Diel feeding behavior and gut evacuation rates were investigated in adult female Acartia tonsa, in laboratory experiments at 20 "C using different concentrations of cultured Thalassiosira weissflogii as food, and in large mesocosms at -13°C using natural plankton as food. Diel feeding pattern was remarkably stable under the different food conditions and temperatures, maintaining about a 3-fold amplitude in gut contents and feeding rate over the diel period. The diel rhythm was retained even when the copepods were food limited and lost about 20 % body carbon during the course of the 24 h experiment. The persistence of the diel feeding rhythm in A. tonsa under starvation implies a strong selective pressure, possibly related to predator avoidance. Daily ingestion rates of copepods fed a high concentration of T. weissflogii at 20°C were about 90 000 cells d-l, or -148 % body carbon and 104 % body nitrogen, compared with 74 and 85 % body carbon and 40 and 45 % body nitrogen in mesocosm experiments at -13 "C with natural food. Daytime and night-time gut evacuation rates at 20 "C were not significantly different, indicating that the diel change in gut contents reflects a real change in feeding rate, rather than a change in the instantaneous gut clearance rate. Instantaneous gut evacuation rates were 0.0907 and 0.0894 min-l in 2 experiments at 20°C and 0.0415 and 0.0432 minp' at 12.7 and 13.3"C respectively. Day-night comparison of gut contents measured in individual A. tonsa, fed T. constricts at 8"C, revealed considerable individual variability. Range in gut contents was about 50-fold during the day (0.17 to 8.97 ng pigment copepod-'), and 100-fold at night (0.0 to 13.66 ng copepod-'). Distribution of gut content values was skewed towards lower values, especially during daytime. However all but 1 copepod in the day/night sequence had a measurable amount of food in the gut, indicating fairly recent feeding.
Maximum ingestion rate (I,,,,,) in Acartja tonsa females from Narragansett Bay, Rhode Island. USA, when measured under standardized conditions of temperature (20 "C) and food (the diatom Thalassiosira weissflogii), varied by a factor of 2 to 3 (22 000 to 50320 cells copepod-' d-', 6.00 to 18.6 pg C copepod-' d-l, and 1.33 to 3.32 pg N copepod-' d-', or 121 to 376% final body C d-'. and 90 to 245% final body N d-l). Overall mean values were 38 200 cells copepod-' d -l , 10 3 pg C copepod-' d-', 1.96 pg N copepod-' d", 203% final body C d -' , and 146% final body N d ' Copepods gained weight during laboratory incubations, and consequently I,,,,, averaged 25.6 % and 19.9 O/o higher as a percentage of initial, than of final, body C and N. I,,,,, was most strongly related to the residual effects of field temperature, and secondarily to in situ food level, and initial body weight and condition factor (CF: weight per unit length). Weight and CF were strongly affected by the degree of food limitation. I,,, was highest in copepods with low initial body weight and CF, and from the poorest food conditions in the field. This compensatory increase in I,,,, resembles the hunger response described for other copepods, and would enable food-limited A. tonsa to more effectively exploit transient plankton blooms. Copepods increased significantly in both weight and CF during the 24 h laboratory incubat i o n~, demonstrating that body size was food limited even during plankton blooms. Mean weight increments over 24 h were 27.8% C, 20.3% N, and 22.6% dry weight. The amount of growth, and the growth efficiency, were inversely related to initial CF. Thus copepods that were most severely food limited in the field not only exhibited h~gher I,,,,, and higher growth rates, but also allotted a greater fract~on of ingested energy to growth, when provided with excess food in the laboratory.
Adult females of the marine copepod Acartia hudsonica Pinhey were fed the unicellular diatom Thalassiosira constricta (10.4 pm equivalent spherical diameter) at 4, 8, 12, and 16 "C, to determine whether there was a threshold food concentration below which feeding ceased or was sharply reduced. Field-collected copepods were acclimated for 24 h at 2000 cells ml-' T. constricta, and then transferred to a range of low food concentrations (15 to 876 cells ml-l). Gut pigments were determined fluonmetrically after a 5 h period of feeding, which was calculated to be sufficient for gut contents to equil~brate to the new food concentration. Experiments were carried out during the same phase of the diel cycle (05:OO to 10:OO h) and were terminated near midday, a time when feeding rate was relatively stable, in order to standardize the effects of the die1 feeding rhythm. Lower feeding thresholds of 280, 127, 282, and 214 cells ml-' respectively at 4, 8, 12, and 16 "C were indicated by segmented linear regression analysis of the relationship between gut pigments and phytoplankton concentration. These thresholds corresponded to carbon concentrations of 30.5, 11.6, 20.7, and 16.4 pg C 1-l, and were not significantly different at the 4 temperatures. Die1 changes in gut pigment content were measured at 8 and 12 "C, at a T. constricta concentration of 2000 cells ml-l. Highest gut contents occurred at night, and were ca 4-fold higher than the lowest measured gut contents. Total daily ingestion rates were respectively l l 460 and 14 430 cells d-l, or 1.34 and 2.40 ~g C and 0.24 and 0.42 pg N d-l, and 33.4 and 56 5 % body C and 23.3 and 38.5 % body N d-l. The instantaneous gut evacuation rates were also determined at the 4 temperatures, with mean values of 0.0239, 0.0494, 0.0518, and 0.0645 min-' respectively at 4, 8, 12, and 16 "C, yielding a Q," of 1.88.
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