Growth of Moina macrocopa (Straus 1820) (Crustacea, Cladocera):
influence of trophic conditions, population density and temperature
A. Benider
1, A. Tifnouti
1,∗& R. Pourriot
21Laboratory of Hydrobiology, Department of Biology, Faculty of Science Semlalia, B.P. 2390, Marrakesh, Morocco
2Laboratory of Applied Geology, B123, University of Paris 6th, Place Jussieu, F-75252 Paris Cedex, France Tel: 4434649. Fax: 4437412. E-mail: fssm.bio@cybernet.net.ma
(∗Author for correspondence)
Received 12 February 1999; in revised form 22 December 2000; accepted 15 January 2001
Key words:Growth, trophic conditions, temperature, population density, cladocera,Moina macrocopa
Abstract
The growth of a strain ofMoina macrocopa (Straus 1820) isolated from an experimental stabilization pond in Marrakesh, was examined at seven concentrations of algae (6.25–6.25×105cells ml−1and at 5 different tem- peratures (15–30◦C)). Feeding conditions influenced the growth rate as well as the maximum size that reached 1.8 mm at 25 ◦C and at the highest algal concentration (6.25 ×105 cell ml−1). The life span and number of moltings reached a maximum (17.4 days and 13 moltings) at average nutrient concentrations (6.25×105 cell ml−1). Juvenile stages varied from 1 to 3 and adult ones from 6 to 8. In the temperature interval tested, growth rate increased with temperature while longevity decreased. Temperature had less effect on maximal size than nutrient availability. Population density (but not crowding) influenced longevity and survival but had no effect on growth.
Introduction
In the experimental stabilization ponds of Marrakesh, Moina macrocopa(Straus, 1820) was the only repres- entative of Cladocera present (Tifnouti, 1987; Tifnouti
& Pourriot, 1989). However, no representative of this species was isolated a month and a half after isolating the species for the first time. Furthermore, Daph- nia magnawhich generally lives in such ecosystems, could not be isolated either. In order to explain the dis- appearance of representativesM. macrocopafrom the ponds, a study of the influence of algal concentration, temperature, density of population and crowding on the growth ofM. macrocopawas carried out.
This study on the demographics ofMoinawas per- formed under laboratory controlled conditions (Allan, 1976; Hebert, 1978; Myrand & De la Noue, 1982).
In natural ecosystems, several environmental factors interact to regulate the development of zo- oplankton populations. The level of resources, thermal conditions, the population density and to a lower ex- tent, crowding are factors that influence their growth
(Montu, 1976; Porter & Orcutt, 1980; Hanazato &
Yasuno, 1984; Jana & Pal, 1985b). Studies of pop- ulation growth that deal with all these variables are scarce. Most are restricted to age, growth and birth rate. Factors such as density, crowding, trophic condi- tions are more difficult to measure and have been less frequently examined.
Similarly, there has been few investigations on spe- cies of the genusMoinaunder laboratory conditions such as the one by Jana & Pal (1985b) and Bonou et al. (1991) onMoina micrura, Hardy (1989) onMoina reticulataand Gordo et al. (1994) onMoina salina.
Here, we analyze the effect of different nutrient concentrations, population density, crowding and tem- perature on the growth of Moina macrocopaunder laboratory conditions.
Materials and methods
The experimental stabilization pond system has been described elsewhere (Tifnouti, 1987).
Table 1. Culture medium composition L + C Composent Concentration mgl−1 Ca (NO3)2. 4 H2O 40 mg l−1
KNO3 100
Mg SO4. 7H2O 30
KH2PO4 40
CuSO4. 5H2O 30 mg l−1 (NH4)6Mo7O24 60 ZnSO4. 7H2O 60 CoCl2. 6H2O 60 Mn(NO3)2. 4H2O 60
Citric Acid 60
H3BO3 60
Ferric citrate 1625 mg l−1
FeSO4 625
Fe Cl3. 6H2O 625
The strain ofMoina macrocopaused in this work was obtained from the hatching of one ephippium collected on January 10, 1989 from the sediments of the second stabilization pond. This strain repro- duced parthenogenetically for the full 20 months of the experiment.
In order to eliminate the effect of the parental age on the growth and reproduction of the descendants, all females used were first generation as recommended by Pourriot & Rougier (1976).
Cultures of algae used for experimental nutrition consisted of pure cultures of Chlorella sorokiniana (Shihira & Krauss), a species repeatedly isolated from the experimental lagoons of Marrakesh (Chifaa, 1987).
The cells ofChlorella sorokinianahad an average diameter of 3µm, and a biovolume of 14µm3. Their weight per cell was estimated at 14.10−6µg.
Algae cultures were maintained on L+C medium (Table 1). Cells (5 ml) in stationary phase of growth were inoculated every week in erlenmeyer vials con- taining 20 ml of fresh medium. Cultures were main- tained at 25 ◦C, under a photoperiod LD = 15:9 and agitation of 120 revolution min−1.
The cells were counted under microscope (OLYM- PUS, magnification×400) in a Thoma cell. The cell suspension was then diluted to the desired concentra- tion and kept in a refrigerator.
Table 2. Mineral composition of culture me- dium
Kations mg l−1 Anions mg l−1 Ca2+ 11.20 HCO3− 48.80
Mg2+ 3.65 Cl− 9.25
Na+ 13.40 NO3− 2.70
K+ 1.70 SO4− 15.80
Table 3. Algal concentration used in culture media
Culture concentration Algal concentration of Abreviations of algal (cells ml−1) culture medium
(cells ml−1)
102 6.25 C0
104 625 C2
105 6250 C3
2.5 105 15 625 C3(2.5)
5 105 31 250 C3(5)
106 62 500 C4
107 625 000 C5
Cultures of Moina were grown using commercial spring water (Sidi Ali) diluted to half with distilled wa- ter in order to eliminate the variability due to the water from the experimental system (nutrients such as or- ganic molecules and minerals vary from one sampling period to another). The final concentrations of the main minerals are given in Table 2. They correspond to the range of concentration generally used for Clado- cera (Pacaud, 1939). Total mineral ions content was around 100µg l−1, with a dominance of bicarbonates.
The females were maintained separated in glass beakers (diameter: 3.8 cm, height: 2.5 cm) contain- ing 15 ml of culture medium and 1 ml ofChlorella suspension (107 cells ml−1). The algal concentration in the medium was, therefore, equivalent to 6.25× 105cells ml−1. The glass beakers were covered with a watch glass in a water-bath at 25◦C in the darkness.
To investigate the effect of trophic conditions on the growth ofMoina, 15–36 new-born animals were placed in the culture conditions mentioned, with algal concentration between 6.25 and 6.25×105cells ml−1 (Table 3).
Regarding temperature, experiments were conduc- ted in water-baths at temperatures between 10 and 35◦C with algal concentration between 6.25 and 6.25
×105cells ml−1.
Before conducting these experiments, the samples ofMoina were – whenever possible – made similar to the formerly described conditions for at least two generations.
The effect of population density and crowding were investigated in the following experiments:
1. Control plot (T): 1 female 16 ml−1 2. Density plot (D): 1 female 5.34 ml−1 3. Crowding plot (G): 3 females 48 ml−1
The culture medium was renewed daily with 6.25× 105algal cells per female, at 25◦C under darkness.
The size of the animals as well as the presence of molting were measured every 24 h under micro- scope (type:WILD, magnification: ×50) with gradu- ated lenses. The descendants were also counted.
The female parents were then transferred to a new culture medium mixed withChlorella cells. Experi- ments were carried out until the animals’ death.
Data analysis
For each experiment, we determined the individual growth, adult average size and average life span.
Results
Influence of trophic conditions
The growth of M. macrocopa depends on the algal concentrations. At birth, the young females ofM. mac- rocopapresent an average sizeL= 0.49±0.03 mm (N
= 36). Their growth is more or less rapid depending on the trophic conditions of the medium.
The growth curve established for this species shows that at low algal concentration (6.25 cells ml−1= C0), there is almost no growth. At concen- trations 100 (= C2) to 1000 (= C3) times higher, the growth is gradual and low. In all cases, the size of the individuals never exceeds 0.85 mm.
For higher nutrient concentrations (1.56 × 104 cells ml−1 = C3 (2.5); 3.12 × 104 cells ml−1 = C3 (5) 6.25 ×104 cells ml−1 = C4), the growth is rapid during the first two days, but decreases for the rest of the cycle (Fig. 1). For these concentrations, the growth remains limited with maximal sizes of 1.07 mm, 1.25 mm and 1.27 mm, respectively (Fig. 2).
Finally, when the level of nutrients is maximal (6.25 × 105 cells ml−1 = C5), the growth of the animals shows two phases:
Figure 1. Curves of growth ofM. macrocopaat different concen- trations of algae (C0–C5, Table 3) at temperature 25◦C.
Figure 2. Maximal size ofM. macrocopaat diferent concentrations of algae (C0–C5, Table 3) at temperature 25◦C.
The first, from birth to two days, is character- ized by fast growth. The size of the females doubles (Fig. 1).
The second, from the second day to death, cor- responds to a slow growth phase. In these trophic conditions (6.25×105cells ml−1= C5), the individu- als ofMoinareach a maximal size of 1.8 mm at the 13th day (Fig. 2).
The growth rate was calculated in three phases:
from birth to the second day, from second to sixth day and sixth to the animal’s death. The growth rate is almost nil for the low algal concentration (C0).
Table 4. Individual growth rate ofM. macrocopaat three phases of life cycle at different concentrations (temperature: 25◦C)
C0 C2 C3 C3 (2,5) C3 (5) C4 C5
(Algal concentration)2(cells ml−1)
Birth – day 2 0.01 0.05 0.05 0.10 0.16 0.20 0.30
Day 2 – day 6 0 0.01 0.02 0.05 0.05 0.04 0.10
Day 6 – dead – 0.04 0.02 0.01 0.01 0.01 0.04
Figure 3. Average size of females as a function of the moltings at different concentrations of algal (C3–C5, Table 3) at temperature 25◦C.
This parameter increases significantly with increasing algal concentrations. The maximal growth rate 0.3 is reached at C5. The growth rate increases by a factor of 30 between the concentration C0 and C5 (Table 4).
This factor is maximal between the birthday and the second day regardless of the algal concentration.
Along with the increase in size, the females per- form many molting cycles in a life time (Fig. 3). Their number varies between 8 (for C3) and 13 (for C3(2.5) and C3(5)) and is related to the length of the growth period and depends on the nutritional resources. The increase in size between two molting cycles is higher during the first stages of the life cycle (until the 3rd or 4th molting). In the adult stages, molting cycles occur after each hatch. The interval time between moltings corresponds to a development stage (Gras & St-Jean, 1978b). For M. macrocopa, we can distinguish, in average 1–3 juvenile stages (at C5, C4 and C3(5) respectively) and 6 to 8 adult stages.
Figure 4. Life span ofM. macrocopaat different concentrations of algae (temperature: 25◦C).
The average life span at different concentrations of algae (Fig. 4) shows that the increase of nutrient concentration corresponds to an increase in average life span from 3.8±1.1 days at C0 to 17.4±4.3 days at C3(2.5). Beyond these concentrations ofChlorella, the longevity of females becomes shorter to reach an average value of 9.5±1.7 days at C5. The ana- lysis of variance (test of Newman & Keuls) shows that the difference between the average life duration for two successive algal concentrations is significant (5% level), except between C4 and C5. The results show the existence of individual variation for the same nutrient level.
The survival curves established at different nutri- ent concentrations can be divided into three groups (Fig. 5): 1. At low algae concentrations (C0, C2 and C3), there is a progressive increase in the maximal longevity and survival as well as a reduction in juven-
Figure 5. Curves of survival ofM. macrocopa, at different concen- trations of algae (temperature: 25◦C).
ile mortality. In fact, there is a decrease of 60% and 20% of the total number of individuals on the 5th day at concentrations C0 and C2, respectively. There is no change at concentration C3. The decrease in the num- ber of individuals can be explained by the low level of resources that is not adequate for the maintenance of the organism. 2. The maximal duration of life is reached for nutrient levels equivalent to C3(2.5) and C3(5). The survival curves follow a similar pattern:
mortality is low during the major fraction of the po- tential duration of life, then increases progressively.
Under these conditions, the level of concentration ne- cessary for maintenance reaches (C3(2.5)) or even (C3(5)). Finally, for algae concentrations between C4 and C5, the number of individuals remain stable until the 6th or 7th day, then decreases rapidly thereafter.
The shape of the two survival curves is similar to the one observed for concentrations lower than the level of maintenance (C3). This could be explained by the fact that, at the algae concentrations considered (C4 and C5), the process of reproduction caused the death of females. This activity creates a reduction of life duration and consequently, an increase of mortality.
Influence of population density and crowding
The growth curves obtained for the two densities of population (1 female/16 ml = T and 1 female/5.34 ml
= D) and for the two tested crowdings (1 female/16 ml
= T, 3 females/48 ml = G), follow a similar pattern:
growth is important during the two first days, a phase during which the size of the animals doubles (Fig. 6).
For the rest of the cycle, the growth is continuous but slower. The maximal size reaches 1.72 mm (12th day) for lot D and 1.8 mm (16th day) for lots T and G. These
Figure 6. Influence of the density of population (D = +), of crowding (G = o) on the growth ofM. macrocopa(temperature:
25◦C).
Table 5. Life span of M. macrocopa as a function of population density and crowding
Average life span (days)
T = 1 female 16 ml−1 12.6±2.6 G = 3 females 48 ml−1 10.8±3.2 D = 1 female 5.34 ml−1 8.0±2.4
results show that growth is identical for the two tested population densities (63 and 190 animal per liter). We observed a slight difference in maximal size between lots T and D, probably related to the shorter life span in the last lot.
The results of life and survival span change with population density and crowding (Table 5) show a reduction of average longevity at the density of 1 fe- male/5.34 ml. At the opposite, gathering three females does not significantly affect this parameter compared to isolated individuals.
Survival curves for the three groups (T, D and G) (Fig. 7) have a rectangular shape and are similar for the three lots. Nevertheless, the longevity ofMoinais 12 days lower for the density 1 female/5.34 ml (lot D) than the density 1 female/16 ml (17 days). The mortality occurs earlier, with higher rates.
However, packing ofMoinaby 3 (lot G) does not really affect the shape of the curves of survival in com- parison with isolated animals (lot T). In the two cases, mortality starts the 9th day and continues until all the animals are dead by the 17th day.
Figure 7. Survival curves ofM. macrocopaas a function of the density of population and the crowding (temperature: 25◦C).
Figure 8. Growth curves ofM. macrocopaat different temperature (concentration of algae: 6.25 105cells ml−1).
Influence of temperature
Before starting an experiment,Moinawas acclimated to the desired temperature for at least two generations except for the plots at 10 and 35◦C. In fact, at 10◦C, only the mature females survive for 4–5 days. The young offspring set free die a few hours after their birth. At 35◦C, no ovigerous female survived for more than 24 h. Between these extreme lethal temperatures, the growth ofMoinawas followed at 15, 18, 20, 25 and 30◦C (Fig. 8).
The growth curves, with a parabolic shape, show two successive phases:
The first, before the first egg, corresponds to the juvenile phase (Table 6). It lasts for 1–2.5 days at 20, 25 and 30◦C, with a fast increase of the individual size of the animals. At 18 and 15◦C, this phase is longer. It lasts for 5–8 days. This phase is also characterized by
Table 6. Growth rate (mm d−1) ofM. macrocopa, juvenile and adult stages, at 5 temperatures (algal concentration = 6.25 105cells ml−1)
Temperature (◦C)
15 18 20 25 30
Juvenile stage 0.08 0.15 0.20 0.30 0.50 Adult stage 0.01 0.02 0.05 0.06 0.08
Figure 9.Maximal size ofM. macrocopaat 5 temperatures (con- centration of algae: 6.25 105cells ml−1).
a maximal growth rate that ranges from 0.08 (at 15◦C) to 0.5 mm/day (at 30◦C).
The second, after sexual maturity of the females, is characterized by a slower growth rate at 20 and 30◦C, and very slow growth rate at 15 and 18◦C. During this phase, the growth rate ranges from 0.01 (at 15◦C) to 0.08 mm/d (at 30◦C, Table 6).
Size measurements show that all females have the same size at maturity (Fig. 8). The size ranges from 1.19 mm (at 18◦C) to 1.11 mm (at 30◦C).
Analysis of the evolution of life span for different temperatures (Fig. 8) shows that the average life span increases between 15 and 18◦C. Beyond these tem- peratures, there is a decrease of average life span of Moina. Maximal average life span of 22 days occurs at 18◦C, while a minimal life span of 6 days appears at 30◦C.
In general, the size of the animals remains small at low temperatures. The maximal sizes range from 1.34 mm at 15◦C to 1.56 mm at 18◦C. At higher tem- peratures (20 and 25◦C), the animals reach a maximal size of 1.8 mm, except for 30◦C where the maximal size is only 1.71 mm (Fig. 9).
Figure 10. Average size of females as a function of the number of moltings at 5 temperatures (concentration of algae: 6.25 105cells ml−1).
In parallel to the growth of size, we observed molt- ing related to the succession of stages. The number of moltings depends on the thermal conditions of the culture. It ranged from 3 at 15 ◦C to 10 at 20 ◦C and 25◦C (Fig. 10). The growth between successive moltings is important up to the 3rd and 4th molting cycle. It becomes lower and constant for the following moltings. Generally speaking, one or two moltings oc- cur during the juvenile stage, the others appear after each hatching. We observed 1–3 juvenile stages on average (between 30 and 18◦C, respectively) and 4–
6 adult stages (between 18 and 30◦C, respectively).
Each stage becomes shorter as temperature increases.
The results show thatM. macrocopagrowth at tem- peratures from 15 to 30◦C, with optimal growth at 20–25◦C, while the temperatures 10 and 35◦C cor- respond to the lower and upper limits of tolerance for this species.
Moreover, size did not change at the temperatures tested (0.78 mm at 15, 20 and 25 ◦C; 0.70 mm at 30◦C). Similar results were described forM. salina (Gordo et al., 1994) with sizes of 1.18 mm (at 20◦C) and 1.05 mm (at 30◦C) (Fig. 11).
At 15, 18 and 20◦C, mortality occurs between the 8th and 12th day and increases progressively. Com- plete extinction off is observed by day 17 at 20◦C, day 21 at 15◦C and day 23 at 18◦C. All survival curves show a similar shape (Fig. 12).
For higher temperatures (25 and 30◦C), mortality starts at the 6th and 7th day. The decrease of the num-
Figure 11. Life span ofM. macrocopaat 5 temperatures (concen- tration of algae: 6.25 105cells ml−1).
Figure 12. Survival curves ofM. macrocopaat temperatures (con- centration of algae: 6.25 105cells ml−1).
ber of animals is then rapid, with complete extinction of all animals at day 14 at 25◦C and day 9 at 30◦C.
Discussion
The development ofM. macrocopais optimal at high algae concentrations. Nevertheless, the females reach maturity at C3(5) concentration. The size at this stage is the same for the concentrations C3(5) and C4 (1.034±0.061 mm and 1.028 ±0.036 mm, respect- ively) while it reaches the significantly higher value of 1.133±0.077 mm in C5.
The lowest concentration (C0) of algae (6.25 cells ml−1) corresponds to a limiting concentration that allows individuals to survive for only a few days.
These results show that the growth of M. mac- rocopais closely controlled by the concentration of nutrients in the culture medium, in accordance with
data gathered in situ on the same population (Tifn- outi, 1987). The latter showed that the variation in the abundance of the different size classes was related to the trophic level (algae concentration fluctuated between 1.8×107and 2.6×109cells liter−1). How- ever, under culture conditions, the size of the females increases reaching 1.8 mm. For higherChlorellacon- centrations of 106 cells ml−1, Hanazato & Yasuno (1984) obtained identical results forM. macrocopa. It seems that 1.8 mm is the maximal size for this species.
Compared to other species of the same genus, (Moina micrura: 1.3 mm – Khalaf & Shihab, 1979; Moina brachiata:1.56 mm – Lazim & Faisal, 1989; Moina reticulata: 1.6 mm – Hardy, 1989),M. macrocopacan be considered as a species of big size.
In general, under the culture conditions used in this study (T=25◦C),M. macrocopashowed a life duration from 2 days (minimal duration at concentration C0) to 22 days (maximal duration observed for C3(2.5) and C3(5)).
Hence, the data show a clear influence of nutri- tion conditions on longevity. Literature data report life spans from 9 to 19 days for the related speciesMoina micrura(Jana & Pal, 1985a).
When trophic conditions are adequate, the shape of the survival curve is rectangular. This model seems to be general to Cladocera (Gras & St-Jean, 1978a;
Bonou et al., 1991; Gordo et al., 1994). Although the effect of nutrition inadequacy slows down growth, the critical level of resources is variable among species.
Duncan (1989) that on Cladocera from the temperate zones (Daphnia magna, Daphnia pulicaria, Daph- nia hyalina) the threshold concentration, defined as the trophic level at which the growth rate is zero, ranges from 0.01 to 0.02 mg DW l−1 (Dry Weight per liter). Higher values (0.06–0.20 mg DW l−1) were observed for tropical species such as Daphnia lum- holtzi, Diaphanosoma brachyurum, Moina micrura andMoina reticulata. For rotifers, the range of con- centration ranges from 0.06 mg DW/l for Keratella cochlearis to 1.03 mg DW.l−1 for Keratella crassa.
The threshold concentration forMoina macrocopain this study (0.13 mg DW.l−1) is significantly higher than that for other species of Cladocera from the tem- perate zone. It corresponds more to the levels found in tropical Cladocera and rotifers. The observed differ- ence betweenMoina macrocopaand other temperate Cladocera could be related to the quality of the food.
The qualitative aspect of nutrition, investigated by Rothhaupt (1990) for cultures of Brachionus rubens and Brachionus calyciflorus, revealed that the same
species can have different threshold concentrations de- pending on the algal species used. Rothhaupt (1990) found a value of 0.142 mg DW.l−1 in Brachionus rubenswithMonorhaphidium minutumas food, and 0.62 mg DW.l−1 withChlamydomonas sphaeroides.
In light of these observations, it seems that threshold trophic conditions defined experimentally are only ap- proximations of their value in natural systems where trophic conditions are variable.
Beyond this threshold concentration level, the use of the energy from ingested food depends on the im- portance of resources and on the species. In the case of nutrition deficiency, this energy is invested exclusively in survival (Lynch, 1980); such was the case ofMoina macrocopa in this study. Under trophic conditions above the threshold, the partition of energy between reproduction and growth depends on the species. For certain Cladocera of small size (Bosmina longirostris, Ceriodaphnia quadrangula, Diaphanosoma brachy- urum,Moina micruraandDaphnia retrocurva) the en- ergy is equally partitioned between the two processes across the whole life cycle (Lynch, 1980; Threlkeld, 1987; Duncan, 1989). On the other hand, other spe- cies of Cladocera (Ceriodaphnia cornuta, Daphnia ambigua,Daphnia pulex,Daphnia magnaandSimo- cephalus acutirostratus) and other species of rotifers (Asplanchna brightwelli, Brachionus rubens) invest, after maturity, most part of their energy in reproduc- tion (Snell & King, 1977; Lynch, 1980, Threlkeld, 1987, 1989; Duncan, 1989). This strategy leads to a decrease in longevity. Moina macrocopa (in the present work) is more similar to the second group be- cause of the reduction (six times) of its growth rate after maturity.
In their work on Moina micrura, Jana & Pal (1985b) demonstrate that growth in this species starts to be affected at a population density below 190 anim- als l−1. They found that the growth processes become density-bound starting at 75 animals l−1, a result re- lated to competition for space, food, and confinement.
In our research on Moina, the daily renewal of the culture medium, the quantity of nutrition and the sys- tematic removal of offspring alleviate competition and waste accumulation. Therefore, it is not surprising that our results are different from those of Jana & Pal (1985b).
Moreover, the absence of the effect of crowding on the growth ofM. macrocopareveals that the culture of several animals together (3 in this case) in a reasonable space (48 ml) with sufficient nutrients (6.25 × 105 cells per female) does not affect their physiology. In-
deed, the analysis of variance shows that the difference in life span between groups T and G is not significant (1% level). The difference for the density groups is, on the contrary, highly significant. These results high- light the negative influence of population density on survival. Nevertheless, the presence of several animals (3 in this case) in the same culture, but at a lower popu- lation density (1 female/16 ml), does not considerably affect survival. Similar observations were described forMoina micruraby Jana & Pal (1985b).
We did not find any influence of crowding on growth. No physiological changes were detected in cultures of three females together, which indicates that there was no production of inhibitors.
These laboratory results confirm in situ observa- tions (Tifnouti, 1989). Moina was found when the temperature of the lagoons was between 18 and 26◦C.
Terrier & Tank (1928), cited by Khalaf & Shihab (1979), found 8 ◦C as the lowest limit. The thermal behavior ofM. macrocopain this study is close to that described forMoina micruraby Hanazato & Yasuno (1985). We report arrested growth at temperatures below 15◦C.
Literature data show that at 30◦C,Moina micrura presents 2–2.5 juvenile stages (Murugan, 1975; Gras
& St-Jean, 1978b) and up to 11 adult stages (Murugan, 1975). Lazim & Faisal (1989) found two juvenile stages and 9 adult stages inMoina brachiataat 20◦C.
It seems that the number of adult stages is lower inM.
macrocopathan in other species investigated by Gordo et al. (1994).M. salinahas 3.2 (at 20◦C) and 6.2 (at 30◦C), with three juvenile stages.
In summary, all results show a decrease in longev- ity as temperature increases forM. macrocopa, and for Moina micrura(Hanazato & Yasuno, 1985). Life span drops from 21 days at 15◦C to 7 days at 30◦C. In an earlier study, Hanazato & Yasuno (1984) found a life span of 11 days inM. macrocopaat 23◦C (using Chlorellaat 106cells per ml and with a photoperiod of 16:8). The best temperature for survival seems to be 18◦C.
The effect of temperature in related to the use of energy. At low temperatures, it allows only growth and survival, with reduced fecundity. At high temper- atures, most energy is used for reproduction leading to the death of the female. The effects of temperature can be summarized as follows:
1. 10 and 35◦C are extreme temperatures. These lim- iting temperatures are different between species.
It is probable that temperature tolerance is com-
bined with an increase of the lower temperature threshold. The value of this threshold is difficult to estimate because it is accompanied with high mortality.
2. in the interval 15–30 ◦C, growth rate has posit- ively related with temperature and negatively re- lated with longevity. This has also been found in Daphnia sp. (Munro & White, 1975; Vijver- berg, 1980; Hanazato & Yasuno, 1985), Dia- phanosoma brachyurum(Herzig, 1984; Hanazato
& Yasuno, 1985),Simocephalus vetulus(De Bern- ardi et al., 1978) andBosmina longirostris(Vijver- berg, 1980).
Conclusions
Studies on trophic variables for Cladocera (Jana & Pal, 1985b) and rotifers (Gilbert, 1963; Clement & Pour- riot, 1975; Pourriot & Rougier, 1977) have shown the complexity of these phenomena and the difficulty of interpretation of the results.
The importance of trophic conditions for growth has led many researchers (Saint-Jean, 1978; Bonou et al., 1991; Gordo et al., 1994) to define growth indexes.
Our results show a clear influence of trophic con- ditions on the growth ofM. macrocopa. The optimal concentration ofChlorellawas 6.25×105cells ml−1. Under these conditions, females reach a maximal size of 1.8 mm. The minimal concentration of Chlorella for survival was 6.25 cells ml−1.
The longevity ofM. macrocopain culture was re- lated to trophic level, with average life span around 10 days, and maximal values of 22 days and a minimum of 2 days.
The two densities of 1 female/16 ml and 1 fe- male/5.34 ml and a crowding of three females/16 ml did not significantly affect the growth of M. mac- rocopa. Longevity decreased at the density of 1 female/5.34 ml.
M. macrocopa did not grow at 10 and 35 ◦C.
Optimum growth was between 20 and 25 ◦C and decreased outside this interval. Average life span de- creased with increasing temperatures. Optimum sur- vival was around 18◦C.
Acknowledgements
This study has been financially supported by the pro- ject S.E.M. 03/204/017. 177/MAR. We thank Mrs C.
Rougier and Mr G. Lacroix for their interest in this work and Ms Soumaya Tifnouti & A. Boussaid for the translation of the text.
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