Copepod Feeding Research Articles

Copepod feeding in a coastal area of active tidal mixing: diel and monthly variations of grazing impacts on phytoplankton biomass

AbstractThis study examined monthly feeding rates and grazing impact on phytoplankton biomass, as well as diel feeding rhythms of four key copepod species in a tidally well mixed estuary (Asan Bay, Korean Peninsula). Monthly ingestion rates estimated based on gut pigment analysis were closely associated with their peak densities, but not with phytoplankton biomass, implying high ingestion may be related to reproductive output for population growth. The three smaller copepods, Acartia hongi, Acartia pacifica and Paracalanus parvus, showed feeding preference for smaller phytoplankton (<20 μm) with higher clearance rates, whereas the larger Calanus sinicus preferred larger phytoplankton. Acartia pacifica and P. parvus showed distinct increased nocturnal feeding rates as measured with gut fluorescence, whereas A. hongi showed no significant day–night differences. Copepod diel feeding patterns were not associated with food quantity, and endogenous physiological rhythm might be hypothesized as responsible for the observed diel feeding patterns. Grazing impact on phytoplankton biomass by the four copepods in the estuary was on average 8% (range 0.2–29.8%) of the phytoplankton standing stock, similar to values reported in other coastal waters. Very high copepod abundances but low daily carbon ration (<20% for all copepods) provided by feeding on phytoplankton indicate that copepods also grazed on other non‐phytoplankton foods in Asan Bay.

Diatom induction of reproductive failure in copepods: The effect of PUAs versus non volatile oxylipins

Several species and strains of diatoms were tested for their effects on egg production, hatching success, faecal pellet production, ingestion rate and food selection in the copepod Temora stylifera. Two of the diatom species tested were well known polyunsaturated aldehyde (PUA) producers: Skeletonema marinoi (SM) and Thalassiosira rotula (TR1) , while the other species tested, Skeletonema pseudocostatum, was a non-PUA producing species. Another T. rotula strain (TR2) was also tested since previous studies had reported contradictory results, due either to absence of PUA production or production in small amounts. Our results showed strong inhibitory effects on reproductive parameters of copepods with all diatom species tested. Although we confirm that S. pseudocostaum did not produce PUAs, this diatom produced large quantities of other oxylipins such as (5Z,8Z,11Z,13E,15S,17Z)-15-hydroxy-5,8,11,13,17-eicosapentaenoic acid (15S-HEPE) and 13,14-13R-hydroxy-14S,15S-trans-epoxyeicosa-5Z,8Z,11Z,17Z-tetraenoic acid (13,14-HEpETE) and 15-oxo-5Z,9E,11E,13E-pentadecatetraenoic acid (15-oxoacid), all of which have already been found in S. marinoi (Fontana et al., 2007a) and Pseudonitzschia delicatissimia (d'Ippolito et al., 2009). Another unidentified oxylipin was also present in high quantities in LC–MS profiles. Some of these oxygenated fatty acid derivatives were also found, together with PUAs, in TR1 and in TR2. Both PUA-producing and non-producing diatoms caused negative effects on hatching success with induction of apoptosis in newly hatched nauplii. This suggests that negative diatom effects can also depend on the production of non volatile oxylipins. Copepod feeding behaviour was found to be non-selective when each diatom species was offered together with the non toxic dinoflagellate Prorocentrum minimum suggesting that grazers are not aware of the toxicity of their food.

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

Summary 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.

Copepod guts as biogeochemical hotspots in the sea: Evidence from microelectrode profiling of Calanus spp

The environmental conditions inside the gut of Calanus hyperboreus and C. glacialis were measured with microelectrodes. An acidic potential hydrogen (pH) gradient was present in the gut of C. hyperboreus, and the lowest pH recorded was 5.40. The gut pH of a starved copepod decreased by 0.53 after the copepod resumed feeding for a few hours, indicating the secretion of acidic digestive fluid. A copepod feeding on Thalassiosira weissflogii (diatom) had slightly lower pH than that feeding on Rhodomonas salina (cryptophyte). Oxygen was undersaturated in the gut of both C. hyperboreus and C. glacialis, with a steep gradient from the anal opening to the metasome region. The central metasome region was completely anoxic. Food remains in the gut led to a lower oxygen level, and a diatom diet induced a stronger oxygen gradient than a cryptophyte diet. The acidic and suboxic–anoxic environments of the copepod gut may support iron dissolution and anaerobic microbial activities that otherwise are not favored in the well‐buffered and oxygenated ambient ocean.

Influence of iron on fatty acid and sterol composition of marine phytoplankton and copepod consumers

We conducted laboratory experiments to determine the influence of iron on the fatty acid (FA) and sterol composition of the diatom Thalassiosira oceanica, the cryptophyte Rhodomonas salina, the prymnesiophyte Isochrysis galbana, and the copepod Acartia tonsa that fed on these algae. Algal cells were cultured with no added Fe or with Fe added at 100 nmol L−1. Fe‐deficient culture medium resulted in lower growth rates, cell volumes, and photosynthetic efficiencies for all algal species. The total mass of FA per cell was 2.2–3.1 times as high in Fe‐replete cells as in Fe‐deficient cells; on a carbon–normalized basis, the FA concentrations of Fe‐replete algal cells were 1.4–1.5 times as high. Differences were noted between Fe treatments for specific saturated, monounsaturated, and polyunsaturated FAs within each algal species. Total C‐normalized 20:5ω3 + 22:6ω3 concentrations declined 18%, 25%, and 35% in Fe‐deficient T. oceanica, R. salina, and I. galbana, respectively. Across all species, another consistent difference in FA composition between Fe treatments was for 18:4ω3, which had a C‐normalized concentration that was ∼ twofold higher in Fe‐replete algae. No consistent pattern for all algal species was evident for Fe effects on algal sterol concentration and composition, and in response to Fe limitation sterols were conserved more than C and FAs in these cells. Copepods feeding on Fe‐replete and Fe‐deficient algae displayed smaller differences in FA and sterol concentrations than in their algal diets. Differences in lipid concentration and composition between Fe‐replete and Fe‐deficient algae may partly explain the lower egg production rate of copepods fed Fe‐deficient algae.

Performance of Larval Florida Pompano Fed Nauplii of the Calanoid Copepod Pseudodiaptomus pelagicus

AbstractThe Florida pompano Trachinotus carolinus is a highly prized marine fish species, the larviculture of which currently includes the feeding of live rotifers and nauplii of brine shrimp Artemia spp. However, no previous studies have evaluated the feeding of copepod nauplii. In this study, the growth and survival of Florida pompano larvae fed nauplii of the calanoid copepod Pseudodiaptomus pelagicus were compared with those of larvae fed the standard reference diet of enriched rotifers Brachionus plicatilis. Experiments were conducted during the first 7–9 d posthatch (DPH), a period preceding the provision of Artemia nauplii. Treatments included feeding only copepod nauplii during the first day, the first three days, and on all days, as well as copepod nauplii mixed with rotifers during the entire experiment. In addition, the dietary effects on larval fatty acid composition were examined. Feeding copepod nauplii at a density of 2.0–3.5 nauplii/mL during the first day or the first three days of feeding had advantages over feeding only rotifers. However, after approximately 3 DPH, increased quantities of nauplii were needed to provide sufficient nutrients for growth. This was demonstrated in larvae fed copepod nauplii for the entire trial, for which survival was significantly higher than for the other treatments but for which growth was significantly reduced. Larvae fed a mixture of rotifers and nauplii for the entire trial had survival similar to that of larvae fed only rotifers (∼40%); however, growth was greater in larvae fed the mixed diet, suggesting that there is a nutritional advantage to including copepods in the diet. Fatty acid analyses revealed that increased levels of docosahexaenoic acid were associated with larvae fed copepods, which probably contributed to the observed higher survival and growth. These results indicate that there are multiple benefits to feeding copepods to Florida pompano larvae.Received June 6, 2010; accepted August 10, 2010

Can copepods be limited by the iron content of their food?

In laboratory experiments, we show that naupliar survival and egg production by the copepod Acartia tonsa was significantly lower when they were fed iron‐depleted algal cells than when they were fed a diet of Fe‐replete cells that had two orders of magnitude higher Fe levels. Naupliar survival after 1 week was reduced from 60% for copepods feeding on Fe‐replete diatoms (Thalassiosira oceanica) to 0% for those feeding on Fe‐depleted cells. The decline of egg production rate was greatest (83%) when T. oceanica was used as prey and smaller (20–30%) when the cryptophyte Rhodomonas salina and the prymnesiophyte Isochrysis galbana were used as food. The assimilation rates of Fe and C from Fe‐replete and Fe‐depleted algae were determined using radioisotopes. Egg production was hyperbolically dependent on the Fe assimilation rate (r2=0.71). The effect of Fe on copepod reproduction rates could not be explained by changes in ingestion rate and the rate of carbon assimilated, because there was no significant difference of ingestion rate and C assimilation between Fe‐replete and Fe‐depleted treatments. Iron might limit zooplankton productivity in high‐nutrient, low‐chlorophyll regions.

Copepod feeding in the ocean: scaling patterns, composition of their diet and the bias of estimates due to microzooplankton grazing during incubations

Here, we report insights from the compilation and analysis of data on marine calanoid copepod feeding rates in the ocean. Our study shows that food availability and body weight are major factors shaping copepod feeding rates in the field, with a relatively minor role of temperature. Although the maximal feeding rates of copepods that are observed in the field agree with the well-known 3/4 of body size scaling rule for animals, copepod feeding in the oceans is typically limited and departs from this rule. Ciliates and dinoflagellates appear to be highly relevant in the composition of copepod diets, and this represents an indirect increase in the flux of primary production that is likely to reach the upper trophic levels; this contribution is higher in the less productive systems and may help to explain accounts of proportionally higher standing stocks of copepods supported per unit of primary producer biomass in oligotrophic environments. Contrary to common belief, diatoms emerge from our dataset as small contributors to the diet of copepods, except in some very productive ecosystems. We have also evaluated the bias in the estimation of copepod grazing rates due to within-bottle trophic cascade effects caused by the removal of microheterotrophs by copepods. This release of microzooplankton grazing pressure accounts for a relevant, but moderate, increase in copepod grazing estimates (ca. 20–30%); this bias has an effect on both the carbon flux budgets through copepods and on our view of their diet composition. However, caution is recommended against the indiscriminate use of corrections because they may turn out to be overestimates of the bias. We advise that both uncorrected and corrected grazing rates should be provided in future studies, as they probably correspond to the lower and upper boundaries of the true grazing rates.

Feeding impacts of ontogenetically migrating copepods on the spring phytoplankton bloom in the Oyashio region

We investigated the feeding habits and grazing rates of the ontogenetically migrating copepods in the Oyashio region to evaluate their grazing impacts on the food web during the spring phytoplankton bloom. The bloom was in progress from early to late April, although chlorophyll a concentrations fluctuated considerably with the frequent exchange of different water masses. Biomass of the copepod community reached a maximum in mid-April when late copepodites of Neocalanus cristatus, Neocalanus flemingeri and Eucalanus bungii contributed to the biomass increase. Gut pigment contents of the predominant copepods were much higher during the bloom compared with the levels in March (pre-bloom). The temporal fluctuations were not correlated with those of mean chlorophyll a concentrations in the 0–50 m layer. Feeding experiments indicated that major food items for the copepods were centric diatoms and flagellates. During the period of lower ambient chlorophyll, the copepods changed their heterotrophic prey from naked ciliates to tintinnids. Apparent clearance rates were positive for naked ciliates and negative for heterotrophic nanoplankton, Cryptophyceae and bacteria when chlorophyll was high, suggesting trophic cascade effects from copepod feeding even during the phytoplankton bloom. The carbon demands of the copepod community were estimated to be 156 mgC m −2 day −1 in early March to 797 mgC m −2 day −1 in mid-April. The grazing rates on phytoplankton reached 480 mgC m −2 day −1, equivalent to as much as 28% of primary production. Non-phytoplankton prey supported 40 to 71% of the copepod carbon requirement. These results suggest that the copepod community does not graze the phytoplankton bloom down, but it does have significant impacts on microbial food webs.

Differential effects of nutrient-limited primary production on primary, secondary or tertiary consumers

Nutritional imbalances between predator and prey are the rule rather than the exception at the lower end of food webs. We investigated the role of different grazers in the propagation of nutritionally imbalanced primary production by using the same primary producers in a three-trophic-level food chain and a four-trophic-level food chain experimental setup. The three-trophic-level food chain consisted of a classic single-cell primary producer (Rhodomonas salina), a metazoan grazer (the copepod Acartia tonsa) and a top predator (the jellyfish Gonionemus vertens), while we added a protozoan grazer (Oxyrrhis marina) as primary consumer to the food chain to establish the four-trophic-level food chain. This setup allowed us to investigate how nutrient-limitation effects change from one trophic level to another, and to investigate the performance of two components of our experimental food chains in different trophic positions. Stoichiometry and fatty acid profiles of the algae showed significant differences between the nutrient-depleted [no N and no P addition (-P), respectively] and the nutrient-replete (f/2) treatments. The differences in stoichiometry could be traced when O. marina was the first consumer. Copepods feeding on these flagellates were not affected by the nutritional imbalance of their prey in their stoichiometry, their respiration rates nor in their developmental rates. In contrast, when copepods were the primary consumer, those reared on the -P algae showed significantly higher respiration rates along with significantly lower developmental rates. In neither of our two experimental food chains did the signals from the base of the food chains travel up to jelly fish, our top predator.

Evaluation of DNA extraction and handling procedures for PCR-based copepod feeding studies

Molecular methods are becoming increasingly common for taxonomic and ecological studies of marine and freshwater plankton. Recently, nucleic acids have been used as target molecules for identification and quantification of prey species in studies of trophic interactions. A critical step in the quantification of mesozooplankton feeding by molecular analysis is the isolation of microalgal DNA from predator guts and in the food environment. It is essential that total genomic DNA extraction provides maximum quantitative yield suitable for downstream analysis. In this study, we compared the efficacy and experimental variability of eight different protocols for total genomic DNA extraction from free-living microalgae and microalgae within the gut of copepods. We also developed and evaluated different sampling procedures for copepods prior to genomic extraction. The optimal protocol was evaluated using real time quantitative polymerase chain reaction (qPCR) and the integrity of the genomic DNA was determined by amplifying PCR targets of increasing size. Considerable variability was observed between purification protocols. Qiagen DNeasy® Blood & Tissue kit was the most efficient of the tested methods for genomic extraction from both free-living microalgae and microalgae inside copepod guts. Furthermore, the appropriate handling of predator copepods prior to genomic extraction was essential for quantitative gut content estimates.

Does the nutrient stoichiometry of primary producers affect the secondary consumer Pleurobrachia pileus?

We investigated whether phosphorus limitations of primary producers propagate upwards through the food web, not only to the primary consumer level but also onto the secondary consumers’ level. A tri-trophic food chain was used to assess the effects of phosphorus-limited phytoplankton (the cryptophyte Rhodomonas salina) on herbivorous zooplankters (the copepod Acartia tonsa) and finally on zooplanktivores (the ctenophore Pleurobrachia pileus). The algae were cultured in phosphorus-replete and phosphorus-limited media before being fed to two groups of copepods. The copepods in turn were fed to the top predator, P. pileus, in a mixture resulting in a phosphorus-gradient, ranging from copepods having received only phosphorus-replete algae to copepods reared solely on phosphorus-limited algae. The C:P ratio of the algae varied significantly between the two treatments, resulting in higher C:P ratios for those copepods feeding on phosphorus-limited algae, albeit with a significance of 0.07. The differences in the feeding environment of the copepods could be followed to Pleurobrachia pileus. Contrary to our expectations, we found that phosphorus-limited copepods represented a higher quality food source for P. pileus, as shown by the better condition (expressed as nucleic acid content) of the ctenophore. This could possibly be explained by the rather high C:P ratios of ctenophores, their resulting low phosphorus demand and relative insensitivity to P deficiency. This might potentially be an additional explanation for the observed increasing abundances of gelatinous zooplankton in our increasingly phosphorus-limited coastal seas.

Lipid dynamics and feeding of dominant Antarctic calanoid copepods in the eastern Weddell Sea in December

Lipid content, fatty acid composition, and feeding activity of the dominant Antarctic copepods, Calanoides acutus, Calanus propinquus, and Metridia gerlachei, were studied at a quasi-permanent station in the eastern Weddell Sea in December 2003. During 3 weeks of the spring phytoplankton development, total lipid levels of females and copepodite stages V (CVs) of C. acutus were almost doubled. Meanwhile, only a slight increase in total lipid content occurred in M. gerlachei, and no clear trend was observed in lipids of C. propinquus females. The pronounced increase of lipids in C. acutus was due to an accumulation of wax esters. The proportion of wax esters in the lipids of M. gerlachei was clearly lower, while triacylglycerols played a more important role. In C. propinquus, triacylglycerols were the only neutral lipid class. There were no pronounced changes in the feeding activity of M. gerlachei, whereas the feeding activity of C. acutus had rapidly increased with the development of the phytoplankton bloom in December, which explains its rapid lipid accumulation. The combination of gut content and fatty acid trophic marker analyses showed that C. acutus was feeding predominantly on diatoms. The typical diatom fatty acid marker, 16:1(n-7), slightly decreased and the tracer for flagellates, 18:4(n-3), increased in females and CVs of C. acutus. This shift indicates the time, when the significance of flagellates started to increase. The three copepod species exhibited different patterns of lipid accumulation in relation to their trophic niches and different duration of their active phases. The investigations filled a crucial data gap in the seasonal lipid dynamics of dominant calanoid copepods in the Weddell Sea in December and support earlier hypotheses on their energetic adaptations and life cycle strategies.

Copepod feeding study in the upper layer of the tropical South China Sea

The South China Sea (SCS) is the world’s largest marginal sea being notable for vertical mixing at various scales resulting in a sequence of chemical and biological dynamics in surface waters. We investigated the ingestion, gut content, evacuation and clearance rates of copepods collected from six stations (including a South East Asia Time Series station) along a transect line in the tropical of a SCS cruise during September 27, 1999 to October 2, 1999. The goal of the present study was to understand the feeding ecology of copepods in the upper water layers (0–5 m) of the northern SCS during autumn. We measured the gut pigment contents of 33 copepod species by the gut fluorescence method. The gut chlorophyll a values of most small size copepods ( 2 mm) had significantly higher values than medium sized copepods (1–2 mm) and smaller sized copepods. The present study shows that the performance of feeding on phytoplankton was variable in different sized copepod groups, suggesting that copepods obtained in the tropical area of the southeastern Taiwan Strait might be opportunistic feeders.

Upslope transport of near-bed zooplankton

Abstract Zooplankton residing just above the deep-sea floor is an important component of the benthic/benthopelagic food chain. Consuming planktonic particulates and organisms, holoplankton and meroplankton are prey for fish and large invertebrates. Mechanisms controlling their abundances have been explored over relatively long time scales (months to years). Here, zooplankton were sampled every 2 h for 2.2 d using a moored, automated, serial zooplankton pump. The physical regime (currents and temperature) 1–100 m above bottom was measured during an inclusive 24-d period. The study site was located on the upper continental slope (750 m) of the Mid-Atlantic Bight, between the productive shelf and more impoverished rise and abyss. The coupled biological and physical records indicated tidally driven, net upslope transport of the holoplankton. The copepod (74.5% of collections) time series showed marked periodicity with a peak frequency of ∼13 h, approximately the diurnal tide (Fourier analysis). Local maxima corresponded with minimal water temperatures. Moreover, tidal cross-slope flow was highly coherent and 90° out of phase with temperature. Thus, maximal copepod concentrations, originating in colder deeper water, would be transported up the slope by the tide. Estimated net displacement of ∼1 km/d would deliver the animals to continental-shelf depths within a couple weeks. Time series of the much less abundant larvaceans (urochordates) (15.3%) and polychaete larvae (8.9%) showed periodicities with peak frequencies of 8–9 h. Statistical significance of the periodic signals could not be determined due to low numbers. Revealing holoplankton dynamics on scales of hours, this study may contribute to understanding of, for example, copepod feeding and aggregation near the deep-sea floor.

The importance of protozooplankton as prey for copepods in the coastal areas of the central Irish Sea

This study evaluates food supply for copepods, highlighting the trophic relationship between copepods and protozooplankton. To test the hypotheses that protozooplankton prey are capable of sustaining the copepod standing stock in the western Irish Sea, the taxonomic and size composition of these two groups and the size-specific predation of copepods on protozooplankton were investigated. Protozooplankton and copepod samples were collected off the southwest coast of the Isle of Man using 1.7 l Niskin water bottles and two nets (64 and 280 μm meshes), respectively. Copepod predation on protozooplankton was calculated using weight-specific clearance rates from the literature, considering the availability of prey that was accessible to a given size of copepod. Low protozooplankton biomass was dominated by small cells (<60 μm), and high copepod biomass was dominated by small species, which were more efficiently collected by a 64-μm mesh net. However, large copepods were only collected by a 280-μm mesh net, suggesting that the combination of the two nets provided a better estimate of copepod biomass. Predation by the copepod assemblage in the Irish Sea removed 1–47% and 0.5–22% of ciliates and dinoflagellates standing stock, respectively, resulting in 1–40% of the copepod feeding requirement per day. Contrary to our hypothesis, copepods could not meet their feeding requirements by grazing only on the microzooplankton prey (15–200 μm), and other food sources (i.e. nanoplankton) must be important additional dietary components to copepods in the Irish Sea.

Decreased astaxanthin at high feeding rates in the calanoid copepod Acartia bifilosa

In marine food webs, copepods are the major producers of a carotenoid pigment astaxanthin, which is an important antioxidant. The availability of astaxanthin for higher trophic levels can be affected by changes in phytoplankton stocks and copepod feeding; however, the functional relationship between food availability and astaxanthin production is poorly understood. We hypothesized that with a given food type and quality, astaxanthin content in copepods is positively related to feeding and egg production rates. The hypothesis was tested by measuring astaxanthin accumulation in concert with ingestion and egg production rates in the copepod Acartia bifilosa exposed to different algal concentrations (Tetraselmis suecica; 0 to 1200 μg C L−1). Egg production and ingestion rates increased with increasing food availability and reached a plateau at ≥400–600 μg C L−1. In contrast, increasing accumulation of astaxanthin with increasing food availability was observed only at concentrations ≤150 μg C L−1. Contrary to our hypothesis, at 600–1200 μg C L−1 copepods had maximal ingestion and egg production rates, but low astaxanthin contents. It is suggested that this low accumulation of astaxanthin at high food concentrations results from a food-dependant decrease in assimilation efficiency. These findings are important for the understanding of astaxanthin dynamics within marine food webs, where increases in phytoplankton biomass may translate to a trade-off between zooplankton quantity and its nutritional quality for zooplanktivores.

Contribution of auto- and heterotrophic protozoa to the diet of copepods in the Ulleung Basin, East Sea/Japan Sea

We estimated the ingestion rates and prey selectivity of two calanoid copepods, Calanus sinicus and Neocalanus plumchrus, on auto- and heterotrophic protozoa in the Ulleung Basin in the East Sea/Japan Sea. Both copepod species preferentially ingested auto- and heterotrophic protozoa. The heterotrophic protozoa comprised between 42.9 and 86.7% of the total carbon ration ingested by the copepods, although they comprised on average 21.5% of the total available prey carbon. In particular, ciliates comprised the major dietary component for the copepods in the study area. The copepod consumption of ciliates was on average 38.2% of the total daily carbon ration of auto-and heterotrophic protozoa. Auto- and heterotrophic protozoa in the 20―50 μm size were the fraction that was most actively consumed by copepods and they comprised on average 45.1% of the total carbon ration ingested. In all experiments, the copepods selected the ciliate and heterotrophic dinoflagellate (HDF) groups over the other protozoa. Our results indicate that the copepod feeding regime was influenced by the composition and size of the potential food and that, in this study area, feeding behavior of copepods may control the populations of ciliates and HDFs that are larger than 10 μm in size. Thus, the selective feeding patterns and higher grazing pressure of copepods on heterotrophic protozoa, compared with autotrophic protozoa, indicate a trophic coupling between copepods and the microbial food web in the Ulleung Basin.

Quantification of copepod gut content by differential length amplification quantitative PCR (dla-qPCR).

Quantification of feeding rates and selectivity of zooplankton is vital for understanding the mechanisms structuring marine ecosystems. However, methodological limitations have made many of these studies difficult. Recently, molecular based methods have demonstrated that DNA from prey species can be used to identify zooplankton gut contents, and further, quantitative gut content estimates by quantitative PCR (qPCR) assays targeted to the 18S rRNA gene have been used to estimate feeding rates in appendicularians and copepods. However, while standard single primer based qPCR assays were quantitative for the filter feeding appendicularian Oikopleura dioica, feeding rates were consistently underestimated in the copepod Calanus finmarchicus. In this study, we test the hypothesis that prey DNA is rapidly digested after ingestion by copepods and describe a qPCR-based assay, differential length amplification qPCR (dla-qPCR), to account for DNA digestion. The assay utilizes multiple primer sets that amplify different sized fragments of the prey 18S rRNA gene and, based on the differential amplification of these fragments, the degree of digestion is estimated and corrected for. Application of this approach to C. finmarchicus fed Rhodomonas marina significantly improved quantitative feeding estimates compared to standard qPCR. The development of dla-qPCR represents a significant advancement towards a quantitative method for assessing in situ copepod feeding rates without involving cultivation-based manipulation.

Active prey selection in two pelagic copepods feeding on potentially toxic and non-toxic dinoflagellates

Grazing on two red tide dinoflagellates, the potentially toxic Karenia mikimotoi and the non-toxic Gyrodinium instriatum, was examined in two species of marine copepods, Pseudocalanus elongatus and Temora longicornis. Both copepods cleared K. mikimotoi at rates that were a little lower but comparable to those at which they cleared the slightly larger G. instriatum when the two dinoflagellates were offered separately. However, when feeding on mixtures of the two prey species, the clearance rates on K. mikimotoi were substantially reduced in both copepods while their clearances of G. instiatum remained unaltered, suggesting active prey selection. Video observations of individual prey capture and feeding events showed prey rejection frequencies (caught and then released cells) that did not differ between mixed and mono-specific diets. This suggests that the selection between prey cells occurs prior to capture and that it is based on remote characterization of the cells.