Effects of small-scale turbulence on feeding rate and gross-growth efficiency of three Acartia species (Copepoda: Calanoida)

Experiments designed to test the effects of small-scale turbulenceon the feeding rates and gross-growth efficiency of calanoidcopepods have been performed within a wide range of controlledfood concentrations. Turbulence significantly enhanced feedingrates only at food concentrations lower than the ingestion saturatinglevel. Gross-growth efficiency (the quotient carbon-egg production/carbon-foodingested) of the different Acartia species studied showed differentpatterns of response to turbulence, in agreement with the hydrodynamiccharacteristics of their habitat. Furthermore, experiments conductedon Acartia clausi at two different intensities of turbulenceindicate a shift in the response, with enhancement of feedingat low intensities of turbulence and negative interference athigher intensities.     

Capture mechanisms used by the lobate ctenophore, Mnemiopsis leidyi, preying on the copepod Acartia tonsa

Although the lobate ctenophore Mnemiopsis leidyi is an influentialplanktonic predator, the mechanisms enabling it to capture itscharacteristically wide range of prey have not been systematicallyexamined. We recorded interactions between free-swimming M.leidyiand two stages (nauplii, adults) of the calanoid copepod Acartiatonsa in order to determine a mechanistic explanation of thisfeeding process. Prey encounter with Mnemiopsis involved twodifferent processes. The first depended on fluid motions createdby the nearly continuous beating of cilia lining the four auricles.These cilia created a low-velocity flow in which A.tonsa naupliiwere entrained (94% of naupliar encounters) and transportedpast the oral lobes onto the tentillae (oral tentacles). Thenauplii, although capable of rapid escape responses, generallyappeared to be insensitive to the current in which they werecarried. The second process relied upon the collision of swimmingprey with the inner surfaces of the oral lobes and was not obviouslyinfluenced by the auricular feeding currents. Adult A.tonsawere rarely entrained in the auricular flow, but, instead, propelledthemselves into contact with the oral lobes (97% of adult encounters).Both prey capture processes functioned simultaneously. The synergisticfunctioning of these processes probably explains the broad patternsof prey ingestion found by in situ studies of Mnemiopsis feeding.     

A Mechanistic Individual-Based Model of the Feeding Processes for Oikopleura dioica

A mechanistic physiological model of the appendicularian Oikopleura dioica has been built to represent its three feeding processes (filtration, ingestion and assimilation). The mathematical formulation of these processes is based on laboratory observations from the literature, and tests different hypotheses. This model accounts for house formation dynamics, the food storage capacity of the house and the gut throughput dynamics. The half-saturation coefficient for ingestion resulting from model simulations is approximately 28 and is independent of the weight of the organism. The maximum food intake for ingestion is also a property of the model and depends on the weight of the organism. Both are in accordance with data from the literature. The model also provides a realistic representation of carbon accumulation within the house. The modelled half-saturation coefficient for assimilation is approximately 15 and is also independent of the weight of the organism. Modelled gut throughput dynamics are based on faecal pellet formation by gut compaction. Model outputs showed that below a food concentration of 30 , the faecal pellet weight should represent a lower proportion of the body weight of the organism, meaning that the faecal pellet formation is not driven by gut filling. Simulations using fluctuating environmental food availability show that food depletion is not immediately experienced by the organism but that it occurs after a lag time because of house and gut buffering abilities. This lag time duration lasts at least 30 minutes and can reach more than 2 hours, depending on when the food depletion occurs during the house lifespan.     

Response of two zooptankton grazers to an ichthyotoxic estuarine dinoflagellate

The dinoflagellate Pfiesteria piscicida (gen. et sp. nov.).a toxic ‘ambush predator’, has been implicated asa causative agent of major fish kills in estuanne ecosystemsof the southeastern USA. Here we report the first experimentaltests of interactions between P.piscicida and estuarine zooplanktonpredators. specifically the rotifer Brachionus plicatilis andthe calanoid copepod Acartia tonsa. Short-term (10 day) exposureof adult B.plicatilis to P.piscicida as a food resource, aloneor in combination with the non-toxic green algae Nannochlorisand Tetraselmis. did not increase rotifer mortality relativeto animals that were given only non-toxic greens Similarly,short-term (3 day) feeding trials using adult A.tonsa indicatedthat the copepods survived equally well on either P.piscicidaor the non-toxic diatom Thalassiosira pseudonana. Copepods giventoxic dinoflagellates exhibited erratic behavior, however, relativeto animals given diatom prey. The fecundity of B.plicatiliswhen fed the toxic dinoflagellate was comparable to or higherthan that of rotifers fed only non-toxic greens We concludethat, on a short-term basis, toxic stages of P.piscicida canbe readily utilized as a nutritional resource by these commonestuarine zooplankters. More long-term effects of P.piscicidaon zooplankton, the potential for toxin bioaccumulation acrosstrophic levels, and the utility of zooplankton as biologicalcontrol agents for this toxic dinoflagellate. remain importantunanswered questions.     

Effects of the red tide dinoflagellate, Karenia brevis, on grazing and fecundity in the copepod Acartia tonsa

Among studies of copepod grazers fed harmful algae, decreasedgrazing and fecundity are the most common results. The causesof decreased grazing (physiological incapacitation, behavioralavoidance or lack of stimulation) and decreased fecundity (toxicversus nutritional effect) vary among studies. This study useda series of controlled laboratory experiments to investigatethe cause of decreased grazing and fecundity in the copepodAcartia tonsa fed sole and mixed diets of the harmful alga,Karenia brevis. Copepods fed K. brevis mixed with the nutritionallyviable dinoflagellate Peridinium foliaceum had higher ingestionrates and offspring production than copepods fed a sole dietof K. brevis (even when K. brevis was virtually nontoxic). Copepodsfed mixtures did not discriminate between P. foliaceum and K.brevis while feeding. The results of this study suggest thatK. brevis is not toxic to A. tonsa but lacks some chemical componentresponsible for stimulating a grazing response in A. tonsa aswell as the nutritional requirements for normal offspring production.     

Prey Detection and Prey Capture in Copepod Nauplii

Copepod nauplii are either ambush feeders that feed on motile prey or they produce a feeding current that entrains prey cells. It is unclear how ambush and feeding-current feeding nauplii perceive and capture prey. Attack jumps in ambush feeding nauplii should not be feasible at low Reynolds numbers due to the thick viscous boundary layer surrounding the attacking nauplius. We use high-speed video to describe the detection and capture of phytoplankton prey by the nauplii of two ambush feeding species (Acartia tonsa and Oithona davisae) and by the nauplii of one feeding-current feeding species (Temora longicornis). We demonstrate that the ambush feeders both detect motile prey remotely. Prey detection elicits an attack jump, but the jump is not directly towards the prey, such as has been described for adult copepods. Rather, the nauplius jumps past the prey and sets up an intermittent feeding current that pulls in the prey from behind towards the mouth. The feeding-current feeding nauplius detects prey arriving in the feeding current but only when the prey is intercepted by the setae on the feeding appendages. This elicits an altered motion pattern of the feeding appendages that draws in the prey.     

Turbulence decreases the hydrodynamic predator sensing ability of the calanoid copepod Acartia tonsa

The copepod Acartia tonsa is very sensitive to hydrodynamicsignals, including those made by approaching predators, andresponds with a vigorous escape jump. Whether the presence ofmoderate turbulence changes this ability to detect hydrodynamicsignals was investigated by comparing the response of copepodsto velocity gradients created by a siphon flow in turbulentand still water. Turbulence decreased the distance at whichA. tonsa initiated escapes from the siphon and increased thecapture rate, indicating decreased sensitivity to hydrodynamicsignals, but did not trigger unnecessary escape reactions thatmight produce fatigue.     

Physiological evaluation of temperature effect on the growth processes of the mysid, Neomysis intermedia Czerniawsky

Ingestion, respiration, and molting loss rates were measuredover the 3 – 29°C range in Neomysis intermedia. Weightspecific rates of these physiological processes ranged from2 to 140% body C day^–1 for ingestion, from 2 to 15% bodyC day^–1 for respiration, and from 0.1 to 5% body C day^–1for molting loss. All weight-specific rates showed a logarithmicdecrease with a logarithmic increase in body weight, and a logarithmicincrease with a linear increase in temperature below 20 or 25°C.The effect of temperature, however, was different between thephysiological rates, with a large temperature dependency foringestion (Q₁₀ = 2.6 –3.9) and molting loss (Q¹⁰ = 2.9– 3.6) and a moderate temperature dependency for respiration(Q₁₀ = 1.9 – 2.1). Calculated assimilation efficiencychanged with body size, but was constant over the temperaturerange examined. Allocation of assimilated materials varied witha change in temperature, reflecting the different temperaturedependence between physiological processes. It was deduced thatthe strong temperature dependency of the growth rate in N. intermediaobserved in the previous studies resulted from the large temperatureeffect on ingestion and assimilation rates, superimposed bythe different allocation of assimilated materials. ¹Present address: Department of Botany, University of Tokyo,Hongo, Tokyo 113, Japan     

Estimation of phosphorus metabolic rates of Daphnia galeata using a three-compartment model

A compartment model to estimate the different phosphorus metabolicrates in Daphnia galeata is presented. The model has three compartments:gut, metabolic pool and structural pool. Existing two-compartmentmodels used for carbon and phosphorus turnover in Daphnia donot allow estimation of ingestion and egestion rates. We extendedexisting two-compartment models with one more compartment, thegut, which allowed us to estimate both the ingestion and theegestion rates. Parameters of the model are estimated from asingle experiment of feeding unlabelled Daphnia with ³²P-labelledScenedesmus obliquus. Separate experiments with juvenile andadult daphnids were carried out in order to compare their metabolisms.This model permits a reliable estimation of the different metabolicrates of Daphrna in a single experiment and discriminates clearlybetween animals of different sizes.     

纺锤水蚤摄食生态学研究进展

纺锤水蚤(Acartia)是温带、亚热带近岸水域优势的小型桡足类,在区域生态系统的物质循环与能量流通中起着重要作用。综述了国内外对纺锤水蚤食性、摄食机制、摄食影响因子及摄食转化效率的研究工作:纺锤水蚤为杂食性,偏爱高营养的动物性饵料,摄食浮游植物和微型浮游动物时分别采取滤食策略和伏击策略,摄食过程受到自身生理状态和环境因子的共同调节。其对食物的利用效率约为60%,其他40%通过Sloppy feeding和排泄等方式以溶解形式释放到海水中。目前摄食研究多以实验模拟为主,自然现场研究手段有限,分子生物学技术有望促进认识其在自然生态系统中的地位与作用。     

Chlorophyll a conversion and gut passage time for the pelagic tunicate Oikopleura vanhoeffeni (Appendicularia)

We determined the chlorophyll (Chl) a conversion efficiency,defined as the conversion of Chi a into pheopigments and non-fluorescentproducts, for the cold-water appendicularian Oikopleura vanhoeffeniusing ⁶⁸Ge as conservative tracer. In both laboratory experimentsand in freshly collected animals, there was enough undegradedChi a present in the gut to provide an index of feeding activityfor individual animals. In laboratory experiments with diatoms,Chi a conversion efficiency was predictable with an averageof 79%. The mean gut passage time (0.8 h) determined using cornstarch and diatoms as markers was not influenced by food concentrationor animal size over a trunk length range of 1.8–5 mm.When experimentally determined Chi a conversion efficiency andgut passage times were applied to O.vanhoeffeni from Logy Bay(Newfoundland), the estimated clearance rates were in closeagreement with previously published values using several differenttechniques. We therefore suggest that the modified gut pigmenttechnique is a useful tool to assess in situ ingestion ratesof O.vanhoeffeni.     

Differential grazing by Acartia tonsa on a dinoflagellate and a tintinnid

The calanoid copepod, Acartia tonsa Dana, ingests both the dinoflagellate,Heterocapsa triquetra (Ehrenberg) Stein, and the tintinnid ciliate,Favella sp. In laboratory experiments its ingestion rate increaseswith increasing dinoflagellate density to a maximum at     

Pigment ingestion and egg production rates of the calanoid copepod Temora longicornisr. implications for gut pigment loss and omnivorous feeding

We examined the relationship between egg production rate (E)and pigment ingestion rate (I, from gut content corrected for33% loss) for adult female Temora longicornis in Long IslandSound on 47 occasions. Linear regression of E on I [both variablesexpressed in mass of nitrogen (N) female^–1 day^–1]was: E_N = 0.0016 + 0.770 x I_N. The slope, 0.77, is the apparentgross efficiency of egg production, equivalent to the grossgrowth efficiency (GGE) assuming that females partition allnitrogen for growth into egg production. Published work suggeststhat a GGE of 0.37 would be expected for herbivorous copepods.The discrepancy between the expected value of 0.37 and observedvalue of 0.77 could result from unquantified losses of gut pigmentor because T.longicomis ingested a significant amount of nitrogenby feeding as a carnivore. We suggest that if T.longicomis femalesderive all of their nitrogen for growth by feeding on phytoplankton,and if no correction for pigment loss is employed, then thegut pigment method underestimates pigment ingestion by no morethan a factor of two.     

Observations on the existence of lower threshold and upper critical food concentrations for the copepod Acartia tons a Dana

Acartia tonsa Dana represents a genus of temperate-tropical inshore copepods which, by virtue of its high biomass and rapid generation times, may be assumed to be an important planktonic primary consumer in terms of total production world-wide; its grazing response to different food concentrations is little known. Using wide ranges of concentrations of phytoplankton cultures, we have found that A. tonsa has a maximum grazing rate of ≈ 10.0 μg chl $\text{a}\text{1}$, decreasing to zero below 1.0 μg chl $\text{a}\text{1}$. This was confirmed using the more limited range of naturally-occurring particulate material. Although grazing rate became progressively reduced above 10 μg chl $\text{a}\text{1}$, ingestion rates continued to increase over the next order of magnitude of food concentration. It appears that A. tonsa is rarely exposed to food concentrations in its natural environment (as measured by conventional techniques) high enough to stimulate the maximum grazing effort. On the other hand, it is suggested that the continued increase of ingestion rates in the laboratory at much higher concentrations may indicate an adaptive mechanism associated with the encountering of ephemeral micro-patches of such concentrations, which would permit rapid filling of an empty gut in an energy-efficient manner. Fecal pellet production was also measured and shown to be a good indicator of ingestion rate.     

Diel variations in gut pigment content, diel vertical migration and estimates of grazing impact for copepods in the southern Benguela upwelling region in October 1987

The gut fluorescence technique was used to estimate ingestionand filtration rates of the adult female copepods Paracalanusparvus, Cenlropages brachiatus and Calanus austrails, and copepoditestages 3, 4 and 5 of C.australis in the southern Benguela upwellingregion. During the study period chlorophyll concentrations withinthe upper 20 m of the water column were high, 5 µg I^–1in mid-shelf waters and 15–30 µg I^–1 in innershelf waters. Copepod gut pigment content was low and constantduring the day then increased sharply during the first 2 h aftersunset. Gut pigment content was 2–6 times higher duringthe night compared with daytime values. Small non-migratingcopepods (Paracalanus parvus) showed the smallest diel differencein gut pigment content and large migrating copepods (Centropagesbrachiatus and Calanus australis) the largest difference. Eggproduction rates were 20 and 50% of maximum at the mid-shelfand inner shelf stations respectively, suggesting food-limitation.Comparison of ingestion rates calculated from egg productiondata with ingestion rates calculated from gut pigment data suggestedthat the copepods were feeding omnivorously at the inner shelfstations but herbivorously at the mid-shelf stations. Assumingthat all of the phytoplankton was available as food, the nearshorecopepod assemblage grazed {small tilde}1% of the standing cropeach day, and the mid-shelf assemblage grazed 5% day^–1.Because of errors and uncertainties associated with the gutfluorescence technique, the feeding impact could be underestimatedby 2–4-fold. We discuss several approaches which couldlead to more precise estimates of feeding rates. ³Present address: Marine Sciences, SUNY, Stony Brook, NY, 11794-5000,USA     

Seasonal feeding and fecundity of the calanoid copepod Acartia tonsa in Long Island Sound: is omnivory important to egg production?

Many suspension-feeding copepods show omnivorous feeding behavior. However, the relative contribution to egg production of herbivorous and heterotrophic feeding in copepods remains an open question. In this study, we quantified pigment ingestion rates and egg production rates of the planktonic calanoid copepod Acartia tonsa from July to November of 1986 in Long Island Sound, a large temperate estuary. Pigment ingestion and egg production rates were better correlated to the > 10-µm chlorophyll size fraction than total chlorophyll. Maximum pigment ingestion and egg production rates were observed during the fall bloom in September. Pigment ingestion and egg production rates were linearly related. However, pigment ingestion rates accounted for only 48% of the variance in egg production rates. If female A. tonsa had fed entirely as herbivores over the course of the season, the observed gross efficiency of egg production, K₁ = egg production ratelpigment ingestion rate, in terms of nitrogen, would have been 0.68. This growth efficiency is considerably higher than the expected value, K ₁ = 0.38, for this species based on laboratory studies of herbivorous feeding. We suggest that the ratio H = K ₁/K₁ is a measure of the fraction of egg production that is due to herbivorous feeding. Thus, we infer that herbivory accounted for 56% (0.38/0.68) of egg production in A. tonsa in this study. The ratio H is an useful tool in examining the relative contribution to egg production of herbivorous and heterotrophic feeding in copepods.     

Effects of prey motility and concentration on feeding in Acartia tonsa and Temora longicornis: the importance of feeding modes

Feeding experiments were conducted with the ambush-feeding copepodAcartia tonsa and the feeding-current-generating copepod Temoralongicornis. The copepods were offered a mixed diet of the dinoflagellateHeterocapsa triquetra and the ciliate Balanion comatum of similarcell size. The dinoflagellate was offered at a constant concentrationof 10–15 cells mL^–1, whereas the ciliate was offeredat a variety of concentrations, ranging from 7 to 57 cells mL^–1.Copepods with different feeding modes possess different mechanismsfor prey detection, suggesting that the two copepods would responddifferently to the two prey types. Both copepods had significantlyhigher clearance rates on the highly motile ciliate than onthe less motile dinoflagellate. In encounters between A. tonsaand its prey, we argue that this is due to the higher hydromechanicalsignal generated by the ciliate. The advection feeding copepodT. longicornis fed on the two prey according to their relativeconcentrations; in this case, we suggest that although B. comatumis capable of detecting feeding-current-generating predators,the feeding current velocity generated by T. longicornis isgreater than the escape velocity of this ciliate.     

Relationship between functional response and gut transit time in the calanoid copepod Acartia clausi: role of food quantity and quality

The relationship between ingestion rate and gut transit timeof the calanoid copepod Acartia clausi was examined in laboratoryexperiments with five different diets: (i) living cells of thediatom Thalassiosira weissflogii, (ii) detrital cells of thesame diatom, (iii) 50:50 mix of the two previous diets on aprotein basis, (iv) dinoflagellate cells of Prorocentrum micansand (v) Prorocentrum minimum. Ingestion followed a Holling type2 response for diets 1 and 4 and a linear one for diets 2, 3and 5. Gut transit time varied with food abundance only whenthe copepods were fed with the living diatom. The gut evacuationrate increased with the concentration of T. weissflogii withvalues of 0.010, 0.020, 0.032, 0.042 min^–1, correspondingto gut transit time of 97, 50, 31 and 24 min, measured at 50,110, 130 and 275 µg protein L^–1, respectively. Copepodsfed with dinoflagellates, mixed and pure detrital diets exhibitedlonger and similar gut transit times ranging from 85 to 166min, depending on diet. The coupling between ingestion rateand gut transit time measurements is discussed in the contextof copepod feeding strategies.     

The effects of food deprivation, prey density and volume on clearance rates and ingestion rates of Diacyclops thomasi

The ingestion rates of the copepod, Diacyclops thomasi, on thesoft-bodied rotifer, Synchaeta pectinata, increased 10-fold(0.07–0.77 Synchaeta h^–1) over the range 50–250prey l^–1. The saturating functional response curve appearedsigmoid but was statistically indistinguishable from a parabola.The response curve was more linear and 10 times lower over thesame range of density when Diacyclops was offered Kerarellacochleans, a species having a stiffened lorica. Diacyclops maximizedits ingestion rate on Synchaeta as a function of the availablegut space. Predation effort, measured as clearance rates, waslinked tentatively to changes in swimming speed of Diacyclopsand was a function of hunger level. Diacyclops, which were starvedfor varying periods of time, increased their ingestion rateson Synchaeta up to a maximum (-3.0 h^–1) after 7–10h of food deprivation. The gut passage time of Diacyclops wasestimated to be 7–8 h. Therefore, ingestion rates (andclearance rates) appeared to be strongly correlated to the volumeof food in the gut.     

The escape behavior of marine copepods in response to a quantifiable fluid mechanical disturbance

The threshold shear values needed to elicit the escape reactionto a quantifiable fluid mechanical disturbance were comparedbetween five free-swimming oceanic copepod species. The resultsindicate a significant difference in the threshold for differentspecies of copepods and between different age groups withina single species. In general, animals captured from more energeticregimes required a higher threshold than those captured frommore pacific locations. Labidocera madurae required the highestshear values with 51.5 s^–1 for 50% of the animals testedto elicit an escape reaction (S₅₀). Acartia tonsa and Euchaetarimana, in contrast, were behaviorally the most sensitive requiringan S₅₀ of only 1.5 and 4.1 s^–1, respectively, to initiatean escape reaction. Pleuromamma xiphias and Oithona requiredintermediate shear values with an S₅₀ of 7.2 and 8.1 s^–1.When compared to literature values, the threshold needed toelicit an escape reaction was consistently higher than averageenvironmental shear values. Threshold shear values also variedsignificantly with developmental stage. Naupliar stages of A.tonsarequired greater than six times the S₅₀ value required by adultsof the same species. This suggests that the higher vulnerabilityto predation of naupliar stages of copepods may not only reflectinferior escape strength, but may also result from the higherthreshold needed to elicit an escape reaction. This study supportsthe hypothesis that selective feeding patterns exhibited bypredators of copepods may be the result of the differentialbehavioral sensitivities of different species and developmentalstages of copepods.