Journal of Plankton Research Vol.18 no.6 pp.1033-1040.1996
Is fecundity modified by crowding in the copepod Centropages
typicusl
A.Miralto, A.Ianora, S.A.Poulet1, G.Romano and M.Laabir1
Stazione Zoologica 'A. Dohrn', Villa Comunale, 1-80121 Naples, Italy and
'Station Biologique, CNRS, F-29682 Roscoff, France
Abstract. Experiments were conducted to test the effect of overcrowding on egg production rates in the
copepod Centropages typiciis. Fertility was not significantly reduced by the presence of other conspecifics when the volume of water was the same for females incubated alone or in the presence of other
females and males. There was no evidence of chemical cues affecting egg production when females
were kept in large (1 female in 100 ml sea water [SW] or small (1 female in 25 ml SW) volumes of water,
with or without partitions, and in the presence or absence of food or males. Also, spawning rate was not
reduced when females were incubated in flow-through experimental chambers which allowed diffusion
of pheromones between females incubated alone or in high densities (100 ind. in 50 ml SW). Only when
females were placed at unnaturally high densities (1 female ml 1 ) was egg production significantly
reduced, probably due to physical disturbance and increased number of collisions rather than chemical
communication among individuals. The results show that chemical cues do not affect spawning behaviour when population densities are close to field conditions. By contrast, overcrowding induced by
swarming or aggregation may potentially limit reproductive success due to reduced feeding, and production and cannibalism of eggs.
Introduction
Behavioural responses of copepods to the presence of other conspecifics are well
documented. Strickler (1975) reported that copepods avoid all encounters a priori
to diminish the chances that an encounter will be a disastrous one, and that they
avoid physical contacts by jumping away. Wong et al. (1986) and van Duren and
Videler (1996) also observed that jumping frequency was altered by the presence
of other zooplankters. Physical disturbance may, in turn, also alter foraging and
reproductive activities. Feeding efficiency has been shown to be affected by the
presence of other animals in several ways. First, food concentration may be
reduced due to increased feeding by other grazers. Second, feeding activity may be
altered due to physical interference between individuals, such as fighting and nervous disturbance (Walker, 1979; Wong, 1988). Hargrave and Geen (1970)
observed that feeding rate declined in four copepod species as the number of animals per litre and the duration of the experiment increased. They concluded that
lowered feeding rates at high zooplankton densities may have resulted from
reduced volume of water available to each individual or the secretion of metabolites such as ammonia as densities increased. Folt and Goldman (1981) showed, for
the first time, that a reduction in feeding activity in the presence of potential competitors was due to the release of a chemical signal by the competitors.
Much less is known on how the presence of conspecifics affects reproductive
activity. There are several reports of reduced fertility due to overcrowding of copepod females in cultures (Walker, 1979; Kimmerer, 1984; Ki0rboe and Johansen,
1986; Davis and Altalo, 1992; Ohman and Runge, 1994; Laabir et al., 1995).
Although most of these authors suggested that this was due to cannibalism of eggs,
Walker (1979) found that conditioned medium reduced the number of ovisacs per
© Oxford University Press
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A.Miralto el al.
female and extended the interval between the production of successive clutches.
Kahan et al. (1988) described inhibition of hatching of mature embryos from eggsacs at high population densities of the copepod Tigriopus japonicus. Lazzarretto
et al. (1990) reported delayed female maturation in T.fulvus in overcrowded conditions and suggested that this strategy may be an adaptation to the severe habitat
occupied by this species. Such alterations in reproductive behaviour support the
hypothesis that population densities may be regulated via negative feedback
mechanisms. In the present study, we examine the effects of overcrowding on the
fecundity of Centropages typicus and show that females do not always respond
negatively to the presence of other conspecifics. This behaviour is discussed in
relation to chemical signals involved in mating encounters (Katona, 1973; Griffiths
and Frost, 1976; Van Leeuwen and Maly, 1991) and predator avoidance reactions
(Legier-Vissier et al., 1986).
Method
Zooplankton was sampled weekly from March to May 1994 and 1995 in coastal
waters of the Gulf of Naples by oblique tows with a 250 u,m mesh net, and was kept
in an insulated box until arrival at the laboratory 1-2 h later. There, adult C.typicus
females and males were sorted into different sized containers, depending on the
experiments, filled with 50 u,mfilteredsea water (SW) collected at the same sampling site.
Different experiments were run at 20°C and 12L:12D conditions for 24 h to test
the effect of overcrowding on egg production rates. The procedure for each of the
experiments is described below. In the case of large volume containers, the water
wasfirstfilteredthrough a 50 u,m sieve and eggs were then rinsed into crystallizing
dishes. After 24 h, counts of both intact eggs and crumpled membranes due to
cannibalism were summed to obtain total egg production per female. All data were
statistically tested for differences in fecundity using both the one-way ANOVA
and f-tests.
Effect of female presence
In afirstgroup of four complementary experiments, females were either incubated
alone or together to verify if the presence of other females influenced egg production rates.
Group 1. To test the effect of overcrowding, 20 females were placed individually in
crystallizing dishes containing 100 ml SW and another 20 females were incubated
together in beakers containing 2 1 SW so that the volume of water per female was
the same as for females incubated alone (100 ml per female). Experiments were
replicated 15 times.
Group 2. Twenty females were placed individually in 25 ml SW and 20 females
were incubated together in beakers with 500 ml SW (25 ml per female). This was
done to verify the effect of container size with respect to the first group of females.
Experiments were replicated 15 times.
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Is fecundity modified by crowding in C.typicas'!
Group 3. To test the effect of increasing female densities in containers, 1,3,5 and
10 females were placed in 25, 75, 125 and 250 ml SW, respectively, so that each
female had the same volume of water available (25 ml per female) as in Group 2.
Experiments were replicated 15 times.
Group 4. To test what happened if we increased the number of females in containers without increasing the volume available to each female, control females
were incubated alone in 25 ml SW, whereas 25 test females were placed in 25 ml
SW (1 ml per female). Since food limitation might have affected egg production,
another 25 females were placed in 20 ml S W and 5 ml of a culture of the dinoflagellate Prorocentrum minimum at a concentration of 5 x 104 cells ml"1. These experiments were replicated five times.
Effect of male presence
These experiments tested if fecundity was affected by the presence of males.
Group 1. Twenty females and 20 males were incubated as single couples in 50 ml
SW (25 ml per individual), and 10 females and 10 males were incubated together in
beakers with 500 ml SW (25 ml per individual). Experiments were replicated 15
times.
Group 2. Twenty females were placed individually in 25 ml SW. Test females were
divided into two batches: containers with one female and nine males together in
250 ml SW and 10 females together in 250 ml SW (25 ml per individual). Experiments were replicated seven times.
Effect of partitions on container bottom
To test if reduced fecundity was due to egg cannibalism, 20 control females were
placed individually in 100 ml SW whereas test females were divided into two
batches: 20 females were incubated together in 100 ml and 20 females were incubated together in containers with 100 ml SW but with a partition on the bottom so
that eggs were not cannibalized. Experiments were replicated seven times.
Effect of conditioned water
An experimental apparatus was designed to verify if fertility was modified due to
the release of chemical cues by other conspecifics. The apparatus (Figure 1) was
divided into two chambers each of which had a 50 ml volume capacity. The chambers were separated by a glassfibrefilter(20 (im pore size), and a peristaltic pump
ensured diffusion of 0.45 u,m filtered SW waterflowingfrom the crowded to the
uncrowded chamber at aflowrate of 8 ml min~'. In afirstset of experiments, one
female was placed in one chamber and 100 females into the second. In another set,
one female was placed in one chamber and 100 males into the other. Control
females were incubated singly in 100 ml unconditioned water. Experiments were
run for 24 h. We assumed that if overcrowding induced the production of chemical
cues which reduced fecundity, this would presumably have lowered egg production rates in test females. Each set of experiments was replicated 20 times.
1035
A.Mlralto el al.
Fig. 1. Flow-through incubation chamber to test the effect of conditioned water on egg production rates
in C.typicus. Water flows from the crowded (100 females or males) to the uncrowded (one female)
chamber allowing for diffusion of chemical cues between conspecifics.
Results
When females were incubated alone or together in the same volume of water (100
ml per female), fecundity was somewhat higher for females incubated alone but
the difference was not significant from when they were placed together (one-way
ANOVA, F = 2, P = 0.11), (Figure 2A). When container volume was reduced to 25
ml per female, egg production was lower than in the first group, but again differences between females incubated alone or together were not significant (one-way
ANOVA, F=\,P = 0.13), (Figure 2B).
To test the effect of increasing female density, 1,3,5 and 10 females were placed
in containers without reducing the volume of water available per female (Figure
2C). There was a 30% reduction in egg production when three or more females
were incubated together compared to when females were placed alone, but the
differences were not significant (one-way ANOVA, F = 2.5, P = 0.08). However,
when the volume of water available per female was reduced, there was a significant
difference between females incubated alone or together (one-way ANOVA, F =
39.07, P = 0.95). This difference was not due to the presence or absence of food
(Figure 3).
We also tested if the presence of males affected egg production. Females and
males incubated as individual couples showed no significant differences in egg production compared to an equal number of couples incubated together (one-way
ANOVA, F = 3, P = 0.09), (Figure 4A,B). Also females showed no significant
differences in egg production when one female was incubated with nine males as
compared to when 10 females were incubated together (one-way ANOVA, F- 2,
P = 0.11), (Figure 4C,D).
Partitions on container bottoms reduced egg cannibalism. However, the number
of eggs per female in containers with dense populations was always significantly
1036
Is fecundity modified by crowding in Ctypicusl
B
a
60
•
I
20 F/500 ml
|
1 K / 25 ml
40
|
a
Fig. 2. Effect of female presence on egg production rates in C.typicus: (A) females incubated alone in
100 ml or 20 females incubated together in 2 1 seawater (i.e. 100 ml per female); (B) same as previous
experiments but reducing the volume available per female (25 ml); (C) increasing female densities
without decreasing the volume available per female (25 ml). Histograms represent means ± SD.
lower than for control females incubated alone indicating that cannibalism was not
the main factor responsible for this difference (Figure 5).
Females incubated in flow-through experimental chambers which would have
allowed for the diffusion of chemicals or pheromones that might interfere with
normal egg laying indicated no significant differences between control females
incubated alone and females exposed to water conditioned by other female and
male conspecifics (/ =£ 0.3, d.f. - 38, a = 0.95), (Figure 6).
Discussion
The results indicate that fertility was not significantly reduced by the presence of
other conspecifics when the volume of water was the same for both groups
of experimental animals. Whether animals were kept in small or large volumes of
water, with or without partitions, and in the presence or absence of food or males,
there was no evidence of chemical cues affecting egg production. Even when
females were incubated in chambers that could have allowed for an exchange of
pheromones, egg production rates were not modified by the presence of other conspecifics. Only when females were placed in small containers at high densities (1
female ml-') was egg production significantly reduced, confirming the results
reported by Kimmerer (1984) and others (see Introduction) who reported a dimin1037
A.Miralto el al.
4)
801
-r
60
40
20
B
C
FED
UNFED
0
Fig. 3. Effect of crowding on egg production rates in Ctypicus: (A) females incubated alone in 25 ml; 25
females incubated together in 25 ml seawater in (B) fed and (C) unfed conditions. Histograms represent means ± SD.
ution in fecundity when females were incubated in unnaturally high densities.
Most likely, this reduction is due to physical disturbance and increased number of
collisions with decreasing container volume rather than chemical communication
among animals. Also, cannibalism could not explain lower egg production rates
when animals were incubated together since partitions on container bottoms only
moderately reduced egg predation by females.
This does not imply that animals are not aware of the presence of others. Chemical cues have been shown to be very important in regulating other aspects of the
reproductive behaviour of copepods such as the search for mates and remating
(e.g. Van Leeuwen and Maly, 1991). Also they have been shown to be important
for both food detection and capture (e.g. Alcaraz et al., 1980) as well as to detect
and avoid potential predators (L^gier-Visser etal., 1986; Bollens etal., 1994). Our
results show, however, that pheromones may not be involved in spawning behaviour since copepods seem to have less behavioural control over egg production
than feeding, which ceases at high densities (Kimmerer, 1984). Also they suggest
that chemical cues are not used by females to warn them of potential predators of
their own eggs implying that egg predation may be a serious problem in broadcast
spawners such as Ctypicus. Recent studies have in fact suggested that high egg
Fig. 4. Effect of male presence on egg production rates in Ctypiaa: couples incubated alone (A) or
together (B) without reducing the volume of water available per individual (25 ml); (C) one female and
nine males incubated together (25 ml ind-') versus (D) 10 females placed together (25 ml ind"1)- Histograms represent means ± SD.
1038
Is fecundity modified by crowding in C.typicus?
50
""S 30
T
0 20
r1
I 40
a>
St.
T
1 ioi
A
B
T
c
Fig. 5. Effect of cannibalism on egg production rates in C.typicus: (A) females incubated alone in 100 ml
and (B) 20 females incubated together in 100 ml seawater in containers without partitions: (C) 20
females incubated together in 100 ml in containers with partitions. Histograms represent means ± SD.
mortality may largely be related to predation by other co-occurring copepod species (Liang et al., 1994; Uye and Sano, 1995). This in turn implies that cannibalism
will be considerably reduced in egg-carrying species as already shown by Ki0rboe
and Sabatini (1994).
On the other hand, fecundity has been shown to be directly affected by high
population density in both calanoids (e.g. Ki0rboe and Johansen, 1986, this study)
and harpacticoids (Walker, 1979). This may be relevant to copepod distribution
patterns at sea. Copepods are in fact known to form aggregates and swarms which
may offer several advantages such as reduced predation stress. In such conditions,
populations are known to reach densities as high as 0.1-10 ind ml 1 (Omori and
Hamner, 1982; Haury and Yamazaki, 1995) which will decrease fecundity, as
shown by our results, as well as the quantity of food available to each individual. In
both cases, overcrowding will therefore potentially limit reproductive success. At
this point, the trade-off between the advantages (i.e. reduced predation, facility of
mate finding) and disadvantages (i.e. reduced feeding and egg laying) of swarming
behaviour remains an open question.
Fig. 6. Effect of conditioned water on egg production rates in C.typicus when females are incubated
alone (1 and 3) or inflow-throughexperimental chambers (2 and 4, see Figure 1) in the presence of
other female (A) or (B) male conspecifics. Histograms represent means ± SD.
1039
A.MIralto et at.
Acknowledgements
We wish to thank F.Esposito and M.Di Pinto for their technical assistance.
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Received on November 21, 1995; accepted on February 2, 1996
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