Ontogenetic Changes in Daphnia Responsiveness to Fish Kairomone
Descrição do Produto
Hydrobiologia 526: 219–224, 2004. J. Pijanowska, P. Dawidowicz, A. Jachner & K. Szeroczyn´ska (eds), Cladocera. Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands.
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Ontogenetic changes in Daphnia responsiveness to ﬁsh kairomone Andrzej Mikulski, Dorota Lipowska & Joanna Pijanowska Department of Hydrobiology, Warsaw University, Banacha 2, 02-097 Warsaw, Poland
Key words: Daphnia, kairomones, life history, ontogenesis
Abstract Exposing Daphnia to the presence of a predator at diﬀerent stages of ontogenesis leads to diﬀerent reactions. Predator-induced changes of life history parameters related to the ﬁrst reproduction schedule, as age and size at ﬁrst reproduction, take place in the stage preceding that one in which vitellogenesis occurs. Determination of the amount of energy allocated to this ﬁrst reproduction, including oﬀspring number, occurs in the ﬁrst 24-h of life. In case of parameters related to reproduction schedule, time since induction until the release of the ﬁrst clutch of oﬀspring is tens of hours; in case of allocation patterns – more than 100. This suggests that Daphnia do not ‘‘remember’’ the information of a potential threat but, instead, the decisions on future development are undertaken once they have estimated the reigning predation regime. Shortening of the period during which eggs are carried in the brood chamber, thus shortening the time when Daphnia are more easily visible, may be another adaptation to reduce danger from ﬁsh predation and so increasing the probability of survival until the ﬁrst successful reproduction.
Introduction Planktonic animals may change their life parameters in the presence of chemical substances released into the water by predators or prey. For example, threatened individuals may descend deeper (Dawidowicz & Loose, 1992; Dawidowicz, 1993) or may change their body size and shape (Dodson & Havel, 1988; Dodson, 1989), in an attempt to avoid a predator attack. They may also change life history patterns, i.e. depending on the dominating pressures, animals may delay or accelerate maturity, mature at a smaller or larger size, release less but larger, or more but smaller neonates (Macha´cˇek, 1991; Stibor, 1992; Riessen, 1993; Weider & Pijanowska, 1993; Sakwin´ska, 2002). These antipredator defenses are costly and the scale of defense should be adjusted to predation risk, to avoid the unnecessary costs of permanent defense. Therefore, the time needed to display various defensive mechanisms may diﬀer, from seconds to the display of a behavioral reaction following a sudden change in light intensity or water move-
ment, to more than one generation needed to display changes in morphology (Agraval et al., 1999) or life history traits (e.g. producing ephippial eggs, Alekseev & Lampert, 2001). The time between initiation of the defense machinery and the moment when these changes become adaptive is crucial for equating the real costs of defense. When this period is extended, the probability of the disappearance of the factor inducing the defensive mechanism is relatively high and, as a consequence, the probability of bearing costs and taking no advantage from defense is also high. On the other hand, when the adaptive features are determined in diﬀerent ontogenetic stages, and environmental conditions change during ontogenesis, some components of life strategy may become maladaptive, or an individual may be forced to adjust parameters determined later to these determined earlier in ontogenesis. The aim of our study was to determine how the response of Daphnia females exposed to the
220 presence of ﬁsh diﬀers in diﬀerent stages of their ontogenesis. Timing of adaptive phenotypic changes in the life history of Daphnia exposed to ﬁsh presence and the length of reaction time needed to display these changes can thus be determined. Additionally, it will become clear whether females can remember information on predator presence, or the changes occur when the proximal factor is acting directly during physiologically determined moments when undertaking decisions on further development.
Materials and methods A clone of Daphnia magna Straus originating from Lake Binnensee (where Daphnia coexist with ﬁsh) was used. Females were cultured individually in glass jars in synthetic medium ADAM (Klu¨ttgen et al., 1994) and fed the green algae Scenedesmus acutus in a concentration equal to 1 mg Corg Æ l)1. Medium with algae was changed daily. Presence of ﬁsh was simulated by exposing Daphnia to water containing ﬁsh kairomone from crucian carp (Carassius carassius – one ﬁsh/7 l/24 h), fed 50 Daphnia per day. Neonates from the third clutch of third generation (great-grand daughters of the single founder female) were split into six groups (10 individuals in each). Females from the ﬁrst group were cultured in ‘ﬁsh water’ from birth to 1st post-embryonic exuvium (for the duration of the ﬁrst instar), then transferred to control water. Females from second group were exposed to ﬁsh kairomone for second instar, from third group – in third instar, etc. Females from the sixth group (control) were not exposed to ﬁsh kairomone. Age at release of eggs from the ovaries to the brood chamber (AFO) and age at ﬁrst reproduction (AFR, i.e. age at releasing the ﬁrst-clutch of oﬀspring) were recorded with 10 min. accuracy. Size at ﬁrst reproduction (SFR, i.e. size after releasing the ﬁrst clutch eggs to the brood chamber) and the size of neonates were measured with 10 lm accuracy. Neonates from the ﬁrst clutch were counted, dried and weighted; second clutch eggs were counted. The weight of adult females was calculated using model obtained for the same clone of Daphnia: W ¼ expð4:1291 þ ð1:8963 lnðLÞÞÞ;
where W is weight (lg) and L is length of an animal (mm). The inﬂuence of ﬁsh kairomone on the life-history traits of Daphnia was tested using a Kruskal– Wallis non-parametrical ANOVA. The diﬀerences between means were tested using a t-test for ranks. Reproductive eﬀort (RE) was calculated as a proportion of weight of the ﬁrst clutch to the total weight of female at ﬁrst reproduction and the ﬁrst clutch eggs: RE ¼ ðWc =Wc þ Wf Þ 100%; where RE is reproductive eﬀort, Wc – dry weight of whole 1st clutch, Wf – dry weight of female at ﬁrst reproduction. Correlation of certain life-history traits was tested using the product-moment (Pearson’s) correlation coeﬃcient (Sokal & Rolf, 1981).
Results Daphnia exposed to ﬁsh kairomone during the third instar released their ﬁrst clutch of eggs from the ovaries to the brood chamber (Fig. 1A, Table 1), and released their ﬁrst oﬀspring (Fig. 1B, Table 1) earlier than their sisters that had not been exposed to this factor, or exposed to this factor in other instars. Time of carrying their ﬁrst eggs was signiﬁcantly shorter (Fig. 1C, Table1), and they were smaller at ﬁrst reproduction than other females (Fig. 1D, Table 1). All females exposed to ﬁsh kairomone in the third instar had four juvenile instars (Fig. 1E), whereas control females as well as those exposed to the kairomone in the ﬁrst instar had ﬁve juvenile instars. Females from other treatments had either four or ﬁve juvenile instars. Daphnia in ﬁsh treatments released, on average, signiﬁcantly fewer ﬁrst-clutch oﬀspring (Fig. 1F, Table 1) and, as a consequence, the total mass of their ﬁrst clutch was lower (Fig. 1G, Table 1), despite the fact that the mass of a single neonate did not diﬀer from that in the control treatment (Fig. 1H, Table 1). Daphnia exposed to ﬁsh kairomone in ﬁrst and second instar exhibited a signiﬁcantly lower reproductive eﬀort than control females (Fig. 1I, Table 1). The number of secondclutch eggs of females exposed to ﬁsh kairomone at the third instar was smaller than in Daphnia
Figure 1. Age at maturity – AFO (A), age at ﬁrst reproduction – AFR (B), time of carrying eggs in brood chamber – THE (C), size at ﬁrst reproduction – SFR (D), number of juvenile instars (E), number of ﬁrst-clutch oﬀspring (F) dry mass of 1st clutch oﬀspring (G), dry mass of single neonate (H), reproductive eﬀort RE – ratio of ﬁrst clutch weight to total weight of egg-carrying female together with ﬁrst clutch eggs in brood chamber (I) and number of second-clutch oﬀspring (J) of Daphnia exposed to ﬁsh kairomone in ﬁrst, second, third, fourth and ﬁfth instars (dark bars), and not exposed to this factor (light bar); means ± 1 SD; ‘‘’’ – signiﬁcant diﬀerences from control treatment (test t for ranks, p < 0.05).
cultured in other ﬁsh treatments and in ﬁsh-free environment (Fig. 1J). Variability of life history traits (measured as coeﬃcients of variation CV ¼ s/x, where s is and x is mean) in females
exposed to ﬁsh kairomone in ﬁrst and third instars, especially age and size at ﬁrst reproduction and the number of juvenile stages, was signiﬁcantly lower than in remaining treatments (Fig. 2).
222 Table 1. Kruskal–Wallis test for diﬀerences in life history parameters between Daphnia not exposed and exposed to ﬁsh kairomones in various instars; signiﬁcant diﬀerences shown with bold font Df
AFR Time of holding eggs in brood chamber
Number of juvenile instars