Phenotypic associations in the Bosminidae (Cladocera): Zoogeographic patterns

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1 Limnol. Oceanogr., 29(l), 1984, , by the merican Society of Limnology and Oceanography, Inc. Phenotypic associations in the Bosminidae (Cladocera): Zoogeographic patterns W. Gary Sprules Department of Zoology, University of Toronto, Erindale College, Mississauga, Ontario L5L lc6 John C. H. Carter Department of Biology, University of Waterloo, Waterloo, Ontario N2L 3Gl Charles W. Ramcharan Department of Zoology, University of Toronto, Erindale College bstract The hypothesis that enlarged antennules and mucrones in Bosminidae are adaptations to invertebrate predation pressure is tested in a range of physically diverse and geographically disparate lakes. In lakes with many abundant invertebrate predators, small bosminids tend to have longer mucrones, for a given body size, than they do in low-predator lakes; larger, less vulnerable bosminids show no such pattern. There is a much weaker, inverse relation between antennule length and predator abundance. Given that many of the study lakes are large and presumably ecologically complex, our data suggest not only that invertebrate predation is a considerable force in freshwater ecosystems but also that there is an enormous survival advantage for small prey species in the modification of their morphology. There has been considerable speculation on the selective advantage of exuberant helmets and tail spines in cyclomorphic freshwater Cladocera. It has been hypothesized that Daphnia with such enlarged structures sink less rapidly in warm (=low viscosity) water (Wesenberg-Lund 198), are less visible to fish predators because core body size is relatively small (Brooks 1965; Green 1967; Jacobs 1961; O Brien et al. 1979), swim faster because they have larger antenna1 muscles (Hebert 19783; O Brien and Vinyard 1978), and have relatively large surface areas for gas exchange (Hebert 1978 b). However, Gilbert (1966, 1967) and Halbath (197) demonstrated that elongated spines developed by the rotifer Brachionus calyczjlorus in the presence of the predatory rotifer splanchna actually reduce the prob- ability of capture or ingestion. Dodson (1974) proposed a similar protective advantage for the exuberant structures of Cladocera, and most of the available evidence is now considered to support the hypothesis that animals with enlarged helmets Financially supported through Natural Sciences and Engineering Research Council of Canada operating grants to W.G.S. and J.C.H.C. and a bursar-y to C.W.R. and tail spines are less vulnerable to invertebrate predators that prefer small prey, without increasing their visibility to vertebrate predators that prefer large prey (Grant and Bayly 1981; Hebert and Loaring 198; Krueger and Dodson 1981; O Brien and Vinyard 1978; O Brien et al. 1979). The mechanism of reduced vulnerability is unclear, but exuberant structures are generally thought to interfere with predator handling of prey. Recent evidence (W. C. Kerfoot pers. comm.) indicates that, for Daphnia, enlarged helmets may increase the probability of escape after the predator is encountered but before the Daphnia is actually handled by the predator. For the genus Bosmina in particular, individuals with enlarged mucrones experience relatively less invertebrate predator mortality than do those with smaller mucrones (Kerfoot 1975a,b, 1977a; Kerfoot and Peterson 198; O Brien and Schmidt 1,979; Wong 1981b). These studies, however, include in total only five lakes and two principal invertebrate predators. It is thus not clear whether cyclomorphosis in the Bosminidae is an adaptation only to some predators in certain habitats or whether it is a more general adaptation over the wide range of lake types (with their diverse pred- 161

2 162 Sprules et al. Fig. 1. taken. Outline.6mm of Bosmina showing measurements ator associations) inhabited by these cladocerans. Our objective is to assess this generality by examining statistical patterns in bosminid morphology over a relatively large sample of chemically and morphometrically diverse lakes in northeastern North merica. We will show that changes in bosminid morphology are more closely related to variations in invertebrate predator associations than they are to any other lake characters measured. We thank P. Chow-Fraser, J. Roff, W. C. Kerfoot, and C. K. Wong for comments on an earlier draft of this paper. Methods We chose lakes for this study from the data set of Roff et al. (198 l), which comprised 522 lakes selected from a large survey of northeastern North merican lakes (Carter et al. 198). Two subsets of lakes were selected according to the presence of the following nine commonly occurring predatory zooplankton species: Epischura lacustris Forbes, Limnocalanus macrurus Sars, canthocyclops vernalis Fisher, Cyclops scutifer Sars, Diacyclops bicuspidatus thomasi Forbes, Mesocyclops edax (Forbes), Leptodora kindtii (Focke), Mysis relicta Loven, and Chaoborus punctipennis (Say). We designated those lakes in which at least seven of these species were relatively abundant as high-predator lakes and those in which only one or two species were present as low-predator lakes. We further con- strained these subsets by requiring that either Bosmina longirostris (. F. Miiller) or Eubosmina longispina (Baird) be present and, within the limits of the data set, that the lake be sampled during the period May through September. In total, 67 lakes were selected (four had been sampled in October, two in November), 25 high-predator and 42 low-predator. Zooplankton species from the lakes were identified in one of our laboratories (see Carter et al. 198). The Bosminidae were classified according to Deevey and Deevey (197 1). Only lakes containing B. longirostris or E. longispina (or in a few instances both) were selected for this study, so that phenotypic changes within species could be studied separately from changes caused by a seasonal succession of several species. However, because a definitive taxonomy of the Bosminidae has probably not yet been established, we cannot rule out the possibility that phenotypic variation within currently accepted species will eventually be viewed as variation among species. We recognize that some of these predators (e.g. Epischura and Mysis) are more voracious than others, and hence that a lake with one of these species as the sole predator should perhaps not be classed as low-predator. In fact this happened rarely, because the vast majority of lakes with either species also contained at least five other species of predator. predator s impact is a complex combination of its density, numerical and functional responses, and spatial and temporal distributions. We thus consider that the procedure we have used to distinguish high- and low-predator lakes, while imperfect, is reasonable given the limitations of the available data set and is adequate for our purposes. Each of these lakes was sampled for zooplankton from bottom to surface at point of maximum depth with a conical townet; physical, chemical, and geographic data were also collected (see Carter et al. 198 for complete sampling details). From the preserved plankton sample for each lake, a random sample of juvenile and adult bosminids was selected. For the majority of lakes at least 3 animals were measured, except in three lakes where only six or nine animals

3 Bosminid morphology 163 Table 1. Physical characteristics of study lakes (n-sample size; SE-standard error), High-predator lakes (n = 25) Low-predator lakes (n= 42) Character Mean SE Range Mean SE Range Elevation (m) 265 Basin area (km2) Mean depth (m) Surface area (ha) Max hypolimnion temp ( C) (n = 24)* ,39.5 (n = 24) 6.4 Secchi visibility (m) 4.2 Epilimnion ph 6.7 Hardness (mgaliter-i) 25.1 % glacial ,359 l ,865 1,Oll.O , l 1.o (n = 41) , (n = 54:) O could be found. Using a Wild-Leitz stereomicroscope fitted with a Sanyo television camera and a Leco monitor, we determined the mucro, antennule, and body lengths of each bosminid (Fig. 1) with a microcomputer-based measuring caliper (Sprules et al. 198). The average values of each of these characters within a particular lake were computed, as were the average ratios of mucro : body and antennule : body. These averages form the basis for the analyses presented below. Discriminant function analysis (Morrison 1976) was used to test our expectation that, of all physical and biotic lake characters measured, bosminid morphology would most strongly and consistently differentiate between high- and low-predator lakes. The discriminating variables for the analysis included the physical and chemical variables, the bosminid morphology variables, and the abundances of the commonly occurring (found in > 1% of the original 522 lakes) zooplankton species exclusive of the nine predators and of Bosmina and Eubosmina. Except for ph, all physical and chemical variables were logarithmically transformed, the species abundances having originally been expressed on a logarithmic scale (Roff et al. 1981). These 3 discriminating variables were directly entered into the analysis; examination of their standardized coefficients on the resultant discrimi- nant function was the basis for interpreting the data (Green 1979). Discriminant function analysis was performed with SPSS (Nie et al. 1975) and other statistical analyses with SS (Helwig and Council 1979), all at the University of Toronto computing center. Results Our study lakes are distributed throughout the area bounded approximately by 4 1 O and 51 N lat and 68 and 81 W long. They span a considerable range of physical and chemical conditions (Table 1). The highpredator lakes resulting from our selection procedure are biased toward those of glacial origin (i.e. once part of the glacial lake systems: Dadswell 1974), because two of their characteristic predators (Limnocalanus and Mysis) are restricted to such glacial lakes (Roff et al. 1981). By contrast, most of the low-predator lakes are nonglacial, tending to be deeper and clearer with smaller drainage basins (Table 1). On average, the total predator abundance in high-predator lakes was three times that of low-predator lakes. The predatory zooplankters that characterize these high-predator lakes are all capable of preying on bosminids - Epischura (Wong 1981 a), Limnocalanus (Wong pers. comm.),. vernalis and D. bicuspidatus (Kerfoot 1978), C. scutzfir (Kerfoot et al. 198), Mesocyclops (Brand1 and Fernando

4 164 Sprules et al..1.4 l O Q. + b OO o BODY LENGTH (mm) Fig. 2. Mean mucro length plotted against mean body length for all lakes. Bosmina lakes form the left group of points, Eubosmina lakes the right group (see text). Solid symbols are high-predator lakes, open symbols low-predator lakes. Circles-lakes with Bosmina only; triangles-lakes with Eubosmina only; diamonds-lakes with both species. 1979), Leptodora (Karabin 1974), Mysis (Langeland 198 l), and Chaoborus (Pastorok 198). Hence, if cyclomorphic changes in bosminids represent adaptations to these predators, there should be clear differences in the morphology of the cladocerans between high- and low-predator lakes. Such differences do exist for bosminids <.53 mm long (Fig. 2, left group) whose mucrones, for a given body size, are larger in high-predator lakes than in low-predator lakes (the lakes fall into two groups on the basis of body size because most of those to the right in Fig. 2, the Eubosmina lakes, contain only the larger E. longispina, whereas most of those to the left, the Bosmina lakes, contain the smaller B. longirostris either alone or with E. longispina). Both the absolute and relative mucro lengths differ significantly between lake types (Table 2)- differences similar to those found by Kerfoot (1975a) for B. longirostris in low- and high-predator environments. The trend is particularly consistent for those lakes in which Bosmina is the sole bosminid (Fig. 2). In the Eubosmina lakes neither the mucro : body ratio nor the length of the mucro differs between high- and low-predator lakes (Table 2; Fig. 2, right group). lthough there are only five high-predator lakes in this group, there is clearly no tendency for them to contain bosminids with relatively longer mucrones. These patterns of variation are consistent with the hypothesis that cyclomorphic increases in mucro length are adaptive for bosminid cladocerans in lakes containing invertebrate predators. Smaller bosminids typical of the Bosmina lakes are within the preferred size range of the predator and have longer mucrones in high-predator environments than they do in low-predator environments. We presume that this longer mucro reduces predator success in reorienting captured prey as demonstrated by Wong (198 1 b). By contrast, the larger E. longispina generally fall outside the prey size range typical of many of the predators included in our study (Brand1 and Fernando 1978; Kerfoot 1977a; Wong 1981a). The absence of any consistent cyclomorphic pat-

5 Bosminid morphology 165 Table 2. Means (SE) of bosminid morphology variables in high-(h) and low-(l) predator lakes. Bosmina lakes with small (~.53 mm) animals and Eubosmina lakes with large (>.53 mm) animals are separated (n-sample size; NS-not significant). Character ntennule length (mm) Mucro length (mm) Body length (mm) ntennule : body Mucro : body Body length (mm) co.53,o 57 H (n = 2) L (n = 26) P H (n = 5) L (n = 16) P.163(.5).197(.7) ~.1.253(.19).255(.6).95 NS.78(.15).62(.2) co. 1.83(.3).79(.2).38 NS.437(.6).464(.5).2.583(. 15).6 1 S(O.8).16 NS.378(.8).4 17(. 11).1.443(.25).422(.9).3 1 NS.179(.8).132(.4) <O.OO 1.146(.5).132(.4).1 1 NS * Two-talled probablhty of obtammg observed result or worse If means are ldentlcal. NOV (Sokal and Rohlf I98 I) terns in mucro length for this species is thus expected under the predation hypothesis and has been independently noted by Deevey and Deevey (197 1). In the Bosmina lakes, it is not clear why body size is smaller in high-predator lakes (Table 2) although this could result from the size-selective feeding behavior of the various predators. lthough the trend is weaker, there is a tendency in the Bosmina lakes for the animals to have both absolutely and relatively smaller antennules in high-predator lakes (Fig. 3, Table 2). gain, the pattern is most evident in lakes that contain only Bosmina. This suggests to us that smaller bosminids may be at a disadvantage in having large antennules in high-predator environments. When bosminids are attacked, they protect their vulnerable swimming antennae by shielding them in a groove behind the stout antennules (Kerfoot 1977a). It is possible that longer antennules would more likely be damaged than shorter ones during attack, and in fact Kerfoot (1977a) notes such injuries. For the larger Eubosmina the lack of consistent relations between antennule length and predation intensity (Table 2) is presumed to reflect the reduced vulnerability of these larger bosminids to invertebrate predation. Our results differ from the general tendency for antennules to be longer in high-predator environments (Brock 198; Kerfoot 1975a; O Brien and Schmidt 1979). Because this trend in our data is comparatively weak, this result should be interpreted with caution, but we must also emphasize that we are dealing with a much greater diversity of predatory zooplankters per lake than is typical of the studies cited above. In fact Kerfoot (1977a) suggested that antennule response may depend on differences in the feeding behavior of the predominant predators in the system, e.g. cyclopoid vs. Calanoid predators. Because of more consistent evidence for increasing mucro length with predation pressure, we feel that this structure plays the more important role in reducing invertebrate predator mortality in bosminid cladocerans. Mucro and antennule lengths also change ontogenetically (Kerfoot 1975h). In most individual bosminid populations in our lakes, older (larger) animals have longer mucrones and antennules (Table 3), a trend reflected in the pattern of means (Figs. 2, 3). In a few instances feature length is unrelated to body size but is rarely inversely related. The only way such ontogenetic changes could confound the patterns discussed above is if high- (or low-) predator lakes consistently contained only young (or old) animals, or if lakes of either type had been consistently sampled during a restricted part of the season when only animals of a particular age predominated. Neither of these conditions applies, for both high- and low-predator lakes consistently contain the full size range of animals, and there is no seasonal bias in the sampling of the two lake types. Our choice of high- and low-predator lakes was based solely on the abundances of the nine predatory zooplankters listed above. If these predators are the primary cause of morphological variations in bosminids, then we should find that no other measured lake characteristic differentiates between the two subsets of lakes as well as the critical cyclo-

6 166 Sprules et al..3 u O P o OCD La.15 : ;@ P & 8 ; (&o O8 1 I, I BODY LENGTH (mm) Fig. 3. s Fig. 2, but for mean antennule length. morphic variables (e.g. mucro length). Discriminant function analysis supports this expectation (Table 4). The absolute value of its standardized discriminant function coefficient indicates a variable s relative importance in discriminating between highand low-predator lakes. Mucro length, body length, and their ratio are the most important discriminating variables. Knowledge of these bosminid features in particular would permit one to classify correctly the lake from which the specimens were taken as high- or low-predator with 95.5% success. ntennule length is considerably less important, less so than some morphometric variables (Table 4). No nonpredatory zooplankton species shows such consistent differences in abundance between high- and low-predator lakes as does morphological variation in bosminids. These results suggest that it is predation intensity, not other lake characters, that affect phenotypic changes in bosminids. Discussion We have shown that small Bosminas tend to have absolutely and relatively longer mucrones in high-predator than in low-predator environments. No such pattern was observed for the larger Eubosmina, which is presumed to be beyond the size of prey that can profitably be exploited by the invertebrate predators characteristic of our study lakes. These results are consistent with the hypothesis that increase in mucro length is an adaptive response that decreases mortality due to invertebrate predation. It is particularly noteworthy that this pattern is manifest over a large number of geographically disparate and physically diverse lakes. Hence, this evolutionary tactic in the Bosminidae appears generally, and not simply locally, adaptive. Our test of this predation hypothesis is conservative in at least two respects. First, many of the lakes are comparatively large and therefore presumably ecologically complex, so that predator-prey interactions could be ameliorated through physical or temporal separation of their populations. Second, the seasonal cycles of bosminid cyclomorphosis and predator abundance typical of most lakes (Black 198; Wong 1981 b) do not appear explicitly in our data, because the lakes were sampled only once between May and October. Both of these conditions would tend to mask ecological patterns; yet, we have detected relationships between bosminid morphology and predator communities. This not only suggests that invertebrate predation is a

7 Bosminid morphology Table 3. Pearson correlations between feature length in each (mucro, antennule) and body size for animals of 15 lakes. Lakes were chosen to represent the four combinations of species and predator abundance were otherwise selected randomly. Sample sizes vary among lakes, but statistically significant correlations are identified. High-predator Low-predator Mucro ntennule Mucro ntennule Bosmina.3*.87* *.13*.54*.6*.3*.44*.47*.51*.31* * Eubosmina *.39* *.56*.34*.42*.59* * Statistically significant. but considerable force in freshwater ecosystems but that there is an enormous survival advantage for small prey species in the modification of their morphology. If there is a selective advantage to longer mucrones in high-predator environments, then, since mucro length regresses when predators are rare, there must be an associated disadvantage in low-predator environments. Evidence suggests that, compared with short-featured morphs, long-featured morphs of cyclomorphic cladocerans have a lower intrinsic rate of population growth (Hebert and Loaring 198; O Brien and Vinyard 1978; Zaret 1969), filter at lower rates (O Brien and Vinyard 1978), or experience greater mortality at ecdysis (Zaret 1972). Kerfoot (19773) also suggested that long-featured morphs produce fewer eggs which are large with more yolk, because their offspring have thicker carapaces and longer features. If our observation that short antennules are adaptive in high-predator environmcnts is generally true, then their disadvantage where predators are rare requires clarification. Clearly experimental work on the role of bosminid antennules in predatory interactions is needed. The specific cyclomorphic response of a prey organism will depend on the feeding behavior of its principal predators. Kerfoot (1977a) noted that snatching predators such Table 4. Standardized discriminant function coefficients of limnological and biological variables. Character Coefficient Mucro length (mm) Mucro : body Body length (mm) Surface area (ha) Lake volume (m3) Max depth (m) Mean depth (m) Senecella calanoides 1.27 Diaptomus oregonensis.939 Epilimnion ph.843 Total hardness (mgmliter- ) Daphnia retrocurva Sida crystallina.699 Diaphanosoma birgei.637 Max hypolimnion temp ( C).581 Color (mgaliter-i).429 Diaptomus minutus Basin area (km2).411 Ceriodaphnia lacustris.384 Daphnia pulex.361 Elevation (m).359 D. galeata mendotae -.35 Hypolimnion ph.26 ntennule : body Secchi depth (m).23 1 ntennule length (mm).29 D. catawba.133 D. dubia -.12 Holopedium gibberum.75 Daphnia longiremis -.49 Canonical correlation =.945 Percentage of lakes correctly classified = 95.5 as Cyclops are best foiled by increased prey size, whereas handling predators such as Epischura are best foiled by modified prey shape. By contrast, there may be very little defense against large, fast predators such as Mysis. Clearly, the cyclomorphic patterns we have noted represent an integrated response to the nine different predators studied. Covariation in our data set is too great for us to be able to examine statistically the effects of subsets of predators with different feeding behaviors. Since the indications in our study are that invertebrate predators can exert considerable selective force on prey populations in lakes, it is clearly of interest to obtain detailed experimental data on the dynamics of these interactions. The genetic basis for the cyclomorphic response in the Cladocera appears to be complex. For both Bosmina (Black 198)

8 168 Sprules et al. and Daphnia (Hebert 19783), there is evidence that seasonal change in the relative frequency of long-featured morphs is actually a succession of genetically distinct clones; Black argued that each clone is optimally adapted to the prevailing predatory regime. By contrast, individuals of Daphnia can undergo cyclomorphic helmet elaboration during their development (O Brien and Vinyard 1978), indicating a purely phenotypic response. The proximate factors inducing cyclomorphosis, whether phenotypically or genotypically based, can be physical conditions such as high temperature alone (Hebert 1978a) or in combination with turbulence (Brooks 1965), or chemical secretions from predators as widely different as notonectids (Grant and Bayly 1981) and Chaoborus larvae (Krueger and Dodson 1981). Investigations of the evolutionary advantages of the indirect response to physical conditions vs. the direct response to chemical conditions and the associated predator-prey combinations involved would be profitable. References BLCK, R. W The nature and causes of cyclomorphosis in a species of the Bosmina longirostris complex. Ecology 61: 1122-l 132. BRNDL, Z., ND C. H. FERNNDO Prey selection by the cyclopoid copepods Mesocyclops edax and Cyclops vicinus. Int. Ver. Theor. ngew. Limnol. Verh. 2: , ND The impact of predation by the copepod Mesocyclops edax (Forbes) on zooplankton in three lakes in Ontario, Canada. Can. J. Zool. 57: BROCK, D Genotypic succession in the cyclomorphosis of Bosmina longirostris (Cladocera). Freshwater Biol. 1: BROOKS, J. L Predation and relative helmet size in cyclomorphic Daphnia. Proc. Natl. cad. Sci. 53: CRTER, J. C,, M. J. DDSWELL, J. C. ROFF, ND W. G. SPRULES Distribution and zoogeography of planktonic crustaceans and dipterans in glaciated eastern North merica. Can. J. Zool. 58: 1355-l 387. DDSWELL, M. J Distribution, ecology and postglacial dispersion of certain crustaceans and fishes in eastern North merica. Natl. Mus. Can. Publ. Zool. 11. DEEVEY, E. S,, JK., ND G. B. DEEVEY The merican species of Eubosmina Seligo (Crustacea, Cladocera). Limnol. Oceanogr. 16: DODSON, S. I daptive change in plankton morphology in response to size-selective preda- tion: new hypothesis of cyclomorphosis. Limnol. Oceanogr. 19: GILBERT, J. J Rotifer ecology and embryological induction. Science 151: 1234-l splanchna and posterolateral spine production in Brachionus calyciforus. rch. Hydrobiol. 64: l-67. GRNT, J. W., ND I.. BYLY Predator induction of crests in morphs of the Daphnia carinata King complex. Limnol. Oceanogr. 26: GREEN, J The distribution and variation of Daphnia lumholtzi (Crustacea: Cladocera) in relation to fish predation in Lake lbert, East frica. J. Zool. Lond. 151: GREEN, R. H Sampling design and statistical methods for environmental biologists. Wiley. HLBCH, U Die Ursachen der temporal Variation von Brachionus calyciflorus Pallas (Rotatoria). Oecologia 4: HEBERT, P. D. 1978a. Cyclomorphosis in natural populations of Daphnia cephalata King. Freshwater Biol. 8: b. The adaptive significance of cyclomorphosis in Daphnia: More possibilities. Freshwater Biol. 8: , ND J. M. LORING Selective predation and the species composition of rctic ponds. Can. J. Zool. 58: HELWIG, J. T., ND K.. COUNCIL SS user s guide. SS Inst., Inc. JCOBS, J Cyclomorphosis in Daphnia galeata mendotae Birge, a case of environmentally controlled allometry. rch. Hydrobiol. 58: KRBIN, Studies on the predatory role of the cladoceran Leptodora kindtii (Focke) in secondary production of two lakes with different trophy. Ekol. Pol. 22: KERFOOT, W. C. 1975a. The divergence of adjacent populations. Ecology 56: 1298-l Seasonal changes of Bosmina (Crustacea, Cladocera) in Frains Lake, Michigan: Laboratory observations of phenotypic changes induced by inorganic factors. Freshwater Biol. 5: a. Implications of copepod predation. Limnol. Oceanogr. 22: Competition in cladoceran commu- nities: The cost of evolving defenses against copepod predation. Ecology 58: Combat between predatory copepods and their prey: Cyclops, Epischura, and Bosmina. Limnol. Oceanogr. 23: 189-l 12. -, D. L. KELLOGG, JR., ND J. R. STRICKLER Visual observations of live zooplankters: Evasion, escape, and chemical defenses. m. Sot. Limnol. Oceanogr. Spec. Symp. 3: New England. -, ND C. PETERSON Predatory copepods and Bosmina: Replacement cycles and further influences of predation upon prey reproduction. Ecology 61: KRUEGER, D.., ND S. I. DODSON Embryological induction and predation ecology in Daphnia pulex. Limnol. Oceanogr. 26:

9 Bosminid morphology 169 LNGELND, Decreased zooplankton density in two Norwegian lakes caused by predation of recently introduced Mysis relicta. Int. Ver. Theor. ngew. Limnol. Verh. 21: MORRISON, D. F Multivariate statistical methods, 2nd ed. McGraw-Hill. NIE, N. H., C. H. HULL, J. G. JENKINS, K. STEIN- BRENNER, ND D. H. BENT Statistical package for the social sciences, 2nd ed. McGraw- Hill. O BRIEN, W. J., D. KETTLE, ND H. RIESSEN Helmets and invisible armour: Structures reducing predation from tactile and visual planktivores. Ecology 6: , ND D. SCIIMIDT rctic Bosmina morphology and copepod predation. Limnol. Oceanogr. 24: , ND G. L. VINYRD Polymorphism and predation: The effect of invertebrate predation on the distribution of two varieties of Daphnia carinata in south India ponds. Limnol. Oceanogr. 23: PSTOROK, R Selection of prey by Chaoborus larvae: review and new evidence for behavioral flexibility. m. Sot. Limnol. Oceanogr. Spec. Symp. 3: New England. ROFF, J. C., W. G. SPRULES, J. C. CRTER, ND M. J. DDSWELL The structure of crustacean zooplankton communities in glaciated eastern North merica. Can. J. Fish. quat. Sci. 38: SOKL, R. R., ND F. J. ROHLF Biometry, 2nd ed. Freeman. - SPRULES, W. G., L. B. HOLTBY, ND G. GRIGGS micro-computer based measuring device for biological research. Can. J. Zool. 59: 16 1 l-l WESENBERG-LUND, C Plankton investigations of the Danish Lakes. General Part: The Baltic freshwater plankton, its origin and variation. Gly- dendalske Boghandel. WONG, C. K a. Predatory feeding behavior of Epischura lacustris (Copepoda, Calanoida) and prey defense. Can. J. Fish. quat. Sci. 38: b. Cyclomorphosis in Bosmina and copepod predation. Can. J. Zool. 59: ZRET, T. M Predation-balanced polymorphism of Ceriodaphnia cornuta Sars. Limnol. Oceanogr. 14: Predators, invisible prey, and the nature of polymorphism in the Cladocera (Class Crustacea). Limnol. Oceanogr. 17: 17 l-l 84. Submitted: 1 September 1982 ccepted: I1 July 1983