Journal of Plankton Research VoL19 no.8 pp.969-978, 1997
Daphnia swimming behavior during vertical migration
Stanley I.Dodson, Ralph Tollrian1 and Winfried Lampert2
Department of Zoology-Birge Hall, University of Wisconsin, 430 Lincoln Drive,
Madison, WI53706-1381, USA, lLudwig Maximilians University, Zoological
Institute, Karlstrasse 25, D-80333, Munchen and 2Max Planck Institutfur
Limnologie, Postfach 165, D-23402 Ploen, FRG
Abstract. We observed the individual swimming behavior of a clone of Daphnia hyalina swimming
freely inside a mesocosm-scale plankton tower. Changes in light intensity and the presence or absence
offishsmell induced vertical migration through -4 m. The results of analysis of video records of individual swimming behavior include the following: when lights were turned on, Daphnia moved down
by fast downward swimming, not by sinking or moderate swimming; when lights were turned off, the
Daphnia rose by fast upward swimming, not by upward moderate swimming (with hops). Moderate
swimming was nearly horizontal and fast swimming was nearly vertical. Fish smell increased the proportion of the population swimming fast in response to a light stimulus, but inhibited the expression
of sinking behavior. These results, interpreted in the light of the predator-avoidance hypothesis of
diel vertical migration, suggest that vertical migration through fast swimming is less dangerous than
vertical migration via either sinking or vertical moderate swimming.
Introduction
Diel vertical migration (DVM), a phenomenon of many kinds of plankton, has
been a favorite study of marine and freshwater ecologists since Cuvier first
described the phenomenon [see Cushing (1951) and Bayly (1986) for reviews of
the early history of DVM].
Some species of small (1-2 mm long) crustacean zooplankton exhibit DVM, as
do a variety of other organisms, including insect larvae,fish,rotifers and protists,
in both freshwater and marine systems (Cushing, 1951; Pearre, 1979; Bayly, 1986;
Larsson and Dodson, 1993). Typically, the population sinks as light increases in
the morning, spends the day deep in the lake in near darkness, and then rises
again toward the surface in the evening, as sunlight fades [but see Cushing (1951)
and Bayly (1986) for examples of organisms that descend at night]. Timing of the
migration is typically related to rates of change of light associated with dawn and
dusk (Cushing, 1951; Ringelberg e/a/., 1967; Pearre, 1973,1979; Ringelberg, 1987,
1993; Ringelberg and Flik, 1994). In freshwater, migration intensity is often
modulated by the presence of fish smell (an ecologically important chemical
signal or 'kairomone'; see Larsson and Dodson, 1993). Predator smell may not
be a necessary cue for marine organisms (Bollens et al, 1994). The amplitude of
migration depends on a constellation of environmental factors such as water
clarity (Juday, 1904; Dodson, 1990; Haney, 1993), the presence of predators
(Zaret and Suffern, 1976; Neill, 1990; Loose, 1993), food supply and temperature
(Stich and Lampert, 1984; Flik and Ringelberg, 1993), organism species (Cushing,
1951), organism size and age, and perhaps nutritional state (Pearre, 1979). The
influence of these environmental factors on DVM is further affected by the
internal physiological state and even past experience of migrating individuals
(Haney, 1993). DVM can be understood as an adaptive strategy (Lampert, 1993)
© Oxford University Press
969
SXDodson, R.ToDrian and W.Lampert
that optimizes the population growth rate by (i) minimizing mortality from visual
predators through daytime retreat to the dark refuge of depth and (ii) maximizing developmental and reproductive rates through night-time exposure to warm
near-surface water.
The mass movements called vertical migration are a population-level phenomenon that is related to individual movement. However, the relationship is not
necessarily straightforward. For example, Pearre (1979) argues that population
movements and individual movements have little in common. Movements of
migrating zooplankton individuals have not previously been directly observed for
animals swimming in a large volume of water.
The mesocosm-scale plankton towers in Ploen provide a unique opportunity
to make direct observations on individual free-swimming animals in large
volumes of water (Lampert and Loose, 1992). Loose (1993) used the Ploen plankton towers, a clone of Daphnia hyalina, and water conditioned with fish to simulate DVM. He showed that the D.hyalina population migrated downward when
lights were turned on, and upward when lights were turned on, if fish-conditioned
water was added to the tower. We used this same system to study individual
Daphnia swimming behavior during vertical migration induced by fish smell and
lights being turned on or off.
Daphnia do not all behave the same at any time, including during vertical
migration. Typically, the individuals may be exhibiting a variety of behaviors,
such as swimming in any direction (at a variety of speeds) or sinking, and populations at different depths may be behaving differently (Pearre, 1979). For the
population to migrate vertically from some average position, there has to be some
change in the swimming behavior of all or some of the individuals. In order to
understand better how individual Daphnia perform vertical migration, we
recorded the direction of movement and measured the amount of time Daphnia
spent swimming doing moderate swimming, fast swimming, or sinking, and the
direction of swimming. We also tested for the effect of fish smell on individual
swimming behavior. Based on previous observations by Loose (1993) and Van
Gool and Ringelberg (1995), we expected the Daphnia to show more extreme
responses in the presence of fish smell compared to water lacking fish smell.
Method
Biological and environmental set-up
We used the D.hyalina clone from Lake Constance (Stich and Lampert, 1984). In
the lake, the Daphnia show a pronounced DVM (up to 35 m). This clone is maintained in culture at the Max Planck Institute for Limnology in Ploen, Germany.
We produced sufficient animals to inoculate the plankton towers by growing
the clone in a 150 1 plastic rain barrel. The clone was regularly fed enough
Scenedesmus acutus to support rapid population growth.
The mass culture was filtered to remove small animals. Animals larger than -1
mm were added to a 64001 plankton tower (Lampert and Loose, 1992)filledwith
filtered lake water. Methods used for feeding the Daphnia and maintaining the
tower's environment were similar to those described in Loose (1993).
970
Daphnia swimming beharior during vertical migration
Thermal stratification was induced in the tower by cooling the water below 3.75
m to ~9°C (Figure 1). The upper 3.75 m (the epilimnion) remained at 19-20°C.
Observations through portholes in the tower confirmed that Daphnia did not
migrate into the colder water below the thermocline.
The food level (S.acutus) in the tower was adjusted to -4.5 mg C I"1. Epilimnetic food levels were monitored and adjusted by use of a particle counter
(CASY, Scha'rfe Systems) and a calibration curve for converting particle volumes
into carbon values.
Light levels were due either to room lights (lights on) or light leaking into the
room (lights off) from nearly covered windows. All experiments were carried out
during the day, between 09:00 and 17:00 h. The higher light level (lights on) was
-150 |xE m"2 s"1; the lower light level was -1.5 u,E nr 2 s"1 at the surface of the
water in the tower. Lights were turned on or off abruptly, as in Loose (1993).
Lights were left on for at least 2 h, then turned off, and the 2 h video record began.
After an hour, the lights were turned on again, followed by another hour of video
recording. This sequence was carried out once a day. The sequence was repeated
three times with fish smell, and twice again after a period of 5 days, during which
fish smell dissipated (Loose, 1993). Fish smell was introduced into the tower by
pumping in water from a culture of the cyprinid fish, Leucaspius delineatus, as
described in Loose (1993).
Temperature (°C)
8
10
12
14
16
18
20
22
8-
10 -
12
J
Fig. L Pattern of vertical thermal stratification in the Ploen plankton tower, with an indication of the
position of the video camera.
971
SXDodson, R.Toflrian and W.Lampert
Video set-up
A video camera was mounted outside the tower, in front of one of the glass ports
at a depth of 2.5 m below the water surface (Figure 1). The Daphnia migrated
between near the surface in dim light to ~4 m when the room lights were turned
on. We saw no Daphnia at windows below 4 m.
An IR light was suspended inside the tower, positioned in line with the camera
and -15 cm behind the center of the tower. The light had a 13 cm face of opaque
plastic and contained 16 IR diodes with a maximum output at -880 nm. This light
is probably invisible to Daphnia, but it provided a bright background to silhouette individual Daphnia for the IR-sensitive camera. The camera was adjusted to
tape a cube 10 cm on a side. Animals inside the cube were in sharp focus.
Data were scored manually from the video image from the two-dimensional
video record. For each animal, we wrote down the apparent angle of swimming
across the screen and the kind of swimming (fast, moderate or sinking). Although
two-dimensional representation of a three-dimensional behavior results in loss of
information (Hamner and Hamner, 1993), our method was necessitated by time
constraints and technical difficulties. A two-dimensional representation is biased
toward scoring individuals as swimming vertically (up or down) that in fact are
swimming straight toward or away from the observer. Similarly, swimming speed
toward or away from the observer is not evident in the two-dimensional record.
Thus, compared to the true three-dimensional behavior, two-dimensional data
will be more variable: some animals that are swimming fast mostly horizontally
will appear to be swimming slowly and vertically. Therefore, what we measured
was not necessarily the true angle of movement or true speed, but was
nevertheless an angle or speed useful for comparative purposes, because the same
biases occurred in all the treatments.
Data collection
Preliminary observations revealed three distinct swimming behaviors: fast swimming, moderate swimming and sinking. In our experiments, swimming behavior
was scored as animals swam through the field of view. These behaviors were
quantified as follows.
Moderate swimming behavior was similar to that observed under high light and
high food levels in Dodson et al. (1997), in which swimming speed averaged -5.4
mm s"1, with a standard deviation of 1.39. The animals hopped about three times
a second. A hop is a strong power stroke upwards, followed by a short bout of
sinking, as shown graphically in Dodson and Ramcharan (1991). Animals can
swim horizontally or upwards or downwards by varying the amount of time spent
sinking between hops (Gorski and Dodson, 19%). A single Daphnia swimming
moderately would take -20 s to cross the field of view.
Fast swimming speed is >8 mm s"1 measured as two-dimensional displacement
distance traveled over a 3 s interval. During fast swimming, the animals did not
appear to hop, and there were no perceptible sinking bouts between power
strokes. Fast swimming was typically in a straight line, in the direction the head
972
Daphnia swimming behavior during vertical migration
is pointing, and was typically vertical (upward or downward). A Daphnia expressing fast swimming would take -10 s to cross the field of view.
Sinking behavior was defined as downward with the head pointed up and with
less than one power stroke per second. Sinking rates were -3-4 mm s"1. Sinking
animals took -30 s to cross the field of view.
The angle of swimming was also quantified, based on the entry and exit points
of thefieldof view. The angle of swimming for each animal was classified as falling
into one of eight possible sectors (Figure 2). Thus, for each animal that swam
through the field of view, we recorded one of three types of behavior and an angle
of movement.
The 2 h set of observations was replicated three times with fish smell present
(one sequence a day on May 13, 14 and 15) and twice with no fish smell (one
sequence a day on May 22 and 23).
Data were combined for each 10 min segment of the 2 h observation period.
For each 10 min period, we calculated the proportion of animals swimming fast
or moderately, or sinking. The proportions were arcsine transformed to improve
normality.
We also calculated the average angle of fast and moderate swimming for each
10 min segment. We did not record the angle of sinking because, at our scale of
observation, there was no variability; the Daphnia always sank straight down.
Our fundamental statistical unit was the set of three transformed proportions
and the two average angles for each 10 min segment of each 2 h sequence.
Results
Figure 3 is a graphical presentation of the average frequency of behaviors and
average angles of movement, by 10 min intervals. It is important to remember
that the data in Figure 3 are for animals swimming through the 10 cmfieldof view.
45°
-45°
•""^
-90°
Fig. 2. The scheme used to score the two-dimensional angle of movement of Daphnia swimming across
the video screen.
973
SJJ>odson, R.TolMan and W.Lampert
5 15 25 35 45 55 65 75 85 95 105115
Fast
I 9 ¥ I
Moderate
o p p1* 8 o
f 1 |
Fasf
15 25 35 45 55, ,65 75 85 95 105115,
Light
Dark
Light
Time (minutes)
Fig. 3. Averages by 10 rain intervals for Daphnia swimming behavior. Dark circles indicate fish smell
present; open circles indicate no fish smell. Averages are based on three sets of 2 h-long observation
periods with fish smell present, and two sets of 2 h-long observation periods with fish smell absent.
Error bars are one standard error of the mean values.
We saw these animals for -10-30 s. At these temporal and spatial scales, we have
no information about how often animals switch behaviors. Therefore, when we
observed that, for example, 45% of the animals observed were performing fast
upward swimming, we cannot distinguish whether only 45% of the population
were exhibiting this behavior, or whether the entire population was switching
back and forth between fast swimming and moderate swimming, swimming
upward fast 45% of the time and horizontally the rest of the time. Because the
entire population tends to move downward (Loose, 1993), it is probable that each
individual switches between fast downward swimming and moderate horizontal
swimming.
Sinking rates did not change significantly during the 2 h observation period,
regardless of lights being turned off or on (one-way ANOVA, 17; P > 0.05; d.f. =
2,17 in observations withfishsmell, d.f. = 1,11 in observations withoutfishsmell).
In the dark, a larger fraction of the population was observed sinking when fish
smell was present than when fish smell was absent (the Sign test, P < 0.05, using
the averages in Figure 3). The same trend was seen in the light, but the difference
was not statistically significant (P = 0.11).
Before the lights were turned off, we observed many D.hyalina swimming
moderately and horizontally, and no animals swimming fast either up or down.
We tested for the effect of a change in light by (i) comparing the swimming
behavior from the last 10 min of the dark phase to the first 10 min of the light
974
Daphnia swimming behavior during vertical migration
phase, and (ii) the last 10 min of the light phase with the first 10 min of the dark
phase. In the second comparison, we assumed that the behavior at the end of the
2 h observation was representative of swimming behavior in light before the
observations began. The observations were analyzed using one-way ANOVAs
of swimming behaviors (dependent variables are type of behavior or angle) with
independent variables before or after the light change. The ANOVAs were
carried out separately for observations with or without fish smell (Table I).
There was a significant change in swimming behavior between the end of the
dark period and the beginning of the light period, and between the end of the
light period and the beginning of the dark period, regardless of whether fish
smell was present or absent. Upward fast swimming appeared when the lights
were turned off and downward fast swimming appeared when the lights were
turned on.
The presence of fish smell increased the percentage of the population expressing fast swimming (the Sign test, P < 0.05, n = 5). Data were analyzed separately
for the dark and light phases.
Moderate swimming was always more or less horizontal, regardless of light or
fish smell conditions (Table I).
Discussion
Daphnia hyalina vertical migration was simulated by our physical set-up which
was patterned on previous studies of DVM in the plankton towers (e.g. Loose,
1993). As in previous experiments, the Daphnia population was observed to move
up when lights were turned off, and down when lights were turned on.
There are several possible ways Daphnia might move downward. Daphnia are
commonly observed to sink when lights are turned on after a period of low light
(Ringelberg, 1987).
Table L Results of the two-way ANOVAs of behavior and angle of movement for the transition from
dark to light (lights on) and from light to dark (lights off)
Lights on
% Sinking
% Fast swimming
% Moderate swimming
Angle of fast swimming
Angle of moderate swimming
Lights off
% Sinking
% Fast swimming
% Moderate swimming
Angle of fast swimming
Angle of moderate swimming
Light
Fish
*
•
•*
• **
OS
•
**
ns
ns
*
*•
'Light' is the effect of the transition; 'Fish' is the fish smell present or absent
ns, P > 0.05; *P < 0.05; **P < 0.01; **•/» < 0.001. There were no significant interaction effects between
light and fish on swimming behavior.
975
SXDodson, R.ToUrian and W.Lampert
We expected the individuals to respond to a sudden increase in light by an
increase in sinking behavior or swimming (at the same moderate speed) in a more
downward angle. To our surprise, the Daphnia did neither. There was no increase
in sinking behavior and there was no change in the direction of moderate swimming. Instead, the Daphnia exhibited a large increase in downward-directed
head-down fast swimming, a behavior that is quite rare during normal swimming
(with no changes in light intensity).
Daphnia also used fast swimming to ascend when the lights were turned off.
Moderate swimming continued to be approximately horizontal.
Fast swimming either up or down declined in frequency over a period of -30
min.
Fish smell affected Daphnia in two ways. The presence of fish smell increased
the fraction of the population exhibiting fast swimming after a change in light,
and sinking behavior tended to be seen less often in the presence of fish smell.
What can we deduce about the adaptive significance of Daphnia swimming
behavior, if we assume that the predator-avoidance and temperature-advantage
DVM model is correct, and that our results accurately represent individual
Daphnia swimming behavior during vertical migration? First, because the
Daphnia did not use either sinking behavior or moderate downward swimming
to migrate downward, and because sinking behavior is inhibited in the presence
offishsmell, we conclude that fast vertical swimming results in less mortality than
either sinking or moderate downward swimming. Fast-swimming Daphnia are
almost certainly more conspicuous than slower-moving Daphnia (O'Brien, 1987).
However, fast-swimming Daphnia spend less time in the brightly lit upper waters
where they are at risk to visual predators. Therefore, the increase in conspicuousness must be more than offset by the decrease in time of exposure.
Fish smell increased the proportion of the population migrating downward.
This observation is consistent with the conclusion that Daphnia are better off
being conspicuous for a short amount of time.
Fast swimming in an upward direction, at low light level, puts the Daphnia at
small risk of fish predation, because the fish have difficulty seeing the Daphnia.
The model predicts that animals move upward to warmer water when visual predation is not a risk (e.g. at night). Fast swimming simply moves the animals to
warmer surface water more quickly than moderate swimming, and thus maximizes the amount of time spent growing and reproducing during the night in
warm water.
Our results pose several questions for future research. We used an on-off light
stimulus to influence swimming behavior. Does a gradual change in light intensity, simulating dawn and dusk, cause the same changes in swimming behavior?
Food was evenly distributed in the epilimnion of the tower. How does food
concentration and distribution affect individual behavior during vertical
migration? Does swimming behavior change with time of day or with the
seasons? Do other Daphnia clones (or species) show the same vertical migration
behavior? This seems unlikely, given the variability seen in Daphnia phototactic
behavior (De Meester, 1996) and the differences seen among different Daphnia
species (Dodson et aL, 1997). What happens at the top and bottom of the
976
Daphnia swimming behavior during vertical migration
migration path? Is there a gradient of fast swimming downward from the top of
the tower just after lights are turned on? That is, do all the individuals near the
surface show fast downward swimming, while deeper in the tower, a smaller proportion of the population swims fast? We observed individual animals for only a
few seconds, as they swam through a 10 cm square video image. What is the longer
time scale pattern of individual behavior during DVM? During the hour or so
that Daphnia were exhibiting fast swimming, were only a few animals swimming
fast (i.e. variability in behavior at the population scale), or did all individuals swim
fast for part of the time (i.e. behavioral variation at the within-individual level)?
Regardless of the pattern (whether variability at the population or individual
level), how does the variation arise, given that we were observing members of a
single clone?
There is clearly much more to know about the relationship between individual
swimming behavior and DVM. Our observations suggest that the tower system is
a powerful tool for studying the linkage between individual and population
behavior of zooplankton.
Acknowledgements
We thank Petter Larsson, Eric von Elert, Matt Brewer and Tom O'Keefe for their
contributions to this paper. Thanks also to H.Hansen (Plon electrician), R.Attoe
and K.Olesen (Madison electricians), F.W.Schdler and F.Nerhoff von Holderberg
(Plon machineshop), R.Gange (Madison machineshop), L.Sch5ler (Plon algae
cultures), H J.Krambeck (Plon computer department), W.Feeny (Madison artist),
D.Leland (Madison accountant and shipping expediter) and to the departmental
managers G.Hinz (Pl3n) and W.Holthaus (Madison) for their assistance in this
project. This research was funded by a research fellowship to S.I.D. by the Max
Planck Society for the Advancement of Science.
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Received on August 20, 1996; accepted on March 25, 1997
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