM. rubrum was 0.55 ng C predator-1 d-1 (1.3 cells predator-1 d-1). The maximum clearance rate ofG. dominans onM. rubrum was 0.14 μL predator-1 h-1.The growth rates of
L. masanensis, O. rotunda , andStrombidium sp. onM. rubrum prey at single prey concentrations (995-1,130 ng C mL-1) at which both growth and ingestion rates ofG. dominans onM. rubrum were saturated were negative.When the abundances of
M. rubrum and small heterotrophicGyrodinium spp. (25-35 μm in cell length) in Masan Bay in 2004-2005 and Shiwha Bay in 2008-2009 (n = 121) were 1-1,014 cells mL-1 and 1-1,356 cells mL-1, respectively, grazing coefficients attributable to small heterotrophicGyrodinium spp. on co-occurringM. rubrum were up to 0.236 h-1 (Fig. 4).Among the heterotrophic dinoflagellates and a ciliate investigated in this study,
G. dominans, L. masanensis, O. rotunda, P. kofoidii , andStrombidium sp. prey onM. rubrum . With respect to feeding mechanisms,G. dominans, P. kofoidii , andStrombidium sp. feed on prey by direct engulfment, butL. masanensis by a peduncle, andO. rotunda by a pallium (Strom and Buskey 1993, Kim and Jeong 2004, Jeong et al. 2007, Yoo et al. 2010). Since organisms with different feeding modalities were able to graze onM. rubrum , we conclude that feeding mechanisms do not generally determine the ability of heterotrophic protists to feed onM. rubrum . In addition, the size range of the predators that can feed onM. rubrum is also wide, and thus this factor is also not a critical determinant of protist feeding onM. rubrum. G. shiwhaense, G. spirale, O. marina, P. piscicida, P. bipes , andS. algicida did not even attackM. rubrum when they encountered the ciliate. Thus,G. dominans, L. masanensis, O. rotunda, P. kofoidii , andStrombidium sp. may have an ability to detectM. rubrum cells by physical and / or chemical cues, while the other organisms may lack this feature.M. rubrum usually stay motionless for a second, but swim or jump quickly. When it jumps, the maximum swimming speeds ofM. rubrum are 2,217-12,000 μm s-1, which are comparable to or greater than that ofG. dominans, O. rotunda, P. kofoidii , andStrombidium sp. (2,533, 420, 1,182, and 4,000 μm s-1, respectively) (Lee, unpublished data) (Barber and Smith 1981 cited by Smayda 2002, Crawford 1992, Buskey et al. 1993, Crawford and Lindholm 1997, Kim and Jeong 2004, Fenchel and Hansen 2006). Therefore,G. dominans, O. rotunda, P. kofoidii , andStrombidium sp. are likely to captureM. rubrum when they are motionless or whenM. rubrum may bump into them and then stun them.G. dominans was the only predator whose growth actually increased when grazing onM. rubrum in this study, even thoughL. masanensis, O. rotunda, P. kofoidii , andStrombidium sp. also fed onM. rubrum . In addition, the mixotrophic dinoflagellatesAmylax triacantha andDinophysis acuminata are known to grow onM. rubrum (Park et al. 2006, 2013b , Kim et al. 2008). Therefore, during red tides dominated byM. rubrum, G. dominans, A. triacantha , andD. acuminata are expected to be present. In contrast,L. masanensis, O. rotunda, P. kofoidii , andStrombidium sp. may be absent due to a lack of co-occurring alternative optimal prey species. The maximum growth rate ofG. dominans onM. rubrum (0.48 d-1) is lower than the mixotrophic growth rates ofA. triacantha andD. acuminata on the same prey (0.68 and 0.91 d-1, respectively) (Table 2). A lower ingestion rate ofG. dominans onM. rubrum (0.55 ng C predator-1 d-1) when compared withA. triacantha (2.54 ng C predator-1 d-1) andD. acuminata (1.30 ng C predator-1 d-1) may be partially responsible for this lower growth rate. DuringM. rubrum red tides,G. dominans may be less abundant thanA. triacantha andD. acuminata . However,G. dominans can grow on diverse algal prey species, whileA. triacantha andD. acuminata can only grow onM. rubrum (Nakamura et al. 1992, 1995, Kim and Jeong 2004, Park et al. 2006, 2013b , Kim et al. 2008, Yoo et al. 2010, 2013b , Jeong et al. 2011a , 2014). Thus, the abundance ofG. dominans in the period of red tides that are not associated withM. rubrum may be greater than those ofA. triacantha andD. acuminata . We suggest that future studies should compare the relative abundances of these three predators, and their grazing impact on prey populations, duringM. rubrum -associated red tides.The maximum growth rate (μmax) of
G. dominans onM. rubrum (0.48 d-1) is comparable to that on the mixotrophic dinoflagellatesHeterocapsa triquetra andKarenia mikimotoi , and the raphidophyteChattonella antique , but higher than that on the mixotrophic dinoflagellateBiecheleria cincta , the cryptophyteRhodomonas salina , and the chlorophyteDunaliella teriolecta (Table 3). However, the μmax ofG. dominans onM. rubrum is lower than that observed with the mixotrophic dinoflagellatesGymnodinium aureolum, Prorocentrum minimum , andSymbiodinium voratum , the euglenophyteEutreptiella gymnastica , and the diatomThalassiosira sp. (Table 3).M. rubrum , these mixotrophic dinoflagellates, and the raphidophyte cause red tides in the waters of many countries (Crawford 1989, Heil et al. 2005, Jeong et al. 2011a , 2013, Park et al. 2013a , Yih et al. 2013).G. dominans is likely to be more abundant duringM. rubrum red tides than duringB. cincta, R. salina , orD. teriolecta red tides, but less abundant duringE. gymnastica, G. aureolum , orP. minimum red tides.The maximum rate at which
G. dominans can ingestM. rubrum is one of the lowest among the algal prey species, with the exception ofB. cincta and comparable to that onR. salina (Table 3). Interestingly,M. rubrum andRhodomonas spp. exhibit jumping behaviors (Fenchel and Hansen 2006, Berge et al. 2008). These jumping behaviors ofM. rubrum may act as an anti-predation behavior. However, the ratio of the maximum growth rate relative to the maximum ingestion rate ofG. dominans onM. rubrum is greater than that on any other algal prey, with the exception ofP. minimum . Therefore,M. rubrum is likely to be the most nutritious algal prey forG. dominans, P. minimum notwithstanding.In the numerical response of
G. dominans to four algal prey species, the feeding threshold prey concentration for growth ofG. dominans onM. rubrum is lower than that ofE. gymnastica orG. aureolum , but higher than that ofS. voratum (Table 3, Fig. 5A). Therefore,G. dominans may preferentially grow onM. rubrum rather than onE. gymnastica orG. aureolum at low prey concentrations. The KGR (the prey concentration sustaining 1/2 μmax) ofG. dominans onM. rubrum is greater than that onG. aureolum , andS. voratum , but lower than that onE. gymnastica . Therefore, the growth ofG. dominans onM. rubrum is more sensitive to a change in prey concentration than the same parameter inE. gymnastica , but less sensitive thanG. aureolum , andS. voratum . The functional response ofG. dominans feeding on diverse algal prey species follows a Holling type II pattern (Holling 1959). With respect to the functional response ofG. dominans to eight algal prey species, the KIR (the prey concentration sustaining 1/2 Imax) when grown onM. rubrum is greater than that obtained withR. salina, P. minimum, D. teriolecta , andH. triquetra , but lower than that obtained withE. gymnastica, G. aureolum , andS. voratum (Fig. 5B). Therefore, the ingestion ofG. dominans onM. rubrum is more sensitive to a change in prey concentration thanE. gymnastica, G. aureolum , andS. voratum , but less sensitive thanR. salina, P. minimum, D. teriolecta , andH. triquetra .To our knowledge, prior to this study, there had been no reports on the impact of protist grazing on
Mesodinium populations. Grazing coefficients derived from studies in Masan Bay in 2004-2005 and Shiwha Bay in 2008-2009 show that up to 21% ofM. rubrum populations can be removed by smallGyrodinium populations in approximately 1 d. Therefore, small heterotrophicGyrodinium spp. can have a considerable grazing impact on populations ofM. rubrum under suitable conditions.G. dominans is one of the few protistan grazers that are able to feed onM. rubrum , and is the only protistan grazer with a documented grazing impact onM. rubrum abundance. This finding should be taken into consideration when developing models to explain the red tide dynamics ofM. rubrum .-
22. Jeong H. J., Ha J. H., Park J. Y., Kim J. H., Kang N. S., Kim S., Kim J. S., Yoo Y. D., Yih W. 2006 Distribution of the heterotrophic dinoflagellate Pfieteria piscicida in Korean waters and its consumption of mixotrophic dinoflagellates raphidophytes and fish blood cells [Aquat. Microb. Ecol.] Vol.44 P.263-278
-
56. Yoo Y. D., Jeong H. J., Kim J. S., Kim T. H., Kim J. H., Seong K. A., Lee S. H., Kang N. S., Park J. W., Park J., Yoon E. Y., Yih W. 2013 Red tides in Masan Bay Korea in 2004-2005: II Daily variations in the abundance of heterotrophic protists and their grazing impact on red-tide organisms [Harmful Algae] Vol.30 P.S89-S101
-
[Table 1.] Conditions for the isolation and maintenance of the experimental organisms, and feeding occurrence by diverse heterotrophic protistan predators
-
[]
-
[]
-
[]
-
[]
-
[]
-
[Fig. 1.] Feeding by heterotrophic protistan predators on Mesodinium rubrum. (A & B) Gyrodinium dominans having 1-2 ingested M. rubrum cells. (C) Polykrikos kofoidii. (D) Strombidium sp. (E) Luciella masanensis. (F) Oblea rotunda. White arrows indicate prey (M. rubrum) materials. Scale bars represent: A-F, 10 μm.
-
[Fig. 2.] Specific growth rate of the heterotrophic dinoflagellate Gyrodinium dominans on Mesodinium rubrum as a function of mean prey concentration (x). Symbols represent treatment means ± 1 standard error. The curves are fitted by the Michaelis-Menten equation [Eq. (2)] using all treatments in the experiment. Growth rate (d-1) = 0.48 [(x - 23.3) / (325.7 + [x - 23.3])], r2 = 0.881.
-
[Fig. 3.] Specific ingestion rates of the heterotrophic dinoflagellate Gyrodinium dominans on Mesodinium rubrum as a function of mean prey concentration (x). Symbols represent treatment means ± 1 standard error. The curves are fitted by the Michaelis-Menten equation [Eq. (3)] using all treatments in the experiment. Ingestion rate (ng C predator-1 d-1 = 0.55 [x / (94.6 + x)], r2 = 0.453.
-
[Fig. 4.] Calculated grazing coefficients of small heterotrophic Gyrodinium spp. (n = 121) in relation to the concentration of co- occurring Mesodinium rubrum (see text for calculation). Clearance rates, measured under the conditions provided in the present study, were corrected using Q10 = 2.8 (Hansen et al. 1997) because in situ water temperatures and the temperature used in the laboratory for this experiment (20℃) were sometimes different. The scales of the circles in the inset boxes are g (h-1).
-
[Table 2.] Growth and ingestion rates of dinoflagellate predators when feeding on Mesodinium rubrum
-
[Table 3.] Comparison of growth and grazing data for Gyrodinium dominans on diverse prey species
-
[Fig. 5.] A comparison of the numerical (A) and functional (B) responses of the heterotrophic dinoflagellate Gyrodinium dominans feeding on diverse prey related to prey concentration. Rates are corrected to 20℃ using Q10 = 2.8 (Hansen et al. 1997). Eg, Eutreptiella gymnastica, euglenophyte; Ga, Gymnodinium aureolum, mixotrophic dinoflagellate; Sv, Symbiodinium voratum, mixotrophic dinoflagellate; Mr, Mesodinium rubrum, mixotrophic ciliate; Ht, Heterocapsa triquetra, mixotrophic dinoflagellate; Dt, Dunaliella tertiolecta, chlorophyte; Pm, Prorocentrum minimum, mixotrophic dinoflagellate; Rs, Rhodomonas salina, cryptophyte. All responses in (A) were fitted to Eq. 2, whereas those in (B) were fitted to Eq. 3.