Selective predation by the carnivorous marine copepod Euchaeta elongata: Laboratory measurements of predation rates verified by field observations of temporal and spatial feeding pattern&y2

The size of copepod prey strongly influenced dietary selectivity by Euchaeta elongata. Adult females (4.2-mm prosome length) showed highest predation rates on adult copepod prey 6501,000 pm long and on intermediate sized copepodid stages of Calanus pacz$cu+s. Selective feeding curves of copepodid stages IV and V of E. elongata showed a slight shift toward smaller prey. The prosome length of the primary prey was 70% the length of the second basipodal segment of the maxilliped for CIV, CV, and adult females of E. elongata. In both single and multispecies experiments at two concentrations, predation rates of adult female E. elongata seemed to be invariably focused on prey 650-1,000 pm long. Because E. elongata can consume adults of small copepod prey and the young copepodid stages of large copepod prey, it causes mortality of the entire copepod assemblage. Apparent recognition of a preferred prey, Pseudocalanus spp., and the nonpreferred cyclopoids seems to depend partly on their activity patterns. Thus, although size is an important determinant of dietary selectivity, species-specific behavioral differences can also influence feeding rates. Variations in predatory feeding activity on copepod prey were closely related to diel vertical migrations of E. elongata. The number of adult females with ingested prey began to increase soon after sunset, when the predatory copepod entered the surface layers containing prey of the preferred size, thus corroborating the selective feeding pattern observed in the laboratory. The timing of the migrations coincided well with sunset (ascent) and sunrise (descent) in winter, spring, and summer. The trophic interaction of predatory zooplankton with other animals has been recognized as important in structuring planktonic communities (Steele and Frost 1977; Zaret 1980). This structuring occurs through the selective impact of a predator on specific prey populations. The selection and rejection of food items by invertebrates can be regulated by various characteristics of both prey and predator, including body size (Anderson 1970; Brand1 and Fernando 1975; Pearre 1980), swimming behavior, predator attack and prey escape tactics (Gerritsen and Strickler 1977; Kerfoot 1978; Pastorok 198 l), and feeding history (Holling 1966; Lonsdale et al. 1979). The specific reaction of a predator to these cues allows distinct l This research was supported by National Science Foundation grants OCE 74-22640, OCE 78-18263, and GA-24385 to B. W. Frost. 2 Contribution 1401 from the School of Oceanography, University of Washington. 3 Present address: Hawaii Inst. Mar. Biol. Dep. Oceanogr., Univ. Hawaii at Manoa, 1000 Pope Rd., Honolulu 96822. feeding modes, resulting in a differential impact on prey. Often these modes of selection can have a strong influence on community interactions within planktonic systems. Selective feeding by an abundant predator can control the diversity of its herbivorous prey by preventing competitive exclusion between prey (Hall et al. 1970). Size-selective feeding can also alter community structure by removal of prey of a preferred size (Brooks and Dodson 196 5) or of a particular morphology (antipredatory adaptations: O’Brien et al. 1979; Kerfoot 1980). I have examined the effects of prey size and concentration on the selective feeding habits of a large (4.2-mm prosome length of adult female, 1.4-mg dry wt.), carnivorous copepod, Euchaeta elongata Esterly from Puget Sound, Washington. Because E. elongata can ingest adult stages of other copepods (Yen 1982a), I determined the predation rates of the three most advanced stages of Euchaeta feeding on different sizes of adult copepods, as well as on the different