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Temperature-dependent effect of food size on the reproductive performances of the small-sized cladoceran Moina micrura
Biochemical Systematics and Ecology 59 (2015) 297e301
Contents lists available at ScienceDirect
Biochemical Systematics and Ecology journal homepage: www.elsevier.com/locate/biochemsyseco
Temperature-dependent effect of food size on the reproductive performances of the small-sized cladoceran Moina micrura Rui Chen, Nuo Xu, Feixi Zhao, Yingyuan Wu, Yuan Huang, Zhou Yang* Jiangsu Key Laboratory for Biodiversity and Biotechnology, School of Biological Sciences, Nanjing Normal University, 1 Wenyuan Road, Nanjing 210023, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 December 2014 Accepted 21 February 2015 Available online
To determine the combined effects of temperature and minimal change in food size on small-sized cladocerans, we fed Moina micrura with unicells and small colonies of Scenedesmus obliquus at different temperatures (20, 25, and 30 C) and evaluated the parameters related to development and reproduction. Results showed that development to maturity was not affected by food sizes but significantly promoted by increasing temperature. The number of broods and total offspring significantly increased at 30 C when M. micrura fed on unicellular S. obliquus but was not significantly different among the three temperatures when fed on colonial S. obliquus, i.e. only at 30 C, the reproductive performance of M. micrura fed on unicellular S. obliquus was significantly higher than those fed on colonial S. obliquus, indicating that the effect of food size on the reproductive performances of M. micrura is dependent on temperature. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Colony formation Food selectivity Life history traits Moina micrura Scenedesmus obliquus
1. Introduction Algae, the main group of primary producers in aquatic ecosystems, usually face the risk of mortality from an entire assemblage of grazers. Since they are small relative to their grazers, an effective way to withstand grazing pressure is through increase in size (Van Donk et al., 2011). Therefore, some algae have evolved induced colony formation against herbivorous grazers (Van Donk et al., 2011). For example, Scenedesmus, a major freshwater phytoplankton taxon, can form induced colonies in response to the absence of certain herbivorous grazers (Hessen and Van Donk, 1993; Yang et al., 2007; Van Donk et al., 2011). As main phytoplankton consumers, cladocerans feed on Scenedesmus and the size of the foods they ingest is directly related to their body sizes (Burns, 1968). Thus, increase in induced colony size of Scenedesmus can limit the filtering rates of small cladocerans (Hessen and Van Donk, 1993), thereby decreasing the intrinsic rate of population increase. It is suggested that the negative influence of colonial Scenedesmus on population growth of Daphnia is determined by algal morphology rather than biochemical composition (Lürling et al., 1997). Lürling and Van Donk (1996) reported that colony formation in Scenedesmus obliquus is induced by infochemicals released by herbivorous zooplankton; these colonies are too large to be grazed by small zooplankton; i.e., changes in the prey size-spectrum substantially impair predator grazing (Burns, 1968).
* Corresponding author. Tel.: þ86 25 85891671; fax: þ86 25 85891526. E-mail address: [email protected] (Z. Yang). http://dx.doi.org/10.1016/j.bse.2015.02.013 0305-1978/© 2015 Elsevier Ltd. All rights reserved.
298
R. Chen et al. / Biochemical Systematics and Ecology 59 (2015) 297e301
Temperature is possibly the most important abiotic environmental factor that influences freshwater ecosystems. Temperature considerably affects ectothermic freshwater organisms because metabolic rates are directly controlled by ambient temperature (Hochmuth and De Schamphelaere, 2014). Given the importance of temperature, climate change-induced shifts continuously receive considerable attention. In the long term, climate change will increase the average temperature in small water bodies by 3e6 C over the century (Houghton, 2005). This increased temperature may directly affect the life history traits and population dynamics of cladocerans (Wojtal-Frankiewicz, 2012). Previous studies showed that clearance rate and population growth rate are significantly low in the small cladoceran Daphnia cucullata fed with large colonial Scenedesmus (Lürling and Van Donk, 1996; Lürling et al., 1997). The smallest colonies used in their experiments were 1.8 cells per colony on the average (Lürling et al., 1997). However, McCauley and Downing (1985) and Bern (1990) demonstrated that food sizes larger than ~18 mm can depress feeding of D. cucullata; thus, the foods with 1.8 cells per colony may not be suitable for D. cucullata, especially for smaller cladoceran Moina. Additionally, in some cases, slightly induced colonies may not be large, and just contain about two cells per colony on the average. Despite various studies on the changing food sizes, minimal information is known regarding the effects of such small changes in prey food size on the performance of smaller cladocerans. Based on the above knowledge, especially, temperature often influences the impact of diet algae on cladocerans (Hietala et al., 1997), therefore, we hypothesized that the life history traits of small cladocerans will be affected by the small change in food sizes and temperature may disturb such effect. To test the hypotheses, we fed a small-sized cladoceran, Moina micrura, with two types of small food sizes, namely, unicells and small colonies (~2.0 cells per colony) of Scenedesmus at different temperatures, and then recorded the parameters related to development and reproduction. The results supported our hypotheses and indicated the temperature-dependent effect of food size on the reproductive performance of M. micrura. 2. Materials and methods 2.1. Algae and zooplankton S. obliquus (FACHB-416) was obtained from the Institute of Hydrobiology, Chinese Academy of Sciences. The alga was axenically cultured in a 1-L Erlenmeyer flask with BG-11 medium at 25 C and illuminated using fluorescent light at 40 mmol photons m2 s1 with a lightedark period of 14:10 h. M. micrura, a laboratory clone cultured in our laboratory for several years, was maintained in a 1-L beaker under culture conditions similar to that used in S. obliquus; the cladoceran was fed with S. obliquus daily. New-born M. micrura (<24 h old) from a single mother (F0) were isolated and individually grown in 50-mL beakers. The first two generations were removed, and the third generation (F3) was used for experiments. 2.2. Life history experiments We used a fullefactorial design for food size and temperature, with each factor containing two and three levels, respectively. The two types of food size, namely, unicells and small colonies, were obtained by normal culturing or adding a small amount of the filtrate of Daphnia cultures (Lürling and Van Donk, 1996; Zhu et al., 2014), respectively. The combinations of treatment were 2 3, and each treatment was performed in ten replicates. For each replicate, newborns from F3 were collected within 6 h of birth and transferred individually into a 50-mL beaker containing 30 mL of unicellular (measured value: ~1.1 cells per colony) or small colonial (measured value: ~2.0 cells per colony) S. obliquus suspension. All these newborns were fed with equal amounts of algae of equivalent density (5 105 cells mL1). The beakers were incubated in temperature-controlled chambers at 20, 25, and 30 C, respectively, under 40 mmol photons m2 s1 with a lightedark period of 14:10 h. The experiment lasted for 10 days. We monitored the moults (i.e., shedding carapaces) of all test individuals and measured their body length (the distance between the top of the head and the base of the tail spine) daily by using the affiliated analytical program (Nikon NIS-elements D3.00, sp1) under an inverted microscope (Nikon Eclipse TieS, Nikon Instruments, Japan). We also recorded daily the offspring production and the following traits: time to the first batch of eggs appearing in brood pouch, time to first brood, body size of each female at the first batch of eggs, body size at first brood, number of broods per female, and total number of offspring per female (Yang et al., 2012; Lyu et al., 2013). 2.3. Statistical analysis All results were presented as means ± 1 SE. The combined effects of food size and temperature on the development and reproduction of M. micrura were analysed by two-way ANOVA using Sigmaplot 11.0 software, followed by Duncan's test to reveal treatment differences. Statistical significance was established at P < 0.05. 3. Results During the 10-day experiment, temperature and food size each significantly affected the number of moults, and there was significant interaction between temperature and food size on this parameter (Table 1). The number of moults was significantly higher at 30 C than those at other temperatures, regardless of food size. The moults of M. micrura fed with unicells and colonies were similar at 25 C, but the food size significantly affected the number of moults at 20 and 30 C (Fig. 1), i.e. the
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Table 1 Summary of two-way ANOVA for the effect of temperature and food size on the life history traits of M. micrura. Traits
Source of variation
DF
SS
MS
F
P
Number of moults
Temperature Food size Temperature Temperature Food size Temperature Temperature Food size Temperature Temperature Food size Temperature Temperature Food size Temperature Temperature Food size Temperature Temperature Food size Temperature
2 1 2 2 1 2 2 1 2 2 1 2 2 1 2 2 1 2 2 1 2
64.240 4.153 2.442 12.700 0.0167 0.0333 13.900 0.267 1.033 31,468.504 5.509 2184.906 37,676.987 3429.670 661.559 44.845 13.267 32.101 38,105.069 4672.837 30,596.236
32.120 4.153 1.221 6.350 0.0167 0.0167 6.950 0.267 0.517 15,734.252 5.509 1092.453 18,838.493 3429.670 330.780 22.423 13.267 16.050 19,052.534 4672.837 15,298.118
208.758 26.993 7.937 381.000 1.000 1.000 48.115 1.846 3.577 16.654 0.00583 1.156 14.236 2.592 0.250 183.560 108.612 131.394 57.262 14.044 45.978
<0.001 <0.001 0.001 <0.001 0.322 0.375 <0.001 0.180 0.035 <0.001 0.939 0.322 <0.001 0.113 0.780 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001
Time to first eggs
Time to first brood
Size at first eggs
Size at first brood
Number of broods
Total offspring per female
Food size
Food size
Food size
Food size
Food size
Food size
Food size
number of moults of M. micrura fed with colonies was higher than those fed with unicells. The results indicated that high temperature and colonial S. obliquus promoted M. micrura moulting. Development to maturity (the time to first eggs and to first brood) was significantly earlier at higher temperature (Fig. 2). In comparison with those at 20 C, the time to first eggs was about one day earlier and the time to first brood was about 0.5 day earlier at 25 and 30 C. The results of two-way ANOVA showed there was significant interactive effect between temperature and food size on the time to first brood. The food size did not significantly affect on the development to maturity (Table 1). The body size at first eggs and first brood increased with increasing temperature, regardless of food size (Fig. 3). The food size did not significantly affect the sizes at first eggs and first brood (Table 1). During the experimental period, the number of broods and total offspring per female significantly increased at 30 C compared with that at the other two temperatures when M. micrura fed on unicellular S. obliquus. No significant difference existed among the three temperatures when M. micrura fed on colonial S. obliquus. At 30 C, the reproductive performances of M. micrura fed on unicellular S. obliquus were significantly higher than those fed on colonial S. obliquus. The results of twoway ANOVA showed there was significant interactive effect between temperature and food size on the number of broods and total offspring per female (Table 1; Fig. 4).
4. Discussion In the experiment, we used a species of small cladocerans, M. micrura to compare the life history traits of animals reared on unicells and small colonies at different temperatures. The results indicated that both temperature and food size had effects on some of the life history traits at different extents. High temperatures significantly promoted the development of M. micrura as indicated by the increased moults and short time to maturity. Similarly, previous study showed that the generation time of
Fig. 1. Effect of temperature and food size on the number of moults in M. micrura. Error bars indicate 1 SE. Some of the error bars are too small to be observed by the naked eye.
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R. Chen et al. / Biochemical Systematics and Ecology 59 (2015) 297e301
Fig. 2. Effect of temperature and food size on the time to first eggs and first brood in M. micrura. Error bars indicate 1 SE. Some of the error bars are too small to be observed by the naked eye.
Moina macrocopa decrease with increasing temperature (Orcutt and Porter, 1984). Therefore, the increased temperature can shorten the development time and maturation time of cladocerans. In the experiments, we specially selected two types of small food size of Scenedesmus, namely, ~1.1 cells per colony and 2.0 cells per colony, and then maintained sufficient food concentrations. Surprisingly, the food size had no significant effect on the four traits: time to first eggs, time to first brood, size at first eggs, and size at first brood (Table 1). This finding indicated that the small colonial food met the energy requirement of the animals in the early stage of growth and development as the unicells did. The effect of food size on the reproduction of the small cladoceran was not significant at 20 and 25 C, but significant at 30 C. Moreover, the number of brood and total offspring per female significantly increased with increasing temperature when M. micrura fed on unicellular Scenedesmus; this finding is similar to that of a previous study, in which Moina obtains higher densities at 30 C than that at 20 C (Nandini et al., 2004). At 20 and 25 C, the two types of food size also did not influence the broods and total offspring, indicating that both food sizes can satisfy the energy requirements to obtain maximum reproduction at these temperatures. However, at 30 C, the broods and total offspring of M. micrura fed on unicells were significantly higher than those fed on small colonies, which indicated the temperature-dependent effect of food size on the number of brood and total offspring per female. This result could be attributed to the increased threshold of fundamental energy requirement in M. micrura at high temperatures (Lampert, 1977), inducing M. micrura to ingest more food to maintain reproduction; otherwise energy allocated to reproduction would decrease. As small cladocerans are less effective in feeding on colonies (DeMott, 1995), M. micrura fed on small colonies cannot ingest sufficient amount of food compared with those fed on unicells; thus, they cannot meet the higher energy requirements at higher temperatures (Paul et al., 2004) and produce lower reproductive output. The results also indicated that the reproductive performance of the small cladoceran M. micrura increased with increasing temperature when fed with optimal-sized food, but not large-sized foods, even small colonies. Previous study showed that the growth and reproduction of cladocerans are dependent on temperature and food availability (Lampert and Trubetskova, 1996). The effect of food availability may be more pronounced than the temperature effect, and the temperature response of Daphnia magna is the most pronounced when the food is not limited (Giebelhausen and Lampert, 2001); this phenomenon was also demonstrated in our study using M. micrura, i.e. the number of broods and total offspring per female significantly differed at different temperatures when M. micrura sufficiently fed on unicells of S. obliquus. Significant interactive effect between temperature and food size on the time to first brood, the number of broods and total offspring per female indicated that the effect of temperature is dependent on food availability, and the temperature affects the sensitivity of small cladocerans to food size. Therefore, the effect of food size on the reproductive performance of small-sized cladocerans is dependent on temperature. The results implied that the population dynamics of small-sized cladocerans may shift under global warming.
Fig. 3. Effect of temperature and food size on the body size at first eggs and first brood in M. micrura. Error bars indicate 1 SE. Some of the error bars are too small to be observed by the naked eye.
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301
Fig. 4. Effect of temperature and food size on the number of broods and total offspring per female in M. micrura. Error bars indicate 1 SE. Some of the error bars are too small to be observed by the naked eye.
Acknowledgments This study was supported by the National Natural Science Foundation of China (31270504) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References Bern, L., 1990. Size-related discrimination of nutritive and inert particles by freshwater zooplankton. J. Plank. Res. 12, 1059e1067. Burns, C.W., 1968. The relationship between body size of filter-feeding cladocera and the maximum size of particle ingested. Limnol. Oceanogr. 13, 675e678. DeMott, W.R., 1995. The influence of prey hardness on Daphnia's selectivity for large prey. Hydrobiologia 307, 127e138. Giebelhausen, B., Lampert, W., 2001. Temperature reaction norms of Daphnia magna: the effect of food concentration. Freshw. Biol. 46, 281e289. Hessen, D.O., Van Donk, E., 1993. Morphological changes in Scenedesmus induced by substances released from Daphnia. Arch. Hydrobiol. 127, 129e140. n-M€ €tta €, C., Walls, M., 1997. Sensitivity of Daphnia to toxic cyanobacteria: effects of genotype and temperature. Freshw. Biol. 37, 299e306. Hietala, J., Laure aa Hochmuth, J.D., De Schamphelaere, K.A.C., 2014. The effect of temperature on the sensitivity of Daphnia magna to cyanobacteria is genus dependent. Environ. Toxicol. Chem. 33, 2333e2343. Houghton, J., 2005. Global warming. Rep. Prog. Phys. 68, 1343e1403. Lampert, W., 1977. Studies on the carbon balance of Daphnia pulex DeGeer as related to environmental conditions. II. The dependence of carbon assimilation on animal size, temperature, food concentration and diet species. Arch. Hydrobiol. Suppl. 48, 310e335. Lampert, W., Trubetskova, I., 1996. Juvenile growth rate as a measure of fitness in Daphnia. Funct. Ecol. 631e635. Lürling, M., Van Donk, E., 1996. Zooplankton-induced unicell-colony transformation in Scenedesmus acutus and its effect on growth of herbivore Daphnia. Oecologia 108, 432e437. Lürling, M., De Lange, H.J., Van Donk, E., 1997. Changes in food quality of the green alga Scenedesmus induced by Daphnia infochemicals: biochemical composition and morphology. Freshw. Biol. 38, 619e628. Lyu, K., Wang, Q.Q., Chen, R., Lu, Q.L., Yang, Z., 2013. Inter-specific differences in survival and reproduction of cladocerans to nitrite gradient and the ecological implications. Biochem. Syst. Ecol. 48, 151e156. McCauley, E., Downing, J.A., 1985. The prediction of cladoceran grazing rate spectra. Limnol. Oceanogr 30, 202e212. Nandini, S., Mayeli, S.M., Sarma, S.S.S., 2004. Effect of stress on the life-table demography of Moina macrocopa. Hydrobiologia 526, 245e254. Orcutt, J.D., Porter, K.G., 1984. The synergistic effects of temperature and food concentration on life-history parameters of Daphnia. Oecologia 63, 300e306. Paul, R.J., Lamkemeyer, T., Maurer, J., Pinkhaus, O., Pirow, R., Seidl, M., Zeis, B., 2004. Thermal acclimation in the microcrustacean Daphnia: a survey of behavioural, physiological and biochemical mechanisms. J. Therm. Biol. 29, 655e662. Van Donk, E., Ianora, A., Vos, M., 2011. Induced defences in marine and freshwater phytoplankton: a review. Hydrobiologia 668, 3e19. Wojtal-Frankiewicz, A., 2012. The effects of global warming on Daphnia spp. population dynamics: a review. Aquat. Ecol. 46, 37e53. Yang, Z., Kong, F.X., Shi, X.L., Xing, P., Zhang, M., 2007. Effects of Daphnia-associated infochemicals on the morphology, polysaccharides content and PSIIefficiency in Scenedesmus obliquus. Int. Rev. Hydrobiol. 92, 618e625. Yang, Z., Lü, K., Chen, Y., Montagnes, D.J., 2012. The interactive effects of ammonia and microcystin on life-history traits of the cladoceran Daphnia magna: synergistic or antagonistic? PLoS One 7, e32285. Zhu, X.X., Nan, H.H., Zhang, L., Zhu, C., Liu, J., Yang, Z., 2014. Does microcystin disrupt the induced effect of Daphnia kairomone on colony formation in Scenedesmus? Biochem. Syst. Ecol. 57, 169e174.
Contents lists available at ScienceDirect
Biochemical Systematics and Ecology journal homepage: www.elsevier.com/locate/biochemsyseco
Temperature-dependent effect of food size on the reproductive performances of the small-sized cladoceran Moina micrura Rui Chen, Nuo Xu, Feixi Zhao, Yingyuan Wu, Yuan Huang, Zhou Yang* Jiangsu Key Laboratory for Biodiversity and Biotechnology, School of Biological Sciences, Nanjing Normal University, 1 Wenyuan Road, Nanjing 210023, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 December 2014 Accepted 21 February 2015 Available online
To determine the combined effects of temperature and minimal change in food size on small-sized cladocerans, we fed Moina micrura with unicells and small colonies of Scenedesmus obliquus at different temperatures (20, 25, and 30 C) and evaluated the parameters related to development and reproduction. Results showed that development to maturity was not affected by food sizes but significantly promoted by increasing temperature. The number of broods and total offspring significantly increased at 30 C when M. micrura fed on unicellular S. obliquus but was not significantly different among the three temperatures when fed on colonial S. obliquus, i.e. only at 30 C, the reproductive performance of M. micrura fed on unicellular S. obliquus was significantly higher than those fed on colonial S. obliquus, indicating that the effect of food size on the reproductive performances of M. micrura is dependent on temperature. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Colony formation Food selectivity Life history traits Moina micrura Scenedesmus obliquus
1. Introduction Algae, the main group of primary producers in aquatic ecosystems, usually face the risk of mortality from an entire assemblage of grazers. Since they are small relative to their grazers, an effective way to withstand grazing pressure is through increase in size (Van Donk et al., 2011). Therefore, some algae have evolved induced colony formation against herbivorous grazers (Van Donk et al., 2011). For example, Scenedesmus, a major freshwater phytoplankton taxon, can form induced colonies in response to the absence of certain herbivorous grazers (Hessen and Van Donk, 1993; Yang et al., 2007; Van Donk et al., 2011). As main phytoplankton consumers, cladocerans feed on Scenedesmus and the size of the foods they ingest is directly related to their body sizes (Burns, 1968). Thus, increase in induced colony size of Scenedesmus can limit the filtering rates of small cladocerans (Hessen and Van Donk, 1993), thereby decreasing the intrinsic rate of population increase. It is suggested that the negative influence of colonial Scenedesmus on population growth of Daphnia is determined by algal morphology rather than biochemical composition (Lürling et al., 1997). Lürling and Van Donk (1996) reported that colony formation in Scenedesmus obliquus is induced by infochemicals released by herbivorous zooplankton; these colonies are too large to be grazed by small zooplankton; i.e., changes in the prey size-spectrum substantially impair predator grazing (Burns, 1968).
* Corresponding author. Tel.: þ86 25 85891671; fax: þ86 25 85891526. E-mail address: [email protected] (Z. Yang). http://dx.doi.org/10.1016/j.bse.2015.02.013 0305-1978/© 2015 Elsevier Ltd. All rights reserved.
298
R. Chen et al. / Biochemical Systematics and Ecology 59 (2015) 297e301
Temperature is possibly the most important abiotic environmental factor that influences freshwater ecosystems. Temperature considerably affects ectothermic freshwater organisms because metabolic rates are directly controlled by ambient temperature (Hochmuth and De Schamphelaere, 2014). Given the importance of temperature, climate change-induced shifts continuously receive considerable attention. In the long term, climate change will increase the average temperature in small water bodies by 3e6 C over the century (Houghton, 2005). This increased temperature may directly affect the life history traits and population dynamics of cladocerans (Wojtal-Frankiewicz, 2012). Previous studies showed that clearance rate and population growth rate are significantly low in the small cladoceran Daphnia cucullata fed with large colonial Scenedesmus (Lürling and Van Donk, 1996; Lürling et al., 1997). The smallest colonies used in their experiments were 1.8 cells per colony on the average (Lürling et al., 1997). However, McCauley and Downing (1985) and Bern (1990) demonstrated that food sizes larger than ~18 mm can depress feeding of D. cucullata; thus, the foods with 1.8 cells per colony may not be suitable for D. cucullata, especially for smaller cladoceran Moina. Additionally, in some cases, slightly induced colonies may not be large, and just contain about two cells per colony on the average. Despite various studies on the changing food sizes, minimal information is known regarding the effects of such small changes in prey food size on the performance of smaller cladocerans. Based on the above knowledge, especially, temperature often influences the impact of diet algae on cladocerans (Hietala et al., 1997), therefore, we hypothesized that the life history traits of small cladocerans will be affected by the small change in food sizes and temperature may disturb such effect. To test the hypotheses, we fed a small-sized cladoceran, Moina micrura, with two types of small food sizes, namely, unicells and small colonies (~2.0 cells per colony) of Scenedesmus at different temperatures, and then recorded the parameters related to development and reproduction. The results supported our hypotheses and indicated the temperature-dependent effect of food size on the reproductive performance of M. micrura. 2. Materials and methods 2.1. Algae and zooplankton S. obliquus (FACHB-416) was obtained from the Institute of Hydrobiology, Chinese Academy of Sciences. The alga was axenically cultured in a 1-L Erlenmeyer flask with BG-11 medium at 25 C and illuminated using fluorescent light at 40 mmol photons m2 s1 with a lightedark period of 14:10 h. M. micrura, a laboratory clone cultured in our laboratory for several years, was maintained in a 1-L beaker under culture conditions similar to that used in S. obliquus; the cladoceran was fed with S. obliquus daily. New-born M. micrura (<24 h old) from a single mother (F0) were isolated and individually grown in 50-mL beakers. The first two generations were removed, and the third generation (F3) was used for experiments. 2.2. Life history experiments We used a fullefactorial design for food size and temperature, with each factor containing two and three levels, respectively. The two types of food size, namely, unicells and small colonies, were obtained by normal culturing or adding a small amount of the filtrate of Daphnia cultures (Lürling and Van Donk, 1996; Zhu et al., 2014), respectively. The combinations of treatment were 2 3, and each treatment was performed in ten replicates. For each replicate, newborns from F3 were collected within 6 h of birth and transferred individually into a 50-mL beaker containing 30 mL of unicellular (measured value: ~1.1 cells per colony) or small colonial (measured value: ~2.0 cells per colony) S. obliquus suspension. All these newborns were fed with equal amounts of algae of equivalent density (5 105 cells mL1). The beakers were incubated in temperature-controlled chambers at 20, 25, and 30 C, respectively, under 40 mmol photons m2 s1 with a lightedark period of 14:10 h. The experiment lasted for 10 days. We monitored the moults (i.e., shedding carapaces) of all test individuals and measured their body length (the distance between the top of the head and the base of the tail spine) daily by using the affiliated analytical program (Nikon NIS-elements D3.00, sp1) under an inverted microscope (Nikon Eclipse TieS, Nikon Instruments, Japan). We also recorded daily the offspring production and the following traits: time to the first batch of eggs appearing in brood pouch, time to first brood, body size of each female at the first batch of eggs, body size at first brood, number of broods per female, and total number of offspring per female (Yang et al., 2012; Lyu et al., 2013). 2.3. Statistical analysis All results were presented as means ± 1 SE. The combined effects of food size and temperature on the development and reproduction of M. micrura were analysed by two-way ANOVA using Sigmaplot 11.0 software, followed by Duncan's test to reveal treatment differences. Statistical significance was established at P < 0.05. 3. Results During the 10-day experiment, temperature and food size each significantly affected the number of moults, and there was significant interaction between temperature and food size on this parameter (Table 1). The number of moults was significantly higher at 30 C than those at other temperatures, regardless of food size. The moults of M. micrura fed with unicells and colonies were similar at 25 C, but the food size significantly affected the number of moults at 20 and 30 C (Fig. 1), i.e. the
R. Chen et al. / Biochemical Systematics and Ecology 59 (2015) 297e301
299
Table 1 Summary of two-way ANOVA for the effect of temperature and food size on the life history traits of M. micrura. Traits
Source of variation
DF
SS
MS
F
P
Number of moults
Temperature Food size Temperature Temperature Food size Temperature Temperature Food size Temperature Temperature Food size Temperature Temperature Food size Temperature Temperature Food size Temperature Temperature Food size Temperature
2 1 2 2 1 2 2 1 2 2 1 2 2 1 2 2 1 2 2 1 2
64.240 4.153 2.442 12.700 0.0167 0.0333 13.900 0.267 1.033 31,468.504 5.509 2184.906 37,676.987 3429.670 661.559 44.845 13.267 32.101 38,105.069 4672.837 30,596.236
32.120 4.153 1.221 6.350 0.0167 0.0167 6.950 0.267 0.517 15,734.252 5.509 1092.453 18,838.493 3429.670 330.780 22.423 13.267 16.050 19,052.534 4672.837 15,298.118
208.758 26.993 7.937 381.000 1.000 1.000 48.115 1.846 3.577 16.654 0.00583 1.156 14.236 2.592 0.250 183.560 108.612 131.394 57.262 14.044 45.978
<0.001 <0.001 0.001 <0.001 0.322 0.375 <0.001 0.180 0.035 <0.001 0.939 0.322 <0.001 0.113 0.780 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001
Time to first eggs
Time to first brood
Size at first eggs
Size at first brood
Number of broods
Total offspring per female
Food size
Food size
Food size
Food size
Food size
Food size
Food size
number of moults of M. micrura fed with colonies was higher than those fed with unicells. The results indicated that high temperature and colonial S. obliquus promoted M. micrura moulting. Development to maturity (the time to first eggs and to first brood) was significantly earlier at higher temperature (Fig. 2). In comparison with those at 20 C, the time to first eggs was about one day earlier and the time to first brood was about 0.5 day earlier at 25 and 30 C. The results of two-way ANOVA showed there was significant interactive effect between temperature and food size on the time to first brood. The food size did not significantly affect on the development to maturity (Table 1). The body size at first eggs and first brood increased with increasing temperature, regardless of food size (Fig. 3). The food size did not significantly affect the sizes at first eggs and first brood (Table 1). During the experimental period, the number of broods and total offspring per female significantly increased at 30 C compared with that at the other two temperatures when M. micrura fed on unicellular S. obliquus. No significant difference existed among the three temperatures when M. micrura fed on colonial S. obliquus. At 30 C, the reproductive performances of M. micrura fed on unicellular S. obliquus were significantly higher than those fed on colonial S. obliquus. The results of twoway ANOVA showed there was significant interactive effect between temperature and food size on the number of broods and total offspring per female (Table 1; Fig. 4).
4. Discussion In the experiment, we used a species of small cladocerans, M. micrura to compare the life history traits of animals reared on unicells and small colonies at different temperatures. The results indicated that both temperature and food size had effects on some of the life history traits at different extents. High temperatures significantly promoted the development of M. micrura as indicated by the increased moults and short time to maturity. Similarly, previous study showed that the generation time of
Fig. 1. Effect of temperature and food size on the number of moults in M. micrura. Error bars indicate 1 SE. Some of the error bars are too small to be observed by the naked eye.
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Fig. 2. Effect of temperature and food size on the time to first eggs and first brood in M. micrura. Error bars indicate 1 SE. Some of the error bars are too small to be observed by the naked eye.
Moina macrocopa decrease with increasing temperature (Orcutt and Porter, 1984). Therefore, the increased temperature can shorten the development time and maturation time of cladocerans. In the experiments, we specially selected two types of small food size of Scenedesmus, namely, ~1.1 cells per colony and 2.0 cells per colony, and then maintained sufficient food concentrations. Surprisingly, the food size had no significant effect on the four traits: time to first eggs, time to first brood, size at first eggs, and size at first brood (Table 1). This finding indicated that the small colonial food met the energy requirement of the animals in the early stage of growth and development as the unicells did. The effect of food size on the reproduction of the small cladoceran was not significant at 20 and 25 C, but significant at 30 C. Moreover, the number of brood and total offspring per female significantly increased with increasing temperature when M. micrura fed on unicellular Scenedesmus; this finding is similar to that of a previous study, in which Moina obtains higher densities at 30 C than that at 20 C (Nandini et al., 2004). At 20 and 25 C, the two types of food size also did not influence the broods and total offspring, indicating that both food sizes can satisfy the energy requirements to obtain maximum reproduction at these temperatures. However, at 30 C, the broods and total offspring of M. micrura fed on unicells were significantly higher than those fed on small colonies, which indicated the temperature-dependent effect of food size on the number of brood and total offspring per female. This result could be attributed to the increased threshold of fundamental energy requirement in M. micrura at high temperatures (Lampert, 1977), inducing M. micrura to ingest more food to maintain reproduction; otherwise energy allocated to reproduction would decrease. As small cladocerans are less effective in feeding on colonies (DeMott, 1995), M. micrura fed on small colonies cannot ingest sufficient amount of food compared with those fed on unicells; thus, they cannot meet the higher energy requirements at higher temperatures (Paul et al., 2004) and produce lower reproductive output. The results also indicated that the reproductive performance of the small cladoceran M. micrura increased with increasing temperature when fed with optimal-sized food, but not large-sized foods, even small colonies. Previous study showed that the growth and reproduction of cladocerans are dependent on temperature and food availability (Lampert and Trubetskova, 1996). The effect of food availability may be more pronounced than the temperature effect, and the temperature response of Daphnia magna is the most pronounced when the food is not limited (Giebelhausen and Lampert, 2001); this phenomenon was also demonstrated in our study using M. micrura, i.e. the number of broods and total offspring per female significantly differed at different temperatures when M. micrura sufficiently fed on unicells of S. obliquus. Significant interactive effect between temperature and food size on the time to first brood, the number of broods and total offspring per female indicated that the effect of temperature is dependent on food availability, and the temperature affects the sensitivity of small cladocerans to food size. Therefore, the effect of food size on the reproductive performance of small-sized cladocerans is dependent on temperature. The results implied that the population dynamics of small-sized cladocerans may shift under global warming.
Fig. 3. Effect of temperature and food size on the body size at first eggs and first brood in M. micrura. Error bars indicate 1 SE. Some of the error bars are too small to be observed by the naked eye.
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Fig. 4. Effect of temperature and food size on the number of broods and total offspring per female in M. micrura. Error bars indicate 1 SE. Some of the error bars are too small to be observed by the naked eye.
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