COMPETITION

BETWEEN SIMOClZEPHALUS CYCLOPS VIRIDIS

VETULUS

AND

Richard A. Parker Department

of Zoology,

Washington

State

University,

Pullman,

Washington

ABSTRACT

Simocmhalus vet&s and CycZops &i&s were raised in pure and mixed cultures under controlled laboratory conditions. Cyclops numbers were depressed markedly in the presence of Simocephnlus, whereas Cyclops had essentially no influence on numbers of Simocephalus.

Cladocerans and copepods are prominant among the zooplankton of freshwater lakes, and as such play an important role in trophic level dynamics. These animals are subject to extreme fluctuations in numbers and because of these have been the object of extensive study by limnologists. Although the effects of certain environmental factors have been evaluated for 1 This investigation was supported by a research grant RG-5241 from the National Institutes of Health, Public Health Service. The assistance of Mrs. Rosana Brousseau is gratefully acknowledged.

some cladocerans, in particular Daphnia, relatively little effort has been concentrated on copepods. The relationship of daphnid cyclomorphosis to temperature has been studied by many investigators, among them Coker and Addlestone ( 1938), Kiang ( 1942)) and Brooks ( 1946). Pratt ( 1943 ) has analyzed Daphnia population development at different temperatures; Ewers ( 1930)) Coker ( 1933), and Aycock ( 1942) discussed the effect of temperature on species of Cyclops. Anderson et nZ. (1955) show that the number of cladocerans in

s, adults 20Low

food

WEEK FIG. 1. Fluctuations in the progressive bi-weekly means of the numbers of Simocephalus vetulus adults in pure and mixed cultures at the low food level. ( Solid line indicates pure cultures, dashed line indicates mixed cultures. )

180

COMPETITION

BETWEEN

S. VETULUS

AND

181

C. VZRZDZS

&

adults

High

food

20-

16.

WEEK 2. Fluctuations in the progressive bi-weekly means of the numbers of Simocephalus vet&s adults in pure and mixed cultures at the high food level. ( Solid line indicates pure cultures, dashed line indicates mixed cultures. ) FIG.

Soap Lake, Washington, is not related to their winter phytoplankton food supply, whereas the number of copepods in Lake Lenore rises after the spring and fall phytoplankton maxima. These results may be atypical since both lakes arc highly saline; also, Comita and Anderson (1959) have found that the Diuptomus population of Lake Washington is correlated positively Ryther with chlorophyll concentration. ( 1954) has shown that Chlor& will inhibit the growth and reproduction of Daphnia; however, Slobodkin (1954) found that the sizes of his Daphnia populations were directly related to the quantity of Chlnmyclomonas available as food. The latter is in support of work by Ingle, Wood, and Banta (1937) and Banta (1939). Slobodkin and Richman (1956) studied the effects of removal of newborn Daphnia and concluded that the resulting increase in reproductive rate was due to the increased

food supply for the remaining individuals. On the other hand, Frank ( 1952) found that when two cladocerans, Daphnia and Simocephnlus, occupy the same medium, the former invariably eliminates the latter, and he concluded that direct density effects were operating. Since cladoceran and copepod population peaks often fail to coincide, one is led to inquire into possible competition, either direct or indirect, which may act as a significant limiting factor. To evaluate this possibility, Simocephalus vetulus and C yclaps viridis (kindly identified by R. W. Kiser ) were collected from Silver Lake near Spokane, Washington, to be raised under controlled laboratory conditions. METHODS

In the spring of 1958, 320 cultures were started in small glass containers, each of which held 50 ml of Silver lake water fil-

182

RICHARD

A. PARKER

WEEK FIG. 3. Fluctuations in the progressive bi-weekly means of the numbers of Simocephalus iuvenilcs in pure and mixed cultures at the low food level. (Solid line indicates pure cultures, iinc indicates mixed cultures. )

tcred through number 25 silk bolting cloth. The cultures were distributed as to content and number according to the following design : Low food High food

Simocephnlus

cyczo)ls

Simocephnlus x cyczops

40 40

40 40

80 80

More cultures were assigned to the mixed category because it was felt that the variability would be greater here. Each pure culture was initiated with one gravid female, whereas each mixed culture received one gravid female of each genus. All cultures were kept in a water bath at 20°C under continuous fluorescent lighting. The animals were fed weekly with a moewusii suspension of Chlamydomonas having a Klett photometer density of 75 with a red filter, Those under low food received I ml and those under high food re-

vetulus dashed

ceived 2 ml. The alga was raised in the following medium: KN03 K2HP04 MgSol: 7Hz0 chelated iron soil extract distilled water

0.250 0.100 0.100 0.005 167 833

g g g g ml ml

Sixteen cultures, two of each pure culture and four of the mixed cultures at each food level, were counted each week, and then were terminated due to the difficulty in making successful transfers of all Cyclops Individuals were recorded in nauplii. arbitrary classes. Zfimocephnlus-juveniles and adults, and Cyclops-nauplii and copcpodids ( including adults), Although culture termination has rather obvious faults, it is apparent that this procedure results in

COMPETlTION

BETWEEN

S. VETULUS

AND

C. VZRZDZS

183

WEEK IFIG. 4. Fluctuations in the progressive hi-weekly means of the nmnbers of Simocephalus uetdus juveniles in pure and mixed cultures at the high food lcvcl. (Solid line indicates pure cultures, dashed line indicates mixed cultures. )

independent weekly observations lend themselves to reliable analysis.

which

RESULTS

Results are summarized graphically using progressive biweekly ( two-week moving) averages of the mean culture sizes for each week. In these figures a maximum will be identified at any point where the slope is positive to the left of that point and negative to the right. Simocephalus adults At low food there appears to be little difference between the magnitude of fluctuations in numbers of adults in the two types of cultures (Fig. 1) , In pure culture, maxima occur near weeks 5, 9, 12, and 15; whereas, in mixed cultures maxima occur near weeks 5, 10, 15, and 17. The mean times between maxima are 3.8 and 4.2

weeks, respectively, thereby suggesting that, if this difference is real, those animals grown with Cyclops develop at a slower rate. When raised at the higher food level, the maxima are grcatcr for the animals in mixed cultures and they occur somewhat in advance of those in pure cultures ( Fig. 2); this is in contrast with the situation cited above. However, as will be shown later, a real difference between total population levels of pure and mixed cultures cannot bc demonstrated. Simocephalus juveniles As was the case with the adults fed at the low food level, little difference between numbers of juveniles in pure and mixed populations can be noted (Fig. 3). Maxima are somewhat larger in the mixed cultures, and there arc fewer of them.

184

RrCHARD

A. PARKER

160-

&

copepodids

Low

food

MO-

120-

IOCK k.i

0

‘=”

6C-

4c-

1 0

IO

15

I

,

1 20

WEEK Fluctuations in the progressive bi-weekly means of the numbers of Cyclops &i&s copepodids FIG. 5. in pure and mixed cultures at the low food level. (Solid lint indicates pure cultures, dashed line indicates mixed cultures. )

When fed at the higher rate, isolated maxima were larger than for animals fed at the lower rate; however, it seems unlikely that there is any real difference between 20-week population means for either culture type (Fig. 4). As was noted for the adults at high food, the maxima in mixed cultures secmcd to have occurred in advance of those in pure cultures. Cyclops copepodids Under conditions of low food, mean copepodid numbers in pure cultures reached a maximum of approximately 160 ani-

mals per culture: twice that of those in mixed cultures ( Fig. 5). Furthermore, it is obvious that population development after three months in the pure cultures was about three weeks ahead of that in the mixed cultures. At the high food level, differences bctween pure and mixed cultures are increased markedly; copepodids in pure cultures reaching 240 individuals and copepodids in mixed cultures reaching 80 individuals ( Fig. 6). As above, there appears to be a lag in population development for those animals in mixed cultures.

COMPETITION

BETWEEN

Analysis

TABLE 1.

S. VETULUS

of variance

AND

185

c. VIRZDIS

of total population

size Cyclops

Simocepholus cl.f.

Source

Culture type (C ) Food level ( F ) Time (T) CF CT FT CFT Residual Total ***

Significant * Significant

SS

1 1 19 1 19 19 19 78

492 131 5237 245 3033 3578 2898 10705

157

26319

SS

d.f.

MS

492 131 276 245 160 188 153 137

1 1 19 1 19 19 19 80

94429 28169 373579 253 167621 73410 103660 346261

159

1187387

MS 94429"

ti: $

2fjl69:':

19662 258 8822 3864 5456 4328

at the 0.001 level. at the 0.05 level.

&.

copepodids

r

FIG. 6. Fluctuations in the progressive bi-weekly in pure and mixed cultures at the high food level. catcs mixed cultures. )

means of the numbers of Cyclops vi&is copcpodids (Solid lint indicates pure cultures, dnshcd line indi-

186

HIC:I-IARD

A.

PAHKER

E

nauplii

WEEK

7. Fluctuations in the progressive bi-weekly means of the numbers of Cyclops &i&s nauplii in pure and mixed cultures at the low food level. (Solid line indicates pure cultures, dashed lint indicates mixed cultures. ) FIG.

Cyclops nauplii When fed at the lower rate, numbers of nauplii reached 85 per container in pure cultures in contrast to approximately 30 in mixed cultures ( Fig. 7). Again, the lag between culture types is evident. A final comparison shows that nauplii fed at the higher food level reached maxima in pure cultures that were approximately twice that of those in mixed cultures ( Fig. 8). Here the lag is not well defined. The lag in population development of both Cyclops copepodids and adults grown in mixed cultures over those raised in pure cultures stems worthy of further comment. As was seen in the discussion of SimocephnZus, there were instances of an apparently beneficial nature for those animals in mixed The opposite effect noted for cultures. Cyclops suggests that any benefit to the Simocephalus may have been at the expense of the Cyclops population.

Although graphical presentations are helpful in analyzing the results of these experiments, they are often misleading since the mean values used give no indication of the dcgrec of sample variation. For those who prefer a more detailed account, an analysis of variance of total population size is given in Table 1. The 320 cultures were randomly divided into two groups, one for the analysis of Simocephnlus populations and one for the analysis of Cyclops populations. Inadvertent loss of two cultures in the former group accounts for the difference in degrees of freedom. Here we see that there is a very highly significant difference between Cyclops population size in pure and mixed cultures, the former being much larger as seen in Figs. 5-8. The only other probable difference occurred between Cyclops populations raised at the two different food levels. In this case, there

COMPETITION

BETWEEN

S. VETULUS

AND

C. VIRZDZS

187

WEEK

8. l?luctuations in the progressive bi-weekly means of the numbers of Cyclops viridis nauplii in pure and mixed cultures at the high food lcvcl. (Solid lint indicates pure cultures, dashed line indicates FIG.

mixed cultures. )

were fewer animals when fed at the lower rate than at the higher rate. DISCUSSION

It has been shown that joint occupancy by Simocephalus and Cyclops results in a depressed population of the latter. The cause of this depression is at this time unknown, however, one may speculate as to its nature. Suspect is food supply although even at the low food level there always appeared to bc a slight excess of the alga at the end of each week. It is possible, of course, that prcvcnts the prcscnce of Simocephalus proper feeding by Cyclops unless large quantities of food are available. In order to make further discussion profitable, it is desirable to determine the part of the Cyclops life cycle affected. It is apparent that the cladoceran population must, in some way, partially inhibit successful

reproduction of the copepod; however, detailed analyses of results from these rapidly fluctuating populations are extremely difficult. One may gain insight into the problem if numbers of SimocephaZus can be correlated in some way with numbers of An approach which may Cz#ops nauplii. yield the necessary information revolves around use of p, the rank correlation coefficient ( Spearman 1906). This quantity behaves like the product-moment correlation coefficient in that it may vary betwccn + 1 and - 1, with 0 indicating For further discussion no relationship. of this statistic the reader is referred to Kendall (1952). It is defined as

oizldi2 1 -n3_n where n is the number of paired observations and di is the difference between the

188

RICHARD A. PARKER

TABLE 2.

Analysis

of lug effect by rank correlation Cyclops

10

11

12

13

.39 .84 .67 .67 .13 -.27 -.61 -.63 -.36 -.57 -.26

.43 .80 .61 .70 .14 -.18 -.50 -.52 -.40 -.55

.31 53 .56 .64 .31 .09 718 -.54 -.53

.25 .49 .58 .65 .39 .20 -.24 -.57

Lag (weeks)

0 1 2 3 4 5 6 7 8 9 10

viridis

nauplii Number 14

.13 -40 .52 .55 .27 .05 -.42

Week

Species 1

1

1

1

4 2

5 4 2 3

2 3 4 5

5, i di” =

Species 2

.26 .47 .54 .58 .40 .17

.31 53 .56 .46 .20

on number

of

17

18.

19

20

.34 .55 .53 .49

.39 .41 .32

.34 .30

.28

10, and p = 0.5.

If we had previously shown that species 1 was depressing species 2, WC would of course be looking for a negative correlation. On this basis, we could simply realign the data and recompute p. To illustrate, let us look for a depressing lag effect by moving the species 2 data upward one week at a time. We might begin by using only data from weeks l-3 for both species, then from weeks l-3 for species 1 and 2-4 for species 9 and finally 1-3 for species 1 and 3-5 for &, species 2. Reranking the data for species 2 each time, WC have: 2 weeks’ lag 1 week lag No lag Species 1 Species 2 Species 1 Species 2 Spccics 1 Species 2

1

1

1

2 3

3 2

2 3 p=-1.0

3 2

1

1

ing our discovery week: Species 1

No lag

Spccics 2

1

1

2 4 3

4 3 2 p=o.4

i=l

p-O.5

of paired observations 15 16

vctulus

-

ranks of members of the it” pair. For cxample, consider five weeks of hypothetical population data for two species occupying a common habitat. Numbers of organisms are replaced by rank in the following table.

Here n =

of number of Simocephalus at high food level

3

2

1 3 2 p=-0.5

Clearly the known detrimental effect of species 1 on species 2 is occurring maximally after a lag of one week. We might have begun with four weeks’ data, thereby limit-

of a lag effect to one 1 week lag Species 1 Species 2

4 3

1

2 4 3

1

2 p=-1.0

Again it is clear that the greater effect is taking place after a one week lag. This tcchniquc has been applied to the number of Simocephnlus individuals and the number of Cyclops nauplii in mixed cultures at the high food level ( Table 2). For example, in the second horizontal line (one WC&S lag) the 10 paired observations comprise the averages of Simocephnlus in weeks 1 to 10 compared with CycZops in weeks 2 to 11; and that, at the extreme right, the 19 pairs are weeks 1-19 compared with 2-20, etc. Since autocorrelations have not been computed and the same data have been utilized to compute all of the p’s, significance considerations are not justified; however, the pattern of the correlation values may prove useful. It suggests that a maxioccurs around mum positive correlation week three, whereas a maximum negative correlation dots not occur until week seven. If this 7-week lag is real, it is obvious that the Cyclops depression is not due to the destruction of Cyclops’ eggs by Simocephnlus. Rather, it would seem that the Simocephnlus intcrfcre with the normal development of a group of Cyclops in such a way that their

COMPETITlON

BETWEEN

future fecundity is reduced. The positive correlation needs to be explained. REFERENCES

ANDERSON, G. C., G. W. COMITA, AND VEIXNA 1955. A note on the ENGSTROM-HEG. phytoplankton-zooplankton relationships in two lakes in Washington. Ecol., 36: 757-759. of tempcraturc on AYCOCK, D. 1942. Influence size and form of Cyclops vernalis Fischer. Jour. Elisha Mitchell Sci. Sot., 58: 84-93. RANTA, A. M. 1939. Studies on the physiology, genetics, and evolution of some Cladocera. Publ. Carnegie Institute Washington, 513 : l-285. BROOKS, J. L. 1946. Cyclomorphosis in Daphnia. I. An analysis of D. retrocurua and D. galeata. Ecol. Monogr., 16: 409-447. COKER, R. E. 1933. Influcncc of temperature on size of fresh-water copepods (Cyclops). Int. Rev. Hydrobiol., 29 : 406-436. -, AND II. H. ADDLESTONE. 1938. Influence of tempcraturc on cyclomorphosis of D. longispina. Jour. Elisha Mitchell Sci. Sot., 54: 45-75. COMITA, G. W., AND G. C. ANDERSON. 1959. The scnsonal development of a population of Diaptomus ashlundi Marsh, and related phytoplankton cycles in Lake Washington. Limnol. Oceanogr., 4 : 37-52. EWERS, L. A. 1930. The larval development of freshwater Copepoda. Ohio State Univ. Contrib. Franz Thcodorc Stone Lab., 3: l43.

S. VETULUS

AND C. VIRZDIS

189

1952. A laboratory study of intraFRANK, P. W. competition in species an d interspccies Daphnia pulicaria (Forbes) and Simocephalus vetulus (0. F. Miillcr). Physiol. Zool., 25: 178-204. INGLE, L., T. R. WOOD, AND A. M. BANTA. 1937. A study of longevity, growth, reproduction, and heart rate in Daphnia longispina as influcnccd by limitations in quantity of food. Jour. Exp. Zool., 76: 325-352. KENDALL, M. G. 1952. The advanced theory of statistics. Vol. I. Hafncr, New York. 457 PP. KIANG, G. II. 1942. Ubcr die Cyclomorphosc dcr Daphnicn cinigcr Voralpensccn. Int. Rev. Hydrobiol., 41: 345-408. PHATT, DAVID M. 1943. Analysis of population development in Daphnia at different tempcraturcs. Biol. Bull., 85: 116-140. RYTHER, JOI-IN H. 1954. Inhibitory effects of phytoplankton upon the feeding of Daphnia pngrzn with rcfcrcncc to growth, reproduction, end survival. Ecol,, 35: 522-533. SLO~~DKIN, L. B. 1954. Population dynamics in Daphnia obtusa Kurz. Ecol. Monogr., 24: 69-88. -, ANI> SUMNI~H RICI-IMAN. 1956. The cffeet of removal of fixed percentages of the newborn on size and variability of Daphnia pulicaria ( Forbes ) populations. Limnol, Oceanogr., 1: 209-237. SPEARMAN, C. 1906. A footrule for measuring correlation. Brit. Jour. Psychol., 2: 89.