Photosensitivity of the calanoid copepod Acartia tonsa

The light intensity and spectral sensitivities of the calanoid copepod Acartia tonsa Dana were determined by measuring phototactic responses. Adult females displayed only positive phototaxis. The dark-adapted copepod, which possesses a single naupliar eye, perceived light at intensities as low as 2.8x10¹¹ photons m^-2 s^-1. The action spectrum for positive phototaxis had no clear maxima but rather showed a broad range of greatest sensitivity from 453 to 620 nm. This sensitivity encompassed those wavelengths that are maximally available at the depth where the copepod is found during the day. This spectral overlap, coupled with the finding that the copepod requires light cues for nocturnal vertical miration, suggests that broad spectral sensitivity is an adaptive mechanism to maximize light intensity sensitivity during migration.     

Evaluation of the effect of antimicrobials in marine cultures,using the copepod Acartia tonsa as a bioindicator

Cultures of the copepod Acartia tonsa are used both in aquaculture and ecotoxicology studies. However, the cultivation of these crustaceans at high densities results in the proliferation of microorganisms that can affect the organisms of interest, leading to illness or death. Antimicrobials inhibit microbial growth and may favour the cultivated species, aiding the development of ecological studies. This study investigated the potential of antimicrobials (antibiotic + antifungal) to inhibit bacteria and fungi when applied to marine zooplankton cultures, using the copepod A. tonsa as a bioindicator of acute toxicity. Treatment with 0.025?g?L^?1 of penicillin G potassium + 0.08?g?L^?1 of streptomycin sulphate + 0.04?g?L^?1 of neomycin sulphate + 0.005?g?L^?1 of nystatin resulted in 95% bacterial inhibition (after 12?h of exposure); however, after this time, the inhibitory effect was lost. The antimicrobial combination tested in this study prevented colonisation by fungi until 168?h after exposure, without causing acute toxicity to A. tonsa. Thus, it has potential for use in marine cultures of less sensitive organisms.     

Effects of suspended sediments on egg production of the calanoid copepod Acartia tonsa

The estuarine copepod Acartia tonsa was collected on several occasions between 4 April and 14 August 1985 from Terrebonne Bay, Louisiana (29°08N; 90°36W) and the effects in its diet of suspended sediments, collected from the same area, were measured at five different concentrations of sediment (100 to 1 000 ppm) and six phytoplankton concentrations (500 to 13 000 cells ml^-1 Thalassiosira weissflogii). Egg production rate was used as an index of diet quality. At low phytoplankton concentrations (500 cells ml^-1), and at intermediate phytoplankton concentrations (2 000 cells ml^-1) for previously starved copepods, egg production was reduced by up to 40% at a sediment concentration of 250 ppm and further reduced at higher sediment concentrations. At higher food concentrations (4 000 to 13 000 cells ml^-1), suspended sediment had no effect on egg production rates at sediment concentrations up to 500 ppm. Rates were reduced only at the highest sediment concentration of 1 000 ppm. Under most natural conditions, suspended sediment would not significantly affect egg production rates in A. tonsa.     

Role of the free amino acid pool of the copepod Acartia tonsa in adjustment to salinity change

The free amino acid pool of the calanoid copepod Acartia tonsa was reduced in proportion to the decrease in external salinity within 24 h, with a corresponding increase in ammonia excretion and a transient rise in oxygen consumption. The free amino acid pool was not increased in response to increased salinity. Catabolism of free amino acids is important in the reduction of cellular osmotic pressure in reduced salinities. Antagonistic demands of osmotic preservation and nutritional metabolism on the free amino acid pool may limit the production of the species in waters of higher salinity.     

Diet and egg production of the copepod Acartia tonsa in Florida Bay. II. Role of the nutritional environment

 As part of an ongoing study of changes in the trophic pathways of Florida Bay's pelagic ecosystem, the nutritional environment (seston protein, lipid and carbohydrate levels), diet (taxon-specific microplankton ingestion rates) and egg production rate of the important planktonic copepod Acartia tonsa were measured off Rankin and Duck Keys in July and September 1997 and in January, March and May 1998. Rankin Key has been the site of extensive sea grass mortality and persistent ultraplankton blooms since 1987. Duck Key has experienced neither of these perturbations. Protist (auto-plus heterotroph) biomass was approximately twice as high off Rankin as off Duck Key. Diatoms, dinoflagellates and heterotrophic protists dominated the food environment off Rankin Key, while cells <5 μm diam often predominated off Duck Key. Protein and carbohydrate concentrations were higher off Rankin Key than Duck Key, while average lipid levels were usually low at both stations. Ingestion rates at both stations frequently approached temperature- and food-dependent maxima for the species, exceeding 100% of estimated body C d⁻¹ on 3 of 5 occasions off Rankin Key. Egg production rates, however, were consistently low (Rankin: 3 to 16 eggs copepod⁻¹ d⁻¹; Duck: 1 to 12 eggs copepod⁻¹ d⁻¹), and gross egg production efficiencies (100% × egg production C/ingested C) averaged <10%. At Duck Key, egg production rate varied with temperature and food concentration, while off Rankin Key, egg production was strongly correlated with seston protein content. The efficiency with which lipids (which were scarce in the seston) were transferred from the diet to the eggs increased exponentially with decreasing seston lipid content. Egg production efficiencies based on protein, however, were independent of seston protein content and never exceeded 10%. Received: 23 December 1998 / Accepted: 23 March 2000     

Feeding of the copepod Acartia tonsa on the diatom Nitzschia closterium and brown algae (Fucus vesiculosus) detritus

Integumental sensilla were examined in 39 species of meso- and bathypelagic shrimps and 6 species of epibenthic shrimps. A tuft organ, consisting of a group of openended, tubular setae and so probably functioning as a chemosensor, is described. A single tuft with ancillary smaller tufts occur on the dorsal median surface of the fourth abdominal segment and two pairs of tufts occur on the proximal dorsal region of the telson of all oplophorid and pandalid shrimps examined. An additional tuft is present on the fifth abdominal segment of the rare bathypelagic Physetocaris microphthalma Chace. No tuft organs were identified in Processa canaliculata Leach, suggesting that they may not occur in all families of caridean shrimps. Acanthephyrid, systellaspid and pandalid shrimps have the integument completely covered by scales. These are extremely delicate and are normally removed during the process of catching the shrimp through abrasion of the integument by the net. The scales are peculiarly orientated, pointing anteriorly in the anterior half and posteriorly in the posterior half of the body. They probably have a sensory function as distance receptors, monitoring water currents and disturbances in the environment surrounding the shrimp.     

Scanning electron microscopy investigation of bacterial colonization of the marine copepod Acartia clausi

Association between copepods and bacteria was observed in many scanning electron micrographs. Particular sites on the copepods were selectively colonized by bacteria; the joints of segments and legs, swimming legs and depressed parts of the body surface were found to be densely covered with bacteria. In comparison, bacterial attachment to copepod skeletons in fecal pellets excreted by chaetognaths was not selective; bacteria were sparsely found all over the copepod. Between 9 to 30% of copepods in Tokyo Bay waters had attached bacteria in January and April 1983.     

Trophic modification of essential fatty acids by heterotrophic protists and its effects on the fatty acid composition of the copepod Acartia tonsa

To test whether heterotrophic protists modify precursors of long chain n−3 polyunsaturated fatty acids (LCn−3PUFAs) present in the algae they eat, two algae with different fatty acid contents (Rhodomonas salina and Dunaliella tertiolecta) were fed to the heterotrophic protists Oxyrrhis marina Dujardin and Gyrodinium dominans Hulbert. These experiments were conducted in August 2004. Both predators and prey were analyzed for fatty acid composition. To further test the effects of trophic upgrading, the calanoid copepod Acartia tonsa Dana was fed R. salina, D. tertiolecta, or O. marina that had been growing on D. tertiolecta (OM-DT) in March 2005. Our results show that trophic upgrading was species-specific. The presence of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in the heterotrophic protists despite the lack of these fatty acids in the algal prey suggests that protists have the ability to elongate and desaturate 18:3 (n−3), a precursor of LCn−3PUFAs, to EPA and/or DHA. A lower content of these fatty acids was detected in protists that were fed good-quality algae. Feeding experiments with A. tonsa showed that copepods fed D. tertiolecta had a significantly lower content of EPA and DHA than those fed OM-DT. The concentration of EPA was low on both diets, while DHA content was highest in A. tonsa fed R. salina and OM-DT. These results suggest that O. marina was able to trophically upgrade the nutritional quality of the poor-quality alga, and efficiently supplied DHA to the next trophic level. The low amount of EPA in A. tonsa suggests EPA may be catabolized by the copepod.     

Effects of food quality on the reproductive success of Acartia tonsa and Acartia hudsonica: laboratory observations

The effect of the chemical composition of food on the reproductive success of the copepods Acartia tonsa Dana and A. hudsonica Pinhey was studied in the laboratory. Laboratory-reared individuals were fed one of three monoalgal diets at different stages of growth: the diatom Thalassiosira weissflogii, the flagellate Rhodomonas lens and the dinoflagellte Prorocentrum minimum. The diet was analyzed for carbon, nitrogen, protein, carbohydrate and fatty acid content. Reproductive success was measured as eggs female^-1 day^-1 (E _r) and as the hatching success of the eggs. The E _r of Acartia spp. was correlated with protein and specific fatty acids [16:17 (negative), 20:53, 22:63, and 18:0 (positive)] and, especially, the fatty acid composition of the algae expressed as the 3:6 and 20:22 fatty acid ratios. The youngest diatom cultures and exponentially-growing flagellates displayed the highest E _r; the lowest E _r was recorded for females fed the senescent diatom cultures. The development time of eggs was affected by the age of the phytoplankton culture fed to the female. Hatching success of eggs decreased with the age of the algal culture, but no correlation was found with the meansured chemical components of the food.     

Orientation affects the sensitivity of Acartia tonsa to fluid mechanical signals

Nearly all organisms show directional bias in sensitivity to environmental signals. In this study, the behavioral sensitivity of a common estuarine copepod, Acartia tonsa, varies significantly with respect to their orientation to a well-characterized fluid mechanical signal. Maximum sensitivity occurs at an angle of 25°–30° and lowest sensitivity occurs at angles of 60°–90° relative to the source. These results support the hypothesis that copepods are not uniformly sensitive to fluid signals and show directional bias in mechanosensitivity. The data also show that large copepods initiate their escape reaction further from the source than small copepods. There is, however, an uncharacteristically large increase in sensitivity at the transition between the nauplii and C1 stage despite being similar in size. This suggests that the mechanosensory system of the naupliar stages is less sensitive to fluid signals and helps to explain the higher predation rates experienced by nauplii.     

Food selection capabilities of the estuarine copepod Acartia clausi

Existing viewpoints and theories of selective grazing by copepods are briefly reviewed in order to formulate explicit hypotheses to be tested experimentally. Based on these hypotheses, a series of grazing experiments was run to determine (1) the extent of the selective ingestion capabilities of Acartia clausi and (2) how these capabilities were affected by previous feeding histories. Groups of copepods were separately preconditioned on a small diatom (Thalassiosira pseudonana), a large diatom (T. fluviatilis), or a plastic sphere. The ingestive behavior was then examined on various combinations of spheres and food particles. Spheres offered alone were not ingested. In mixtures of diatoms and spheres, the copepods avoided ingesting spheres intermediate in size between the sizes of the diatoms. The copepods either ingested particles on either side of the spheres, or ignored all particles less than the size of the largest spheres. The pattern observed depended upon the size of the preconditioning food. However, if the spheres were larger than the largest food particles, the copepods still selectively ingested the food particles. The above results demonstrate that A. clausi has a complex grazing behavior consisting of (1) more efficient grazing on larger particles within its particle-size ingestion range; (2) the ability to alter effective setal spacing to optimize feeding behavior (i.e., the ability to increase efficiency of capture of food particles, and to avoid non-food particles); and (3) the ability for post-capture rejection of non-food particles when they interfere with the ingestion of food particles on which the copepod has been preconditioned. The behavioral patterns observed depend heavily on the food preconditioning and the presence or absence of non-food particles. These results clearly indicate that a simple mechanistic explanation of selective grazing is insufficient.     

Growth and development of nauplii and copepodites of the estuarine copepod Acartia tonsa from southern Europe (Ria de Aveiro, Portugal) under saturating food conditions

A temperature-dependent growth model is presented for nauplii and copepodites of the estuarine calanoid copepod Acartia tonsa from southern Europe (Portugal). Development was followed from egg to adult in the laboratory at four temperatures (10, 15, 18 and 22°C) and under saturating food conditions (>1,000 μg C l⁻¹). Development times versus incubation temperature were fitted to a Belehradek’s function, showing that development times decreased with increasing incubation temperature: at 10°C, A. tonsa need 40.3 days to reach adult stage, decreasing to 8.9 days when reared at 22°C. ANCOVA (homogeneity of slopes) showed that temperature (P<0.001) and growth phase (P<0.01) had a significant effect on the growth rate. Over the range of temperatures tested in this study, highest weight-specific growth rates were found during naupliar development (NI–NVI) and varied from 0.185 day⁻¹ (10°C) to 0.880 day⁻¹ (22°C) with a Q ₁₀ equal to 3.66. During copepodite growth (CI–CV), the weight-specific growth rates ranged from 0.125 day⁻¹ (10°C) to 0.488 day⁻¹ (22°C) with a Q ₁₀ equal to 3.12. The weight-specific growth rates (g) followed temperature (T) by a linear relationship and described as ln g=−2.962+0.130 T (r ²=0.99, P<0.001) for naupliar stages and ln g=−3.134+0.114T (r ²=0.97, P<0.001) for copepodite stages. By comparing in situ growth rates (juvenile growth and fecundity) for A. tonsa taken from the literature with the temperature-dependent growth model defined here we suggest that the adult females of A. tonsa are more frequently food limited than juveniles.     

Experimental study on acute effects of the combined exposure to temperature increase and chlorination upon the marine copepod Acartia omorii

The acute effects of combined exposure to temperature increase and chlorination on a neritic marine copepod, Acartia omorii Bradford (collected offshore of Onjuku, Japan in 1982), were investigated in the laboratory. Continuous flow exposure and batch exposure modes were compared. Based on the results of continuous flow experiments, the 24-h median lethal concentration (24-h LC₅₀, in mg l^-1) of total residual chlorine was estimated using the multiple regression equation below, with a multiple correlation coefficient of 0.955: 24-h LC₅₀=2.988-0.034 dT-1.611 log₁₀ t where dT is temperature rise (°C) and t is exposure duration (min). In batch experiments, the predictive power of the multiple regression equation was reduced, probably due to variations in chlorine concentration during exposure duration.     

The diatom Thalassiosira rotula affects reproductive success in the copepod Acartia clausi

Fecundity, egg viability and fecal pellet production are reported for Acartia clausi females collected in the Bay of Naples, Italy, from April to October 1992 and fed either with a diatom (Thalassiosira rotula) or dinoflagellate (Prorocentrum minimum) diet, at food saturated conditions. The diatom diet significantly reduced both egg and fecal pellet production as well as hatching success. Blockage of egg development occurred with both axenic and non-axenic cultures of T. rotula, suggesting that inhibitors were provided by the diatoms and not by the bacteria associated with diatom cultures. Low hatching success was also artificially induced by exposing newly spawned A. clausi eggs to high concentrations of diatom extracts, indicating the presence of deleterious, inhibitory compounds blocking copepod embryogenesis. Fecundity and hatching success diminished significantly with female age. In contrast, female longevity was not significantly modified by food type. The presence of males did not significantly alter fecundity or egg viability. Females continued to produce viable eggs throughout the period of incubation, with and without males, in both food conditions, indicating that remating is infrequent and not necessary to sustain viable egg production in this species. The succession in low and high population densities may therefore be the outcome of variations in survival rates of eggs, rather than reproductive protential perse; such variations may strongly depend on the adult copepod diet.     

Relationship between growth rate and egg production in the copepod Acartia clausi hudsonica

Egg production by Acartia clausi hudsonica ceases at low concentrations of Isochrysis galbana as food and at high levels reaches a maximum that increases with temperatures in the natural range. This increase parallels the rate of production of fecal pellets. Females without males can produce about 400 eggs before entering a generally short postreproductive period. Weight increments between copepodite stages are exponential and, assuming isochronal durations of stages and that development time of older stages is the same mutiple of embryonic duration at all temperatures, a temperature-dependent rate is estimated for pre-adult growth. We demonstrate that this growth rate also predicts the observed maximal rate of egg production by the nongrowing adult females. Published data for other copepod species (except Pseudocalanus) are inadequate for wider testing of this hypothesis, but available data do suggest that no such simple rule governs total output of eggs by females of different species.     

Discriminate feeding of the calanoid copepod Acartia clausi in mixtures of phytoplankton and inert particles

Adult female Acartia clausi were allowed to feed in the laboratory on each of three algae, the small green alga Dunaliella tertiolecta (5.6 m ESD), the diatoms Thalassiosira decipiens (13.4 m ESD) and T. nordenskioldii (17.7 m ESD), singly or in mixtures with polystyrene beads (15.7 m ESD). The ingestion rate on beads was much lower than that on cells, even when the supply of beads was one order of magnitude higher than that of cells. Beads offered singly were not ingested. In the experiments with T. nordenskioldii, the more cells adult females ingested, the higher the ingestion rate on beads. In addition, a linear regression equation describing the relationship between bead concentration and ratio of ingested beads to ingested cells was highly significant. These results suggest that beads are ingested by chance in feeding bouts on T. nordenskioldii. The bead interference on the ingestion rate on cells was observed in the experiments with D. tertiolecta and T. decipiens, but not with T. nordenskioldii. In a mixture of beads and T. decipiens, juvenile A. clausi discriminated beads from cells more successfully than adults. The presence of beads did not affect the ingestion rate of juveniles on cells. Starved adult females tended to ingest more beads than well fed individuals. The possible effects of body size and hunger on the discriminate feeding of copepods are discussed.     

Critical thermal maxima and upper lethal temperatures for the calanoid copepods Acartia tonsa and A. clausi

Determination of the critical thermal maxima and upper lethal temperatures for Acartia tonsa Dana and A. clausi Giesbrecht revealed the relative temperature tolerance of the two species. A. tonsa exhibited a wider range of acclimation temperatures compared to A. clausi. The response of A. tonsa is related to its ubiquity in Atlantic Ocean estuaries (temperate and tropical), and its capacity to live in temperate waters throughout most of the year. Differences between upper lethal temperatures and critical thermal maxima were very narrow for A. tonsa from tropical and subtropical areas. This suggests that, in warm climates, this copepod lives close to its upper lethal temperature limits.Based in part on a dissertation submitted in partial fulfillment for the Ph.D. degree at the University of Rhode Island.     

Diel periodicity of photosynthesis in marine phytoplankton

Short-term changes in photosynthesis were documented for 17 of 24 marine phytoplankton species, representing a range of taxonomic groups. Periodicity in phytoplankton photosynthesis on light-dark cycles (diel periodicity) was widespread but not universal for the species studied. The centric diatoms Lauderia borealis, Ditylum brightwellii, Stephanopyxis turris, Coscinodiscus rex, Chaetoceros gracile, and Biddulphia mobiliensis had strong diel periodicity in photosynthetic capacity (P _max). Amplitudes of the daily variations ranged from 2.9 to >50, with maxima in the morning or near midday, and with minima during the dark period, and these variations were not dependent on changes in cell pigmentation. There was some evidence for sustained photosynthetic periodicity in constant conditions in several diatoms, and an endogenous rhythm may have been present. The photosynthesis-irradiance (P-I) relationship was time-dependent for representative marine diatoms, with both the initial slope () and the asymptote (P _max) of P-I curves exhibiting significant synchronous diel oscillations. Moreover, detailed studies of the amplitude and timing of photosynthetic periodicity for the diatoms L. borealis and D. brightwellii demonstrated large temporal variations in photosynthesis with morning maxima. These P-I oscillations are discussed with reference to models of primary production which use the relationship between photosynthesis and light as a component of predictive equations for phytoplankton growth in the sea.     

Effects of parental acclimation temperature and egg-incubation temperature on egg-hatching time in Acartia tonsa (Copepoda: Calanoida)

Experimental results indicate that both parental acclimation temperature and egg-incubation temperature have significant effects on the egg-hatching time of Acartia tonsa Dana between 15° and 25°C. Effects of parental acclimation temperature and egg-incubation temperature are additive if the long-term parental acclimation temperature is constant. A. tonsa are sensitive to temperature and changes of temperature during development (in vivo) and after they are laid (in vitro), up to the time of hatching. A temperature change of the parent culture for time periods of 86 h to 8 d is sufficient to change egg-hatching times. The time required for a temperature change to affect egg-hatching times depends on the magnitude and direction of the temperature change. Field collected A. tonsa demonstrate that changes in egg-hatching times occur in the field populations and can be predicted by mean weekly water temperature.