Effects of food limitation on the life history of Simocephalus expinosus Koch ( Cladocera : Daphniidae )

Life history features of the Cladocera Simocephalus expinosus were examined at ambient room temperature in the laboratory at three different food concentrations of Saccharomyces cerevisiae: high food (12 x 10cells/ml), intermediate food (65 x 10cells/ml) and low food (5 x 10cells/ml). Culture media and food were renewed each day. As food level increased, there was a reduction in the age at maturity. Early reproducing females at larger sizes are accompanied by the production of larger broods which generate an important number of neonates per reproductive female. On the contrary, when food level diminished, there was an increase in the age at maturity. Later reproducing females at smaller sizes induced smaller broods and consequently, generate a lesser number of neonates.


INTRODUCTION
Zooplankton plays an important role in food webs of aquatic ecosystems.It occupies a key position, thus allows the transfer of energy and matters from lower trophic levels to higher trophic levels (Banse 1995, Lampert & Somer 1997, Levinsen & Nielsen 2002).In addition, zooplankton has control of "top-down" on primary producers and actively participates in the remineralization of organic matter (Banse 1995).
On the other hand, zooplankton communities are very sensitive and react to a wide variety of environmental stresses (Harris et al. 2000) including the quantity of nutrients (McCauley & Kalff 1981, Pace 1986, Dodson 1992).This constitutes a major force in the structure, abundance and zooplankton diversity.Outside the temperature (Allan 1977, Threlked 1979, Sharma & Pant 1982, Samraoui & Touati 2002) and predation (Brooks & Dodson 1965, De Bernardi & Guissani 1975, Zaret 1980), the food is the most important single factor controlling the population dynamics of Cladocera.In addition, it influences their fecundity (Clark & Carter 1974).
Different studies on the influence of nutritional deficiency in Cladocera report a negative effect on growth and reproduction.In extreme conditions, Cladocera may produce resting eggs and disappear from the plankton.These findings were reported from both laboratory (Hall 1964, Lampert 1977, Porter et al. 1983) and field studies (Hebert 1977, Threlked 1979, Taylor 1981).Thus, this study was therefore conducted to test the influence of food ability in the laboratory on the life-history traits of a cladoceran Simocephalus expinosus Koch, a species reported in Northeast Algeria by previous works of Gauthier (1928), Samraoui et al. (1998) and Samraoui (2002).

MATERIALS AND METHODS
Females of S. expinosus were isolated from a temporary pond, reared in an aquarium containing filtered pond water and oxygenated continuously.Pond water was filtered through a 0.45 µm to remove bacteria, algae and fine detritus.Females are kept at room temperature and fed daily with Saccharomyces cerevisiae.The Simocephalus are acclimatized to the conditions to be tested.The stock of cultures of S. expinosus has been maintained in this laboratory for about two years.Genetically, identical neonates from the first brood and aged less than 16 h were isolated and placed individually in tubes of 20 ml of filtered pond water.Three treatments were applied.In the first treatment, neonates are fed daily of S. cerevisiae at a concentration of 12 x 10 5 cells/ml.This concentration of food will be called high food.In the second treatment, females are fed daily with S. cerevisiae at a concentration of 65 x 10 4 cells/ml.It will be called the intermediate food, while in the third treatment neonates receive food daily at a concentration of 5 x 10 4 cells/ml and will be called the low food.Culture medium was prepared each day.Neonates in the three treatments are monitored daily from birth to death.The medium was changed every day.Before each change, the tubes are prepared, and individuals are transferred one by one using a pipette.The cells of S. cerevisiae were estimated using a hemacytometer.Room temperature, which was measured daily at 12 a.m, was about 19.99 ±0.61.Size (body length) was measured with an ocular micrometer, to the nearest of 0.05 mm from the top of the head to the base of the body (Perrin 1988).We determined the following parameters: -age at maturity (the first day of appearance of eggs in the brood pouch); -size at maturity; -First brood size and successive broods in order to determine the mean brood size per female; -Total number of broods per female; -Total number of neonates produced during the life cycle of the female.
We used analysis of variance to compare means.All analyses were performed with Minitab.

RESULTS
Data revealed that age at maturity of females reared at low food level is extended.By contrast, it is shortened under high food conditions (Fig. 1).Statistical analyses showed that the differences are significant (Table 1).The results showed that females reared under low food conditions grew significantly at smaller sizes than those reared under high food level (Fig. 2, Table 2).First brood size and mean brood size per reproductive female are smaller in females reared under low food conditions compared to those reared under high food conditions (Fig. 3).Statistics reveal a significant effect of food level (Table 3).According to the data reported in figure 4 and table 4, we established that the number of broods per female is not affected by food level.On the contrary, the number of neonates produced by reproductive female increases significantly with increasing food level (Fig. 5, Table 5).

DISCUSSION
It is recognized that the life history traits of Cladocera change depending on the quality and quantity of the resource (Urabe & Strener 2001).The results of this study confirm this finding.The food quantity affects life history parameters of the cladoceran S. expinosus.Indeed, females of this species extend their sexual maturity when food concentration decreases.Late maturity observed in these females is accompanied by a significant decrease in their size.Many studies with various Daphnia species reported that the age and the size at maturity increase and decrease respectively in response to food limitation (Vijverberg 1976, Orcutt & Porter 1984, Tillman & Lampert 1984, Taylor 1985, Foran 1986, Lynch 1989, Guisande & Gliwicz 1992, Riessen & Sprules 1992).There appear to be not exceptions to this pattern.
The results show clearly the influence of food conditions on brood size.Thus, females treated with low food produce smaller broods.The reduction in brood size with food depressions observed in this study is consistent with certain data collected both in the laboratory and in natural conditions on Daphnia pulex (Taylor 1985, Lynch 1989, Ebert & Yamposky 1992), Daphnia longispina (Lampert 1978), on Daphnia magna (Ebert & Yamposky 1992) and on Daphnia pulicaria (Gliwicz & Boavida 1996).It should be noted that the preliminary experiment of our study revealed the existence of a threshold concentration of growth and reproduction corresponding to 5 x 10 4 cells/ml.
In general, the overall result of this study shows that there is a relationship between brood size and food resources.Thus, brood size could act as an indicator of trophic levels.Previous works have developed the index of the nutritional status in Cladocera based on the number of eggs in the brood pouch (Hutchinson 1967).
The results of this study indicate that females of S. expinosus reared under low food level matured slowly, at smaller size produced smaller broods and consequently generate a lesser number of neonates per female.
On the above evidence it may be concluded that the low food level exercises a negative influence on the reproduction in three different ways: by causing an increase in the age at maturity, by lowering the size at maturity and by decreasing the brood size.
Overall, this study showed that the food deficiency is a major cause of reduced fertility of S. expinosus.Applied to natural populations, nutritional quantity therefore, determines fertility, in extreme cases of insufficient food resources, populations of S. expinosus may suppress any phenomenon of reproduction which lead to its disappearance.
Fig. 1: Influence of food concentration on the age at maturity of Simocephalus expinosus.

Fig. 2 :
Fig. 2: Size at maturity of Simocephalus expinosus in relation to food concentration.

Fig. 3 :
Fig. 3: Effect of food concentration on the 1 rst brood size and mean brood size per female in Simocephalus expinosus.

Fig. 4 :
Fig. 4: Number of brood per female in relation to food concentration in Simocephalus expinosus.
Fig. 5: Total number of neonates per reproductive female in function of food concentration in Simocephalus expinosus.

Table 1 :
One way ANOVA testing for the effect of food concentration on the age at maturity of Simocephalus expinosus.
DF: degrees of freedom, SS: sum squares, MS: mean squares.

Table 2 :
One way ANOVA testing the difference in the size at maturity of Simocephalus expinosus in relation to food concentration.
DF: degrees of freedom, SS: sum squares, MS: mean squares.

Table 3 :
One way ANOVA testing for the influence of food concentration on the 1 rst brood size and mean brood size during the life cycle of reproductive female of Simocephalus expinosus.
DF: degrees of freedom, SS: sum squares, MS: mean squares.

Table 4 :
One way ANOVA testing for the effect of food concentration on the number of broods per reproductive female of Simocephalus expinosus.
DF: degrees of freedom, SS: sum squares, MS: mean squares.

Table 5 :
One way ANOVA testing for the difference in the total number of neonates produced per reproductive female of Simocephalus expinosus in relation to food concentration.