Journal of Plankton Research Vol.18 no. 11 pp.2179-2196, 1996
Effects of food quality on individual growth and development in
the freshwater copepod Boeckella triarticulata
Saran Twombly1 and Carolyn W.Burns
Department of Zoology, University ofOtago, PO Box 56, Dunedin, New Zealand
1
Present address: Department of Biological Sciences, University of Rhode Island,
Kingston, RI02881, USA
Abstract. To quantify the demographic effects of food quality, and specifically of the 'poor-quality'
cyanobacterium Anabaena flos-aquae, we reared individual Boeckella triarticulata (Copepoda,
Calanoida) on two diets (monospecific Cryptomonas sp. versus mixed Cryptomonas-Anabaena diets)
and quantified individual growth and developmental trajectories by examining exuviae produced at
each molt, from hatching to maturity. Size at molting was less variable among individuals, within and
between diets, than age. Food quality had significant effects on male sizes at molting and on stagespecific daily growth rates of both sexes; these effects were strongest during late naupliar and all copepodite stages. The mixed Cryptomonas-Anabaena diet significantly slowed development, particularly
of copepodite stages. As a consequence of these effects, individuals raised on the mixed diet were
smaller and older at maturity. Within a given diet, individual differences explained much, if not most,
of the variation exhibited in growth and development. By following growth and development of a large
number of individuals throughout their life cycles, we show that individual females produce variable
offspring, indicative of a bet-hedging life-history strategy, and that Rtnarticulata (like other calanoids)
can grow, develop and survive on diets that include 'poor quality' cyanobacteria.
Introduction
Growth and development have important effects on life-history strategies, population dynamics and secondary production, as they interact to determine an individual's size and age at particular life cycle transitions (e.g. metamorphosis and
maturity), rates of recruitment and of biomass accumulation. For many ectotherms,
temperature and food are the primary environmental factors affecting growth and
developmental rates. Mean developmental times increase hyperbolically as temperature declines, and when food supplies are inadequate, many organisms grow
and develop slowly, survive poorly, achieve smaller sizes and produce few offspring
(e.g. Price, 1985). As a consequence, population sizes decline and organisms may
disperse (e.g. Dingle, 1972) or enter resting stages (e.g. Stewart et aL, 1967; Rolley
et aL, 1974; Istock et al, 1975; Tauber et aL, 1986; Santer, 1991). These general
responses are complicated by extensive individual variation in growth and development rates (e.g. Burdon and Harper, 1980; Bamstedt, 1988). Understanding the
effects of growth and development on life-history strategies, population dynamics
and secondary production requires information on the extent and nature of individual variation, as well as on the effects of environmental variables on mean rates
Growth and development have been studied extensively among aquatic invertebrates, particularly zooplankton. Data on these two processes in copepods are
limited to population mean values, primarily for late copepodite stages, as individual growth and developmental rates are difficult or impossible to measure
(particularly for naupliar stages; e.g. Thompson, 1982; Hart, 1990; van den Bosch
and Gabriel, 1994). In copepods, mean developmental times and stage durations
© Oxford University Press
2179
S.Twombly and GW.Bums
decrease with increasing temperatures and over a range of food concentrations
(e.g. Weglenska, 1971; Heip, 1974; Vidal, 1980a,b; Santer and van den Bosch, 1994),
and adult body sizes increase with increasing food concentration (e.g. Santer,
1994). Food quality also affects growth and development, although its effects have
been less well studied than those of food quantity. Poor food quality reduces
molting frequencies (Mullin and Brooks, 1970; Paffenhofer, 1970; Huntley et al,
1987; Vijverberg, 1989), decreases growth rates and body sizes (e.g. Xu and Burns,
1991) and, if poor enough, can prevent growth and development completely
(Huntley et al, 1987; Xu and Burns, 1991; Hart and Santer, 1994). Because copepods have longer generation times than most other zooplankton taxa, both the
quantity and the quality of food available are likely to change over an individual's
life cycle. In addition, successive developmental stages may differ in food requirements (Santer, 1994; Hansen and Santer, 1995). Thus, food quality, as well as food
quantity, has important consequences for copepod growth and development.
Food quality for primarily herbivorous zooplankton is assessed most often by
physical (size and shape) and chemical (nitrogen, carbon, protein or lipid contents) characteristics of the algae or by short-term response parameters such as
rates of ingestion or selection (reviewed by DeMott, 1990; Vanderploeg et al,
1990). Algal species that have high levels of essential biochemical constituents and
that are selected readily or ingested at high rates are considered to be of high
quality, while low-quality algae are usually those ingested at low rates or avoided
(except at low food levels), and that are low in carbon, nitrogen or other major
compounds (e.g. Butler et al, 1989; DeMott, 1989).
From a demographic perspective, assessment of food quality based on these criteria, and particularly on short-term feeding responses, is informative only if the
resulting 'high-quality' algae sustain growth and development throughout an
organism's life cycle, and support reproduction. Recent consideration of the
demographic consequences of food quality has yielded some surprising results.
Algal species considered to be of high quality, based on their ingestion or selection by copepods, have sometimes proven inadequate for juvenile growth and
development (Hart and Santer, 1994) or for adult survival and reproduction
(Chen and Folt, 1993). In contrast, some copepods grow, develop and reproduce
(e.g. Burns and Xu, 1990a; Xu and Burns, 1991; Santer, 1994) on phytoplankton
that are usually considered to be of low quality, such as cyanobacteria (e.g. Porter
and McDonough, 1984; Fulton and Paerl, 1987; Lampert, 1987).
Copepods often ingest substantial quantities of cyanobacteria in the field
(reviewed by Haney, 1987) and in the laboratory (e.g. Burns and Hegarty, 1994).
Calanoid copepods of the genus Boeckella, common in lakes throughout New
Zealand, persist during dense cyanobacterial blooms and may be particularly able
to ingest, assimilate, grow and and reproduce on cyanobacteria (Burns et al, 1989;
Burns and Xu, 1990b; James and Forsyth, 1990). However, naupliar and copepodite growth and development in the laboratory were severely limited when
individuals were raised on monospecific cyanobacterial diets (Xu and Burns, 1991;
Santer, 1994). Thus, the demographic consequences of cyanobacteria are
uncertain, particularly when this 'poor-quality' food is ingested together with
other algal species (as it would be in the field).
2180
Effects of food quality on A triarticulata
We designed the current study to determine the effects of food quality on
growth and development over the entire life cycle of the freshwater calanoid
copepod Boeckella triarticulata Thomson. We reared individuals on two diets: one
consisting solely of the high-quality (e.g. Chen and Folt, 1993; Hart and Santer,
1994) alga Cryptomonas sp. and the second consisting of a combination of Cryptomonas sp. and the low-quality (Xu and Burns, 1991) cyanobacterium Anabaena
flos-aquae. We used a new technique, which we call exuvium analysis (Twombly
and Bums, 1996), to quantify individual growth and developmental trajectories.
The resulting data allowed us to quantify the effects of food quality on individual
growth and development (age and size at successive stages, instar durations, stagespecific growth rates) from birth to maturation, and to examine the extent and
nature of variation in these important processes.
Method
Ovigerous female B.triarticulata were collected from Saddle Hill Quarry Pond,
20 km west of Dunedin, New Zealand, isolated in -30 ml spring water, and kept
at 15°C and 14:10 h light:dark photoperiod. Females were observed approximately every 12 h until their eggs hatched. Eleven newborn nauplii from each of
five sibships were reared on each of two diets: (i) log phase Cryptomonas sp. at a
concentration of 0.4 mg dry weight (DW) I"1 (henceforth referred to as CR), and
a combination of Cryptomonas sp. and A.flos-aquae (1:1 by DW) at the same
concentration (i.e. 0.2 mg DW H of each species, referred to as C-A). Nauplii were
reared individually in plastic vials containing 15 ml of a mixture (1:1) of modifed
MBL medium (Stemberger, 1981) and spring water, at 15°C and 14:10 h photoperiod. Boeckella triarticulata inhabits shallow lakes and ponds where it commonly achieves high densities and readily forages on settled material. Our rearing
conditions (15 ml standing water without constant resuspension of cyanobacteria)
were appropriate for this copepod, although they may not be for other species
living in different ecological conditions.
All individuals were examined daily for shed exuviae using a stereomicroscope
with darkfield illumination at 15-40 X. Exuviae were measured at 100 X (Nl-CI)
or 40 X (CII-CV) under a compound microscope using a calibrated ocular
micrometer; they provided a quantitative record of individual growth and
development, from birth to maturity. The lengths of naupliar exuviae were
measured from the anterior mid-dorsal margin to the base of the caudal spines,
and lengths from the anterior to mid-dorsal posterior margins of the prosome
(cephalothorax) were measured for all copepodite exuviae. The developmental
stage of each exuvium was verified at measurement.
Food and medium were replaced every second day. When nauplii metamorphosed to the first copepodite stage, food concentrations were increased to 1.2 mg
DW I"1; food concentrations were increased again, when animals reached
maturity, to 2.0 mg DW H (Burns and Xu, 1990b; C.W.Burns and B.Hegarty
(unpublished) determined this concentration to be above the incipient limiting
concentration for adult B.triarticulata). These food concentrations are higher than
those required by many marine copepods (e.g. Mullin and Brooks, 1970;
2181
S.Twombiy and GW.Burns
Paffenhofer and Harris, 1976; Vidal, 1980a,b), but similar to levels necessary for
development and reproduction in other freshwater copepods (e.g. T6th and Kato,
1996). Adult copepods were reared in 30 ml water. Microscopic observations
when medium and food were changed confirmed the presence of excess food for
adults, assuring that they were never food limited.
Cryptomonas sp. stock cultures were maintained in modified MBL medium
(Stemberger, 1981) at 20°C in dim light at 12:12 h light:dark cycle; A.flos-aquae cultures were grown in WC medium (Guillard and Lorenzen, 1972) and kept at
ambient temperature and photoperiod. Algal stock concentrations were determined turbidimetrically and translated to DW per liter using a previously determined DW regression. Appropriate volumes of each algal stock culture were added
to culture medium (modified MBL medium and spring water) to obtain the final
food concentration required, depending upon individual developmental stage.
Statistics
Our data on growth and development consisted of sequential measurements of
size and age at successive molts, on each of 55 individuals (11 nauplii from five sibships), raised on two separate diets. Size and age were log10 transformed to equalize variances. Age and size at each molt, together with the derivative parameters
of stage duration (time in days between two successive molts) and stage-specific
daily growth rates (change in size between two successive molts divided by stage
duration), were analyzed using repeated measures multivariate analysis of variance (MANOVA), in which the response variable (size, age, duration, growth
rate/day) for each stage was treated as a separate dependent variable. Multivariate analysis was chosen for repeated measures data because the assumption of circularity (evaluated by Mauchly's criterion for sphericity) was not met (von Ende,
1993). MANOVA was followed by profile analysis to examine the patterns of
response between diets and among sibships (von Ende, 1993). Hypotheses concerning the 'flatness' of the response variables were relatively unimportant in
analyses of age and size, as all individuals increased in size or age with each successive stage and response curves always had highly significant slopes. The 'flatness' of the response of stage durations and stage-specific growth rates over
successive stages was more interesting and allowed us to test the hypothesis that
instar durations or daily growth rates are constant over the entire life cycle. We
were also interested in hypotheses concerning the shape or parallelism of the
response curves. When stage-diet interaction terms in MANOVA analyses were
significant, profile analysis was used to identify those stages where diet had the
largest effects. For profile analysis, we examined differences in size and age
between three naupliar stages (N2, N4, N6) and three copepodite stages (CI, CIII,
CV) rather than all 11 stages, and adjusted the resulting significance levels accordingly (a = 0.05/4 = 0.0125). Profile analysis of instar durations involved pairs of all
successive developmental stages; for analysis of growth rates, contrasts included
N3-N6 and CII-CV stages. All significance levels were adjusted accordingly (von
Ende, 1993). Analysis of variance was then used to identify sources of variation
(diet, sibship, diet-sibship interaction) in stage-specific contrasts.
2182
Effects of food quality on B. triarticulata
Females and males were analyzed separately in repeated measures analysis of
age, size and stage durations, as exploratory paired /-tests revealed significant
differences in these parameters between sexes for older copepodite stages. Sexes
did not differ in stage-specific growth rates, and females and males were combined
for this analysis. Growth rate analysis included only stages N3-N6 and CII-CV.
We did not find Nl exuviae for all individuals, as the duration of the first naupliar
stage is very short and many individuals had already molted to the N2 stage before
we separated newly hatched nauplii into vials. As a result, we could not calculate
growth rates for N2 individuals. We also could not calculate CI growth rates
because we measured body length for N6 stages and prosomal lengths for CI
stages.
In addition to repeated measures, mixed-model ANOVAs were used to
examine sources of variation (diet, sibship, diet-sibship interaction) in age and
size at metamorphosis (N6 molt) and maturity (CV molt). We calculated variance
components for each random effect (sibship, sibship-diet interaction, and the
error term or individual variation) to determine the contribution of individual
variation to the variation documented in each parameter. Females and males were
analyzed separately. The relationship between age or size at metamorphosis and
age or size at maturation was investigated using Pearson product moment correlations. Statistical Analysis Systems (SAS) procedures were used for all statistical
analyses (SAS Institute, 1985).
Results
Eleven percent (or six individuals) of the nauplii initiated in each diet died before
maturity, and we followed the growth and development of the remaining individuals. Not all exuviae, for all stages of all of the surviving individuals, were recovered, and most of our statistical analyses were based on those individuals for
which we had complete growth trajectories (n = 46 for CR; n = 47 for C-A; Figures
1 and 2). In both diets, most or all pre-adult mortality (83% for CR, 100% for CA) occurred during copepodite stages, and primarily during the CI stage.
Growth
Two aspects of growth were analyzed: size at a given molt and the rate at which a
particular size was achieved (stage-specific growth rates). Size at successive molts,
averaged over all individuals in a diet, showed limited variation (Table I; coefficients of variation 3-6% with one exception) and was equally variable between
diets. This limited size variation, particularly in comparison with age (see below),
suggests that molting is size determined in this species.
The trajectories in Figure 1 show that individuals responded differently to
identical rearing conditions. The most marked variation was exhibited by individuals reared on C-A: although growth to metamorphosis (N6 molt) was fairly
uniform, growth achieved during the copepodite stages differed among siblings
(nauplii from the same clutch) and over all individuals reared on this diet. Diet
had a significant effect on male size at molting (F = 3.45, P = 0.012), and profile
2183
S.Twombry and CW.Bunis
Tabk L Mean (x), standard deviation (SD), coefficient of variation (CV) and sample size (n) for size
at molting for successive developmental stages of B.triarticulata raised on Cryptomonas alone (CR)
and a 1:1 mixture of CR and A.flos-aquae (C-A). The asterisk marks a change in the molt skin dimension measured for all copepodite stages
Stage
SD
(tun)
CV
(%)
n
(M.m)
135.0
166.6
214.1
265.7
336.1
423.2
414.7
540.5
739.1
974.3
1273.0
1131.5
8.4
7.8
10.0
15.7
16.0
15.4
16.0
21.3
36.9
61.3
52.4
44.6
6.2
4.7
4.7
5.9
4.7
3.6
3.8
3.9
5.0
6.3
4.1
3.9
25
46
46
46
46
46
46
46
46
46
23
24
X
Nl
N2
N3
N4
N5
N6
a*
CII
an
CIV
CV-F
CV-M
C-A
CR
X
SD
((im)
CV
(%)
n
(jim)
136.6
170.4
2125
262.7
338.6
417.2
403.9
5323
718.2
935.2
1218.8
11123
7.7
6.5
11.2
12.8
11.6
15.9
56.5
193
39.1
56.5
46.2
57.0
5.6
3.8
5.3
4.9
3.4
3.8
13.0
3.6
5.4
6.0
3.8
5.1
23
47
47
47
47
47
47
47
47
47
23
26
analysis identified the early copepodite stages (CI-CIII contrast, F = 7.74,
P = 0.OO8) as the stages during which this effect was largest. The effect of diet on
female sizes was only marginally significant (F= 2.59, P = 0.04) and profile analysis was not performed for females There was no significant interaction between
diet and sibship for either gender.
Mean stage-specific growth rates averaged over all individuals raised on a given
diet and coefficients of variation are shown in Table II. Growth rates were highest
for CIII and CIV stages, and declined for the CV stage; extensive individual variation in growth rates is reflected in the large (>25%) coefficients of variation.
Sexes were pooled for repeated measures analysis of growth rates (n = 92), which
showed that growth rates varied significantly among stages (F- 88.53, P = 0.0001)
and that diet affected growth rates (F = 6.38, P = 0.0001). Sibship also affected
growth rates (F = 3.80, P = 0.0001), primarily because individuals from sibship 4
had lower growth rates than those from other sibships. There was, in addition, a
significant interaction between diet and sibship over all stages (F= 1.72, P = 0.03).
Profile analysis of specific contrasts showed that diet had a significant effect
Tabk IL Mean stage-specific growth rates (fun d a y 1 ) and coefficients of variation (%, in parentheses) for successive stages of B.triarticulata raised on CR (n = 45) and C-A (n = 47) diets
Stage
CR
C-A
N3
N4
N5
N6
CII
43.88 (35.7)
47.48 (35.9)
6331 (36.8)
74.24 (38 5)
6133 (41.2)
95.95 (26.9)
107.60 (29 5)
65.84 (33.6)
37.26
44.83
69.66
58.06
5430
73.74
72.59
49.98
cm
CTV
CV
2184
(38.3)
(41.6)
(31.9)
(44.4)
(33.6)
(35.2)
(37.1)
(43.0)
88
to
.
8
l - M
«•
Kg. 1. Growth trajectories for individual B.triarticulata from five sibships reared on two separate diets: Cryptomonas sp. alone (CR) and a 1:1 combination ol
CR and A.flos-aquae (C-A). Each trajectory tracks the size an individual achieved at successive molts, over time (in days post-hatching). Total lengths of naupliar exuviae and prosomal lengths of copepodite exuviae were measured; this difference accounts for the apparent discontinuity (or step) in each growth trajectory.
-
i
I
S-TWombly and CW.Bmiis
(adjusted a = 0.05/6 = 0.008) on late (N5-N6) naupliar stages (F = 8.22, P = 0.005)
and copepodite stages CII-CIV (F = 10.45, P = 0.002).
Both females and males raised on C-A were significantly smaller at maturity
(CV molt) than individuals raised on CR (F= 7.25, P = 0.01 for females; F= 5.7,
P - 0.02 for males). Sibship accounted for a significant amount of size variation at
metamorphosis in both sexes and for male size at maturity; the diet-sibship interaction was insignificant in all cases. Variance component analysis showed that a
substantial portion of size variation at metamorphosis and maturity was explained
by individual differences (the error term; Table III). Size at metamorphosis was
inversely, but insignificantly, correlated with size at maturity for individuals reared
on both diets (r = -0.05, P = 0.74, n = 43 for CR; r = -0.22, P = 0.13, n = 46 for CA). This result indicates that any size advantage achieved by metamorphosis was
not maintained throughout the life cycle, and that growth patterns during the
copepodite (juvenile) phase differed from those exhibited earlier in the life cycle
(among naupliar stages; see Figure 1).
Development
Age at a given molt was always more variable than size (Table IV; coefficients of
variation 8-16%), for all stages after the second naupliar stage, and this variation
was fairly uniform over all stages. Females and males at maturity (CV molt)
reared on C-A exhibited the most variation in age. This variability is reflected in
Table IIL Results of mixed-model analysis of variance for age and size at metamorphosis (N6 molt)
and maturity (CV molt), and variance components: F statistics, significance (P) and variance components (VC, % ) . The error term represents individual variation
Source
Females
Males
P
VC
1.93
3.61
1.17
-
0.17
0.01
034
-
20.0
0.0
80.0
Diet
Sibship
Diet x sibship
Error
7.25
1.94
0.45
-
0.01
0.12
0.77
-
AGE - metamorphosis
Diet
Sibship
Diet x sibship
Error
3.31
1.48
0.74
-
F
SIZE - metamorphosis
Diet
Sibship
Diet x sibship
Error
F
P
VC
022
4.13
0.61
-
0.64
0.007
0.65
-
26.0
0.0
74.0
_
14.2
0.0
85.8
5.7
9.98
1.53
-
0.02
0.0001
0.21
-
—
49.6
5.7
44.6
0.08
0.23
0.57
-
_
5.7
0.0
943
5.0
11.7
0.09
-
0.03
0.0001
0.98
-
_
523
0.0
47.7
0.0001
0.17
0.93
-
_
19.14
0.0
80.86
41.6
10.8
3.18
-
0.0001
0.0001
0.02
-
_
29.9
22.7
47.3
SIZE-maturity
AGE -maturity
Diet
Sibship
Diet X sibship
Error
2186
22.4
1.7
0.21
-
Effects of food quality on Rtrtartiailata
Table IV. Mean (jr), standard deviation (SD), coefficient of variation (CV) and sample size (n) for age
at successive molts for Rtriarticulata raised on CR and C-A diets
Stage
CR
SD
(days)
CV
(%)
n
(days)
1.0
2.0
3.2
43
5.5
6.8
9.0
11.2
13.3
15.6
20.3
18.3
0.0
0.0
0.4
0.5
0.5
0.5
0.8
1.0
1.1
1.2
1.7
1.5
0.0
0.0
12.1
11.1
9.1
7.5
9.1
9.4
8.5
7.8
8.7
83
25
46
46
46
46
46
46
46
46
46
23
24
X
Nl
N2
N3
N4
N5
N6
CI
CII
an
CIV
CV-F
CV-M
C-A
SD
(days)
CV
(%)
n
(days)
1.0
2.0
33
4.5
5.7
12
95
11.8
14.5
17.8
24.2
22.4
0.0
0.0
0.4
0.6
0.5
0.6
0.7
1.1
1.2
1.9
3.7
3.5
0.0
0.0
13.6
12.2
9.3
8.7
7.9
9.2
8.2
10.9
15.4
15.6
23
47
47
47
47
47
47
47
47
47
23
26
X
the resulting developmental trajectories (Figure 2): individuals within a sibship
and treatment varied in age at each successive stage. For example, individuals
from sibship 1 that were reared on C-A ranged from 15 to 26 days old at the penultimate copepodite stage (CIV molt). As with growth, developmental trajectories
diverged most among older, copepodite stages.
Repeated measures MANOVA showed significant effects of diet on age at
molting in both sexes (females: F = 4.69, P = 0.003; males: F = 8.93; P = 0.0001), as
well as a significant effect of sibship on males (F= 5.57, P = 0.0001). Profile analysis for both sexes showed an effect of diet on development over all copepodite
stages (females: CI-CIII contrast F =7.93, P = 0.008 and CIII-CV contrast
F= 10.20, P = 0.003; males: CI-CIII contrast F= 17.22, P = 0.002 and CIII-CV contrast F - 16.47, P = 0.0002), but not for naupliar stages. Sibship effects were highly
significant (P = 0.0001) for all contrasts for males (i.e. throughout development),
but were significant only for early naupliar stages (N2-N4) in females.
Stage durations were fairly uniform over all naupliar stages, although coefficients of variation for each stage were large (>30%; Table V). Stage durations
were longer, but similar for the first three copepodite stages. The CV stage exhibited the longest duration. Durations of older copepodite stages raised on the CR
diet were less variable (coefficients of variation 16-18%) than those for individuals reared on C-A (38-54% coefficients of variation). Repeated measures
MANOVA showed that both diet (females: F = 3.25, P = 0.008; males F = 5.22,
P = 0.0002) and sibship (females: F = 2.54, P = 0.0001; males: F = 2.60, P = 0.0001)
affected stage durations, particularly for older copepodite stages (females:
F = 3.93, P = 0.05; males: F = 7.87, P = 0.008).
Both females and males reared on C-A were significantly older at maturity than
individuals reared on CR (F= 22.4, P - 0.0001 for females: F=41.6, P = 0.0001 for
males). Age at metamorphosis was positively and significantly correlated with age
at maturity for individuals reared on CR (r = 0.66, P = 0.0001, n = 43): some
individuals consistently developed more rapidly than others and the first
2187
88
to
Fig.2. Developmental trajectories for individual B.triarticulata from five sibships, reared on two separate diets (CR versus C-A). Each trajectory tracks the age
of an individual (days since hatching) at successive molts, from N2 to CV.
i:
i:
i
P
a
f
Effects of food quality onB. triarticulata
Table V. Mean stage durations (in days) and coefficients of variation (%, in parentheses) for
Rtriarticulataraisedon CR and C-A diets
Stage
CR
Females
1.26 (35.6)
1.26 (35.6)
1.13 (30.5)
1.08 (26.5)
1.48 (34.5)
2.08 (24.6)
a
2.35 (20.7)
en
2.17 (22.6)
cm
2.43 (20.8)
crv
4.09 (32.9)
cv
1
Sexes differ at P < 0.02.
N2
N3
N4
N5
N6
C-A
Males
Females
Males
1.12 (30.0)
1.13 (30.0)
1.17 (32.6)
1.21 (34.3)
1.21 (34.3)»
2.17 (32.4)
2.17 (32.4)
1.96(183)
2.12 (15.9)'
3.21 (183)
1.18 (33.4)
1.23 (34.9)
1.23 (34.9)
1.14(30.9)
1-59(31.6)
2.36 (22.5)
230 (23.9)
239 (24.2)
3.23 (20.1)
6.41 (57.3)
1.28 (35.8)
1.28 (35.8)
1.24 (35.2)
1.20 (34.0)
132 (333)
2.16(21.9)
2.24 (26.7)1
2.80 (25.3)
3.44 (54.4)
4.48 (38.2)1
individuals to metamorphose were also usually the first to mature. Among individuals reared on the C-A diet, however, age at metamorphosis was not correlated
with age at maturation (r = 0.17, P = 0.25, n = 46). Developmental trajectories of
these individuals diverged after metamorphosis (Figure 1) and variation in age at
maturity increased (Table IV), so that the first individuals to metamorphose were
often not the first to mature.
Discussion
Food quality affected both growth and development among individual B.triarticulata in this study. The inclusion of the 'poor-quality' cyanobacterium, A.flosaquae, in a diet containing Cryptomonas slowed development and prolonged
stage durations, so that individuals raised on the mixed diet were older at maturity
than their siblings raised on Cryptomonas alone. Growth rates of individuals
raised on the C-A diet were also reduced and these copepods were smaller at
maturity than siblings raised on CR. The effects of diet were pronounced in older
copepodite stages, but were usually not detected among nauplii. We found no
effect of food quality on mortality or the pattern of mortality. Age at a given molt
was always more variable than size, implying that development is more variable
than growth. Coefficients of variation, growth and developmental trajectories, and
variance components all indicated that individual variation for all parameters was
substantial, and often the largest source of variation documented. Aspects of
these results are discussed below.
Effects of food quality on growth and development
Reduced growth and delayed development exhibited by B.triarticulata fed on the
mixed C-A diet indicate that this diet was of poorer quality, demographically, than
the monospecific CR diet. Similar effects of poor food quality have been reported
for a variety of other copepods raised on either unialgal or mixed diets (Mullin
and Brooks, 1970; Paffenhofer, 1970; Huntley et al, 1987; Ban, 1994; Hart and
2189
S.Twombly and CW.Bnms
Santer, 1994; Santer and van den Bosch, 1994; Hansen and Santer, 1995). For
example, Huntley et al. (1987) found rapid naupliar development and high survival of Calanus pacificus fed on several unialgal diets, while other algal species
resulted in development and survival indistinguishable from that on filtered seawater. Effects of diet, in B.triarticulata as well as in other species, are more apparent on copepodite stages than on nauplii (Harris and Paffenhofer, 1976;
Paffenhbfer and Harris, 1976; Vidal, 1980a,b; Santer and van den Bosch, 1994;
Hansen and Santer, 1995). Nauplii of B.triarticulata and of Boeckella hamata
(S.Twombly and C.W.Burns, unpublished data) appeared less affected by food
quality than older stages. Santer (1994) suggested that nauplii of Eudiaptomus
gracilis avoided ingesting toxic Microcystis so that mortality rates on a mixture of
Microcystis and Cryptomonas were low. Eudiaptomus gracilis nauplii fed a
mixture otAnabaena and Cryptomonas survived well, suggesting some nutritional
benefit of the cyanobacterium when offered as part of a mixed diet. In our study,
B.triarticulata nauplii may have been less affected by a diet that included
Anabaena because they avoided ingesting this species, because they derived
adequate nutrition from the Cryptomonas portion of the diet alone, or because
the negative effects of this cyanobacterium are ameliorated when offered in a
mixed diet. In contrast to some other studies (e.g. Santer, 1994; Santer and van den
Bosch, 1994), we found nauplii to have lower food requirements than older
developmental stages (see also Vidal, 1980a,b). Taken together, these studies
underscore the importance of food quality to copepod growth and development,
and support the suggestion that food has a more important effect on these processes than temperature (Klein Breteler et al, 1982; Hart, 1990; Ban, 1994).
We used data on age at molting to calculate individual stage durations, and to
assess the effects of diet on stage durations. Stage durations were similar among
all naupliar stages in each diet, increased but similar among the first three or four
copepodite stages, and were prolonged for the last (CV, CR diet) or last two (CIV
and CV, C-A diet) copepodite stages. Coefficients of variation were high (>20%)
for all stages, indicating considerable individual variation, and were particularly
high for the last (CV) stage for individuals reared on C-A. The resulting 'pattern'
of stage durations in B.triarticulata does not conform particularly well to the
'general' pattern outlined by Landry (1983) for marine copepods (see also Peterson and Painting, 1990): thefirstfeeding naupliar stage (N2) was not prolonged in
B.triarticulata, and naupliar stage durations were uniformly shorter than those of
copepodite stages. Fryd et al (1991) reported unequal stage durations for Centropages hamatus and Ctypicus, and stage durations of Eurytemora affinis (determined by rearing copepods individually) varied depending on temperature and
food concentrations (Ban, 1994). The pattern outlined by Landry (1983) does not
appear to apply to all copepods.
We limited our consideration of food quality in this study to one species of
cyanobacterium, A.flos-aquae, and fed copepods a mixed diet consisting of a nontoxic strain of this organism and a nutritious alga. We chose this combination
because cyanobacteria are typically considered poor-quality food for zooplankton
(e.g. references in Lampert, 1987), yet many copepods readily ingest cyanobacteria in the field (e.g. Moriarty et al., 1973; Haney, 1987) and in the laboratory, even
2190
Effects of food quality on B. triartiailata
when offered together with Cryptomonas (Burns and Xu, 1990b; Burns and
Hegarty, 1994). Xu and Burns (1991) showed that a monospecific diet of
A.flosaquae caused high mortality among both nauplii and copepodites, and prevented development in B.triarticulata [Santer (1994) found similar results for
Eudiaptomus gracilis]. However, when raised on a mixed diet that included a
cyanobacterium, B.triarticulata survived and reproduced (S.Twombly and
C.W.Burns, unpublished data) as well as individuals raised on Cryptomonas alone,
and negative effects of the cyanobacterium consisted of slower development and
lower growth. A similar amelioration of negative effects of cyanobacteria when
offered in mixed diets has been reported for cladocerans (Lampert, 1981; Holm
and Shapiro, 1984) and rotifers (Gilbert and Durand, 1990), as well as copepods
(Santer, 1994). Nauplii of Eudiaptomus gracilis did not metamorphose when
raised on cyanobacteria alone, but developed to copepodites on mixed cyanobactena-Cryptomonas diets (Santer, 1994). Santer's data on mortality patterns
suggest that cyanobacteria may, in fact, confer an advantage when provided in
mixed diets: survival of nauplii on CR-Microcystis aeruginosa and CR-Anabaena
variabilis diets was higher than when raised on CR alone. Whereas copepodites
ingest large quantities of cyanobacteria in natural diets, the failure of monospecific cyanobacterial diets to promote naupliar growth and development in the
laboratory has cast doubt on the ability of cyanobacteria to sustain copepod
growth and development. Our results, and those of others, suggest that cyanobacteria, when consumed along with algae in field situations, may promote growth,
survival and development, and may also enhance reproduction (Gilbert and
Durand, 1990; S.Twombly and C.W.Burns, unpublished) in a diversity of zooplankton.
Individual variation
Quantifying individual variation in growth and development as a consequence of
diet was a major focus of this study. Most examinations of copepod growth and
development have relied on population mean values to estimate development
(e.g. Vidal, 1980b; Peterson, 1986; Santer, 1994; Hansen and Santer, 1995) and to
calculate stage-specific growth rates (e.g. Paffenhofer, 1970; Vidal, 1980a), and
have not emphasized individual variation. Peterson (1986) suggested that individual variation in developmental times increased with successive stages based
on changes in slopes of the regression of percent-frequency of stage over time
(days after hatching). Carlotti and Nival (1991) raised copepodites of Temora
stylifera individually, and reported extensive individual variation in instar durations (coefficients of variation 20-68%), largely as a consequence of the co-existence of rapid and slow developers. We did not find that individual variation in
age at molting (comparable to Peterson's measures) increased over successive
stages; coefficients of variation were fairly uniform and increased only in later
copepodite stages raised on C-A diet. Variation in stage durations in B.triarticulata was similar in magnitude to that recorded by Carlotti and Nival for T.stylifera, although never as large as 68%. In addition to these studies,Twombly (1993,
1995,1996) found that individual differences in age and size at metamorphosis
2191
S.Twombly and CW.Bnms
accounted for a large portion of the variance observed in these traits when individuals were raised under controlled laboratory conditions. Whenever documented, individual variation in growth and development is substantial.
Processes other than growth and development vary among individuals; for
example, Bamstedt (1988),Kleppel etal. (1988) and Rodriguez and Durbin (1992)
recorded coefficients of variation in gut fullness that ranged from 35 to 170%, and
attributed this variation to asynchronous and intermittent feeding activity. Paffenhofer (1994; see also Paffenhofer et ai, 1995) quantified fecal pellet production
as another way to evaluate individual variation in feeding activity and found lower
coefficients of variation (usually <30%). Bamstedt (1988) also reported intrastage
variation in body length and mass, reproductive parameters (gonad length, egg
production) and metabolic activity. Extreme individuals often differed by factors
of 4-5 in protein or lipid contents, gonad length and oxygen consumption.
Kankaala and Johansson (1986) developed a technique to weigh individual copepods and found high interindividual variation in length-biomass relationships
(and thus growth rates). One result of all of these studies is clear: estimates or predictions based on mean values are misleading.
Individual variation in growth (size at molting, stage-specific growth rates) and
development can be quantified by measuring naupliar and copepodite exuviae
(Twombly and Burns, 1996). This quantification is an important step towards
understanding growth and development, because individual patterns, particularly
of growth and especially for naupliar stages, have been impossible to document
(e.g. Thompson, 1982; Hart, 1990; van den Bosch and Gabriel, 1994). Growth and
developmental trajectories (Figures 1 and 2), coefficients of variation and variance components estimates provide convincing evidence that individuals from the
same clutch (sibship), raised in identical conditions, vary in age and size at each
molt as well as in stage durations and growth rates. Although variance component
analysis partitions variance in a trait only among random effects (the effect of diet
was not included), it showed that individual differences were often the largest
source of variation in either age or size at molting. Thus, 48% (males) and 94%
(females) of the variation observed in age at metamorphosis was due to differences between individuals (within the same sibship or family), while 74 and 80%
of the variance in size at metamorphosis was due to individual differences. A similarly high proportion of variance in age and size at maturity was due to individual
differences. Although diet (food quality) had significant effects on age, size, stage
duration and growth rates, almost all of the variation observed was accounted for
by individual differences, either among or within families.
Thus, there is considerable evidence that individual variation in diverse physiological and demographic processes is a persistent and widespread phenomenon
(Bamstedt, 1988; Carlotti et aL, 1993; this study). This variation is maintained in
controlled laboratory conditions and does not appear to be due entirely to genetic
factors (Peterson, 1986); it is due in part to the fact that consistently fast and slow
developers occur within the same population or sibship (Carlotti and Nival, 1991;
this study), as well as to the fact that some individuals change developmental or
growth rates during development (e.g. Peterson, 1986; Carlotti and Nival, 1991;
this study). In B.triarticulata, these options were related to diet. Among
2192
Effects of food quality on B. triarticulata
individuals raised on CR, there was a significant, positive correlation between age
at metamorphosis and age at maturity, reflecting consistently rapid and slow
developers, whereas age at metamorphosis was not correlated with age at maturation among individuals reared on C-A, suggesting that rapid early developers
slowed later in life. Correlations between size at metamorphosis and size at
maturity were negative and insignificant for both diets, suggesting that an early
size advantage (or disadvantage) was not maintained, and that growth patterns
differed between naupliar and copepodite phases, irrespective of diet. Clearly,
individuals from the same clutch, raised under identical (or nearly identical) conditions, do not all follow the same growth 'rules', and rules based on mean values
(e.g. Miller et ai, 1977; references in Hart, 1990) most likely do not represent what
the majority of individuals in a population do. These 'rules' may, in addition,
change with food quantity or quality. With techniques to quantify individual
growth and development, the resulting variation must be incorporated into our
studies of population dynamics, life histories and secondary production. Techniques to quantify growth and development for a single individual over its entire
life cycle also provide the data needed to examine and to modify existing 'rules'
for copepod growth (see van den Bosch and Gabriel, 1994).
Long-term demographic studies contribute information that is essential to the
evaluation of food quality and its effects on zooplankton population ecology. Previous studies of copepods have shown (i) that some readily ingested algae cannot
sustain the growth and development of particular life stages, (ii) ontogenetic
differences in assessments of food quality and (iii) species-specific differences in
the quality of particular algal foods (e.g. Hansen and Santer, 1995). Our study
reinforces these results and extends them in some interesting ways. First, by
measuring individual growth and development of a large number of individuals,
we show that some females produce variable offspring, suggestive of a bethedging strategy in response to a variable or uncertain larval environment
(Stearns, 1976). There was often significant variation among families in growth
and development, suggesting a genetic basis for some of the variation observed.
Second, our results contribute to gathering evidence that many calanoid copepods, including members of the genus Boeckella inhabiting lakes in New Zealand,
are able to ingest, assimilate, grow, survive and reproduce on cyanobacteria when
cyanobacteria are ingested along with other algae. This ability is probably not
restricted to New Zealand boeckellids (see Santer, 1994), but the mechanism
allowing improved growth or survival on mixed diets containing non-toxic
cyanobacteria is still unknown. These studies expand our understanding of
copepod ecology, and particularly of the link between short-term feeding
behavior and longer-term demographic responses
Acknowledgements
This research was supported by grants to C.W.B. from the Division of Sciences,
University of Otago, and was completed while S.T. was on sabbatical leave from
the University of Rhode Island. S.T. is grateful to Colin Townsend and the Department of Zoology, University of Otago, for their support and hospitality. Caryn
2193
S-TwomMy and CW.Bumj
Thompson, Department of Mathematics and Statistics, University of Otago, provided generous statistical advice, and Nancy Clancy and one reviewer improved
earlier versions of the manuscript.
References
Bamstedt.U. (1988) Ecological significance of individual variability in copepod bioenergetics. Hydrobiologia, 167/168,43-59.
Ban.S. (1994) Effect of temperature and food concentration on post-embryonic development, egg production and adult body size of calanoid copepod Eurytemora affinis. J. Plankton Res., 16,721-735.
BurdonJJ. and HarperJ.L. (1980) Relative growth rates of individual members of a plant population.
J.EcoL, 68,953-957.
Burns,C.W. and Xu,Z. (1990a) Utilization of colonial cyanobacteria and algae by freshwater calanoid
copepods: survivorship and reproduction of adult Boeckella spp. Arch. HydrobioL,117, 257-270.
Burns,C.W. and Xu,Z. (1990b) Calanoid copepods feeding on algae and filamentous cyanobacteria:
rates of ingestion, defaecation and effects on trichome length./ Plankton Res., 12,201-213.
Burns,C.W. and Hegarty3- (1994) Diet selection by copepods in the presence of cyanobacteria. J.
Plankton Res.,16,1671-1690.
Burns,C.W., Fbrsyth.DJ., HaneyJ.E, James,M.R., Lampert.W. and Pridmore.R.D. (1989) Coexistence
and exclusion of zooplankton by Anabaena minutissima var. attenuata in Lake Rotongaio, New
Zealand. Arch. Hydrobiol. Beih. Ergebru LimnoL, 32,63-82.
ButlerJM.M., Suttle.C.A. and Neill.W.E. (1989) Discrimination by freshwater zooplankton between
single algal cells differing in nutritional status. Oecologia (Berlin), 78,368-372.
Carlotti,F. and Nival.S. (1991) Individual variability of development in laboratory-reared Temora
stylifera copepodites: consequences for the population dynamics and interpretation in the scope of
growth and development rules. /. Plankton Res., 13,801-813.
Carlotti,E, Krause,M. and Radach,G. (1993) Growth and development of Calanus finmarchicus
related to the influence of temperature: experimental results and conceptual model. LimnoL
Oceanogr.,3S, 1125-1134.
Chen.C.Y. and Fblt,CL. (1993) Measures of food quality as demographic predictors in freshwater
copepods. J. Plankton Res.,15,1247-1261.
DeMott.W.R. (1989) Optimal foraging theory as a predictor of chemically-mediated food selection by
suspension-feeding copepods. LimnoL Oceanogr., 34,140-154.
DeMott,W.R. (1990) Retention efficiency, perceptual bias, and active choice as mechanisms for food
selection by suspension-feeding zooplankton. In Hughes,R.N. (ed.), Behavioral Mechanisms of
Food Selection. Springer-Verlag, New York, pp. 569-594.
Dingle,H. (1972) Migration strategies of insects. Science, 175,1327-1335.
Fryd,M., Haslund.O.H. and Wohlgemuth.O. (1991) Development, growth and egg production of the
two copepod species Centropages hamatus and Centropages typicus in the laboratory. J. Plankton
Res., 13,683-689.
Fulton^R.S., Ill and Paerl^H.W. (1987) Effects of colonial morphology on zooplankton utilization of
algal resources during blue-green algal (Microcystis aeruginosa) blooms. LimnoL Oceanogr., 32,
634-664.
GilbertJJ. and Durand,M.W. (1990) Effect of Anabaena flos-aquae on the abilities of Daphnia and
Keratelh to feed and reproduce on unicellular algae. Freshwater BioL, 24,577-596.
Guillard,R.R.L. and Lorenzen.CJ. (1972) Yellow-green algae with chlorophyUide c J. PhycoL, 8,
10-14.
HaneyJ.F. (1987) Field studies on zooplankton-cyanobacteria interactions. N.Z. J. Mar. Freshwater
Res.,21,461-415.
Hansen,A.-M. and Santer.B. (1995) The influence of food resources on the development, survival and
reproduction of the two cyclopoid copepods: Cyclops vicinus and Mesocyclops leuckarti. J. Plankton Res.,17,631-646.
Harris,R.P. and Paffenhofer, G.-A. (1976) Feeding, growth and reproduction of the marine planktonic
copepod Temora longicomis Muller. /. Mar. BioL Assoc UK, 56,675-690.
Hart,R.C (1990) Copepod post-embryonic durations: pattern, conformity, and predictability. The realities of isochronal and equiproportional development, and trends in the copepodid-naupliar duration ratio. Hydrobiologia, 206,175-206.
Hart^.G and Santer,R (1994) Nutritional suitability of some uni-algal diets-for freshwater calanoids:
unexpected inadequacies of commonly used edible greens and others. Freshwater BioL, 31,109-116.
2194
Effects of food quality on B.triarticulata
Heip,C (1974) A comparison between models describing the influence of temperature on the development rate of copepods. BioL Jaarb. Dodonaea, 42,121-125.
Holm,N.P. and ShapiroJ. (1984) An examination of lipid reserves and the nutritional status of Daphnia
pulcx fed Aphanizomenonflos-aquae.LimnoL Oceanogr.,29,1137-1140.
Huntley.M.E., Ciminiello,P. and Lopez,M.D.G. (1987) Importance of food quality in determining
development and survival of Calanuspacificus (Copepoda: Calanoida). Mar. Biol., 95,103-113.
Istock.GA., Wasserman,S.S. and Zimmer,H. (1975) Ecology and evolution of the pitcher-plant mosquito: 1. Population dynamics and laboratory responses to food and population density. Evolution,
29,296-312.
James^M.R and ForsythJDJ. (1990) Zooplankton-phytoplankton interactions in a eutrophic lake. /.
Plankton Res., 12,455-472.
Kankaala,P. and Johansson.S. (1986) The influence of individual variation on length-biomass regressions in three crustacean zooplankton species. / Plankton Res., 8,1027-1038.
Klein Breteler.W.C.M., FranzJI.G. and Gonzalez,S.R. (1982) Growth and development of four
calanoid copepod species under experimental and natural conditions. Neth. J. Sea Res., 16,195-207.
Kleppel.G.S., Pieper,R.E. and Trager.G. (1988) Variability in the gut contents of individual Acartia
tonsa from waters off southern California. Mar. BioL, 97,185-190.
Lampert.W. (1981) Inhibitory and toxic effects of blue-green algae on Daphnia. Int Rev. Ges. HydrobioL, 66,285-298.
Lampert.W. (1987) Laboratory studies on zooplankton-cyanobacteria interactions. N.Z. J. Mar. Freshwater Res., 21,483-490.
Landry,M.R. (1983) The development of marine calanoid copepods with comment on the isochronal
rule. LimnoL Oceanogr.,2&, 614-624.
Miller,CB., Johnson J.K. and HeinleJD.R. (1977) Growth rules in the marine copepod genus Acartia.
LimnoL Oceanogr., 22,326-335.
Moriarty.DJ.W., DarlingtonJ.P.E., Dunn.I.G., Moriarty.CM. and Tevlin,M.P. (1973) Feeding and
grazing in Lake George, Uganda. Proc R. Soc London Ser. B, 184,299-319.
Mullin,M.M. and BrooksJE.R. (1970) Growth and metabolism of two planktonic, marine copepods as
influenced by temperature and type of food. In SteeleJ.H. (ed.), Marine Food Chains. University of
California Press, Berkeley, CA, pp. 74-95.
Paffenhofer,G.-A. (1970) Cultivation of Calanus helgolandicus under controlled conditions. HelgoL
Wiss. Meeresunters., 20,346-359.
Paffenhofer,G.-A. (1994) Variability due to feeding activity of individual copepods. / Plankton Res.,
16,617-626.
Paffenhofer.G.-A. and HarrisJt.P. (1976) Feeding, growth and reproduction of the marine planktonic
copepod Pseudocalanus elongatus Boeck. / Mar. BioL Assoc UK, 56,327-344.
Paffenhofer,G.-A., Bundy,M.H., Lewis^C.D. and Metz, C. (1995) Rates of ingestion and their variability between individual calanoid copepods: direct observations. / Plankton Res, 17,1573-1585.
Peterson.W.T. (1986) Development, growth and survivorship of the copepod Calanus marshallae in
the laboratory. Mar. EcoL Prog. Ser., 29,61-72.
Peterson.W.T. and Painting,SJ. (1990) Developmental rates of the copepods Calanus australis and
Calanoides carinatus in the laboratory, with discussion of methods used for calculation of development time. /. Plankton Res, 12,283-293.
Porter,K.G. and McDonough^R. (1984) The energetic cost of response to blue-green algal filaments by
cladocerans. LimnoL Oceanogr.,29,365-369.
Price.T. (1985) Reproductive responses to varying food supply in a population of Darwin's finches:
clutch size, growth rates and hatching synchrony. Oecologia (Berlin), 66,411-416.
Rodriguez,V. and Durbin^E.G. (1992) Evaluation of synchrony of feeding behavior in individual
Acartia hudsonica (Copepoda, Calanoida). Mar. EcoL Prog. Ser.,9H,7-13.
Rolley,F., Hodek.I. and Iperti.G. (1974) Influence de la nourriture aphidienne (selon I'fige de la planteh6te a partir de laquelle les pucerons se multiplient) sur l'induction de la donnance chez Semiadalia
undecimnotata Schn. (Coleop., CoccineUidae). Ann. ZooL EcoL Anim.,6,53-60.
Santer3- (1991) The role of food limitation in summer diapause of cyclopoid copepods. Verh. Int. Ver.
LimnoL, 24,2851 (Abstract).
Santer.B. (1994) Influences of food type and concentration on the development of Eudiaptomus
gracilis and implications for interactions between calanoid and cyclopoid copepods. Arch. HydrobioL, 131,141-159.
Santer.B. and van den Bosch J7. (1994) Herbivorous nutrition of Cyclops vidnus: the effect of a pure
algal diet on feeding, development, reproduction and life cycle. / Plankton Res., 16,171-195.
Statistical Analysis Systems institute, Inc. (1985) SAS User's Guide: Statistics, 5th edn. SAS Institute,
Cary.NC
2195
S.TWombly and CW.Buras
Stearns,S.C. (1976) Life-history tactics: a review of the ideas. Q. Rev. BioL, 51,3—47.
Stemberger.R.S. (1981) A general approach to the culture of planktonic rotifers. Can. J. Fish. Aquat
ScL, 38,721-724.
StewartJ.W., Whitcomb.W.H. and BeUJC.O. (1967) Estivation studies of the convergent lady beetle in
Arkansas. / Eton. Entomol., 60,1730-1735.
Tauber.M J., Tauber.C. A. and Masaki,S. (1986) Seasonal Adaptations of Insects. Oxford University
Press, New York.
Thompson3-M. (1982) Growth and development of Pscudocalanus elongatus and Calanus sp. in the
laboratory. / Mar. BioL Assoc UK, 62,359-372.
T6th,L.G. and Kato.K. (1996) Development of Eodiaptomus japonicus Burckhardt (Copepoda,
Calanoida) reared on different sized fractions of natural plankton. /. Plankton Res., 18,819-834.
Twombly.S. (1993) Inter- and intrapopulational variation in time to metamorphosis in a freshwater
copepod. Freshwater BioL, 30,105-118.
Twombly.S. (1995) Phenotypic variation in metamorphosis in four species of freshwater copepods.
Freshwater BioL, 34,29-38.
Twombly.S. (1996) Tuning of metamorphosis in a freshwater crustacean: comparison with anuran
modeli Ecology, 77,1855-1866.
Twombly.S. and Burns,GW. (1996) Exuvium analysis: a nondestructive method for analyzing copepod
growth and development. LAmnoL Oceanogr., in press,
van den Bosch J7. and Gabriel.W. (1994) A model of growth and development in copepods. LimnoL
Oceanogr., 39,1528-1542.
Vanderploeg,H.A., Paffenhofer,G.-A. and LiebigJ.R. (1990) Concentration-variable interactions
between calanoid copepods and particles of different food quality: observations and hypotheses. In
Hughes,R.N. (ed.), Behavioral Mechanisms of Food Selection. Springer-Verlag, New York,
pp. 595-614.
VidalJ. (1980a) Physioecology of zooplankton. I. Effects of phytoplankton concentration, temperature, and body size on the growth rate of Calanus pacificus and Pseudocalanus sp. Mar. BioL, 56,
111-134.
VidalJ. (1980b) Physioecology of zooplankton. II. Effects of phytoplankton concentration, temperature, and body size on the development and molting rates of Calanus pacificus and Pseudocalanus
sp. Mar. BioL,56,135-146.
VijverbergJ. (1989) Culture techniques for studies on the growth, development and reproduction of
copepods and cladocerans under laboratory and in situ conditions: a review. Freshwater BioL, 21,
317-373.
von Ende.C.N. (1993) Repeated-measures analysis: growth and other time-dependent measures. In
Scheiner,S-M. and GurevitchJ. (eds), Design and Analysis of Ecological Experiments. Chapman and
Hall, New York, pp. 113-137.
Weglenska,T. (1971) The influence of various concentrations of natural food on the development,
fecundity and production of planktonic crustacean filtrators. EkoL PoL, 19,427-473.
XuJZ. and Bums,GW. (1991) Development, growth and survivorship of juvenile calanoid copepods on
diets of cyanobacteria and algae. Inl Rev. Ges. HydrobioL,76,73-87.
Received on February 26,1996; accepted on July 5,1966
2196