Food ingestion by the marine planktonic copepod Paracalanus in relation to abundance and size distribution of food

Juveniles and adult females were presented a food spectrum of three algae of different sizes (4.5, 12 and 20 m cell width). The increase in rate of ingestion of the medium-sized alga with an increase in copepod size was significantly greater than the increase in rate of ingestion of the small alga. It is hypothesized that the perception of chemical signals from the small alga by a copepod decreases as the copepod moults from stage to stage. The rate of ingestion of the large alga by copepod stage V (CV) and adult females was lower than the rate of ingestion of the medium-sized alga at mid- and high phytoplankton concentrations. The amount of nitrogen ingested when the medium-sized alga alone was offered was either higher than (stage C II) or not significantly different from that when the three algae were offered together (stage C IV). Ingestion rates are reduced when there is a multialgal food source. This implies that there is increased stability in the ocean because multiparticle food sources are more slowly depleted than unialgal foods. Weight-specific ingestion rates of copepods fed the three algae simultaneously increased from nauplius to stage C III and then decreased as adulthood approached. The contribution of the small alga to the total amount of nitrogen ingesied was greatest for naupliar stages while the contribution of the medium-sized cells was greater for later stages. The largest alga was readily ingested by stage C V and adult females but never contributed more than 25% of the nitrogen ingested. Eight to 12% of the nitrogen ingested by adult females was from the small alga. It is hypothesized that the algal cell size for maximum nitrogen ingestion in upwelled waters is relatively small, round or square and close to the size threshold below which adult females do not sense individual cells.     

Impact of copepod grazing on the red-tide flagellate Chattonella antiqua

Although planktonic copepods are major suspension feeders in the sea, the impact of their grazing pressure upon red-tide flagellates has not been fully investigated. In the present study, the grazing of adult females of several copepod species is examined using three food types: viz. natural suspended particles, natural suspended particles mixed with cultured Chattonella antiqua, and cultured C. antiqua. The functional response on C. antiqua was investigated for five species of copepods (Acartia erythraea, Calanus sinicus, Centropages yamadai, Paracalanus parvus and Pseudodiaptomus marinus). Ingestion rates increased linearly with increasing cell concentrations until a maximum level was reached, beyond which the rates were constant. This cell concentration was higher for larger copepods. The weight-specific maximum ingestion rates were higher in the small species. In general, copepods tended to feed selectively on larger particles when feeding on natural particles. This tendency was strongest in a simulated red-tide environment. Thus, it can be surmised that copepods may selectively graze on C. antiqua during the outbreak of a red tide. Grazing pressure by the natural copepod community in Harima Nada, the Inland Sea of Japan, was calculated by integration of the laboratory determined feeding rates and field measurements of zooplankton biomass. The daily removal rate was 3.4 to 30.8% (mean: 12.3%) of C. antiqua biomass at 20 cells ml^-1 and decreased to 0.6–4.3% (mean: 1.8%) at 500 cells ml^-1. Therefore, the grazing pressure by the copepod community is important at the initial stage of the red tide.     

Effects of feeding experience in the copepod Eucalanus pileatus: a cinematographic study

High-speed microcinematography was used to examine the effects of prior experience with particular cell types on the feeding efficiency of a calanoid copepod. Female Eucalanus pileatus were fed monocultures of either the 5-m diatom Thalassiosira pseudonana or the 11-m diatom T. weissflogii during a 2-to 3-d preconditioning period. The smaller diatoms are accumulated passively by the second maxillae while the larger diatoms are detected and actively captured as individual cells. Four females from each preconditioning culture were transferred to a monoculture of the large cells and their behavior filmed at five intervals over a 24-h period to determine whether a loss of efficiency occurs when the copepods must shift capture modes. Ingestion rates for females experienced with the larger cells were approximately 2.5 times higher than those of inexperienced females. Six sequential behavioral steps in the feeding process could alter ingestion rates: (1) amount of time spent flapping the feeding appendages. (2) rate of flapping of the feeding appendages, (3) ability to detect individual cells, (4) success rate of capture attempts, (5) capture and handling time per cell and (6) rejection rate of captured cells. An increased ability to detect cells and a decreased rejection rate contributed significantly to the higher ingestion rate of experienced feeders, indicating that copepods have the ability to learn during the feeding process. Grazing rates may be seriously underestimated in experiments which do not include a preconditioning period, especially those which calculate ingestion over short time intervals. Such effects may also influence the feeding of copepods in the field when encountering changes in particle spectra through vertical migration or horizontal displacement.     

Long-term food modification by Acartia clausi: A preliminary view

After acclimation, the copepod Acartia clausi was allowed to graze for 5 days in a mixed suspension of two discrete size classes (species) of the chaining diatoms Thalassiosira spp. derived from continuous culture. Total particle numbers and particle size distributions of Thalassiosira spp. were stable throughout the 5 days, indicating that the effects of algal removal and modification due to grazing were balanced by algal growth. Grazer ingestion is the predominant process affecting all size classes of the smaller diatom population (T. nordenskioldii); however, both ingestion and chain modification are observed with the larger diatom (T. gravida). Although the greatest percentage removal of algal volume occurs in the largest algal size classes for each algal species, the greatest volume removal occurs at the modal peak (T. nordenskioldii) or just to the right of the modal peak (T. gravida). Flask-to-flask replicability of experiments was tested with the single-celled T. fluviatilis, and these tests were compared to the long-term experiments with T. nordenskioldii and T. gravida. Net particle removal occurs on both the large and small algal species in mixed suspension, not just on the larger-cell-side of the T. gravida distribution. Although 80% of the particles ingested are from the T. nordenskioldii distribution, 80% of the volume ingested is from the T. gravida distribution. The apparent multi-peak selection observed in our data is discussed in reference to two separate hypotheses and in light of other recent work pertaining to selective grazing by copepods.     

Ingestion of natural particulate organic matter and subsequent assimilation,respiration and growth by tropical lagoon zooplankton

Rates of ingestion of natural particulate organic matter and subsequent assimilation and respiration by zooplankton at Enewetak Atoll lagoon (Marshall Islands) were measured using a flow-through system. Maximum daily ingestion rates of carbon and nitrogen, expressed as a percentage of the body content, were 79 and 37%, respectively, for the large copepod Undinula vulgaris; 112 and 65%, respectively, for a group of mixed small copepods; and 61 and 34%, respectively, for the pteropod Creseis acicula. Daily metabolic carbon losses, expressed as above, were 63% for U. vulgaris, 88% for the small copepods, and 50% for C. acicula. Assimilation efficiences of carbon and nitrogen ranged from about 86 to 91%. The above rates are generally higher than in previous reports for similar sized zooplankton in temperate waters, while the daily growth increments, expressed as a percentage of the body carbon content (4.8% for U. vulgaris, 8.6% for the small copepods, and 2.6% for C. acicula), are comparable. It appears that the high rates of ingestion and assimilation of organic matter are compensated by high metabolic losses. These results indicate that at least for carbon, tropical zooplankton may have low growth efficiencies ranging from 4 to 9%.     

Grazing by adult estuarine calanoid copepods of the Chesapeake Bay

Grazing by adult female Eurytemora affinis, Acartia tonsa and A. clausi on natural distributions of particles from the Chesapeake Bay has been investigated. During the course of a year's sampling, a wide variety of particle size-biomass distributions were observed as seasonal shifts in detritus, and over 150 algal species occurred. These distributions were grouped into 5 basic types in the analyses of feeding. All three species demonstrated similar capabilities for feeding over a broad range of particle size with selection (higher filtering rates) on larger particles and on biomass peaks. Feeding on multiple-peak distributions resulted in strong selection or tracking of each biomass peak with reduced filtering rates between peaks. Evidence is presented which suggests that the copepods first feed on large particles and then successively switch to biomass peaks of the smaller size categories. Comparisons of the feeding behavior of Eurytemora affinis and the Acartia species showing that the Acartia species have greater capabilities for taking large particles may be associated with modifications of their mouth parts for raptorial feeding. The results suggest considerable flexibility in copepod feeding behavior which cannot be explained solely by the mechanism of a fixed sieve.     

Selective particle ingestion by oyster larvae (Crassostrea virginica) feeding on natural seston and cultured algae

I investigated selective particle ingestion by oyster larvae (Crassostrea virginica) feeding on natural seston from Chesapeake Bay and laboratory-cultured algae of different sizes or chemical content. In 15 of 16 experiments with complex natural suspensions as food, small(<150 m) and large (>150 m) larvae selected most strongly for small (2 to 4 m) food particles, but in the presence of a large (>10 m)-cell dinoflagellate bloom, large larvae strongly selected much larger (22 to 30 m) food material (presumably dinoflagellates). When fed simplified mixtures of four cultured algal species (Synechococcus bacillaris, Isochrysis sp., Dunaliella tertiolecta, and Prorocentrum minimum) ranging in size from 1 to 11 m, small larvae preferred 1 m algae while large larvae preferred 11 m algae. In experiments with algal mixtures, and with suspensions of natural particles and added algae, large larvae preferred algal species harvested from exponential-phase cultures over other species from stationary-phase cultures. Larval ingestion rates of the cultured alga Thalassiosira pseudonana were about three times higher for cells with a low carbon:nitrogen ratio (7.2:1) than for high C:N ratio (16.2:1) cells when these cells were offered separately in suspensions of equal concentration. As a result, more algal cells, algal C, and algal N was ingested by larvae fed low C:N cells. However, larvae did not show a significant preference for either type of cell when they were offered in a 1:1 cell mixture. Feeding patterns of C. virginica larvae in natural food suspensions can vary with the composition of these complex suspensions, and ingestion seems dependent not only on the size, but on the growth rate and chemical quality of food particles.     

Omnivorous feeding behavior of the Antarctic krill Euphausia superba

Feeding experiments were conducted at Palmer Station from December 1985 to February 1986 to examine the potential role of copepod prey as an alternative food source for Euphausia superba. Copepod concentration, copepod size, phytoplankton concentration, the duration of krill starvation and the volume of experimental vessels were altered to determine effects on ingestion and clearance rates. Krill allowed to feed on phytoplankton and copepods in 50-litre tubs showed greatly increased feeding rates relative to animals feeding in the much smaller volumes of water traditionally used for krill-feeding studies. Clearance rates on copepods remained constant over the range of concentrations offered, but clearance rates on phytoplankton increased linearly with phytoplankton concentration. Feeding rates increased when larger copepods were offered and when krill were starved for two weeks prior to experiments. Clearance rates of krill feeding on copepods were higher than, but not correlated with, their clearance rates on phytoplankton in the same vessel. E. superba may have a distinct mechanism for capturing copepods, perhaps through mechanoreception. Although our observed clearance rate of 1055 ml krill^-1 h^-1 indicates that krill can feed very efficiently on copepod prey, such feeding would meet less than 10% of their minimum metabolic requirements at the typical copepod concentrations reported for Antarctic waters. However, substantial energy could be gained if krill fed on the patches of high copepod concentrations occasionally reported during the austral summer, or if krill and copepods were concentrated beneath the sea ice during the winter or spring months. Our results, indicating efficient feeding on zooplankton and higher clearance rates on phytoplankton than previously believed, represent a step towards balancing the energy budget of E. superba in Antarctic waters.     

Nitrate increases zooxanthellae population density and reduces skeletogenesis in corals

Very little information exists on the effects of nitrate on corals, although this is the major form in which nitrogen is prescrit in tropical eutrophie coastal waters. In this study we incubated nubbins ofPorites porites and explants ofMontastrea annularis in laboratory photostats illuminated by halide lamps, with concentrations of nitrate of 0, 1, 5 and 20 M, for 40 and 30 d, respectively, At the end of this period it was found that the population density of the zooxanthellae had increased significantly with increased nitrate concentration, suggesting nitrogen limitation of the growth rate of zooxanthellae in the control group. There were also significant increases in the amount of chlorophylla ande ₂ per algal cell, in the volume of the algal cells, and in the protein per cell. Overall, the protein per unit surface increased, but this was attributable solely to increased algal protein: there was no significant change in host protein. Maximum gross photosynthesis normalized to surface area was enhanced by nitrate addition, reflecting the increase in algal population density. There was no change when normalized on a per cell basis. Respiration rate normalized to protein content was decreased by nitrate. The most dramatic change was in the rate of skeletogenesis, which decreased by in both species when exposed to nitrate enrichment. A model is presented which suggests that the diffusion-limited supply of CO₂ from surrounding seawater is used preferentially by the enlarged zooxanthellae population for Photosynthesis, thereby reducing the availability of inorganic carbon for calcification. It is concluded that enhanced nitrate levels in tropical coastal waters will have a hitherto unrecognized effect on the growth rate of tropical coral reefs.     

Comparison of four methods to estimate meiobenthic copepod Amonardia normani ingestion rates

Four different methods were used in the control conditions of laboratory to estimate the ingestion rate of a female meiobenthic harpacticoid copepod Amonardia normani: (1) reduction of algal biomass, (2) the quantification of total pigments in fecal pellets, (3) the gut fluorescence method, (4) the percentage of assimilation and the total egestion rate. The food used during all experiments was the diatom Nitzschia constricta in an axenic condition at the concentration of 0.13 μg Chl-a mL^?1 at stationary growth phase. All experiments were made at 20 °C and 30 salinity. All tested methods excepted the quantification of total pigments in fecal pellets resulted in similar estimatives. The gut fluorescence method indicated that during the day gut contents are smaller than during the night but the gut passage time was faster, resulting in similar ingestion rates during the day and the night. The reduction of algal biomass and the percentage of assimilation and the total egestion rate also indicated similar ingestion rates in the day and in the night. The daily ingestion rate represents 107 % of female carbon weight per day (903 ng C cop^?1day^?1).     

Characteristics of feeding on dinoflagellates by newly hatched larval crabs

The zoeal larvae of brachyuran crabs must feed soon after hatching on a diet that includes large micro- and mesozooplankton in order to satisfy nutritional requirements. However, newly hatched larvae have been shown to ingest a variety of dinoflagellates, perhaps using microbial carbon sources to sustain them until they encounter more favored prey. Ingestion of dinoflagellates by larval crabs has been documented previously under conditions in which the larvae were exposed to algae provided in monoculture or in defined mixtures of cells. We report here on experiments conducted on the hatching stage of five crab species to determine if ingestion of dinoflagellates occurred when they were provided in combination with Artemia sp. nauplii or after a period of feeding on mesozooplankton. Quantitative measurements of chl a in the larval guts provided evidence of ingestion of algal cells. Active ingestion of the dinoflagellate Prorocentrum micans at specified intervals during an extended feeding period was determined on larvae of two crab species using fluorescently labeled cells provided for brief periods at prescribed time intervals. Stage 1 larvae of four of the five crab species ingested dinoflagellates when they were provided in combination with nauplii and larvae of all five species ingested cells after feeding solely on nauplii for 24 h. Ingestion of algal cells was first evident in the larval guts after 6 h of feeding at both low (200 cell ml⁻¹) and high (1,000 cells ml⁻¹) prey densities. Higher prey densities resulted in higher gut chl a. Larvae continuously exposed to dinoflagellates actively ingested cells at every 3 h interval tested over a 36 h period. Results confirm previous studies that larvae will ingest dinoflagellates even when they are encountered in a mixed prey field or when having previously fed. Ingestion of cells may occur on a continual basis over time.     

Biologie d'un copépode des mares temporaires du littoral méditerranéen français: Eurytemora velox

The feeding activity of Eurytemora velox, a brackish copepod from temporary lakes of the south of France, was studied in 1978–1979 using various foods (natural particles, monospecific algal cultures, and artificial food) under different conditions of temperature and salinity. Experiments with Amphidinium sp. or Tetraselmis maculata as food showed that the ingestion rate increased with food concentration according to an asymptotic or a linear relationship. Although of slightly smaller size, T. maculata was ingested at a higher rate than Amphidinium sp. Large maximum daily rations (up to 150% of body carbon with Amphidinium sp. and up to 250% with T. maculata) were attained. These values, which greatly exceed those generally obtained with marine copepods, could result from adaptation of the feeding processes of this copepod to its very rich trophic environment. A significant correlation was demonstrated between ingestion rate and fecal pellet production using T. maculata as food. Therefore, daily fecal production was used as an index of feeding activity in experiments carried out with natural food, T. maculata cultures and artifical food (Tetramin). Increased temperature generally resulted in an activation of grazing and filtration rates and of fecal production at low temperatures (10° to 15°C), but a strong decrease was observed over 22°C. Differences of 10 S over or under the natural salinity level led to a decrease in fecal production, suggesting unachieved acclimatization to salinity variation due to a too short acclimation period before the experiments. Fecal pellet production was higher during the day than during the night. It depended also on the quality of food used: high values were obtained with T. maculata, Phaeodactylum tricornutum, Rhodomonas sp. and Chlamydomonas sp., low values with Chlorella sp. and Amphidinium sp., and medium values with natural food material. The assimilation rate (A) was calculated by Conover's methods. A significant negative correlation was obtained between A and the ash content of the food. High assimilation rates were attained with chlorophycean algae, while natural particulate food produced variable assimilation rates, depending on the amount of inorganic material present.

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Biologie d'un copépode des mares temporaires du littoral méditerranéen français: Eurytemora velox

     

Zooplankton grazing on the coccolithophore Emiliania huxleyi and its role in inorganic carbon flux

Grazing and faecal pellet production by the copepods Calanus helgolandicus and Pseudocalanus elongatus, feeding on the coccolithophore Emiliania huxleyi, were measured under defined laboratory conditions, together with the chemical characteristics and sinking rates of the faecal pellets produced. Ingestion rates of both copepods were equivalent at comparable cell concentrations, the relationship between ingestion rate (I, cells copepod^-1 h^-1) and food concentration (C, cells ml^-1), being I=0.558C for both species. P. elongatus produced a larger number of smaller faecal pellets than C. helgolandicus, but egested a larger volume of material per individual. Only between 27 and 50% of the ingested coccolith calcite was egested in the faecal pellets, and it is possible that acid digestion in the copepod gut is responsible for these considerable losses. Average sinking rates of faecal pellets containing E. huxleyi coccoliths, produced by both species, were >100 m d^-1. The implications of the quantitative laboratory estimates for the vertical flux of inorganic carbon are considered using recently studied shelf-break and oceanic E. huxleyi blooms in the N. E. Atlantic as examples.     

Nutritional effect of five species of marine algae on the growth,development, and survival of the brine shrimp Artemia salina

Five species of unicellular algae of the same age, cultured bacteria-free under standard growth conditions, were analyzed for chemical composition and fed to different size classes of Artemia salina. The green algae Chlamydomonas sphagnicolo, Dunaliella viridis, Platymonas elliptica and Chlorella conductrix had significantly higher percentages of protein and lipid than did the diatom Nitzschia closterium. Total ash value was highest in populations of N. closterium. Shrimp fed Chlamydomonas sphagnicolo cells assimilated highest percentages of organic matter, while those fed Chlorella conductrix had lowest assimilation rate. Respiration rates were inversely proportional to animal size (weight) and algal cell volume. Growth, survival, rate of sexual maturtion, and sex ratio were dependent on the growth and assimilation efficiencies obtained from each respective algal food. Shrimp fed Chlamydomonas sphagnicolo, D. viridis, or P. elliptica cells displayed highest growth and assimilation efficiencies.     

Potential effect of phytoplankton colony breakage on the calculation of zooplankton filtration rates

Substantial net production of short chains of the diatom Skeletonema costatum occurred in grazing experiments conducted with Acartia hudsonica (=A. clausi). A general model was then constructed to evaluate the influence of colony breakage during grazing on the calculation of zooplankton filtration and ingestion rates. Breakage can lead to either over-or underestimation of these rates. The magnitude of error is related to breakage probability, particle size distribution, size of the largest colonies, length of the experiment, copepod concentration and actual filtration rates. Size distributions similar to those described in the literature as manifestations of a complex selection for size and particle abundance can be generated by the grazing model under conditions of a uniform filtration rate accompanied by colony breakage. Unless the rate of breakage exceeds the rate of removal and causes net particle production, the occurrence of breakage may go undetected.     

Effects of Olisthodiscus luteus on the growth and abundance of Tintinnids

The effects of the red tide flagellate Olisthodiscus luteus Carter on the growth of two tintinnid species, Tintinnopsis lubulosoides Meunier and Favella sp. (Clap. & Lach.) Jorg., were measured in batch culture. T. tubulosoides and Favella sp. grew at rates equivalent to 1.2 (10°C) and 2.0 (20°C) population doublings per day, respectively, when offered nutritionally adequate phytoplankton species. The growth rates of both tintinnid species were reduced in the presence of 10²–10³ O. luteus cells · ml^-1 in multialgal treatments. Growth rate inhibition was proportionately greater at higher O. luteus densities. Lethal effects were observed for both tintinnid species at O. luteus concentrations of 5x10³ cells · ml^-1 in multi-algal treatments. T. tubulosoides mortality occurred at all O. luteus concentrations in unialgal culture. O. luteus-conditioned medium did not substantially inhibit tintinnid growth when combined with acceptable food species, suggesting that toxicity is induced by ingestion or direct contact with O. luteus cells, or by exposure to a short-lived exudate. In agreement with these results, an inverse relationship between O. luteus concentration and tintinnid abundance was observed in Narragansett Bay, Rhode Island, over a two year period. The small lorica diameter of the species apparently inhibited by these O. luteus blooms suggests a detrimental effect independent of cell ingestion. In addition to the absolute concentration of O. luteus cells, the availability of nutritionally adequate algal food may be an important factor determining the impact of O. luteus blooms on tintinnid populations.Contribution no. 5048 from the Woods Hole Oceanographic Institution     

Electrophoretic survey of genetic variation in Pecten maximus,Chlamys opercularis,C. varia and C. distorta from the Irish Sea

Food selection by young larvae of the gulf menhaden (Brevoortia patronus) was studied in the laboratory at Beaufort, North Carolina (USA) in 1982 and 1983; this species is especially interesting, since the larvae began feeding on phytoplankton as well as microzooplankton. When dinoflagellates (Prorocentrum micans), tintinnids (Favella sp.), and N1 nauplii of a copepod (Acartia tonsa) were presented to laboratory-reared, larval menhaden (3.9 to 4.2 mm notochord length), the fish larvae ate dinoflagellates and tintinnids, but not copepod nauplii. Larvae showed significant (P<0.001) selection for the tintinnids. Given the same mixture of food items, larger larvae (6.4 mm notochord length) ate copepod nauplii as well as the other food organisms. These feeding responses are consistent with larval feeding in the northern Gulf of Mexico, where gulf menhaden larvae between 3 and 5 mm in notochord length frequently ate large numbers of dinoflagellates (mostly P. micans and P. compressum) and tintinnids (mostly Favella sp.), but did not eat copepod nauplii. As larvae grew, copepod nauplii and other food organisms became important, while dinoflagellates and tintinnids became relatively less important in the diet. Since the tintinnids and nauplii used in the laboratory feeding experiments were similar in size as well as carbon and nitrogen contents, the feeding selectivity and dietary ontogeny that we observed were likely due to a combination of prey capturability and larval fish maturation and learning.Contribution No. 5575 of the Woods Hole Oceanographic Institution     

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.     

Carbon and nitrogen metabolism by Monochrysis lutheri: Measurement of growth-rate-dependent respiration rates

Dark respiration rates were measured and carbon-excretion rates calculated for a nitrate-limited population of the marine chrysophyte Monochrysis lutheri grown in continuous culture at 20°C on a 12 h light-12 h dark cycle of illumination and over a series of 4 growth rates. A significant (P<0.05) positive correlation was found between dark respiration rate and growth rate. From a simple linear fit to the data, the respiration rate at maximum growth rate was estimated to be roughly 10.5% of the maximum gross-carbon-production rate, and more than three times higher than the extrapolated respiration rate at zero net-growth rate. Carbon-excretion rates showed no significant correlation with growth rate, and averaged less than 5% of the maximum gross-carbon-production rate. Mean cell nitrogen to carbon ratios were correlated in a virtually linear manner (r=0.994) with growth rate, and at a given growth rate were consistently higher than nitrogen to carbon ratios for the same species grown on continuous light. A comparison of carbon and nitrogen quotas as a function of growth rate for M. lutheri and other species suggests that the increase of cellular nitrogen at high growth rates under nitrate-limited growth conditions may be associated with the storage of cellular protein or amino acids rather than the presence of an inorganic nitrogen reservoir. The maximum nitrate uptake rate per cell during the day changed very little over the range of growth rates studied, and was comparable to the maximum uptake rate found for cells grown on continuous light. However, the cell nitrogen quota increased steadily with growth rate, causing a reduction in the maximum specific-uptake rate of nitrate during the day at high growth rates. The dark nitrate-uptake capacity of the population was clearly exceeded by the supply rate at the two higher growth rates, leading to a buildup of nitrate during the night which amounted to as much as 21% of the particulate nitrogen in the growth chamber by morning.Hawaii Institute of Marine Biology Contribution No. 478.     

Predatory feeding of two marine mysids

Predatory feeding of the marine mysids Mysidopsis bigelowi and Neomysis americana on several species of co-occurring copepods was examined in laboratory experiments. M. bigelowi exhibited a curvilinear functional response; there was a negative logarithmic relationship between prey density and clearance rates. N. americana also exhibited higher clearance rates at lower prey densities. Increased clearance rates at lower prey densities were probably due to increased swimming speed or reaction distance as hunger increased. This response occurred only when mysids could visually locate prey; in complete darkness clearance rates were significantly lower and independent of prey density. Feeding rates on different prey species were only partially dependent on prey size; prey movement patterns and escape behavior also strongly affected feeding rates. M. bigelowi showed active prey selection when offered a choice of different prey species. Estimates of predation rates of estuarine mysid populations indicate that they could have a significant effect on co-occurring copepod populations.