Effects of Fish Chemical Cues on Vertical Migration Behavior of Chaoborus

Effects of Fish Chemical Cues on Vertical Migration Behavior of ChaoborusAuthor(s): Sara F. TjossemSource: Limnology and Oceanography, Vol. 35, No. 7 (Nov., 1990), pp. 1456-1468Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2837733 .

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Limnol. Oceanogr., 35(7), 1990, 1456-1468 ? 1990, by the American Society of Limnology and Oceanography, Inc.

Effects of fish chemical cues on vertical migration behavior of Chaoborus

Sara F. Tjossem Ecology and Systematics, Corson Hall, Cornell University, Ithaca, New York 14853-2701

Abstract I quantified the diel vertical distributions of Chaoborus (phantom midge) larvae in water that

had been conditioned by fish or not exposed to fish, using flow-through laboratory columns and reciprocal transfers of larvae between a pond with fish and another without fish. The larvae that received fish-conditioned water underwent a significantly greater intensity of migration than did those that received fish-free water. Results support the hypothesis that Chaoborus larvae alter their vertical migration behavior in response to the presence of planktivorous fish, in particular, to associated chemical cues. Their behavioral flexibility in migrating permits response to a patchy environment that is variable both seasonally and between habitats.

Among the many hypotheses for the adaptive significance of vertical migration (see Kerfoot 1985; Bayly 1986), recent the- oretical (Gabriel and Thomas 1988) and field studies (e.g. Stich and Lampert 1981; Gli- wicz 1986) favor predator evasion to explain why zooplankton avoid surface water during the day. Predatory invertebrates are likely to respond to the migration of their zooplankton resources and to the risk en- countered from their own predators (Ger- ritsen 1980). Populations of phantom midge larvae show great variation in migratory behavior (Luecke 1986). Field observations show that only migratory larvae coexist with fish, and within migrating populations individual larvae in the benthos may not take part in a daily pattern (LaRow 1976). High- ly visible species either are eliminated from lakes by fish predation or they do not occur when fish are present (Stenson 1978; von Ende 1979). In lakes without fish, Chaobor- us larvae tend to be larger species that undergo little or no migration, such as Cha- oborus americanus (Fedorenko and Swift 1972), although Swift (1976) found a migrating population in the absence of fish.

Acknowledgments Work was partially supported by the American Mu-

seum of Natural History Roosevelt Fund, USDA Hatch Project NY (C)-1 83424 to N. Hairston, the Society of Sigma Xi, and a Mellon grant-in-aid ofresearch. Access to the Amot Pond courtesy of the Amot Teaching and Research Forest.

N. G. Hairston, Jr., C. D. Harvell, B. Peckarsky, G. T. Epp, K. D. Hambright, P. Dawidowicz, and M. L. Dini helped to revise this manuscript.

The underlying mechanisms for this variability are just beginning to be understood.

Luecke (1986) studied the migration of Chaoborusflavicans in Lake Lenore, Wash- ington-a lake known to have been fishless until 1979 when cutthroat trout were introduced. In 1976 the larvae were nonmigra- tory, but by 1982 the population had begun migration. He suggested two possible mechanisms underlying this change: natural selection and behavioral changes. He noted that a subpopulation isolated from fish in a nearby pool continued to migrate and con- cluded that selective predation by trout on nonmigrating individuals was responsible for the change in larval migration pattern. It is likely that fish can exclude populations that are unable to migrate (von Ende 1979), but another possibility is that chaoborids possess phenotypic plasticity by which they can adjust migration behavior in response to the presence or absence of predatory fish. I have addressed this question of induced behavioral response in both field and laboratory experiments with C. flavicans and Chaoborus albatus. If migration behavior can be altered, the larvae must be able to detect the presence of fish. My experiments indicate that Chaoborus can detect the presence of fish by some waterborne cue, re- sulting in migrating populations of chaoborids increasing their intensity of migration when exposed to water conditioned by fish.

Aquatic invertebrates have been shown to respond to chemicals exuded by their predators (Havel 1986; Harvell 1990). Al-

1456



Chemically induced migration 1457

though much of the work has been on the induction of morphological defenses, behavioral flexibility is also found in aquatic organisms (Peckarsky 1980; Sih 1986; Dod- son 1988). Many of these results are, however, responses to the proximity of predators, whereas the migration response of Chaoborus seems to be qualitatively different because the larval behavior changes without the direct presence of predatory fish.

Methods Chaoborus adults live < 6 d, with females

laying up to 500 eggs in rafts on the water; most eggs hatch in 2-4 d. They pass quickly through the first two instars and can develop to the 4th instar in 6-8 weeks. The fraction of a population that migrates may depend on the depth of the water column, the sea- son, and the instar (Parma 1971). Juday (1921) estimated that an average of 33% of the benthic larvae became planktonic each night, but this average does not indicate an individual's activity pattern. LaRow (1976) noted that, at least under laboratory conditions, some individuals may undergo frequent emergence and re-entry to the sediments at night. Temperate species of phantom midges generally have one gen- eration per year, and the larvae can over- winter as 3rd or 4th instars (Borkent 1981). Pupation occurs the following spring and takes from 1 d to 2 weeks. Migration appears to be of highest intensity in the 3rd and 4th instars and pupae (Parma 1971).

Two chaoborids, C. flavicans and C. albatus, were used in experiments to compare the vertical distribution of larvae exposed to treatments of water from ponds that had fish or were fish-free. Each species was cen- sused in the field with a 32-liter Schindler- Patalas trap to quantify both density and extent of migration. For all censuses and experiments the intensity of migration was measured by the difference between midnight and noon values of the number of larvae per liter. Because C. flavicans oc- curred both in a pond with fish and in a fish- free pond, it could be used in both laboratory and reciprocal transfer experiments. The larvae were collected from the Cornell University Experimental Pond Facility, a group of 0.1 -ha ponds near Ithaca. The two

ponds were virtually identical in morphom- etry (0.1 ha, 2.3 m deep), stratification (1.5 m), and oxygen (range over depth, 11.0-0.5 mg liter-1); both had an anoxic hypolimnion and both were classified as eutrophic (Wet- zel 1983).

In early June both ponds had similar numbers of zooplankton (+ fish pond = 291 liter-1, -fish pond = 273 liter-'), while in early August the pond with fish had more (+fish = 1,068 liter-1, -fish = 380 liter-1) (K. D. Hambright pers. comm.). Zooplank- ton were collected by vertical tows of a 64- jum mesh net. The pond with fish contained a mixed assemblage of fathead minnows (Pimephales promelas) and pumpkinseed sunfish (Lepomis gibbosus). Both fish species readily ate Chaoborus larvae (pairs of fathead minnows each ate an average of 7.8 larvae in 5 min, SD = 4.4, n = 6). In June 1989 field trials, each pond held an essen- tially pure population of C. flavicans, although some C. americanus were found in the fish-free pond. I attempted to exclude these large C. americanus from all experimental populations. During the August 1989 trials, the pond with fish held 77% C. flavicans and the fish-free pond held 94% C. flavicans; the balance of larvae was Cha- oborus punctipennis.

Chaoborus albatus was found only in a pond with fish and so it could be used only for laboratory experiments, not for a reciprocal transfer experiment in the field. The pond with fish (-0.3 ha, 2.5 m deep) was in Arnot Forest, a Cornell Research forest near Newfield, New York, and the larvae were collected with a 300-,um plankton net. Abundances of the larvae were documented at three depths (0.5, 1.25, and 2.5 m) in the water column at 1800 (before dusk), 2400, and 0600 hours (after dawn) on four dates in summer 1988 with a 32-liter Schindler- Patalas trap. The daytime census samples from Amot Forest were taken at 0600 hours, and the Cornell Experimental Ponds were sampled at noon.

Field experiments-Reciprocal transfer experiments with C. flavicans from the two Cornell ponds were carried out once in early summer (15-17 June 1989) and again in late summer (7-9 August 1989). Six wooden- framed enclosures (2.5 x 0.75 x 0.75 m),



1458 Tjossem

each with its bottom and two sides covered with seamless, 6-mil, clear plastic sheeting and the remaining two sides covered with 750-,um nylon mesh, were anchored in the center of each pond. As the enclosures sank, they slowly filled with pond water and small zooplankton that passed through the mesh sides. A preliminary trial had shown that this mesh size allowed access to some small plankton but prevented the escape of Cha- oborus larvae. Thus the larvae put into the +fish pond enclosures were exposed to the chemical presence of fish without being fed upon by the fish. The tops of all enclosures were covered with 0.5-cm mesh to prevent fish from jumping into them. The enclosures protruded from the water -0.3 m, giving a volume of 1,210 liters per enclosure in the -fish pond and 1,320 liters per enclosure in the +fish pond. Because the enclosures were left in the ponds between trials, I brushed the sides to free them of attached algae and to encourage water exchange before the second field trial, pumped out the previous larvae, and then added a new batch of phantom midge larvae taken from the ponds.

Once the enclosures were in place, I collected Chaoborus at night from each pond with a 300-,um mesh plankton net, including animals from all depths to minimize any bias in migratory tendency while sampling the population. I poured the larvae into a large container filled with water from the fish-free pond, mixed them by gentle aera- tion, and withdrew subsamples for a final density of 1.0 larva liter-1 for all enclosures. This density was chosen to approximate the average density of larvae found during the daytime census of the fish-free pond. About 75% of introduced larvae were 3rd and 4th instars early in summer, but by late summer the population was 90% 3rd and 4th instars. Chaoborus larvae returned to the same pond from which they came are called source larvae, those transferred to the other pond are called treatment larvae. The six enclosures in each pond were assigned animals randomly so that three enclosures held source animals and three held treatment animals. Three stations outside the enclosures were also sampled.

After 2 d of acclimation, larvae were sam-

pled at noon and midnight for 3 d at three depths (0.3 m, 1.2 m from the surface, and 0.25 m off the bottom) with a hand-oper- ated bilge pump modified with extensions to reach each depth from the side of a boat. A valve prevented water from entering the sampler until the desired depth was reached, and the sampler was cleared after each sample. All enclosure samples were taken at the center of each enclosure, and the maximum sampling depth was 0.25 m above the bottom to avoid excessive depletion of animals from the cages, because most larvae were near the bottom during the day. Each sample of 3.5 liters of water was concentrated with an 85-,um collecting filter, rinsed into a sample bottle, and preserved with 75% ETOH. To safeguard against selective re- moval of animals with a particular migration behavior, I removed < 10% of all stocked larvae in field enclosures during sampling.

General laboratory procedure -Experi- ments in the laboratory were carried out in June and August to test the effect of fish chemical cues on the vertical distribution of larvae. I made three separate tests: C. albatus, C. flavicans from a pond with fish, and C. flavicans from a fish-free pond. A peristaltic pump circulated treatments of +fish or -fish aquarium water through six Plexiglas columns (1 10 x 9 x 9 cm). Three columns were used for each treatment in a trial. Although these three columns were not completely independent because they were attached to the same aquarium, each trial was repeated three times over a week, giving a total of nine columns per treatment and three fully independent replicates. The water in the +fish aquarium was conditioned by placing eight (- 5 cm long) pumpkinseed sunfish in it a day before trials began; the fish remained in the aquarium during the trials to provide fresh chemical cue to the columns. The larvae in the columns were isolated from direct fish contact by the re- circulating water system that was turned on 1 d before each set of observations began. During this time the fish were fed mixed zooplankton that had been collected from and rinsed in fish-free pond water.

The water from each aquarium was pumped through the columns at a water re-




placement rate of 15 h. Outlfows at the tops maintained 1.0 m of water in each column to give a total volume of 9.2 liters per column. The placement of columns was randomized during each trial to prevent any systematic bias in treatment. Inflow tubes were at the bottom of each column and out- flow tubes at the top ensured turnover of water. Each column was marked in 10-cm increments, with the bottom 10 cm wrapped in black plastic to provide a light gradient for the larvae. As a refuge from light, the bottom of each column was covered with a 1.0-cm layer of washed, black aquarium pebbles. The columns were placed in a walk- in incubator with the photoperiod set to the natural daylength (1 4L: 1 OD) of early summer; the temperature was kept at 1 9?C. Three banks of daylight fluorescent lamps ensured even illumination across all columns as measured by a LiCor light meter submerged at depths in the water columns. The laboratory lights were much less intense than sunlight, but provided a gradient of intensity sufficient to maintain diel vertical migration.

Larvae were collected at night from the field and immediately brought back to the laboratory and counted out in sets of 30 under dim illumination. Sets of larvae (30 larvae per column) were randomly assigned to treatments of either + fish or - fish water. Chaoborus flavicans larvae were collected with a 300-,um plankton net at night from a + fish and a - fish pond; the C. albatus came only from a pond with fish during census samples. A mix of 3rd and 4th instar larvae was sorted from each population. The larvae in the columns were fed a rinsed, mixed assemblage of zooplankton (exclud- ing Chaoborus larvae) from the fish-free pond. I recorded the number of larvae observed in each 1 0-cm increment of each column the following noon, then again at midnight and the next noon. Each time I randomized the order of observing the columns. At the end of each trial I emptied and rinsed the columns with deionized water and began again with fresh water and newly caught animals. Because Chaoborus larvae show least light sensitivity at wavelengths of 620 nm or longer (Swift and Forward 1980), I minimized light disturbance by

conducting night counts with a flashlight whose dim beam was covered with a red gelatin filter (Kodak No. 25 Wratten) to al- low only wavelengths of 6 10 nm and longer to pass. Under this dim light the larvae did not show any noticeable change in behavior while I counted them.

Because C. albatus came only from a pond with fish, I had to remove fish chemical cues from their home pond water to create a fish- free water treatment. I reasoned that if the fish cue were proteinaceous, it would be de- natured by high heat (Haschemeyer and Haschemeyer 1973). I had also observed in preliminary trials that the effect of fish-conditioned water was short-lived (1 d) if the water was not continuously renewed with + fish aquarium water. In 1988 1 autoclaved (45 min at 250?C) the Arnot +fish pond water and then aerated it. In 1989 experiments on both C. flavicans and C. albatus, I used water from the fish-free Cornell pond filtered through 75-,um mesh instead of autoclaved water.

The significance of differences in vertical distributions between treatments was tested with ANOVA and planned comparisons of the means (Snedecor and Cochran 1980).

Results Field experiments-The vertical distri-

butions of both C. flavicans and C. albatus from field samples show that the larvae undergo a distinct pattern of diel vertical migration, since the greatest numbers of animals were in the water column at night (Fig. 1). Both populations of C. flavicans underwent migration, but the larvae from the pond with fish showed a much greater intensity of migration than did those from the fish- free pond. The C. flavicans larvae in the pond with fish were not up in the water column during the day, whereas larvae from the fish-free pond showed a lower intensity of migration and some were found throughout the water column both day and night. The intensity of migration was defined as the difference between midnight and noon values of the number of larvae per liter. The C. albatus in the pond with fish showed a marked pattern of vertical migration.

For maximal depth values, the sampling pump collected animals 0.25 m above the



1460 Tjossem

ARNOT FOREST

C. albatus 12

10

. 8- Night

6 -

25 Jun 8 ~ul 31 Jul 22 Aug 24 Sep

CORNELL PONDS C. flavicaits

-4

6 -

42- O+ F nih

2-

" ^ ^ ~~~~~~~- F night E--_ -- --- ---Day -Fday

0 --*-+ dv

11 Jul 18 Jul 3 Aug DATE

Fig. 1. Field census data for Chaoborus albatus ( 1 988) and Chaoborus flavicans (1 9 89) . Schindler-Pa- talas samples at three depths were pooled for a whole- water-column estimate. Chaoborus albatus densities are C 1 SE (n = 2); C flavicans samples are not repli- cated.

bottom of the enclosures, and missed those larvae on the very bottom. The number of larvae right on the bottom has been roughly calculated by knowing that the initial stocking densities were 1.0 larva liter-' and as- suming that mortality was negligible over the 3 d. If the samples at each depth are representative of densities in the water column, one can take the average of the densities at the three depths and calculate the density over all depths except for the very bottom. The three depths are then consid- ered to represent the water column. For ex- ample, in early summer the average number of larvae over the surface, middle, and maximal depths was 0.3 liter-v in 1,069 liters.

With a stocking density of 1.0 liter-', 320 larvae were in the upper 1,069 liters of water, and the remaining 890 larvae were in the bottom 141 liters of the enclosures (see Figs. 2-3).

I then pooled the number of larvae in the surface and middle ofthe water column from the three sampling days and compared the difference between mean numbers at midnight and noon to determine the intensity of migration (Table 1). These depths were chosen because the light levels at both were determined to be high enough for fish to locate larvae (Vinyard and O'Brien 1976). Vinyard and O'Brien (1976) calculated light levels typically limiting to planktivorous fish. Although the light levels for fathead minnows are unknown, I assumed that bluegills are representative of pumpkin- seeds, which are in the same genus. Light levels at maximal sampling depth in the pond with fish were equivalent to the larvae being hidden from view on the bottom of the enclosures, with only 1.0 x 10-7 % surface light in early summer and 0.2% in late summer. The overall pattern in the field trials was that larvae from both the +fish and the -fish ponds showed a significantly greater intensity of migration under the fish treatment than in the fish-free treatment.

In June both the source of the larvae and the treatment of presence or absence of fish cue had a significant effect on larval distribution in the water column (Table 2). I used further analyses of variance, with noon and midnight means as repeated measures, to compare the mean number of larvae in the water column between treatments. Al- though there was no difference in the number of larvae from the fish-free pond up at noon between treatments (Table 1), significantly more of these larvae (3 x) were up at night in the pond with fish compared to the fish-free pond (P < 0.01). The larvae from the fish-free pond seemed to spend more time at the surface at noon in the + fish treatment than in the -fish treatment, but the treatment means did not differ significantly. The larvae from the pond with fish in June were 4 times more likely to be up at noon in the -fish treatment compared to those in the +fish treatment (P < 0.05) (Table 1). At midnight, larvae from both




June:

No-Fish Pond No Fish -> No Fish No Fish-> Fish # larvae / liter

O 1 2 3 4 0 1 2 3 4 6 7 8 0 1 2 3 4 5 6 7

Sur 0 Day . Night

Mid

Max

Inferred Bottom

Fish Pond Fish -> Fish Fish -> No Fish O 1 2 3 4 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

. * . * . * | I . . * ., * . I . * . * . l . . . . .

Sur

Mid

Max

Inferred Bottom __._

Fig. 2. A comparison of the early summer vertical distribution of the average number of larvae outside cages with the number inside cages across treatments. For the treatments, each bar is the mean (+ 1 SE) of three replicate cages. The bottom layer is an extrapolation from the three sample layers, given the stocking density of 1.0 larva liter-'. The bottom layer could not be extrapolated for the samples outside the enclosures because absolute density was not known.

ponds were at least twice as abundant in the water when exposed to the +fish treatment than those in the -fish treatment (P < 0.00 1). As expected, no larvae were up in the water column at noon outside the enclosures in the pond with fish (Table 1). The densities and vertical distributions within the enclosures are similar to those outside in the ponds, suggesting that there was no confounding effect of enclosures on Cha- oborus migration behavior (Fig. 2).

The late-summer enclosures were again stocked with 1.0 larva liter-'. In August, the early summer pattern of greater intensity of migration in +fish treatments was still present regardless of the source of the larvae (Table 1). Unlike early summer, however, the source of the larvae was no longer a

significant factor, and the larvae from the fish-free pond showed no significant difference in vertical distribution between treatments at night (Table 2). In addition, the larvae from the pond with fish did not show a significant increase in the water column at noon in the -fish treatment. Further ANOVA with noon and night as repeated measures showed significantly more larvae were up in the +fish treatment at night (P < 0.01) compared to the -fish treatment.

In late summer the vertical distribution pattern outside the cages was still similar to that inside the cages. Larval abundance above the sediments increased outside the enclosures compared to that inside the enclosures (Fig. 3), despite the census data (Fig. 1) showing a slight decrease in the overall



1462 Tjossem

August:

No-Fish Pond No Fish-> No Fish No Fish-> Fish # larvae/ liter

0 1 2 3 4 5 0 1 2 3 4 0 1 2 3 4

Sur ODay . Night

6 Mid

Max

Inferred Bottom

Fish Pond Fish-> Fish Fish-> No Fish 0 1 2 3 4 0 1 2 3 4 0 1 2 3 5 6

Sur

Mid

Max

Inferred Bottom

Fig. 3. As Fig. 2, but for late summer.

outside population size. The number per liter outside the enclosures suggests that a decreased proportion of the pond population may have stayed close to the pond bottom (Fig. 3).

I see an interesting pattern emerging from the proportion of larvae up in the water

(Table 3), as calculated from the average number of larvae up in the water column over all depths (Figs. 2-3). In early summer, more chaoborids were up in the water column in the +fish treatment than in the -fish treatment, regardless of their source pond. The probability that larvae from the fish-

Table 1. Mean number of larvae per liter (? 1 SE, n = 9) up in the water column in the cages compared with the number of larvae per liter outside the cages for the reciprocal transfer experiment with Chaoborusflavicans. Source (S) refers to the pond from which the larvae came; treatment (T) refers to the pond in which they were placed.

Avg No. up inside cages Outside cages

S T Day Night Day Night

Early summer -fish +fish 0.56(0.19) 1.27(0.14) -fish 0.40(0.09) 0.34(0.08) 0.34(0.09) 0.43(0.18)

+fish +fish 0.06(0.04) 2.50(0.38) 0.00(0.00) 2.56(0.71) -fish 0.59(0.12) 0.99(0.20)

Late summer -fish +fish 0.12(0.10) 1.85(0.33) -fish 0.28(0.11) 1.39(0.21) 1.48(0.73) 3.21(0.51)

+fish +fish 0.12(0.07) 1.94(0.44) 0.03(0.03) 5.34(0.89) -fish 0.12(0.07) 1.11(0.18)




Table 2. Two-factor ANOVA using the difference between midnight and noon values as a measure of intensity of migration for Chaoborus flavicans in the reciprocal transfer experiment.

Source of variation df MS F P

Early summer Source pond 1 34.4 26.8 0.0008 Treatment

(+F, -F) 1 73.4 57.3 0.0001 Source x

treatment 1 10.1 7.9 0.023 Error 8 1.3

Late summer Source pond 1 0.009 0.004 0.95 Treatment

(+F, -F) 1 20.5 8.9 0.02 Source x

treatment 1 0.44 0.19 0.67 Error 8 2.3

free pond were up in the water column (surface, middle, and maximum depths combined), day and night, was about two times higher in the +fish treatment than in the -fish one (Table 3). They were more evenly distributed by depth in the -fish treatment than in the +fish treatment, but most larvae were still on the bottom (Fig. 2). All populations were mostly on the bottom, even though larvae from the fish-free pond migrated less than the larvae from the pond with fish. In late summer, the proportion of larvae in the water column increased to at least 3/4 of the larvae at all times (Table 3). Nevertheless, most were at the maximal sampling depth during the day and rose in the column at night (Fig. 3).

Laboratory trials-The density of larvae up in the water column did not differ sig-

Table 3. The average proportion of larvae up in the water column in the enclosures.

Proportion up

Source Treatment Day Night

Early summer -fish +fish 0.6 0.5 -fish 0.3 0.2

+fish +fish 0.7 1.0 -fish 0.4 0.4

Late summer -fish +fish 0.75 0.75 -fish 0.75 0.75

+fish +fish 0.75 1.0 -fish 0.75 1.0

nificantly within treatments between trials (ANOVA, P > 0.05), thus results of trials were pooled for further analysis. The presence of fish chemical cues affected the migratory behavior of Chaoborus larvae in both the lab and the field by altering the intensity of migration. In contrast to the field, however, a somewhat greater fraction of larvae were up in the water column in the -fish treatment than in the +fish treatment. Al- though the fish cue did not measurably alter the behavior of Chaoborus from the fish- free pond (Fig. 4A), the lack of cue did alter the behavior of the larvae from the pond with fish (Fig. 4B), so that more larvae were up in the water column during the day, and intensity of migration decreased. The laboratory experiments cannot rule out the possibility that the lack of mechanical stimulus (fish movement) produced this response, but it can be ruled out by the field trials because larval behavior changed in the field where it was unlikely that any mechanical cue came through the large enclosures. Migration was quantified by com- paring the number of larvae per liter in the top 90 cm of the column with the number in the bottom 10 cm covered in black plastic, but the treatment effects reported were also evident at 30- or 50-cm divisions.

For C. flavicans, either the source or the treatment of the larvae significantly affected the number of animals up in the water columns in the laboratory. Regardless of water treatment, the C. flavicans larvae from the fish-free pond were 2-3 times more abundant at noon than larvae from the pond with fish. The mean numbers of larvae from the fish-free pond that were up (Fig. 4A) were not different between treatments during the day, but the night averages showed slightly more up in the +fish treatment (P > 0.05). The larvae appeared to be sinking at night when compared to daytime densities, per- haps due to their having abundant food in the day. The C. flavicans larvae from the fish-free pond were a third more abundant in the water column in the day than at night. At all times, more larvae from the fish-free pond than from the pond with fish were up in the water column. In contrast, for larvae from the pond with fish (Fig. 4B), the presence of fish cue produced a significant de-



1464 Tjossem

Laboratory Columns:

A. Fishless Pond C. flavicans

4- C0 Noon E * Midnight

03-

2 2

0

With Fish Without Fish

B. Fish Pond C. flavicans

4 El Noon E Midnight

3 3-

.2

0


Fig. 4. Treatment effects on the mean number (+ 1 SE) of Chaoborusflavicans larvae liter-' in the upper, lighted layer (top 90 cm) of replicate (n = 9) laboratory columns containing 30 larvae each.

crease (P < 0.005) in the number of larvae up in the water column during the day, with no difference between treatments at night. The larvae from both the fish and fish-free sources were similar in the number up at night.

Chaoborus albatus in the laboratory experiments from both 1988 and 1989 (Fig. 5) responded in a manner similar to C. flavicans in the pond with fish. Removing the fish cue resulted in more larvae up in the water during the day and thus a lesser intensity of migration. Significantly more larvae (2-3 x) stayed up in the water at noon when exposed to fish-free water than when they were treated with water circulated through the aquarium with fish (P < 0.05). At night there was no significant difference

Laboratory Columns:

Amot Forest 1988

7- O Noon

6- Midnight

t 5-

. 4-

3 3

2-

* I

0-


Arnot Forest 1989

7 El [ 1Noon

8 6- Midnight

. 5 4

4 2

2-5

*! 1X 0


Fig. 5. Treatment effects on the mean number (+ 1 SE) of Chaoborus albatus larvae up in the laboratory water columns at noon and midnight. The 1988 means are from five replicate columns; the 1989 histograms are based on n = 9.

between the water treatments. These results were thus consistent from year to year and between species.

Discussion This study documents an inducible mod-

ification of vertical migration behavior in response to changes in some quality of the water associated with the presence or absence of fish. The larvae used in the reciprocal transfer experiments came from source ponds that were similar except for the presence or absence of fish. Both populations of larvae showed a greater intensity of migration when exposed to fish water. Within this similar pattern of response, the two populations showed different distributions in the water column between day and night. The




larvae from the pond with fish reacted as predicted when transferred to a fish-free pond by decreasing the intensity of their migration. Nevertheless, when deprived of fish cue they only lessened the intensity of their migration rather than stopping it al- together. Why do chaoborids bother to migrate at all in the absence of a specific cue? One might think that, given the costs of vertical migration, natural selection would favor those larvae that migrate only when a certain stimulus is present. The cost of migration, however, is unknown, and if the chemical cue is variable in its reliability, the best compromise might be to maintain the ability to migrate. This decline in migration intensity suggests that in the absence of fish the larvae gain some benefit, such as increased feeding, from staying up in the water column during the day.

When the larvae from the fish-free pond were transferred to the pond with fish, they did not immediately disappear from the water column during the day in response to fish cues. Instead, they showed a difference in migration only at night, with more larvae up in the water column in the +fish treatment compared to the -fish treatment. The difference in migration response between the two populations of C. flavicans suggests that the long-term association of a population with fish has a strong indirect or direct in- fluence on migratory behavior. The fish could indirectly affect migration behavior by altering food availability for the chaoborids, but this interpretation seems unlikely for two reasons. First, when the enclosures were stocked with larvae they also received some other zooplankton at the same time, but this allocation was equal across treatments. Second, during both field trials the zooplankton densities outside the enclosures were very similar in the fish-free pond, and increased in August in the pond with fish. Because food levels for the larvae were sufficient in both ponds during both trials, a difference in food supply does not appear likely to alter migration patterns over the 3-d trials.

A direct response to predators is suggested by Dorazio et al. (1987), who found that vertical migration of Daphnia in Lake Michigan increased between 1983 and 1985

when predation intensity was thought to be greatest. Such a quick response required either selection or a combination of selection and an induced behavioral response. Be- cause Chaoborus in the pond with fish had been subject to selection pressure by fish predation, an underlying genetic difference in the two populations may have combined with behavioral flexibility to produce the observed different responses of the two populations of larvae. Distinct genotypes could, however, be maintained only by genotypic- specific habitat selection by the females when they lay their eggs. Swift (1976) dismissed such a process when he observed that Cha- oborus trivittatus in Eunice Lake, British Columbia, migrated without having any fish in the lake at the time of the study. He dis- counted avoidance of fish predation as the proximal cause for migration, although he did not compare the intensity of migration of the population in the fish-free lake to that of a lake with fish. Instead, he suggested that there was enough genetic exchange among neighboring lakes with fish to maintain the genetic basis for a migration pattern. The relative importance of adaptation and behavioral response may differ between species or depend on local conditions. These dynamics remain to be explained.

In the laboratory columns, an addition of fish cue does not result in an increased intensity of migration for C. flavicans from the fish-free pond, although an increase is observed for larvae from the pond with fish. The differences between laboratory and field results may be due to differences in the con- centration of fish cue, light regime, and depth of the water column.

This study suggests that environmental cues can change migratory behavior without an attendent increase in mortality. The presence of fish chemical cues acts as a signal to the larvae to maintain a strong migratory response or suffer high predation. In contrast, a loss of fish cue may signal a decline in the risk of predation and thus a decreasing need to remain hidden in the sediments. Induced behavioral responses can have a short response time and act only when a specific predator is present. Sih (1987) reviewed major categories of antipredator traits and their evolution and sug-



1466 Tjossem

gested that the degree of environmental variability may determine when a species relies more on behavioral than on morphological defenses. When the cost of a defense is high, one would expect fixed morphological defenses mainly in an environment of consistent predation, but flexible behavioral responses under variable predation pressure. The larvae of the phantom midge are so small relative to many planktivorous fish that morphological defenses such as spines would be largely ineffective. An induced behavioral response may still be costly, with lowered reproduction due to the energy re- quirement of the behavior or because avoidance of predators causes the organisms to move to areas of lowered food con- centrations. In addition, more time spent in colder water may increase development times (Swift 1976).

Generations of Chaoborus larvae may be exposed to frequent fluctuations in selection pressure because the flying adult females may oviposit at ponds both with and without fish. They have not yet been found to show any oviposition site preference that could lead to adaptations to local conditions. Regardless of this possibility, any ability of the larvae to escape predation by altering their water-column migration behavior would increase their chances of sur- vival and eventual reproduction. In lakes with fish, vertically migrating larvae are safest from fish predation, while in a fish-free lake, larvae with a lower intensity of migration may encounter more prey in the surface waters or possibly develop sooner. The latter possibility is countered by the obser- vation that most larvae still remain at the greatest depth. The cost of possible exposure to fish predation may be greater than the cost of lost growth and reproduction due to migration. The safest behavior is, thus, for larvae to show some degree of migration behavior that can be accentuated when trig- gered by fish chemical cues.

My experiments cannot determine whether the behavioral differences between the two populations of C. flavicans are ge- netically based. They do, however, strongly support the hypothesis that a population can have a significant amount of behavioral

flexibility in migratory behavior in response to an environmental cue.

The lack of effect of source pond on the vertical distribution of C. flavicans in late summer enclosures (Table 2) may have sev- eral causes. Because the larvae from the pond with fish had been exposed to predation all summer, those larvae that did not have a strong migratory response probably suffered high mortality. This selective predation could have reduced the number of larvae up in the water, leaving only strong migrators in late summer. Yet if selection were the only explanation, one would expect that the intensity of migration would increase only in the pond with fish and not also in the fish-free pond as observed (Table 1). The similar migration patterns between ponds in late summer may result from increased water transparency or earlier emergence of those larvae that had stayed up in warm water during both day and night in the fish-free pond (P. Dawidowicz pers. comm.). A second possibility is that the pro- longed exposure to +fish water during summer had a lasting effect, causing larvae from the pond with fish to continue to migrate even when fish were absent. Thus larvae that were exposed to the fish factor at a particular stage migrated throughout their lives. A third possibility is that as the risk of visual predation grew larger throughout the summer with older instars, the cost of migration outweighed the earlier benefits of being up in the water. Early in summer the migrators traded the risk of being eaten at dawn and dusk for the benefit of finding prey. By late summer, they had less need for food because they had reached a stage where they could decrease food intake. Al- though older instars eat more prey per individual predator (Fedorenko 1975), they are also the stages that are capable of over- wintering with very little available prey. Predation risks could have remained the same while the nutritional needs of the larvae decreased, leading to a reduction in the number of larvae up in the water column during the risky daylight hours. Lastly, it should be pointed out that these factors could act in concert.

The predation hypothesis for vertical migration often depends on the assumption




that changes observed in vertical migration result from changes in the abundance of specific genotypes brought about by selective mortality (Stich and Lampert 198 1; Luecke 1986). Luecke (1986) found that C. flavicans in Lake Lenore migrated after the intro- duction of trout and he invoked natural selection to account for the change in distribution. On the basis of my results with the same species, however, environmental induction would explain equally well the pattern he observed.

The experiments reported here quantified a population response, not the movement of individuals. Understanding individual responses would help to clarify how variation in migration is expressed. The differences in vertical distribution could be due either to some individuals staying up in the water while others stay deep or to a contin- ual shifting among all larvae between the water column and the bottom. The marked tendency for a large proportion of larvae to stay down at all times may indicate that they feed near the bottom or that migration may have a periodicity > 1 d. Further comparisons of populations of the same species that vary in their rate of encounter with fish may reveal interesting variations in the ability of larvae to respond to the presence of fish.

Chaoborus has been found to escape visual predation because of its great transparency, its low activity when not migrating, and its migrations out of well-lit water. It now appears that the ability to migrate can be influenced by fish chemical cues (or their absence) inducing a change in behavior. This trait seems to be adaptive for the midges, because over generations individuals may encounter habitats with or without predatory fish. Behavioral modifications that decrease the chances of being eaten are likely to persist in the populations.

References BAYLY, I. A. E. 1986. Aspects of diel vertical migra-

tion in zooplankton, and its enigma variations, p. 349-368. In P. DeDeckker and W. D. Williams [eds.], Limnology in Australia. Monogr. Biol. 61. Junk.

BoRKENT, A. 1981. The distribution and habitat pref- erences of the Chaoboridae (Culicomorpha: Dip- tera) of the Holarctic region. Can. J. Zool. 59: 122- 133.

DODSON, S. I. 1988. The ecological role of chemical stimuli for the zooplankton: Predator-avoidance behavior in Daphnia. Limnol. Oceanogr. 33: 1431- 1439.

DoRAzIo, R. M., J. A. BOWERS, AND J. T. LEHMAN. 1987. Food web manipulation influences grazer control of phytoplankton growth rates in Lake Michigan. J. Plankton Res. 9: 891-899.

FEDORENKO, A. Y. 1975. Feeding characteristics and predation impact of Chaoborus (Diptera, Cha- oboridae) larvae in a small lake. Limnol. Ocean- ogr. 20: 250-258.

,AND M. C. SwIFr. 1972. Comparative biology of Chaoborus americanus and Chaoborus trivittatus in Eunice Lake, British Columbia. Limnol. Oceanogr. 17: 721-730.

GABRIEL, W., AND B. THOMAS. 1988. Vertical migration of zooplankton as an evolutionarily stable strategy. Am. Nat. 132: 199-216.

GERRITSEN, J. 1980. Adaptive responses to encounter problems. Am. Soc. Limnol. Oceanogr. Spec. Symp. 3: 52-62. New England.

GLIWICZ, M. Z. 1986. Predation and the evolution of vertical migration in zooplankton. Nature 320: 746-748.

HARVELL, C. D. 1990. The ecology and evolution of inducible defenses. Q. Rev. Biol. 65: 323-340.

HASCHEMEYER, R. H., AND A. E. V. HASCHEMEYER. 1973. Proteins. A guide to study by physical and chemical methods. Wiley.

HAVEL, J. 1986. Predator-induced defenses: A re- view, p. 263-279. In W. C. Kerfoot and A. Sih [eds.], Predation: Direct and indirect impacts on aquatic communities. New England.

JUDAY, C. 1921. Observations on the larvae of Cor- ethra punctipennis Say. Biol. Bull. 40: 271-286.

KERFOOT, W. C. 1985. Adaptive value of vertical migration: Comments on the predator hypothesis and some alternatives. Contrib. Mar. Sci. 27 (suppl.): 91-113.

LARow, E. J. 1976. Population interaction: A syn- chronizer for the persistent rhythmicity of Cha- oborus larvae. Hydrobiologia 48: 85-88.

LUECKE, C. 1986. A change in the pattern of vertical migration of Chaoborus flavicans after the intro- duction of trout. J. Plankton Res. 8: 649-657.

PARMA, S. 1971. Chaoborusflavicans (Meigen) (Dip- tera, Chaoboridae). An autecological study. Ph.D. thesis, Univ. Groningen. 128 p.

PECKARSKY, B. L. 1980. Predator-prey interactions between stoneflies and mayflies: Behavioral observations. Ecology 61: 932-943.

SIH, A. 1986. Antipredator responses and the per- ception of danger by mosquito larvae. Ecology 67: 434-441.

1987. Predators and prey lifestyles: An evo- lutionary and ecological overview, p. 203-224. In W. C. Kerfoot and A. Sih [eds.], Predation: Direct and indirect impacts on aquatic communities. New England.

SNEDECOR, G. W., AND W. G. COCHRAN. 1980. Sta- tistical methods, 7th ed. Iowa State.

STENSON, J. A. E. 1978. Differential predation by fish



1468 Tjossem

on two species of Chaoborus (Diptera: Chaobor- idae). Oikos 31: 98-101.

STICH, H. B., AND W. LAMPERT. 1981. Predator evasion as an explanation of diurnal vertical migration by zooplankton. Nature 293: 396-398.

SwIFr, M. C. 1976. Energetics of vertical migration in Chaoborus trivittatus larvae. Ecology 57: 900- 914.

, AND R. B. FORWARD, JR. 1980. Photore- sponses of Chaoborus larvae. J. Insect Physiol. 26: 365-37 1.

VINYARD, G. L., AND W. J. O'BRIEN. 1976. Effects of light and turbidity on the reactive distance of blue- gill (Lepomis macrochirus). J. Fish. Res. Bd. Can. 33: 2845-2849.

VON ENDE, C. N. 1979. Fish predation, interspecific predation, and the distribution of two Chaoborus species. Ecology 60: 119-128.

WETZEL, R. G. 1983. Limnology, 2nd ed. Saunders.

Submitted: 12 February 1990 Accepted: 8 May 1990

Revised: 6 August 1990



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Effects of Fish Chemical Cues on Vertical Migration Behavior of Chaoborus