Feeding by Heterotrophic Dinoflagellates and Ciliates on the Free‐living Dinoflagellate Symbiodinium sp. (Clade E)

To investigate heterotrophic protists grazing on Symbiodinium sp., we tested whether the common heterotrophic dinoflagellates Gyrodinium dominans, Gyrodinium moestrupii, Gyrodinium spirale, Oblea rotundata, Oxyrrhis marina, and Polykrikos kofoidii and the ciliates Balanion sp. and Parastrombidinopsis sp. preyed on the free‐living dinoflagellate Symbiodinium sp. (clade E). We measured the growth and ingestion rates of O. marina and G. dominans on Symbiodinium sp. as a function of prey concentration. Furthermore, we compared the results to those obtained for other algal prey species. In addition, we measured the growth and ingestion rates of other predators at single prey concentrations at which these rates of O. marina and G. dominans were saturated. All predators tested in the present study, except Balanion sp., preyed on Symbiodinium sp. The specific growth rates of O. marina and G. dominans on Symbiodinium sp. increased rapidly with increasing mean prey concentration < ca. 740–815 ng C/ml (7,400–8,150 cells/ml), but became saturated at higher concentrations. The maximum growth rates of O. marina and G. dominans on Symbiodinium sp. (0.87 and 0.61/d) were much higher than those of G. moestrupii and P. kofoidii (0.11 and 0.04/d). Symbiodinium sp. did not support positive growth of G. spirale, O. rotundata, and Parastrombidinopsis sp. However, the maximum ingestion rates of P. kofoidii and Parastrombidinopsis sp. (6.7–10.0 ng C/predator/d) were much higher than those of O. marina and G. dominans on Symbiodinium sp. (1.9–2.1 ng C/predator/d). The results of the present study suggest that Symbiodinium sp. may increase or maintain the populations of some predators.     

Interactions between the mixotrophic dinoflagellate Takayama helix and common heterotrophic protists

The phototrophic dinoflagellate Takayama helix that is known to be harmful to abalone larvae has recently been revealed to be mixotrophic. Although mixotrophy elevates the growth rate of T. helix by 79%–185%, its absolute growth rate is still as low as 0.3 d⁻¹. Thus, if the mortality rate of T. helix due to predation is high, this dinoflagellate may not easily prevail. To investigate potential effective protistan grazers on T. helix, feeding by diverse heterotrophic dinoflagellates such as engulfment-feeding Oxyrrhis marina, Gyrodinium dominans, Gyrodinium moestrupii, Polykrikos kofoidii, and Noctiluca scintillans, peduncle-feeding Aduncodinium glandula, Gyrodiniellum shiwhaense, Luciella masanensis, and Pfiesteria piscicida, pallium-feeding Oblea rotunda and Protoperidinium pellucidum, and the naked ciliates Pelagostrobilidium sp. (ca. 40 μm in cell length) and Strombidinopsis sp. (ca. 150 μm in cell length) on T. helix was explored. Among the tested heterotrophic protists, O. marina, G. dominans, G. moestrupii, A. glandula, L. masanensis, P. kofoidii, P. piscicida, and Strombidinopsis sp. were able to feed on T. helix. The growth rates of all these predators except Strombidinopsis sp. with T. helix prey were lower than those without the prey. The growth rate of Strombidinopsis sp. on T. helix was almost zero although the growth rate of Strombidinopsis sp. with T. helix prey was higher than those without the prey. Moreover, T. helix fed on O. marina and P. pellucidum and lysed the cells of P. kofoidii and G. shiwhaense. With increasing the concentrations of T. helix, the growth rates of O. marina and P. kofoidii decreased, but those of G. dominans and L. masanensis largely did not change. Therefore, reciprocal predation, lysis, no feeding, and the low ingestion rates of the common protists preying on T. helix may result in a low mortality rate due to predation, thereby compensating for this species’ low growth rate.     

Differential interactions between the nematocyst-bearing mixotrophic dinoflagellate Paragymnodinium shiwhaense and common heterotrophic protists and copepods: Killer or prey

To investigate interactions between the nematocyst-bearing mixotrophic dinoflagellate Paragymnodinium shiwhaense and different heterotrophic protist and copepod species, feeding by common heterotrophic dinoflagellates (Oxyrrhis marina and Gyrodinium dominans), naked ciliates (Strobilidium sp. approximately 35 μm in cell length and Strombidinopsis sp. approximately 100 μm in cell length), and calanoid copepods Acartia spp. (A. hongi and A. omorii) on P. shiwhaense was explored. In addition, the feeding activities of P. shiwhaense on these heterotrophic protists were investigated. Furthermore, the growth and ingestion rates of O. marina, G. dominans, Strobilidium sp., Strombidinopsis sp., and Acartia spp. as a function of P. shiwhaense concentration were measured. O. marina, G. dominans, and Strombidinopsis sp. were able to feed on P. shiwhaense, but Strobilidium sp. was not. However, the growth rates of O. marina, G. dominans, Strobilidium sp., and Strombidinopsis sp. feeding on P. shiwhaense were very low or negative at almost all concentrations of P. shiwhaense. P. shiwhaense frequently fed on O. marina and Strobilidium sp., but did not feed on Strombidinopsis sp. and G. dominans. G. dominans cells swelled and became dead when incubated with filtrate from the experimental bottles (G. dominans + P. shiwhaense) that had been incubated for one day. The ingestion rates of O. marina, G. dominans, and Strobilidium sp. on P. shiwhaense were almost zero at all P. shiwhaense concentrations, while those of Strombidinopsis sp. increased with prey concentration. The maximum ingestion rate of Strombidinopsis sp. on P. shiwhaense was 5.3 ng C predator⁻¹d⁻¹ (41 cells predator⁻¹d⁻¹), which was much lower than ingestion rates reported in the literature for other mixotrophic dinoflagellate prey species. With increasing prey concentrations, the ingestion rates of Acartia spp. on P. shiwhaense increased up to 930 ng C ml⁻¹ (7180 cells ml⁻¹) at the highest prey concentration. The highest ingestion rate of Acartia spp. on P. shiwhaense was 4240 ng C predator⁻¹d⁻¹ (32,610 cells predator⁻¹d⁻¹), which is comparable to ingestion rates from previous studies on other dinoflagellate prey species calculated at similar prey concentrations. Thus, P. shiwhaense might play diverse ecological roles in marine planktonic communities by having an advantage over competing phytoplankton in anti-predation against potential protistan grazers.     

Mixotrophy in the sand-dwelling dinoflagellate Thecadinium kofoidii

Thecadinium kofoidii is a marine sand-dwelling dinoflagellate that sometimes forms dense blooms. This species was previously thought to be an exclusively autotrophic dinoflagellate, and its mixotrophic ability has not been explored yet. By investigating its ecophysiology, its trophic mode should be revealed. We explored the mixotrophic ability of T. kofoidii by examining its protoplasm under light and transmission electron microscopes with diverse algal prey species. Furthermore, the feeding mechanism of T. kofoidii and prey species on which it feeds were investigated. In addition, the growth and ingestion rates of T. kofoidii as a function of prey concentration were determined when feeding on the benthic cryptophyte Rhodomonas salina. Thecadinium kofoidii was able to feed on R. salina and the dinoflagellate Symbiodinium voratum, which had equivalent spherical diameters (ESDs) ≤ 10.1?µm, while it did not feed on the benthic dinoflagellates Levanderina fissa, Prorocentrum concavum or Ostreopsis cf. ovata, which had ESDs ≥ 15?µm. Thecadinium kofoidii fed on the edible prey cells using the peduncle. The maximum ingestion rate of T. kofoidii on R. salina was 1.3 cells predator^?1 d^?1. However, feeding on R. salina did not significantly increase the growth rate of T. kofoidii. The low ingestion rate of T. kofoidii on R. salina may have partially resulted in the lack of significant increase in its growth rate due to mixotrophy. The present study discovered predator–prey relationships between T. kofoidii and R. salina and S. voratum, which may change our view of the energy flow and carbon cycling in marine benthic food webs.     

Grazing impact of heterotrophic dinoflagellates and ciliates on common red-tide euglenophyte Eutreptiella gymnastica in Masan Bay, Korea

The euglenophyte Eutreptiella gymnastica is a common red tide causative species. However, there have been no studies on the grazing impact of heterotrophic protists on this species. To investigate the grazing impact of heterotrophic protists on E. gymnastica, we measured daily the abundances of E. gymnastica and co-occurring potential heterotrophic protistan grazers in Masan Bay, Korea, in August 2004 when an E. gymnastica red tide occurred. In addition, we tested whether the common heterotrophic dinoflagellates Gyrodinium dominans, Oxyrrhis marina, Pfiesteria piscicida, Polykrikos kofoidii, Protoperidinium bipes, and Stoeckeria algicida and the naked ciliates Strobilidium sp. (30–40 μm in cell length) and Strombidinopsis sp. (70–100 μm in cell length) were able to feed on E. gymnastica. We also measured their growth and ingestion rates on E. gymnastica as a function of prey concentration. Finally, we calculated the grazing coefficients by combining field data on the abundance of the heterotrophic dinoflagellate and ciliate grazers and co-occurring E. gymnastica with laboratory data on ingestion rates obtained in this study. The maximum abundance of E. gymnastica in Masan Bay in August, 2004 was 7575 cells ml⁻¹, while those of Gyrodinium spp., P. kofoidii, P. bipes, the naked ciliates (≤50 μm in cell length), and naked ciliates (>50 μm in cell length) were 50, 9, 58, 32, and 3 cells ml⁻¹, respectively. The maximum growth rate of G. dominans on E. gymnastica (1.13 d⁻¹) was higher than that of O. marina (0.81 d⁻¹) or P. bipes (0.77 d⁻¹). However, E. gymnastica did not support positive growth of P. kofoidii, Strobilidium sp., and Strombidinopsis sp. (−0.04 ∼ −2.8 d⁻¹). The maximum ingestion rates of G. dominans, P. kofoidii, P. bipes, O. marina, and Strobilidium sp. on E. gymnastica (2.1–2.7 ng C predator⁻¹ d⁻¹) were similar, but they were much lower than that of Strombidinopsis sp. (156 ng C predator⁻¹ d⁻¹). The calculated grazing coefficients for P. bipes, small heterotrophic Gyrodinium spp. (25–35 μm in cell length), naked ciliates (≤50 μm in cell length), P. kofoidii, and naked ciliates (>50 μm in cell length) on E. gymnastica were up to 0.77, 0.61, 0.22, 0.07 and 0.03 d⁻¹, respectively (i.e., up to 54%, 46%, 20%, 7%, and 3% of E. gymnastica populations were removed by the population of each of these heterotrophic protistan grazers in 1 d, respectively). The results of the present study suggest that P. bipes, small heterotrophic Gyrodinium spp., and naked ciliates (≤50 μm in cell length) sometimes have considerable potential grazing impact on the populations of E. gymnastica.     

Feeding by the newly described heterotrophic dinoflagellate Stoeckeria changwonensis: A comparison with other species in the family Pfiesteriaceae

The feeding ecology of the newly described heterotrophic dinoflagellate Stoeckeria changwonensis was explored. The feeding behavior of S. changwonensis, and the kinds of prey species that it feeds on were investigated with several different types of microscopes and high-resolution video-microscopy. Additionally, the growth and ingestion rates of S. changwonensis as a function of prey concentration for perch (Lateolabrax japonicus) blood cells, the raphidophyte Heterosigma akashiwo, the cryptophytes Rhodomonas salina and Teleaulax sp., and the phototrophic dinoflagellate Amphidinium carterae prey were measured. S. changwonensis feeds on prey through a peduncle, after anchoring the prey by using a tow filament. This type of feeding behavior is similar to that of Stoeckeria algicida, Pfiesteria piscicida, and Luciella masanensis in the family Pfiesteriaceae; however, S. changwonensis feeds on various kinds of prey species different from those of the other heterotrophic dinoflagellates. S. changwonensis ingested perch blood cells and diverse algal species, in particular, the large thecate dinoflagellate Lingulodinium polyedrum which are not eaten by the other peduncle feeders. H. akashiwo and the perch blood cells supported positive growth of S. changwonensis, but R. salina, Teleaulax sp., and A. carterae which support positive growth of P. piscicida and L. masanensis did not support positive growth of S. changwonensis. With increasing mean prey concentration the growth rates for S. changwonensis on H. akashiwo and the perch blood cells increased rapidly and then slowly or became saturated. The maximum growth rates of S. changwonensis on H. akashiwo and the perch blood cells were 0.376 and 0.354 d⁻¹, respectively. Further, the maximum ingestion rates of S. changwonensis on H. akashiwo and the perch blood cells were 0.35 ng C predator⁻¹ d⁻¹ (3.5 cells predator⁻¹ d⁻¹) and 0.27 ng C predator⁻¹ d⁻¹ (29 cells predator⁻¹ d⁻¹), respectively. These maximum growth and ingestion rates of S. changwonensis on H. akashiwo, the perch blood cells, R. salina, Teleaulax sp., and A. carterae differed considerably from those of S. algicida, P. piscicida, and L. masanensis on the same prey species. Thus, the feeding behavior of S. changwonensis may differ from that of other species in the family Pfiesteriaceae.     

Interactions between the Newly Described Small‐ and Fast‐Swimming Mixotrophic Dinoflagellate Yihiella yeosuensis and Common Heterotrophic Protists

The mixotroph Yihiella yeosuensis is a small‐ and fast‐swimming dinoflagellate. To investigate its protistan predators, interactions between Y. yeosuensis and 11 heterotrophic protists were explored. No potential predators were able to feed on actively swimming Y. yeosuensis cells, which escaped via rapid jumps, whereas Aduncodinium glandula, Oxyrrhis marina, and Strombidinopsis sp. (approximately 150 μm in cell length) were able to feed on weakly swimming cells that could not jump. Furthermore, Gyrodinium dominans, Luciella masanensis, and Pfiesteria piscicida were able to feed on heat‐killed Yihiella cells, whereas Gyrodinium moestrupii, Noctiluca scintillans, Oblea rotunda, Polykrikos kofoidii, and Strombidium sp. (20 μm) did not feed on them. Thus, the jumping behavior of Y. yeosuensis might be primarily responsible for the observed lack of predation. With increasing Yihiella concentration, the growth rate of O. marina decreased, whereas that of Strombidinopsis did not change. However, with increasing Yihiella concentration (up to 530 ng C/ml), the ingestion rate of Strombidinopsis on Yihiella increased linearly. The highest ingestion rate was 24.1 ng C per predator per d. The low daily carbon acquisition from Yihiella relative to the body carbon content of Strombidinopsis might be responsible for its negligible growth. Thus, Y. yeosuensis might have an advantage over its competitors due to its low mortality rate.     

Feeding by Phototrophic Red-Tide Dinoflagellates on the Ubiquitous Marine Diatom Skeletonema costatum

ABSTRACT We investigated feeding by phototrophic red‐tide dinoflagellates on the ubiquitous diatom Skeletonema costatum to explore whether dinoflagellates are able to feed on S. costatum, inside the protoplasm of target dinoflagellate cells observed under compound microscope, confocal microscope, epifluorescence microscope, and transmission electron microscope (TEM) after adding living and fluorescently labeled S. costatum (FLSc). To explore effects of dinoflagellate predator size on ingestion rates of S. costatum, we measured ingestion rates of seven dinoflagellates at a single prey concentration. In addition, we measured ingestion rates of the common phototrophic dinoflagellates Prorocentrum micans and Gonyaulax polygramma on S. costatum as a function of prey concentration. We calculated grazing coefficients by combining field data on abundances of P. micans and G. polygramma on co‐occurring S. costatum with laboratory data on ingestion rates obtained in the present study. All phototrophic dinoflagellate predators tested (i.e. Akashiwo sanguinea, Amphidinium carterae, Alexandrium catenella, Alexandrium tamarense, Cochlodinium polykrikoides, G. polygramma, Gymnodinium catenatum, Gymnodinium impudicum, Heterocapsa rotundata, Heterocapsa triquetra, Lingulodinium polyedrum, Prorocentrum donghaiense, P. micans, Prorocentrum minimum, Prorocentrum triestinum, and Scrippsiella trochoidea) were able to ingest S. costatum. When mean prey concentrations were 170–260 ng C/ml (i.e. 6,500–10,000 cells/ml), the ingestion rates of G. polygramma, H. rotundata, H. triquetra, L. polyedrum, P. donghaiense, P. micans, and P. triestinum on S. costatum (0.007–0.081 ng C/dinoflagellate/d [0.2–3.0 cells/dinoflagellate/d]) were positively correlated with predator size. With increasing mean prey concentration of ca 1–3,440 ng C/ml (40–132,200 cells/ml), the ingestion rates of P. micans and G. polygramma on S. costatum continuously increased. At the given prey concentrations, the maximum ingestion rates of P. micans and G. polygramma on S. costatum (0.344–0.345 ng C/grazer/d; 13 cells/grazer/d) were almost the same. The maximum clearance rates of P. micans and G. polygramma on S. costatum were 0.165 and 0.020 μl/grazer/h, respectively. The calculated grazing coefficients of P. micans and G. polygramma on co‐occurring S. costatum were up to 0.100 and 0.222 h, respectively (i.e. up to 10% and 20% of S. costatum populations were removed by P. micans and G. polygramma populations in 1 h, respectively). Our results suggest that P. micans and G. polygramma sometimes have a considerable grazing impact on populations of S. costatum.     

The Newly Described Heterotrophic Dinoflagellate Gyrodinium moestrupii,an Effective Protistan Grazer of Toxic Dinoflagellates

Few protistan grazers feed on toxic dinoflagellates, and low grazing pressure on toxic dinoflagellates allows these dinoflagellates to form red‐tide patches. We explored the feeding ecology of the newly described heterotrophic dinoflagellate Gyrodinium moestrupii when it fed on toxic strains of Alexandrium minutum, Alexandrium tamarense, and Karenia brevis and on nontoxic strains of A. tamarense, Prorocentrum minimum, and Scrippsiella trochoidea. Specific growth rates of G. moestrupii feeding on each of these dinoflagellates either increased continuously or became saturated with increasing mean prey concentration. The maximum specific growth rate of G. moestrupii feeding on toxic A. minutum (1.60/d) was higher than that when feeding on nontoxic S. trochoidea (1.50/d) or P. minimum (1.07/d). In addition, the maximum growth rate of G. moestrupii feeding on the toxic strain of A. tamarense (0.68/d) was similar to that when feeding on the nontoxic strain of A. tamarense (0.71/d). Furthermore, the maximum ingestion rate of G. moestrupii on A. minutum (2.6 ng C/grazer/d) was comparable to that of S. trochoidea (3.0 ng C/grazer/d). Additionally, the maximum ingestion rate of G. moestrupii on the toxic strain of A. tamarense (2.1 ng C/grazer/d) was higher than that when feeding on the nontoxic strain of A. tamarense (1.3 ng C/grazer/d). Thus, feeding by G. moestrupii is not suppressed by toxic dinoflagellate prey, suggesting that it is an effective protistan grazer of toxic dinoflagellates.     

Bioenergetics and growth in the ctenophore <Emphasis Type="Italic">Pleurobrachia pileus</Emphasis>

Knowledge of how energetic parameters relate to fluctuating factors in the natural habitat is necessary when evaluating the role of gelatinous zooplankton in the carbon flow of coastal waters. In laboratory experiments, we assessed feeding, respiration and growth of the ctenophore, Pleurobrachia pileus, and constructed carbon budgets. Clearance rates (F, l d⁻¹) of laboratory-reared Acartia tonsa as prey increased as a function of ctenophore polar length (L, mm) as F = 0.17L ^1.9. For ctenophores larger than about 11 mm, clearance rate was depressed in containers of 30–50 l volume. Clearance rates on field-collected prey were highest on the copepod, Centropages typicus, intermediate on the cladoceran, Evadne nordmanni and low on the copepods, Acartia clausi and Temora longicornis. Specific growth rates of 8–10 mm P. pileus increased with increasing prey concentrations to a maximum of 0.09 d⁻¹ attained at prey carbon densities of 40 and 100 μg C l⁻¹ of Artemia salina and A. tonsa, respectively. Weight-specific respiration rates increased hyperbolically with prey concentration. From experiments in which growth, ingestion and respiration were measured simultaneously, a carbon budget was constructed for individuals growing at maximum rates; from the measured parameters, the assimilation efficiency and net growth efficiency were estimated to be 22 and 37%, respectively. We conclude that the predation rates of P. pileus depend on ctenophore size, prey species, prey density and experimental container volume. Because the specific growth rates, respiration, assimilation and net growth efficiencies all were affected by food availability, knowledge of the ambient prey field is critical when evaluating the role of P. pileus in the carbon flow in coastal waters.     

Feeding by the Newly Described Mixotrophic Dinoflagellate Paragymnodinium shiwhaense: Feeding Mechanism,Prey Species,and Effect of Prey Concentration

ABSTRACT. To investigate the feeding by the newly described mixotrophic dinoflagellate Paragymnodinium shiwhaense (GenBank accession number=AM408889), we explored the feeding process and the kinds of prey species that P. shiwhaense is able to feed on using several different types of microscopes, including a transmission electron microscope and high‐resolution video‐microscopy. In addition, we measured the growth and ingestion rates of P. shiwhaense on its optimal algal prey Amphidinium carterae as a function of prey concentration. We also measured these parameters for edible prey at a single concentration at which the growth and ingestion rates of P. shiwhaense on A. carterae were saturated. Paragymnodinium shiwhaense feed on algal prey using a peduncle after anchoring the prey by a tow filament. Among the algal prey offered, P. shiwhaense ingested small algal species that had equivalent spherical diameters (ESDs) ≤11 μm (e.g. the prymnesiophyte Isochrysis galbana, the cryptophytes Teleaulax sp. and Rhodomonas salina, the raphidophyte Heterosigma akashiwo, and the dinoflagellates Heterocapsa rotundata and A. carterae). However, it did not feed on larger algal species that had ESDs ≥12 μm (e.g. the dinoflagellates Prorocentrum minimum, Heterocapsa triquetra, Scrippsiella trochoidea, Alexandrium tamarense, Prorocentrum micans, Gymnodinium catenatum, Akashiwo sanguinea, and Lingulodinium polyedrum) or the small diatom Skeletonema costatum. The specific growth rates for P. shiwhaense feeding upon A. carterae increased rapidly with increasing mean prey concentration before saturating at concentrations of ca. 350 ng C/ml (5,000 cells/ml). The maximum specific growth rate (i.e. mixotrophic growth) of P. shiwhaense on A. carterae was 1.097/d at 20 °C under a 14:10 h light–dark cycle of 20 μE/m²/s, while its growth rate (i.e. phototrophic growth) under the same light conditions without added prey was ?0.224/d. The maximum ingestion and clearance rates of P. shiwhaense on A. carterae were 0.38 ng C/grazer/d (5.4 cells/grazer/d) and 0.7 μl/grazer/h, respectively. The calculated grazing coefficients for P. shiwhaense on co‐occurring Amphidinium spp. was up to 0.07/h (i.e. 6.7% of the population of Amphidinium spp. was removed by P. shiwhaense populations in 1 h). The results of the present study suggest that P. shiwhaense can have a considerable grazing impact on algal populations.     

Prey features affecting ingestion rates by Acanthocyclops robustus (Copepoda: Cyclopoida) on zooplankton

Summary (1) Ingestion rates by adult female and juvenile Acanthocyclops robustus on a number of prey types were measured at a prey concentration of 100/l in experimental volumes of 300–400 ml. (2) For the adult predator, Synchaeta pectinata was most vulnerable (22.3, standard error 1.4, prey ingested per predator per day) as compared to Brachiomus calyciflorus, Brachionus diversicornis, Keratella cochlearis (two morphs), Asplanchna priodonta, Polyarthra major, Synchaeta kitina, Pompholyx sulcata, Daphnia spec., and Bosmina longirostris. For these latter prey, the lowest ingestion rate was on one morph of K. cochlearis and the highest on A. priodonta, being, respectively, 1.0, SE 0.5, and 11.3, SE 1.0, prey per predator per day. (3) With regard to the juvenile predator (mostly copepodite stages I, II and III), ingestion rates on K. cochlearis and P. sulcata were low (respectively 1.2, SE 0.7, and 0.3, SE 0.1, prey per predator per day) but quite high on S. kitina (5.7, SE 0.6). (4) In addition, the effect of increasing prey concentration on the ingestion rate (functional response) by the adult female predator was examined for B. calyciflorus, K. cochlearis, S. pectinata, S. kitina and Daphnia spec.. Increases in ingestion rate with prey density were minimal for B. calyciflorus and K. cochlearis, greater for Daphnia spec., still greater for S. pectinata and of greatest magnitude for S. kitina. (5) The reasons for these results are discussed with particular reference to prey features.     

Dinoflagellate resting cysts as factors in phytoplankton ecology of the North Sea

The occurrence and distribution of dinoflagellate resting cysts were investigated at 11 locations in the south-eastern part of the North Sea. Twenty-six known cyst species and 7 unknown cyst types, which may act as seed population for planktonic dinoflagellate blooms, have been recorded for the first time in the area. The most common cysts in recent sediments were those ofScrippsiella trochoidea, Zygabikodinium lenticulatum, Peridinium dalei, Scrippsiella lachrymosa, Protoceratium reticulatum, Protoperidinium denticulatum, andP. conicum. At all stations,S. trochoidea dominated the cyst assemblages with a maximal abundance of 1303 living cysts/cm³ in the uppermost half centimetre. Cysts of the potentially toxic dinoflagellatesAlexandrium cf.excavatum andA. cf.tamarense were scarce. In the upper 2-cm layer of sediment, dinoflagellate cysts were found in concentrations of 1.8 up to 682 living cysts/cm³. Empty cysts constituted 22–56% of total cyst abundance. The comparative distribution of the cysts showed a general increase in abundance from inshore sites to the offshore area, whereby sandy stations exhibited the lowest cyst abundance and diversity. The wide distribution of living and empty cysts ofScrippsiella lachrymosa suggests that its motile form, which has not been officially recorded in the area until now, is a common plankton organism in German coastal waters. The relatively high abundance of cysts in recent sediments demonstrates the potential importance of benthic resting stages for the initiation of dinoflagellate blooms in the study area.     

Newly discovered role of the heterotrophic nanoflagellate Katablepharis japonica,a predator of toxic or harmful dinoflagellates and raphidophytes

Heterotrophic nanoflagellates are ubiquitous and known to be major predators of bacteria. The feeding of free-living heterotrophic nanoflagellates on phytoplankton is poorly understood, although these two components usually co-exist. To investigate the feeding and ecological roles of major heterotrophic nanoflagellates Katablepharis spp., the feeding ability of Katablepharis japonica on bacteria and phytoplankton species and the type of the prey that K. japonica can feed on were explored. Furthermore, the growth and ingestion rates of K. japonica on the dinoflagellate Akashiwo sanguinea—a suitable algal prey item—heterotrophic bacteria, and the cyanobacteria Synechococcus sp., as a function of prey concentration were determined. Among the prey tested, K. japonica ingested heterotrophic bacteria, Synechococcus sp., the prasinophyte Pyramimonas sp., the cryptophytes Rhodomonas salina and Teleaulax sp., the raphidophytes Heterosigma akashiwo and Chattonella ovata, the dinoflagellates Heterocapsa rotundata, Amphidinium carterae, Prorocentrum donghaiense, Alexandrium minutum, Cochlodinium polykrikoides, Gymnodinium catenatum, A. sanguinea, Coolia malayensis, and the ciliate Mesodinium rubrum, however, it did not feed on the dinoflagellates Alexandrium catenella, Gambierdiscus caribaeus, Heterocapsa triquetra, Lingulodinium polyedra, Prorocentrum cordatum, P. micans, and Scrippsiella acuminata and the diatom Skeletonema costatum. Many K. japonica cells attacked and ingested a prey cell together after pecking and rupturing the surface of the prey cell and then uptaking the materials that emerged from the ruptured cell surface. Cells of A. sanguinea supported positive growth of K. japonica, but neither heterotrophic bacteria nor Synechococcus sp. supported growth. The maximum specific growth rate of K. japonica on A. sanguinea was 1.01 d⁻¹. In addition, the maximum ingestion rate of K. japonica for A. sanguinea was 0.13 ng C predator⁻¹d⁻¹ (0.06 cells predator⁻¹d⁻¹). The maximum ingestion rate of K. japonica for heterotrophic bacteria was 0.019 ng C predator⁻¹d⁻¹ (266 bacteria predator⁻¹d⁻¹), and the highest ingestion rate of K. japonica for Synechococcus sp. at the given prey concentrations of up to ca. 10⁷ cells ml⁻¹ was 0.01 ng C predator⁻¹d⁻¹ (48 Synechococcus predator⁻¹d⁻¹). The maximum daily carbon acquisition from A. sanguinea, heterotrophic bacteria, and Synechococcus sp. were 307, 43, and 22%, respectively, of the body carbon of the predator. Thus, low ingestion rates of K. japonica on heterotrophic bacteria and Synechococcus sp. may be responsible for the lack of growth. The results of the present study clearly show that K. japonica is a predator of diverse phytoplankton, including toxic or harmful algae, and may also affect the dynamics of red tides caused by these prey species.     

Growth,Grazing, and Starvation Survival in Three Heterotrophic Dinoflagellate Species

To assess the effects of fluctuating prey availability on predator population dynamics and grazing impact on phytoplankton, we measured growth and grazing rates of three heterotrophic dinoflagellate species—Oxyrrhis marina, Gyrodinium dominans and Gyrodinium spirale—before and after depriving them of phytoplankton prey. All three dinoflagellate species survived long periods (> 10 d) without algal prey, coincident with decreases in predator abundance and cell size. After 1–3 wks, starvation led to a 17–57% decrease in predator cell volume and some cells became deformed and transparent. When re‐exposed to phytoplankton prey, heterotrophs ingested prey within minutes and increased cell volumes by 4–17%. At an equivalent prey concentration, continuously fed predators had ~2‐fold higher specific growth rates (0.18 to 0.55 d^?1) than after starvation (?0.16 to 0.25 d^?1). Maximum specific predator growth rates would be achievable only after a time lag of at least 3 d. A delay in predator growth poststarvation delays predator‐induced phytoplankton mortality when prey re‐emerges at the onset of a bloom event or in patchy prey distributions. These altered predator‐prey population dynamics have implications for the formation of phytoplankton blooms, trophic transfer rates, and potential export of carbon.     

Scrippsiella hexapraecingula sp. nov. (Dinophyceae), a tida pool dinoflagellate from the Northwest Pacific

Specimens of dinoflagellate collected in tide pools along the Pacific coast of central and southern Japan are described as a new species,Scrippsiella hexapraecingula Horiguchi et Chihara, of the Peridiniaceae (Class Dinophyceae). The plate formula is pp, x, 4′, 3a, 6″, 6c, 5‴, 2″" and, 5s, the same as that of other species ofScrippsiella, except in lacking one precingular plate. The genus must be emended, therefore, as having either six or seven precingular plates. This dinoflagellate migrates diurnally. In the morning motile cells are released from non-motile cells attached to the substrate and in the evening the motile cells swim down to settle on the bottom of the tide pool. Attached non-motile cells form either motile mono- or bispores. Sexual reproduction was not observed.     

Distribution and feeding of chaetognaths in the epipelagic zone of the Lazarev Sea (Antarctica) during austral summer

This study seeks to determine the effects of local hydrography on the distribution, abundance and feeding of chaetognaths in the Lazarev Sea, an area strongly controlled by physical processes which has been held responsible for initiating the Weddell Polynya. Zooplankton samples were taken at 39 stations on four transects located between 6°W and 3°E and from 60°S to 70°S between surface and 350 m. The dominant species, Eukrohnia hamata, accounted for 86.5% of all chaetognaths, followed by Sagitta gazellae (8.1%) and Sagitta marri (5.4%). These three species showed distinct vertical and horizontal distribution patterns. While E. hamata and S. marri had maximum abundances below 250 m depth, S. gazellae showed a narrow distribution band in the upper 150 m depth. The distribution pattern was strongly modified at the Greenwich meridian with an upward transport of a high abundance of deep dwelling organisms (S. marri and E. hamata) and a displacement of S. gazellae to the surface, likely coupled with the rise of the warm, saline halo around the Maud Rise. Small copepods were the main prey of all three chaetognath species. Feeding rates (FR) varied among species and depth. Sagitta marri showed the highest FR with 0.38 prey d⁻¹, followed by S. gazellae and E. hamata (0.22 and 0.07 prey d⁻¹). Feeding rates were usually highest in the 25–80-m stratum. Size distribution and maturity of E. hamata revealed a dominance of small and immature organisms along all depths and stations, suggesting that this area might be acting as an important source of recently spawned organisms to the surface.     

Predator–prey interactions between a planktonic ciliate Strombidium sp. (Ciliophora, Oligotrichida) and the dinoflagellate Pfiesteria piscicida (Dinamoebiales, Pyrrophyta)

The functional response of a planktonic ciliate, Strombidium sp. feeding on the dinoflagellate Pfiesteria piscicida non-toxic zoospores (NTZ) was experimentally studied with four different prey concentrations (43–3153 cells ml⁻¹). Data from direct observations (NTZ inside individual Strombidium sp.) was used to calculate predator–prey specific ingestion and clearance rates. The ingestion rates varied between 0.68 and 14.26 NTZ ind⁻¹ h⁻¹, and with the predator–prey specific handling time of 2.83 min the U_max was 21.18 NTZ ind⁻¹ h⁻¹. The increase in the prey concentration between approximately 700 and 3000 NTZ ml⁻¹ did not increase the uptake of prey, and at the lowest Pfiesteria NTZ concentrations the feeding efficiency of Strombidium sp. was lowered, possibly indicating a situation of threshold feeding. When data from direct observations of ingested Pfiesteria NTZ were compared with values of total NTZ loss from the experimental water during the experiment, ingestion was found to represent only a fraction of the total NTZ loss in the presence of ciliates. This discrepancy was concluded to be due to other grazer related factors than actual ciliate grazing. The control of the initial growth of Pfiesteria community, in a pre-bloom situation, would require only a small ciliate abundance (less than 5 ml⁻¹), but when the Pfiesteria NTZ are scarce, relatively more ciliates are needed to limit the population growth of the dinoflagellate community because of the apparent feeding threshold. It is concluded that the formation of non-toxic P. piscicida blooms require periods of low grazing pressure or a means to escape grazing.     

Dinoflagellate cyst production in one-liter containers

Methods for the production of dinoflagellate cysts in two types of 1 L containers have been developed. Using these methods, dinoflagellate cysts can be produced in amounts large enough for shellfish grazing experiments or whenever large amounts of cysts are needed. The species used were Scrippsiella lachrymosa (B-10) and toxic Alexandrium fundyense (CB501 and GTM25). Cultures of S. lachrymosa yielded 628 ± 74 cysts mL^–1 and A. fundyense cultures yielded 350 ± 98 cysts mL^–1. Findings suggest that aspects of the boundary layer between the media and the wall of the container are important for gamete mating; especially, the slope of the container wall appears to be relevant, which offers some explanation of previous observations that the shape of the container is important in the formation of dinoflagellate resting cysts. These observations may support the theory that physical interfaces in nature facilitate dinoflagellate encystment.     

Growth and Grazing Rates of the Heterotrophic Dinoflagellate Polykrikos kofoidii on Red-Tide and Toxic Dinoflagellates

We investigated growth rates, grazing rates, and prey selection of Polykrikos kofoidii when feeding on several species of red-tide and/or toxic dinoflagellates. Polykrikos kofoidii ingested all prey species used in this study, exhibiting positive growth on Lingulodinium polyedrum, Scrippsiella trochoidea, Ceratium furca, Gymnodinium catenatum, Gyrodinium impudicum, Prorocentrum micans, and the toxic dinoflagellate Amphidinium carterae, but not on P. minimum. Specific growth rates of P. kofoidii increased rapidly with increasing density of L. polyedrum, S. trochoidea, C. furca, and G. catenatum before saturating between 500-2,000 ng C ml(-1). Specific growth rates increased continuously when P. kofoidii was fed the other prey species. Maximum specific growth rates of P. kofoidii on G. catenatum (1.12 d(-1)), S. trochoidea (0.97 d(-1)), and L. polyedrum (0.83 d(-1)) were higher than those on C. furca (0.35 d(-1)), A. carterae (0.10 d(-1)), P. micans (0.06 d(-1)), G. impudicum (0.06 d(-1)), and P. minimum (-0.03 d(-1)). Threshold prey concentrations (where net growth = 0) were 54-288 ng C ml(-1). Maximum ingestion and clearance rates of P. kofoidii on these dinoflagellates were 5-24 ng C pseudocolony(-1) d(-1) and 1.0-5.9 microl pseudocolony(-1) h(-1), respectively. Polykrikos kofoidii strongly selected L. polyedrum over S. trochoidea in prey mixtures. Polykrikos kofoidii exhibited higher maximum growth, ingestion, and clearance rates than previously reported for the mixotrophic dinoflagellate Fragilidium cf. mexicanum or the heterotrophic dinoflagellates Protoperidinium cf. divergens and P. crassipes, when grown on the same prey species. Grazing coefficients calculated by combining field data on abundances of Polykrikos spp. and co-occurring red-tide dinoflagellate prey with laboratory data on ingestion rates obtained in the present study suggest that Polykrikos spp. sometimes have a considerable grazing impact on prey populations.