Selective feeding by protozoa: model and experimental behaviors and their consequences for population stability

Selective feeding by zooplankton can have profound consequencesfor the stability of grazer and prey populations, as demonstratedby the behavior of plankton dynamics models. We present an analyticalapproach—calculation of prey ratio trajectories—thatreveals unambiguously whether selective feeding behavior isstabilizing (i.e. provides a refuge for preferred prey speciesat low prey concentrations) or destabilizing (i.e. results inelimination of prey populations). Prey ratio trajectories werecalculated for three modeled selective feeding behaviors. Constantselection was consistently destabilizing, while selection behaviorsthat changed inresponse to either prey ratio or prey abundancecould be stabilizing. Batch culture experiments with four protozoangrazer species (three ciliates, one heterotrophic dinoftagellate)demonstrated that protozoa fed selectively in every case, weaklypreferring the larger of the two algal species offered. Stabilizingselection was observed only in the experiment with Favella sp.,however, meaning that only this species altered its selectionbehavior in response to changing experimental conditions. Becauseprotozoa are the major grazers of phytoplankton in many planktonicsystems, our findings indicate that the use of selective feedingbehaviors to stabilize plankton dynamics models needs carefulevaluation. The modeling and graphical techniques presentedhereare a tool for linking further exploration of selective feedingbehaviors with the development of planktondynamics models.     

Comparative growth rates and yields of ciliates and heterotrophic dinoflagellates

Growth rates, ingestion rates and grazer yields (grazer volumeproduced/prey volume consumed) were measured for six protozoanspecies (ciliates: Favella sp., Strombidinopsis acuminatum,Uronema sp.; heterotrophic dinoflagellates: Amphidinium sp.,Gymnodinium sp., Noctiluca scintillans) in laboratory batchculture experiments. Comparative growth data indicate that theprymnesiophyte Isochrysis galbana, the prasinophyte Mantoniellasquamata, two cryptophyte species and several autotrophic dinoflagellatespecies were suitable foods for these grazers. When grown onoptimized diets at 13C, maximum ciliate growth rates (range0.77–1.01 day^–1 uniformly exceeded maximum heterotrophicdioflagellate growth rates (range 0.41–0.48 day^–1).A compilation of published data demonstrates that this growthrate difference persists across a range of ciliate and dinoflagellatetaxa and cell sizes. Comparison of volume-specific ingestionrates and yields for the six species studied here showed thatthere was no single explanation for this growth rate disparity.Heterotrophic dinoflagellates exhibited both low ingestion ratesand, in one case, low yields; ciliates were able to achievehigher growth rates via either higher ingestion rates or higheryields, depending on ciliate species. Volume yield increasedover time throughout the exponential growth phase in nearlyall experiments, suggesting variation in response to changingfood concentrations or long-term acclimation to culture conditions.Higher maximum ciliate growth rates mean that these grazershave the potential to exercise tighter control over incipientblooms of their prey than do heterotrophic dinoflagellates.     

On assessment of prey ingestion by copepods

The consumption of photosynthetic and heterotrophic cells byan abundant calanoid copepod species feeding on natural planktoncommunities was quantified with a state-of-the-art image-analysissystem. Late copepodid stages of Eucalanus pileatus did notingest bactena, small photosynthetic and heterotrophic nanoplankton,or the abundant Ceratium spp. in quantifiable amounts Althoughdiatoms were by far the most abundant cells (in terms of POC1^–1), the copepods ingested a higher percentage of ciliatesin relation to their abundance than of diatoms and small heterotrophicdinoflagellates in the first experiment, and ingested a higherpercentage of dinoflagel lates and ciliates compared to diatomsin the second experiment Heterotrophic cells sufficiently largeto be captured were repeatedly preferred by E.pileatus overautotrophs of similar or larger size. More over, among the cellswhich could be individually perceived by this calanoid, largerones were not pre ferred over smaller cells, implying that someaspect of food quality can be as significant as prey size. Theseresults support the notion (e.g. Kleppel, Mar. Ecol Prog. Ser.,99, s183–195, 1993) that feeding by copepods will be underestimatedif ingestion of heterotrophic food organisms is not quantified.While the proposed microscope-based method is comparativelyslow (     

Top-down effects of crustacean zooplankton on pelagic microorganisms in a mesotrophic lake

A series of single-factor in situ experiments was conductedin a mesotrophic lake in Brandenburg, North Germany, to studythe predatory impact of Eudiaptomus graciloides (adults, copepodites,nauplii), cyclopoid copepods (adult Diacyclops bicuspidatus,Thermocyclops oithonoides) and daphnids (adult Daphnia hyalina,Daphnia cucullata) on the microbial community (bacteria, autotrophicpicoplankton, flagellates, ciliates). All zooplankton speciestested reduced the ciliate community significantly and ingestionrates were always higher for ciliates in the 20–55 µmsize category as compared to smaller ciliates (10–20 µm).Adult E.graciloides, which exhibited the highest predatory impacton ciliates, differed from cyclopoids and daphnids by theirability to decimate ciliates to very low abundances. Ingestionrates of ciliates by the crustacean zooplankton followed thesequence E.graciloides > daphnids = cyclopoids = copepodites.While top-down control was evident for ciliates, top-down effectsdown to the autotrophic picoplankton and flagellates were mostlyrestricted to Daphnia-dominated treatments. Top-down effectswere never strong enough to produce negative bacterial growthrates. For all zooplankton tested, clearance rates for ciliatesexceeded those for phytoplankton. Besides the potential of thecrustacean zooplankton to influence the structure of ciliatecommunities, ciliates may contribute to the energy demands ofcopepods and daphnids, especially when phytoplankton resourcesare limited.     

The feeding strategies of two large marine copepods

We compared the feeding behaviour of the two copepods Paraeuchaetanorvegica and Chiridius armatus, allowing them to prey on othercalanoids in small-scale laboratory experiments. Several differenceswere found. When fed either live, free-swimming or dead, non-movingprey, P.norvegica seemed unable to locate the dead prey itemswhile C.armatus foraged heavily on them. When starved, P.norvegicaincreased its feeding rate while C.armatus reduced its feedingafter an initial increase. None of the predators changed theirfeeding rates when exposed to light. Nighttime versus daytimefeeding was tested only with P.norvegcia, which seemed to possessan endogenous feeding rhythm with increased rates at night.Chiridius armatus infected with epizooic ciliates appeared tohave increased feeding rates.     

Feeding characteristics of two tintinnid ciliate species on phytoplankton including harmful species: effects of prey size on ingestion rates and selectivity

The feeding abilities of two tintinnid ciliates, Favella ehrenbergii and Favella taraikaensis, on 10 species of flagellates including harmful marine algae were examined under single prey conditions, and selective feeding of F. taraikaensis on two species of algae of different sizes was investigated under mixed prey conditions. Ingestion rates calculated from the rate of increases of auto-fluorescent particles inside food vacuoles in individuals ranged from 0.5 to 22.1 cells ind(-1) h(-1) for F. ehrenbergii and from 0.8 to 44.9 cells ind(-1) h(-1) for F. taraikaensis on nine species of prey algae. Significant ingestion rates of both Favella species could not be detected on Heterosigma akashiwo, although some individuals of both species were observed ingesting H. akashiwo in the initial incubation period. The relationship between prey cell volume and ingestion rate could be expressed mathematically for both Favella species, indicating that it is possible to estimate the potential feeding activity of each Favella species on specific algae using an equation, and may be applicable to evaluate the food value of prey alga for both Favella species. When F. taraikensis was fed mixtures of H. circularisquama and Pavlova lutheri, significant ingestion rates of H. circularisquama by F. taraikaensis could not be measured when H. circularisquama accounted for less than 20% of the other prey biomass. However, clear selectivity for H. circularisquama was observed when H. circularisquama reached and exceeded 34% of the other prey biomass. Under mixed prey conditions, it is likely that the selectivity of F. taraikaensis is stronger for larger prey compared to prey algae with a size near the lower limit on which F. taraikaensis can feed.     

Predaceous feeding habits of Limnocalanus macrurus

Limnocalanus macrurus, a large, glacial-relict copepod, hasbeen assumed an omnivore or a herbivore; predaceous habits ofthe species are unknown. The predaceous feeding habits of Limnocalanusfrom Lake Michigan were studied in the laboratory using naturalprey. Predation rates were highest on copepod nauplii. Copepoditesof Diaptomus spp. and Cyclops spp. were preyed upon at lowerrates. Limnocalanus preyed selectively upon nauplii <300µm. Small cyclopoid copepodites (<–750 µm)were also selected over large copepodites. Experiments usingtwo prey types showed that nauplii were selected over all copepodites,and that no selectivity existed for either diaptomid or cyclopoidcopepodites. Predaceous feeding habits began in the fourth copepoditestage of Limnocalanus. Predaceous feeding rates of Limnocalanuschanged seasonally being highest in late spring and autumn andlowest in summer and early winter. Since Limnocalanus also feedson net-phytoplankton, predation rate changes may be relatedto changes in the relative abundance of large phytoplanktonand naupliar prey in nature. Limnocalanus predation may be animportant factor in structuring the zooplankton community. Present address: Great Lakes Research Division, University Michigan,Ann Arbor, MI 48109, USA     

Effects of rotifers and ciliates on the growth and survival of Daphnia

Daphnia can suppress ciliates and rotifers through predationand interference competition, but it is not known whether thisproduces any direct benefit to Daphnia. We conducted survivorshipand cohort lifetable experiments to determine whether Daphniacan utilize ciliates and rotifers as food. Three species ofoligotrich ciliates (Halteria grandinella, Strobilidium gyransand Strobilidiumvelox) and one rotifer (Keratella cochlearis)were used. Lifetable experiments were conducted with a basallevel of algae (Cryptomonas sp.), plus either ciliates or rotifers,while survivorship experiments had only the rotifers or ciliates.Densities of 30 H.grandinella ml^–1, 50 S.gyrans ml^–1and 15 S.velox ml^–1 enhanced Daphnia pulex's populationgrowth rate 35–50% over controls with only algae. TenS.gyrans ml^–1 did not produce a significant change inDaphnia's growth rate. Densities of 100 and 300 K.cochlearis^–1 increased Daphnia population growth rates by II and10%, respectively. Both 10 and 50 S.gyrans ml^–1 enhancedDaphnia's survivorship compared to starved controls, but neither100 nor 300 K.cochlearis l^–1 enhanced its survivorship.The amount of enhancement of Daphnia growth rates by rotifersand ciliates is roughly proportional to the death rates imposedby Daphnia. The death rate imposed by Daphnia on rotifers isa function of both algal density and Daphnia size. Per unitbiomass, neither ciliates nor Keratella appear to be as nutritiousfor Daphnia as is Cryptomonas.     

Predation by ciliates on a metalimnetic Cryptomonas population: feeding rates, impact and effects of vertical migration

Deep algal maxima are frequently overlayed by dense populationsof ciliates, rotifers and crustaceans. This has been interpretedas evidence of heavy predation on the algae, although the impactof this predation has never been determined experimentally.We determined the vertical and seasonal distribution of thealga Cryptomonas phaseolus and its most relevant predators,the ciliates Coleps sp. and Prorodon sp., forming metalimneticmaxima in Lake Cisó. On several dates, in situ feedingrates of ciliates were determined by three independent methods:(i) epifluorescence counts of ingested algal cells togetherwith estimates of the food turnover time of the ciliates; (ii)in situ incubations with radioactively labeled algae: (iii)HPLC determination of alloxanthine content in the predator sizefraction. Feeding rates varied between 0.07 and 0.64 Cryptomonasciliate^–1 h^–1. We then calculated integrated predationon the algae. using the functional response of the ciliatesand the vertical distribution of each population. We found thateven though the ciliates were always food saturated, their predationimpact on Cryptomonas was not very large: as an average, 5–25%of the biomass of Cryptomonas was removed daily by the ciliates.Finally, we studied the effects of the diel vertical movementsof these populations on predation impact. By migrating intothe sulfide-rich hypolimnion during the night, Cryptomonas couldreduce its predation losses by 38%. Thus, the algae were protectedfrom predation during several hours of each diel cycle and maintaineda very large biomass throughout stratification, although thisresulted in a very slow growth. Slow growth, coupled to largebiomass, seems to be a general feature of metalimnetic accumulationsof organisms.     

The influence of experimental scale on estimating the predation rate of Gammarus lacustris (Crustacea: Amphipoda) on Daphnia in an alpine lake

We examined the effect of experimental scale on the predationrate of Gammarus lacustris preying on Daphnia middendorffianain 20 l cubitainers, 2800 l mesocosms and diurnal whole-lakesurveys. The predation rate differed significantly among experimentalscales. It was highest in the 20 l cubitainers, intermediatein the 2800 l mesocosms and lowest in the lake. At all experimentalscales, the predation of G.lacustris on D.middendorffiana increasedas a linear function of prey density (Type I functional response).Very different predation rates were calculated for the wholelake depending on whether D.middendorffiana densities were estimatedfrom whole-water column hauls or stratified depth hauls. Thishighlights the importance of accurately determining the spatialdistribution of prey for estimating reliable in situ predationrates. We also determined prey selection by G.lacustris. Daphniamiddendorffiana was strongly selected over the calanoid copepodHesperodiaptomus arcticus. When presented with different sizedDaphnia, G.lacustris preyed size selectively on individuals2 mm. Our results show that the predation rate of small invertebratepredators is sensitive to a wide range of experimental conditionsand needs to be corroborated at a variety of scales. We concludethat the impact of G.lacustris on zooplankton communities wouldbe strongest in small fishless lakes or ponds.     

Ontogenetic changes in copepod behaviour: an ambush cyclopoid predator and a calanoid prey

The behaviour of the predatorMesocyclops thermocyclopoides andprey Eodiaptomus japonicus was observed in their several developmentalstages. Late copepodid M.thermocychpoides preyed on late naupliarand early copepodid E.japonicus. M.thermocyclopoides showedambush behaviour, and was not detected by E.japonicus beforeattack. Detection frequency by M.thermocyclopoides increasedas development of E.japonicus advanced. However, late copepodidsof E.japonicus were avoided by juvenile and adult female M.thermocyclopoides,and were probably recognized as mates by adult males. Neithertemperature nor the presence of other food affected the predator-preybehaviour Present address: Department of Biology, Faculty of Education,Kanazawa University, Marunouchi, Kanazawa, 920 Japan     

A study of feeding in predacious ciliates using prey ciliates labeled with fluorescent microspheres

Feeding in predacious estuarine ciliates was investigated ina series of laboratory experiments using a new method of preylabeling which facilitates microscopic indentification of ingestedprey items. Ingestion rates of Mesodinium pulex, Euplotes vannusand E.woodruffi were estimated using the appearance, insidethe predator, of bacteriovorous ciliates (Metanophrys sp., Cyclidiumsp.and Pleuronema sp ) labeled with fluorescent microspheres. Preyremain motile and have presumably unaltered surface characteristics.Ingestion rates of log-growth phase predators increased withprey density. Mesodinium pulex ingested 0 15–0.32 cellsh^–1 over a prey concentration of 60–2300 ml^–1.Maximum ingestion rates of E. woodruffi and E. vannus were 4.5and 3.4 cells h^–1 respectively, estimated at prey abundancesof 75 and 172 cells ml^–1 respectively. Comparisons offeeding rates on prey of different sizes, and the effects ofstarvation, indicated that ingestion is likely limited by differentfactors in ‘raptorial’ (M pulex) and ‘filterfeeding’ (Euplotes spp.) predators.     

Do transparent exopolymer particles (TEP) inhibit grazing by the euphausiid Euphausia pacifica?

The hypothesis that ubiquitous, sticky transparent exopolymerparticles (TEP) formed from phytoplankton exudates will adhereto and coat the feeding structures of marine zooplankton grazers,and thus depress feeding on phytoplankton, was tested usingthe euphausiid, Euphausia pacifica, as a model organism. Duringtwo feeding experiments, E.pacifica were offered cells of thediatom Thalassiosira weissflogii, TEP, or both TEP and T.weissflogiicells. Ingestion rates on cells were lower in the presence ofTEP. However, contrary to the hypothesis, grazing on cells wasnot inhibited by TEP. Rather, TEP-clusters, aggregates whichformed from TEP and nano-sized particles normally too smallfor the filtering apparatus of E.pacifica to retain, servedas an alternative food source for E.pacifica, reducing theiringestion of cells. These clusters were very similar in formto the TEP actually available to marine grazers in nature. TEP-clusterswere similar to cells in size and food quality, and were grazedat similar rates. When feeding on TEP-clusters, euphausiidsshort circuit the food web by feeding on nano- and picoplanktondirectly, bypassing the microbial loop. Thus, the presence ofTEP appears to enhance, rather than depress, macrozooplanktongrazing.     

The life cycle of Gyrodinium instriatum (Dinophyceae) in culture*

The life cycle events of an unarmored dinoflagellate Gyrodinium instriatum Freudenthal et Lee has been investigated using clonal cultures. After the inoculation of vegetative cells into fresh medium, clumping of gametes was observed after a period of 10 days. In the clumps, a number of gametes were observed to be swimming in close contact with each other, pairing successively and then forming a plano-zygote. This clumping behavior is considered to be useful in the performance of sexual reproduction, particularly in the event of low cell density, because it increases the chance of fusion. Gametes of this species were most often isogarnous, although apparent amsogamous fusions were occasionally observed. When planozygotes were isolated and placed in fresh medium, they enlarged their size and finally divided into two cells. The daughter cells continued to multiply by binary fission and produced vegetative cells. Thus, G. instriatum has an alternative cycle between vegetative cells and zygotes without a hypnozygote stage. However, cysts of this species were transformed from large motile cells (pre-cyst cells) which are oblong and dorso-ventrally flattened in shape. These mottle pre-cyst cells have two longitudinal flagella, which may indicate that cysts of this species are of zygote origin. On the basis of these results, the relationship between zygotes and pre-cysts of G. instriatum is discussed. Excystment was enhanced by dark and cold treatments prior to the incubation for germination experiments with a germination success rate of 26–64%. Encystment was greatly inhibited by the lack of dark and cold treatments.     

Feeding response of a benthic copepod to ciliate prey type,prey concentration and habitat complexity

1. Protozoans are important consumers within microbial food webs and, in turn, they represent potential prey for small metazoans. However, feeding interactions within these food webs are rarely characterised and this is especially true for freshwater sediments. 2. We aimed to quantify the feeding links between a freshwater meiofaunal copepod and ciliates in two laboratory experiments. The first experiment addressed the response of Eucyclops serrulatus towards ciliate density and type (two ciliate species of the same genus differing in terms of body size). A second experiment assessed the effect of habitat structure on feeding rates by introducing different structural complexity into the feeding arena. In contrast to the first experiment, which was run only for one time period, this experiment also tested three different total feeding times (4, 7 and 9 h). 3. Eucyclops serrulatus exhibited high ingestion rates, with 3–69 ciliates copepod^?1 h^?1 consumed depending on food concentration, food type and habitat complexity. Copepods exhibited a preference for the smaller ciliate when total ciliate concentration was low, but selected both ciliates equally when food concentrations were medium or high. However, at very high food concentration, Eucyclops preferred the larger ciliate (which was 1/3 of its own body size), suggesting that the longer handling times of the larger prey are rewarding when the large prey is present in high numbers. In terms of total numbers consumed, copepods fed on more small ciliates, but in terms of carbon units both ciliates were selected equally when total prey concentration was low or medium. However, copepods derived more carbon from the larger prey at high and very high prey concentrations (up to 0.7 μgC out of a maximum of 1.1 μgC copepod^?1 h^?1). Habitat complexity influenced the feeding of copepods when it was observed over time. 4. The copepod–ciliate link is well known from the pelagic zone of both marine and freshwater habitats. We have shown its potential importance within the benthos, where it can be influenced by food identity, food quantity and possibly by habitat complexity.     

Succession of bacterivorous protists on laboratory-made marine snow

Colonization and succession over time by bacterivorous protistson laboratory-made marine snow were analysed in five assaysduring 1994. Marine snow was made hom natural seawater usingrolling tanks. In all experiments, the macroaggregates werestable in size and consistency after the fourth day, and thecolonization and succession processes were similar. Newly formedmacre aggregates became colonized by heterotrophic nanoflagellateson the fourth day, mmt of them kinete plastids (Bodo designisand Rhynchomonns nasuta) and bicosoecids (Pseudobodo tremulansand Bicosoeca sp.). Sarcodines and ciliates appeared 1 day later.Among the former, the most abundant genus was Vannella sp.,while scuticociliates (Uronema marinum) and hypotrichs (Euplotesvannus and Aspidisca steini) were the most abundant ciliates.Most of the species observed in the study were more common tobenthic habitats than to pelagic ones. The planktonic existenceof the genera Bodo, Rhynchomonas, Bicosoeca, Euplotes and Aspidiscadepends on the presence of surfaces because they are poor swimmersor immotile, and Pseudobodo and Vannella need attachment forfeeding. The only pelagic protist observed was Uronema, probablybecause its opportunistic behaviour leads it to exploit enrichedenvironments such as marine snow. Flagellate and ciliate abundancesin laboratory-made macroaggregates were much higher than insurrounding water, which indicates that marine snow representsan enhanced habitat for protist growth.     

Indirectly fluorescently labelled flagellates (IFLF): a tool to estimate the predation on free-living heterotrophic flagellates

A procedure was developed to estimate the direct grazing impacton free-living heterotrophic nanoflagellates (HNF). Culturedflagellates were labelled by feeding on brightly fluorescingbacteria (FLB) and then offered as indirectly fluorescentlylabelled flagellates (IFLF) to potential predators of HNF. Thenumber of FLB in the predators' food vacuoles could be convertedinto IFLF uptake and consumption of HNF. This new techniquewas used to study the HNF-ciliate relationship in the pelagiczone of Lake Constance. Three groups of ciliates were detectedas HNF grazers: small representatives of the genus Strobilidium.a small Haltena-like ciliate (probably Halteria grandinella)and a Codonella sp. Tintinnidium sp. group The ingestion ofHNF by these groups of ciliates ranged between 3 and 15, 3 and39, and 3 and 7 HNF ciliate^–1 h^–1; respectively.The IFLF method allows the direct determination of ingestedflagellate prey in the food vacuoles of their predators. Becauseindigenous living prey organisms were used, tracer discriminationcan be reduced.     

Feeding by larval and post-larval ctenophores on microzooplankton

Feeding by the coastal ctenophorc, Mnemiopsis leidyi, on microplanktonwas investigated. Larval ctenophores (tentaculate stage) grewbest and had the highest survival rates when offered a mixtureof ciliates and copepod nauplii. Larvae did not survive whenoffered phytoplankton alone. Clearing of planktonjc ciliatesby post-larval ctenophores was a function of the ciliate speciesand the size of the predator. Removal of small ciliates (<20µm in size) and phytoplankton was negligible. Small post-larvalctenophores (volume <4 cm³) had higher biovolume-specificclearing rates (0.5–1.5 1 cm^–3 day^–1) thandid larger ctenophores fed the same ciliate species. Duringin situ incubations, adult M. leidyi removed ciliates, rotifersand copepod nauplii from natural microplankton assemblages.The data indicate that non-crustacean microzooplanlctoo arean important component of the diet of larval and post-larvallocate cteoophores, particularly when copepod standing stocksare low.     

Nutritional suitability of the dinoflagellate Ceratium furcoides for four copepod species

The nutritional suitability of Ceratium furcoides for Eudiaplomusgracilis, Mesocyclops leuckarti, Thermocyclops oithonoides andCyclops abyssorum was studied by observing ingestion and assimilation.The dinoflagellate was ingested by female adults of all species.The calanoid copepod E.gracilis could not utilize Ceratium;mortality was high and no egg production was observed when Ceratiumwas the only food source. Mortality was low for the adult cyclopoidson this food and reproduction indicated that Ceratium was assimilated.The first two copepodite instars of M.leuckarti were not ableto handle Ceratium, while older stages preyed on them. The dinoflagellatewas not ingested by female M.leuckarti when its densities werelow. Advanced copepodite stages of C.abyssorum developed intoadults on a diet of Ceratium only. Mesocyclops leuckarti femalesingested Ceratium when offered a mixed food source of Ceratiumand the rotifer Brachionus rubens, but the rotifer was positivelyselected, even if its density was low. The results show thatCeratium provides a suitable food source for advanced copepoditeinstars and adult cyclopoid copepods, although it is not a preferredfood source.     

Monsoonal and lunar variability in microzooplankton abundance and community structure in the Terusan mangrove creek (Malaysia)

This study documents the monsoonal and lunar effects on species composition and abundance of microzooplankton in a tropical estuary. We investigated microzooplankton abundance in relation to the various environmental and biotic parameters, sampled in the Matang mangrove (Malaysia) from April 2013 to February 2014. A total of 39 microzooplankton taxa comprising four major groups, i.e. loricate ciliates (37.72%), aloricate ciliates (29.46%), dinoflagellates (24.33%) and meroplanktonic nauplii (8.49%) were identified. The loricate ciliates were the most diverse group with 31 taxa recorded. Four major species of loricate ciliates were identified, i.e. Tintinnopsis beroidea, Tintinnopsis rotundata, Stenosemella avellana and Tintinnidium primitivum, while Strombidiidae and Strobilidiidae dominated the aloricate ciliates. Although small loricate ciliates were ubiquitous, redundancy analysis shows marked shifts in microzooplankton community structure, from one that was dominated by loricate ciliates during the drier SW monsoon, to aloricate ciliates at the onset of the wet NE monsoon, and then to dinoflagellates towards the end of the drier NE monsoon period. These shifts were associated with rainfall, dissolved inorganic nutrients, salinity, temperature and microbial food abundance. There was no clear lunar effect on abundance of microzooplankton except for Favella ehrenbergii and copepod nauplii, which were more abundant during neap than spring tides.