Notes: SUMMARY. 1. Differential use of food resources by instars I-1V of Chaoborus punctipennis was examined in a mesotrophic New Hampshire lake during midsummer when all instars coexisted. Diet composition and prey preference were quantified first, at night when all instars were feeding at the same depth, and second, during the day when the early instars (I-II) and instar III were segregated by depth. Relative abundance, per cent biomass, and per cent frequency of occurrence of soft and hard-bodied rotifers, flagellated phytoplankton, protozoans, and crustaceans in C. punctipennis diets were quantified using crop content analyses.2. All four instars ingested large, flagellated phytoplankton (mainly Dinobryon and Ceratium). This unexpected result suggests that the effects of phytoplankton on Chaoborus growth and survival should be investigated. All instars also consumed rotifers (mainly Kellicottia, Gastropus, Polyarthra), but only instars III and IV fed upon crustaceans (mainly Daphnia, Bosmina and Diaphanosoma). Small rotifers (Gastropus spp., Keratella cochlearis, Trichocerca similis) occurred more frequently and were more abundant in early instar diets than late instar (III and IV) diets, whereas large rotifers (Asplanchna priodonta and Keratella crassa) were eaten only by instars III and IV. Zooplankton with gelatinous sheaths (e.g. Ascomorpha, Collotheca and Holopedium) were rarely ingested.3. Developmental increase in gape diameter of C. punctipennis seemed to be the major proximate mechanism causing dietary differences among instars. Body widths of hard-bodied prey in C. punctipennis crops were always less than gape diameter. The relationship between prey body width and Chaoborus gape diameter, coupled with knowledge of prey escape behaviour, should be useful for predicting the presence of hard-bodied prey taxa in diets of other Chaoborus species.4. All four instars of C. punctipennis selected soft-bodied, or weakly loricate, rotifer prey over crustaceans and phytoplankton. Early instars preferred the small rotifer T. similis and the protozoan Difflugia sp. to other rotifers and phytoplankton when feeding in the epilimnion or thermocline during the day or night. Late instars positively selected the rotifer, Asplanchna. Prey value (prey weight ingested per unit handling

Notes: 〈list xml:id="l1" style="custom"〉1 An experiment was conducted to investigate potential impacts of food limitation and copepod predation on juvenile survival of Chaoborus purtctipennis. We tested the hypotheses that: (i) juvenile survival of Chaoborus is influenced more by copepod predation than by starvation in a productive environment, and (ii) food limitation and predation interact to affect survival.2 Effects of food concentration (approximately 800, 1400 and 2300 microzooplankton 1-−1) and predator density (0, 1 and 2 Mesocydops edax 1-−1) on Chaoborus development and survival were evaluated using a 3 × 3 factorial design. Jars containing lake water, the appropriate food and predator treatments, and two Chaoborus (〈12h old) were rotated on a plankton wheel at 25°C. Survival and developmental stage were monitored daily until all individuals had either died or moulted to instar II.3 Predation by Mesocydops was the major source of mortality, causing 87.5% of Chaoborus deaths over all treatments. Chaoborus mortality was significantly higher in treatments with Mesocydops (67–100%) than in predator-free treatment (0–13%).4 Development time was significantly longer in the low-density food treatment than in the highest food treatment.5 No significant interaction between food limitation and predation was detected.6 These results suggest that predation by copepods may limit recruitment of juvenile Chaoborus in productive lakes.

Notes: SUMMARY. 1. This is the first study to examine predator-prey interactions between Chaoborm instars and rotifer prey. The predatory behaviour of instars I–III of Chaoborus pimctipennis and the diet selectivity of instars I—IV feeding on rotifers were examined in the laboratory. Prey used in direct observations of predatory behaviour included a variety of rotifers (Symhacta pectlnata, S. ohUmga, Polyarthra remata, Asplanchna girodi, Keratella crassa, spined and unspined forms of Keratella cochlearis) and two crustaceans (Bosmitia longirostris, Mesocyclops edax nauplii.2. In general, strike efficiencies (percentage of strikes resulting in inges- tion) increased in successive instars I—III. Early instar (I and II) strike efficiencies were low when compared with other invertebrate predators. For a given instar. mean prey handling times varied among prey species more than strike efficiencies. Mean handling times for small, soft-bodied rotifers were lowest and those for wide, hard-bodied prey were highest.3. Instar I exhibited significantly greater selectivity for the small, soft- bodied S. obUmga than for the larger S. pectinata, hard-bodied K. crassa, and spined and unspined forms of K. cochlearis. Instars II—IV positively selected both the large and small Symhaeta species over all Keratella species. The relationship between Chaobortts selectivity and prey value (weight of prey per unit handling time) can be described by a power function. Ingestion rates of rotifers by older instars (III and IV) are among the highest reported for invertebrate predators.4. Rotifer vulnerability to Chaoborus predation probably depended on rotifer cuticle texture, body width, and hydrodynamic disturbances. Spined rotifers were not necessarily protected from Chaoborus predation because Chaohorus can manipulate and swallow them. Giguere et al.'s 1982) encounter rate model must be modified to predict encounter rates of slow-moving rotifer prey with Chaohorus.