The marine copepod Acartia hudsonica was shown to be adapted to dinoflagellate prey, Alexandrium ... more The marine copepod Acartia hudsonica was shown to be adapted to dinoflagellate prey, Alexandrium fundyense, which produce paralytic shellfish toxins (PST). Adaptation to PSTs in other organisms is caused by a mutation in the sodium channel. Recently, a mutation in the sodium channel in A. hudsonica was found. In this study, we rigorously tested for advantages, costs, and trade-offs associated with the mutant isoform of A. hudsonica under toxic and non-toxic conditions. We combined fitness with wild-type: mutant isoform ratio measurements on the same individual copepod to test our hypotheses. All A. hudsonica copepods express both the wild-type and mutant sodium channel isoforms, but in different proportions; some individuals express predominantly mutant (PMI) or wild-type isoforms (PWI), while most individuals express relatively equal amounts of each (EI). There was no consistent pattern of improved performance as a function of toxin dose for egg production rate (EPR), ingestion rat...
production on fertilization was evident in the field. During 2 years of weekly collections, at no... more production on fertilization was evident in the field. During 2 years of weekly collections, at no point in time were all females in the population fertilized. We conclude that low rates of spermatophore production over a lifetime and the short reproductive period of males contribute to the low frequencies of mated females in field populations.
Deep-sea Research Part Ii-topical Studies in Oceanography, 1995
A study was undertaken to evaluate the role of exocellular polysaccharides in the flocculation of... more A study was undertaken to evaluate the role of exocellular polysaccharides in the flocculation of a marine diatom bloom in a large tank mesocosm. Surface-active organic matter was extracted from 1.0 μ-filtered tank water by bubble adsorption each day for 7 days of the experiment. In agreement with past studies, particles (3–51 urn equivalent spherical diameter) were readily formed by
The relationship between the rates of ingestion and defecation of the calanoid copepod Acartia to... more The relationship between the rates of ingestion and defecation of the calanoid copepod Acartia tonsa was examined in laboratory experiments with different diets: the diatom Thalassiosira weissflogii, the autotrophic dinoflagellate Prorocentrum minimum, the heterotrophic dinoflagellate Oxyrrhis sp., the flagellate Dunaliella tertiolecta, and the bacterivorous scuticociliate Uronema sp. Experiments were run typically using between 6 and 8 concentrations ranging from 10 to 1150 µg C l -1 . Both diet and food concentration had significant effects on ingestion rate, pellet production rates and pellet volume. Ingestion and pellet production rates increased curvilinearly with food concentration. Copepods achieved the highest pellet production rates, the largest pellets and the shortest estimated gut passage times with the diatom diet. For all diets, gut passage time decreased with food concentration in the range 0 to ca. 250 µg C l -1 , but little thereafter. In addition, for all diets, but most obviously in the case of the diatom, pellet volume increased curvilinearly with food concentration. Maximum pellet volume was achieved at food concentrations ranging from 50 to 150 µg C l -1 . The defecation and ingestion rates were best correlated when the copepods fed on the diatom and the autotrophic dinoflagellate. Likewise, a decrease in estimated assimilation efficiency with increasing food concentration was apparent only for these 2 diets. This latter pattern is primarily due to an increase in pellet production rate, instead of pellet volume, relative to ingestion rate at the high food concentrations. We conclude that a diatom diet is the most likely one to result in attributes that enhance zooplankton-mediated export flux: high fecal pellet production rates, large pellet size and relatively low assimilation efficiency.
... duration and frequency) Buoyancy and sediment sources (magnitude and distribution) Tidal rang... more ... duration and frequency) Buoyancy and sediment sources (magnitude and distribution) Tidal range Wave ... moorings with meteorological and water column measurements of temperature, salinity and velocity at ... Lower: An example of the mesoscale variability in the SST field within ...
To test the hypothesis that the sex ratio of the copepod Acartia tonsa is biased at birth, adult ... more To test the hypothesis that the sex ratio of the copepod Acartia tonsa is biased at birth, adult sex ratios of the offspring of families (21 families in 2011 and 72 in 2013) produced by field-caught females were determined in the laboratory under controlled conditions. Sex ratios at birth were estimated from the adult sex ratio, and after applying a correction in which all dead/missing individuals were counted as the rarer sex (males). Before correction the overall population sex ratio was female biased during both years, with over 42% of mothers producing clutches that were significantly different from unity. After correction, 25% to 33% (2011) of mothers still produced significantly biased sex ratios. The ratio of deaths of males to females had to be 1.75 to 1 (2011) and 7.25 to 1 (2013) for sex ratios not to be biased at birth. There was no evidence of sex reversal in any individual from juvenile (C4 stage) to adult, implying that juvenile sex ratios were also significantly biased. Hence, neither sex reversal nor differential mortality of the sexes is a likely explanation for the observed skewed sex ratios. The sex ratios at birth varied widely from the expectation from Mendelian inheritance of sex chromosomes, suggesting that another mechanism is responsible for skewed sex ratios at birth. Since the majority of families with biased sex ratios at birth were female dominated as adults, biased sex ratio at birth may contribute to the skewed sex ratio of adults observed in the field.
Mating in many copepod species is not random; moreover, individuals have preferences for certain ... more Mating in many copepod species is not random; moreover, individuals have preferences for certain qualities in their mates. This study tested whether female mating status (virgin versus mated) affects mate choice in males in the copepod species Acartia tonsa and Acartia hudsonica, both in laboratory and field populations. In addition, the costs and benefits associated with mating twice versus once for females of both species were tested in laboratory studies. In both A. tonsa and Acartia hudsonica, males mated more frequently with virgin females compared with females that had already mated, suggesting that female reproductive status is important for mating and mate choice. Frequencies of females carrying double spermatophores in the field (at three sites and seasonally within a site) were consistently low, implying that mate choice for female reproductive status is present in natural populations as well. In Acartia hudsonica, the frequency of females carrying double spermatophores was positively related to spermatophore volume and male and female sizes, but negatively related to temperature. No such relationships were found for A. tonsa. However, the frequency of females carrying double spermatophores decreased as the season progressed. In both copepod species, there was no difference in the number of nauplii produced, length of fertilization or lifespan between females that had mated twice compared with those that had mated only once. Hence, there was no particular cost or benefit to mating a second time. Our findings suggest that female reproductive status is important for mating success in these two coastal copepod species and that this is most likely due to male mate-choice.
In this study we hypothesized that the sexes of the copepod Acartia hudsonica experience differen... more In this study we hypothesized that the sexes of the copepod Acartia hudsonica experience different mortalities when exposed to the toxic dinoflagellate Alexandrium fundyense. In laboratory experiments, we manipulated toxin dose and measured sex-specific mortality. We also used a split-family design to determine whether there were genetic influences on sex determination, and therefore on sex ratio. When the dose of the toxic alga was high (25% of diet by carbon content), population mortality was high, and the fraction of males surviving to adulthood was low (20-30% of the adult population). No such effect was observed when A. fundyense comprised only 10% of the diet. Adult male copepods were also more susceptible to the toxic alga, though prior selection for resistance to toxins also had an effect on mortality. No genetic variation for sex ratio was detected in the split-family experiment. Males of A. hudsonica are more sensitive to the toxins of A. fundyense than are females. Resistance may be sex-linked, and copepod populations exposed to a bloom of toxic Alexandrium sp. may experience shifts in sex ratios away from males and toward females. Differential mortality and skewed sex ratios are feedback mechanisms potentially affecting both the population dynamics of grazers and the development of toxic algal blooms.
D~atorn aggregation can increase the sedimentdtion rate of the Intense pulses of carbon formed du... more D~atorn aggregation can increase the sedimentdtion rate of the Intense pulses of carbon formed durlng d~a t o m blooms. Laboratory studies of aggregation dynarn~cs have been troubled by the unavoidable d~sruption of aggregates in laboratory flocculators due to subsampl~ng and electronic part~cle countlng Aggregate disruption prcvcnts accurate measurement of aggregation rate and makes observat~on of part~cle structure impossible. This led us to develop a new non-disruptive method to quantify diatom aggregation using high-resolut~on video and image analysis. The system easily resol\~ed cells of Chaetoceros neogracile 6 to 8 pm in diameter, and facilitated observations of aggregate morphology. C. neogracile made either ball-like or net-like aggregates depending on culture denslty and physiological state. Only the net-llke aggregates reached a substantial slze ( > l mm). A Silimlted batch culture expenment indicated that stickiness of C. neogracile increased significantly with increas~ng severity of nutrient limitation. T h~s increase was not detectable by the subsampling and electronic particle counting method. It is likely that spine formation, cell-surface bound sugars, and transparent exopolymer particles all played d role in determining aggregate structure and changes in stickiness. The stickiness calculation model assumes cultures are initially suspensions of single cells which form doublets as they aggregate. This assumption was tested by comparing mean particle size in a culture with the percentage of cells in aggregates a s determined using nearest neighbor distances, allowing us to assess the mean number of cells per aggregate. C. neograc~le commonly formed aggregates contaming anywhere between 2 and 10 cells showing that model assumptions can be violated under normal experimental conditions. A larger field of view was necessary to estimate the larger particle sizes typlcal of coastal diatom blooms. K E Y WORDS Aggregation Adhesion . Video Image analysis Dlatom Chaetoceros neogracile O Inter-Research 1997 Resale of full article not permitted Mar Emi Prog Ser 155: 77-88, 1997 van Du uren FA (1 968) Defined veloclty grad~ent model floccu-Feb 1997 Amencan Soclety of Limnology and Oceanogralator J S a n~t Eng DIV Proc Am Soc CN Eng94(SA4) 671-682 phy. Waco. TX Waite A, Gordon ME, Vreeland V (1997) Enzyme med~ated Walte A. Olson RJ. Dam HG, Passow U (1995) Measunng sugadheslon and cell-surface sugar locahzation In rnanne ars on the surfaces of marine d~atoms uslng Concanaval~n d~atoms Abstract ASLO Aquatic Snences Mtg, Santa Fe, A and flow cytornetry J Phycol31.925-933
We document for the first time toxin-resistant reproductive phenotypes of copepods and we describ... more We document for the first time toxin-resistant reproductive phenotypes of copepods and we describe a novel procedure to identify these phenotypes. Individual copepods of the species Acartia hudsonica were raised on two diets: a standard nontoxic diet and a diet containing the toxic dinoflagellate Alexandrium fundyense, both offered at nonlimiting concentrations. Resistant individuals were defined as those that survived on the toxic diet. We examined several life-history characters including survivorship, age at metamorphosis, age at maturity, fecundity, and fitness. During this study, we discovered five resistance-related reproductive phenotypes that appeared as discrete classes in a frequency distribution of fecundity. After grouping the data according to these phenotypes, we calculated the fitness of each phenotype on each diet. We also calculated the cost and advantage associated with resistance. On the standard diet, one phenotype had 46% lower fitness than the phenotype with the highest fitness, indicating that possessing resistance alleles can carry a substantial cost. A different phenotype showed maximum relative fitness on the toxic diet and reduced relative fitness on the standard diet. From these results, we argue that resistance is conferred by a simple genetic system showing heterozygote advantage and leading to a polymorphism for resistance. Such a polymorphism will prevent the fixation of resistance alleles in natural populations. It may also confound the interpretation of typical experiments that measure average population responses.
Over the course of a bloom, the quantity and quality of algae available for copepods to eat may v... more Over the course of a bloom, the quantity and quality of algae available for copepods to eat may vary dramahcally The effects of varying phytoplankton concentration and quality on fecal pellet produchon rates and fecal pellet charactenstics of the calanoid copepod Acartia tonsa were examined in the laboratory Bloom conditions were siinulated by feeding copepods cells of the diatom Thalassioslra weissflogii grown in batch cultures Experiments were run when algae were rapidly growing (early bloom) and when algae were in stationary growth phase (late bloom) at concentrations ranging from 0 to 3200 cells ml-' Daily pellet production rates and volume, density, and carbon and nitrogen content of pellets were measured Results show that for a given algal growth stage pellet production rate and pellet volume increase linearly w~t h cell concentration up to a crit~cal point beyond whlch no further increases in these parameters occur In general, (1) maximum pellet volumes were higher when copepods ate late-bloom cells, (2) pellet densities were higher when copepods ate early-bloom cells and (3) volume-specific pellet carbon was higher when copepods ate early-bloom cells Sinking rates of pellets were estlrnated from an empincal model based on pellet volume and density Sinking rates of pellets were lowest at low concentralons (200 cells ml-l) concentrahons of early-bloom cells Sinking rates were highest at high concentrations (2000 cells ml-l) of early-bloom cells S i n l n g rates of fecal pellets were relatively high regardless of food concentration when copepods ate late-bloom cells Daily volume-speclfic fecal matter produced per copepod was also calculated Maxlmum values of total fecal matter index were h~g h e r when copepods ate early-bloom cells Our results indicate that copepod fecal pellets change in a predictable manner over the course of a bloom We conclude that copepod fecal pellets may play different roles in recychng and vertically transporting carbon over the course of a phytoplankton bloom
An efficient RNA-extraction method is crucial for transcriptomic studies of ecologically importan... more An efficient RNA-extraction method is crucial for transcriptomic studies of ecologically important organisms like copepods. In this study, we used the copepod Acartia hudsonica as a test species to evaluate existing methods, and formulated several improved protocols to consistently and efficiently isolate high-quality RNA from both pooled and individual copepod samples. Our protocols recommend copepod preservation in a phenol/guanidine thiocyanate reagent, followed by bead-beating or micropestle homogenization. To obtain high-quality RNA from pooled samples, one initial chloroform extraction followed by multiple phenol/chloroform extractions are needed before the raw RNA samples are further purified with the Qiagen RNeasy Mini Kit. When analyzing individual samples, one chloroform extraction followed by purification using the Zymo Research Direct-zol™ MiniPrep Kit gives the best results. Using these protocols, we isolated RNA from pooled and individual A. hudsonica samples with consistent recovery rate (individual samples: 95 ± 12 ng/male; 272 ± 57 ng/female). The cDNAs synthesized using these RNAs were proven to be of high quality by successful amplification of transcripts including the >7 kb alpha subunit of the voltage-gated sodium channel gene (SCG). For individual A. hudsonica samples, our protocols yielded significantly higher success rates (95%, n = 456) in reverse transcription quantitative PCR of SCG compared to existing methods (0-85%, n = 150). Applications of these protocols to nine other copepod species from seven families have led us to achieve high-quality RNAs and cDNAs from pooled and/ or individual samples. As a result of testing the protocols, we obtained cDNA sequences of various genes in ten copepod species, some of which are the first record in copepods. Our protocols can easily be adopted by laboratories to facilitate molecular work on copepods.
In coastal ecosystems with decades of eutrophication and other anthropogenic stressors, the impac... more In coastal ecosystems with decades of eutrophication and other anthropogenic stressors, the impact of climate change on planktonic communities can be difficult to detect. A time series of monthly water temperatures in the Central Basin of Long Island Sound (LIS) from the late 1940s until 2012 indicates a warming rate of 0.03°C year −1 . Relative to the early 1950s, there has been a concurrent decrease in the mean size of the dominant copepod species Acartia tonsa and Acartia hudsonica, an increase in the proportion of the small copepod Oithona sp., and the disappearance of the two largest-sized copepod genera from the 1950s. These changes are consistent with predictions of the impact of climate change on aquatic ectotherms. This suggests that even in eutrophic systems where food is not limiting, a continued increase in temperature could result in a smaller-sized copepod community. Since copepods dominate the zooplankton, which in turn link primary producers and upper trophic levels, a reduction in mean size could alter food web connectivity, decreasing the efficiency of trophic transfer between phytoplankton and endemic larval fish.
The dinoflagellate, Alexandrium fundyense, produces paralytic shellfish toxins and co-occurs with... more The dinoflagellate, Alexandrium fundyense, produces paralytic shellfish toxins and co-occurs with populations of the copepod, Acartia hudsonica, from Maine, but not New Jersey. The hypothesis that history of cooccurrence between predator and prey effects the ability of prey to recognize and respond to predators with increased toxin production was tested for this copepod-alga interaction. When A. fundyense was exposed to waterborne cues released by copepods from Maine (indirect exposure) that were either starved or fed toxic cells, cell toxin quota increased by 35% compared to unexposed controls. The induced response was significantly less for cells exposed to waterborne cues of copepods from New Jersey, and induction (20%) was only elicited by this population when fed toxic cells. These results suggest that A. fundyense responded to a kairomone from copepods from Maine, but required a feeding cue from copepods from New Jersey. An increase of approximately 300% in cell toxin quota, however, occurred when cells were directly exposed to grazing, and was independent of copepod population. Evolutionary history, therefore, had no apparent effect when induction was underlain by feeding cues. In assays with a mixture of toxic and nontoxic cells, selection for the latter was evident, and also independent of copepod population. Selectivity for nontoxic cells, however, could not account for changes in cell toxin content in the mixture experiments. When A. fundyense was exposed to extracts of toxic or nontoxic Alexandrium, toxin production increased significantly (23%), suggesting modest induction by an alga-to-alga alarm signal.
The marine copepod Acartia hudsonica was shown to be adapted to dinoflagellate prey, Alexandrium ... more The marine copepod Acartia hudsonica was shown to be adapted to dinoflagellate prey, Alexandrium fundyense, which produce paralytic shellfish toxins (PST). Adaptation to PSTs in other organisms is caused by a mutation in the sodium channel. Recently, a mutation in the sodium channel in A. hudsonica was found. In this study, we rigorously tested for advantages, costs, and trade-offs associated with the mutant isoform of A. hudsonica under toxic and non-toxic conditions. We combined fitness with wild-type: mutant isoform ratio measurements on the same individual copepod to test our hypotheses. All A. hudsonica copepods express both the wild-type and mutant sodium channel isoforms, but in different proportions; some individuals express predominantly mutant (PMI) or wild-type isoforms (PWI), while most individuals express relatively equal amounts of each (EI). There was no consistent pattern of improved performance as a function of toxin dose for egg production rate (EPR), ingestion rat...
production on fertilization was evident in the field. During 2 years of weekly collections, at no... more production on fertilization was evident in the field. During 2 years of weekly collections, at no point in time were all females in the population fertilized. We conclude that low rates of spermatophore production over a lifetime and the short reproductive period of males contribute to the low frequencies of mated females in field populations.
Deep-sea Research Part Ii-topical Studies in Oceanography, 1995
A study was undertaken to evaluate the role of exocellular polysaccharides in the flocculation of... more A study was undertaken to evaluate the role of exocellular polysaccharides in the flocculation of a marine diatom bloom in a large tank mesocosm. Surface-active organic matter was extracted from 1.0 μ-filtered tank water by bubble adsorption each day for 7 days of the experiment. In agreement with past studies, particles (3–51 urn equivalent spherical diameter) were readily formed by
The relationship between the rates of ingestion and defecation of the calanoid copepod Acartia to... more The relationship between the rates of ingestion and defecation of the calanoid copepod Acartia tonsa was examined in laboratory experiments with different diets: the diatom Thalassiosira weissflogii, the autotrophic dinoflagellate Prorocentrum minimum, the heterotrophic dinoflagellate Oxyrrhis sp., the flagellate Dunaliella tertiolecta, and the bacterivorous scuticociliate Uronema sp. Experiments were run typically using between 6 and 8 concentrations ranging from 10 to 1150 µg C l -1 . Both diet and food concentration had significant effects on ingestion rate, pellet production rates and pellet volume. Ingestion and pellet production rates increased curvilinearly with food concentration. Copepods achieved the highest pellet production rates, the largest pellets and the shortest estimated gut passage times with the diatom diet. For all diets, gut passage time decreased with food concentration in the range 0 to ca. 250 µg C l -1 , but little thereafter. In addition, for all diets, but most obviously in the case of the diatom, pellet volume increased curvilinearly with food concentration. Maximum pellet volume was achieved at food concentrations ranging from 50 to 150 µg C l -1 . The defecation and ingestion rates were best correlated when the copepods fed on the diatom and the autotrophic dinoflagellate. Likewise, a decrease in estimated assimilation efficiency with increasing food concentration was apparent only for these 2 diets. This latter pattern is primarily due to an increase in pellet production rate, instead of pellet volume, relative to ingestion rate at the high food concentrations. We conclude that a diatom diet is the most likely one to result in attributes that enhance zooplankton-mediated export flux: high fecal pellet production rates, large pellet size and relatively low assimilation efficiency.
... duration and frequency) Buoyancy and sediment sources (magnitude and distribution) Tidal rang... more ... duration and frequency) Buoyancy and sediment sources (magnitude and distribution) Tidal range Wave ... moorings with meteorological and water column measurements of temperature, salinity and velocity at ... Lower: An example of the mesoscale variability in the SST field within ...
To test the hypothesis that the sex ratio of the copepod Acartia tonsa is biased at birth, adult ... more To test the hypothesis that the sex ratio of the copepod Acartia tonsa is biased at birth, adult sex ratios of the offspring of families (21 families in 2011 and 72 in 2013) produced by field-caught females were determined in the laboratory under controlled conditions. Sex ratios at birth were estimated from the adult sex ratio, and after applying a correction in which all dead/missing individuals were counted as the rarer sex (males). Before correction the overall population sex ratio was female biased during both years, with over 42% of mothers producing clutches that were significantly different from unity. After correction, 25% to 33% (2011) of mothers still produced significantly biased sex ratios. The ratio of deaths of males to females had to be 1.75 to 1 (2011) and 7.25 to 1 (2013) for sex ratios not to be biased at birth. There was no evidence of sex reversal in any individual from juvenile (C4 stage) to adult, implying that juvenile sex ratios were also significantly biased. Hence, neither sex reversal nor differential mortality of the sexes is a likely explanation for the observed skewed sex ratios. The sex ratios at birth varied widely from the expectation from Mendelian inheritance of sex chromosomes, suggesting that another mechanism is responsible for skewed sex ratios at birth. Since the majority of families with biased sex ratios at birth were female dominated as adults, biased sex ratio at birth may contribute to the skewed sex ratio of adults observed in the field.
Mating in many copepod species is not random; moreover, individuals have preferences for certain ... more Mating in many copepod species is not random; moreover, individuals have preferences for certain qualities in their mates. This study tested whether female mating status (virgin versus mated) affects mate choice in males in the copepod species Acartia tonsa and Acartia hudsonica, both in laboratory and field populations. In addition, the costs and benefits associated with mating twice versus once for females of both species were tested in laboratory studies. In both A. tonsa and Acartia hudsonica, males mated more frequently with virgin females compared with females that had already mated, suggesting that female reproductive status is important for mating and mate choice. Frequencies of females carrying double spermatophores in the field (at three sites and seasonally within a site) were consistently low, implying that mate choice for female reproductive status is present in natural populations as well. In Acartia hudsonica, the frequency of females carrying double spermatophores was positively related to spermatophore volume and male and female sizes, but negatively related to temperature. No such relationships were found for A. tonsa. However, the frequency of females carrying double spermatophores decreased as the season progressed. In both copepod species, there was no difference in the number of nauplii produced, length of fertilization or lifespan between females that had mated twice compared with those that had mated only once. Hence, there was no particular cost or benefit to mating a second time. Our findings suggest that female reproductive status is important for mating success in these two coastal copepod species and that this is most likely due to male mate-choice.
In this study we hypothesized that the sexes of the copepod Acartia hudsonica experience differen... more In this study we hypothesized that the sexes of the copepod Acartia hudsonica experience different mortalities when exposed to the toxic dinoflagellate Alexandrium fundyense. In laboratory experiments, we manipulated toxin dose and measured sex-specific mortality. We also used a split-family design to determine whether there were genetic influences on sex determination, and therefore on sex ratio. When the dose of the toxic alga was high (25% of diet by carbon content), population mortality was high, and the fraction of males surviving to adulthood was low (20-30% of the adult population). No such effect was observed when A. fundyense comprised only 10% of the diet. Adult male copepods were also more susceptible to the toxic alga, though prior selection for resistance to toxins also had an effect on mortality. No genetic variation for sex ratio was detected in the split-family experiment. Males of A. hudsonica are more sensitive to the toxins of A. fundyense than are females. Resistance may be sex-linked, and copepod populations exposed to a bloom of toxic Alexandrium sp. may experience shifts in sex ratios away from males and toward females. Differential mortality and skewed sex ratios are feedback mechanisms potentially affecting both the population dynamics of grazers and the development of toxic algal blooms.
D~atorn aggregation can increase the sedimentdtion rate of the Intense pulses of carbon formed du... more D~atorn aggregation can increase the sedimentdtion rate of the Intense pulses of carbon formed durlng d~a t o m blooms. Laboratory studies of aggregation dynarn~cs have been troubled by the unavoidable d~sruption of aggregates in laboratory flocculators due to subsampl~ng and electronic part~cle countlng Aggregate disruption prcvcnts accurate measurement of aggregation rate and makes observat~on of part~cle structure impossible. This led us to develop a new non-disruptive method to quantify diatom aggregation using high-resolut~on video and image analysis. The system easily resol\~ed cells of Chaetoceros neogracile 6 to 8 pm in diameter, and facilitated observations of aggregate morphology. C. neogracile made either ball-like or net-like aggregates depending on culture denslty and physiological state. Only the net-llke aggregates reached a substantial slze ( > l mm). A Silimlted batch culture expenment indicated that stickiness of C. neogracile increased significantly with increas~ng severity of nutrient limitation. T h~s increase was not detectable by the subsampling and electronic particle counting method. It is likely that spine formation, cell-surface bound sugars, and transparent exopolymer particles all played d role in determining aggregate structure and changes in stickiness. The stickiness calculation model assumes cultures are initially suspensions of single cells which form doublets as they aggregate. This assumption was tested by comparing mean particle size in a culture with the percentage of cells in aggregates a s determined using nearest neighbor distances, allowing us to assess the mean number of cells per aggregate. C. neograc~le commonly formed aggregates contaming anywhere between 2 and 10 cells showing that model assumptions can be violated under normal experimental conditions. A larger field of view was necessary to estimate the larger particle sizes typlcal of coastal diatom blooms. K E Y WORDS Aggregation Adhesion . Video Image analysis Dlatom Chaetoceros neogracile O Inter-Research 1997 Resale of full article not permitted Mar Emi Prog Ser 155: 77-88, 1997 van Du uren FA (1 968) Defined veloclty grad~ent model floccu-Feb 1997 Amencan Soclety of Limnology and Oceanogralator J S a n~t Eng DIV Proc Am Soc CN Eng94(SA4) 671-682 phy. Waco. TX Waite A, Gordon ME, Vreeland V (1997) Enzyme med~ated Walte A. Olson RJ. Dam HG, Passow U (1995) Measunng sugadheslon and cell-surface sugar locahzation In rnanne ars on the surfaces of marine d~atoms uslng Concanaval~n d~atoms Abstract ASLO Aquatic Snences Mtg, Santa Fe, A and flow cytornetry J Phycol31.925-933
We document for the first time toxin-resistant reproductive phenotypes of copepods and we describ... more We document for the first time toxin-resistant reproductive phenotypes of copepods and we describe a novel procedure to identify these phenotypes. Individual copepods of the species Acartia hudsonica were raised on two diets: a standard nontoxic diet and a diet containing the toxic dinoflagellate Alexandrium fundyense, both offered at nonlimiting concentrations. Resistant individuals were defined as those that survived on the toxic diet. We examined several life-history characters including survivorship, age at metamorphosis, age at maturity, fecundity, and fitness. During this study, we discovered five resistance-related reproductive phenotypes that appeared as discrete classes in a frequency distribution of fecundity. After grouping the data according to these phenotypes, we calculated the fitness of each phenotype on each diet. We also calculated the cost and advantage associated with resistance. On the standard diet, one phenotype had 46% lower fitness than the phenotype with the highest fitness, indicating that possessing resistance alleles can carry a substantial cost. A different phenotype showed maximum relative fitness on the toxic diet and reduced relative fitness on the standard diet. From these results, we argue that resistance is conferred by a simple genetic system showing heterozygote advantage and leading to a polymorphism for resistance. Such a polymorphism will prevent the fixation of resistance alleles in natural populations. It may also confound the interpretation of typical experiments that measure average population responses.
Over the course of a bloom, the quantity and quality of algae available for copepods to eat may v... more Over the course of a bloom, the quantity and quality of algae available for copepods to eat may vary dramahcally The effects of varying phytoplankton concentration and quality on fecal pellet produchon rates and fecal pellet charactenstics of the calanoid copepod Acartia tonsa were examined in the laboratory Bloom conditions were siinulated by feeding copepods cells of the diatom Thalassioslra weissflogii grown in batch cultures Experiments were run when algae were rapidly growing (early bloom) and when algae were in stationary growth phase (late bloom) at concentrations ranging from 0 to 3200 cells ml-' Daily pellet production rates and volume, density, and carbon and nitrogen content of pellets were measured Results show that for a given algal growth stage pellet production rate and pellet volume increase linearly w~t h cell concentration up to a crit~cal point beyond whlch no further increases in these parameters occur In general, (1) maximum pellet volumes were higher when copepods ate late-bloom cells, (2) pellet densities were higher when copepods ate early-bloom cells and (3) volume-specific pellet carbon was higher when copepods ate early-bloom cells Sinking rates of pellets were estlrnated from an empincal model based on pellet volume and density Sinking rates of pellets were lowest at low concentralons (200 cells ml-l) concentrahons of early-bloom cells Sinking rates were highest at high concentrations (2000 cells ml-l) of early-bloom cells S i n l n g rates of fecal pellets were relatively high regardless of food concentration when copepods ate late-bloom cells Daily volume-speclfic fecal matter produced per copepod was also calculated Maxlmum values of total fecal matter index were h~g h e r when copepods ate early-bloom cells Our results indicate that copepod fecal pellets change in a predictable manner over the course of a bloom We conclude that copepod fecal pellets may play different roles in recychng and vertically transporting carbon over the course of a phytoplankton bloom
An efficient RNA-extraction method is crucial for transcriptomic studies of ecologically importan... more An efficient RNA-extraction method is crucial for transcriptomic studies of ecologically important organisms like copepods. In this study, we used the copepod Acartia hudsonica as a test species to evaluate existing methods, and formulated several improved protocols to consistently and efficiently isolate high-quality RNA from both pooled and individual copepod samples. Our protocols recommend copepod preservation in a phenol/guanidine thiocyanate reagent, followed by bead-beating or micropestle homogenization. To obtain high-quality RNA from pooled samples, one initial chloroform extraction followed by multiple phenol/chloroform extractions are needed before the raw RNA samples are further purified with the Qiagen RNeasy Mini Kit. When analyzing individual samples, one chloroform extraction followed by purification using the Zymo Research Direct-zol™ MiniPrep Kit gives the best results. Using these protocols, we isolated RNA from pooled and individual A. hudsonica samples with consistent recovery rate (individual samples: 95 ± 12 ng/male; 272 ± 57 ng/female). The cDNAs synthesized using these RNAs were proven to be of high quality by successful amplification of transcripts including the >7 kb alpha subunit of the voltage-gated sodium channel gene (SCG). For individual A. hudsonica samples, our protocols yielded significantly higher success rates (95%, n = 456) in reverse transcription quantitative PCR of SCG compared to existing methods (0-85%, n = 150). Applications of these protocols to nine other copepod species from seven families have led us to achieve high-quality RNAs and cDNAs from pooled and/ or individual samples. As a result of testing the protocols, we obtained cDNA sequences of various genes in ten copepod species, some of which are the first record in copepods. Our protocols can easily be adopted by laboratories to facilitate molecular work on copepods.
In coastal ecosystems with decades of eutrophication and other anthropogenic stressors, the impac... more In coastal ecosystems with decades of eutrophication and other anthropogenic stressors, the impact of climate change on planktonic communities can be difficult to detect. A time series of monthly water temperatures in the Central Basin of Long Island Sound (LIS) from the late 1940s until 2012 indicates a warming rate of 0.03°C year −1 . Relative to the early 1950s, there has been a concurrent decrease in the mean size of the dominant copepod species Acartia tonsa and Acartia hudsonica, an increase in the proportion of the small copepod Oithona sp., and the disappearance of the two largest-sized copepod genera from the 1950s. These changes are consistent with predictions of the impact of climate change on aquatic ectotherms. This suggests that even in eutrophic systems where food is not limiting, a continued increase in temperature could result in a smaller-sized copepod community. Since copepods dominate the zooplankton, which in turn link primary producers and upper trophic levels, a reduction in mean size could alter food web connectivity, decreasing the efficiency of trophic transfer between phytoplankton and endemic larval fish.
The dinoflagellate, Alexandrium fundyense, produces paralytic shellfish toxins and co-occurs with... more The dinoflagellate, Alexandrium fundyense, produces paralytic shellfish toxins and co-occurs with populations of the copepod, Acartia hudsonica, from Maine, but not New Jersey. The hypothesis that history of cooccurrence between predator and prey effects the ability of prey to recognize and respond to predators with increased toxin production was tested for this copepod-alga interaction. When A. fundyense was exposed to waterborne cues released by copepods from Maine (indirect exposure) that were either starved or fed toxic cells, cell toxin quota increased by 35% compared to unexposed controls. The induced response was significantly less for cells exposed to waterborne cues of copepods from New Jersey, and induction (20%) was only elicited by this population when fed toxic cells. These results suggest that A. fundyense responded to a kairomone from copepods from Maine, but required a feeding cue from copepods from New Jersey. An increase of approximately 300% in cell toxin quota, however, occurred when cells were directly exposed to grazing, and was independent of copepod population. Evolutionary history, therefore, had no apparent effect when induction was underlain by feeding cues. In assays with a mixture of toxic and nontoxic cells, selection for the latter was evident, and also independent of copepod population. Selectivity for nontoxic cells, however, could not account for changes in cell toxin content in the mixture experiments. When A. fundyense was exposed to extracts of toxic or nontoxic Alexandrium, toxin production increased significantly (23%), suggesting modest induction by an alga-to-alga alarm signal.
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Papers by Hans Dam