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Case Construction, Movement, Spatial Distribution and Substrate Selection in the Larva of Chironomus Riparius Meigen

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CASE CONSTRUCTION,

MOVEMENT, SPATIAL DISTRIBUTION AND

SUBSTRATE SELECTION IN THE LARVA OF

CHIRONOMUS RIPARIUS MEIGEN

BY W. D. EDGAR AND P. S. MEADOWS Department of Zoology, University of Glasgow, Scotland

{Received 1 July 1968)

INTRODUCTION

Little experimental evidence is available on the factors affecting movement, sub-strate selection and dispersal of invertebrate larvae found in fresh waters (Macan,

1963; Macfadyen, 1963). An opportunity to study these factors occurred when a population of larvae of the case-building chironomid Chironoinus riparius Meigen became established in a large stoneware sink (110 x 50x20 cm.) in the vicinity of Glasgow University field station on the west bank of Loch Lomond. The sink was filled with loch water to a depth of 4 cm. and had a tarpaulin cover 110 cm. above it. Larvae used in the experiments described below were obtained from this sink, which will subsequently be referred to as the stock sink.

Observations were made on the behaviour of the larvae, particularly with regard to case building, and experiments were conducted on the spatial distribution of larvae, substrate selection and the relationship between these and larval age. Experimental methods will be described in the appropriate sections.

LARVAL BEHAVIOUR

Examination of the cases of the larvae indicated that they consisted mainly of algae. Case construction was observed by placing a larva in a watch-glass containing a film of algae under a binocular microscope.

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248 W. D. EDGAR AND P. S. MEADOWS

Larvae built cases successfully from algae, mud, and a mixture of mud and algae, although with algae the cases were better constructed and built more quickly than with the other two materials. Larvae could not build cases from a mixture of sand and gravel. When presented with this substrate, they exhibited a violent wriggling and swimming movement which they stopped from time to time to rest on the substrate. Similar behaviour has been noted for the larvae of marine invertebrates (Knight-Jones, 1951; Crisp & Meadows, 1963), and is likely to provide opportunities for substrate selection. Experiments on the ability of larvae of C. riparius to select a substrate are described below.

In a number of experiments larvae were removed from their cases before experi-mentation. It proved possible to predict whether cases contained a pupa or larva before they were opened, because the pupal cases contained more sand grains and were more compact than the fluffy cases of younger larvae. This suggests that before pupation larvae selectively incorporate sand grains into their cases, which would give them greater structural support during pupation.

Larvae in the stock sink used algae as a source of food as well as for building then-cases, and clear grazing areas could usually be distinguished around each case. Larvae often emerged from their cases and, whilst attached only by their posterior prolegs, fed on the algae. The gut was clearly visible as a black line in the young larvae due to the presence of algae. This line was absent in the larvae about to pupate which suggested that feeding ceases at some point before pupation. Dissection confirmed that the gut was empty in such animals.

Fig. 1. Larval distribution in stock sink. The outlines of larval cases were traced from 1 photograph. The length of the cases ranged between i-o and 2-5 cm.

SPACING OUT

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An attempt was made to replicate this spacing out under experimental conditions. Twenty of the youngest larvae from the stock sink were removed from their cases, and placed in dishes that had an algal coating. When they had built new cases they were placed in a central area, 36 cm.2, of a plastic basin that measured 27 cm. by 22 cm. The plastic basin was filled with loch water to a depth of 2-25 cm. and its outer surfaces were checkered off into squares of side 4 cm. The squares could be seen through the thickness of the basin, and were used to record the distribution of the larvae after 10-5, 72 and 95-5 hr. During the period of the experiment larvae moved out of-the central area, and after 95-5 hr. appeared to be spaced out in the same way as those in the stock sink.

The larval distributions in the stock sink (Fig. 1) and in the laboratory experiment after 10-5, 72, and 95-5 hr., were analysed by the nearest-neighbour method (Morisita, 1954; Clark & Evans, 1954; Southwood, 1966). For each distribution, the mean distance to the nearest neighbour was computed by taking larvae in turn, measuring the distance to the nearest neighbour, summing these values, and dividing by the number of measurements. The mean distance to the nearest neighbour in a randomly distributed population of the same density (re) was calculated as

where p = population density (Clark & Evans, 1954). Population density was calcu-lated by dividing the number of larvae in the basin by the total area of the bottom of the basin (127-3 c m-2) a nd n° t by the area initially occupied by the larvae. The signifi-cance of the difference between the observed mean nearest-neighbour distance (r0)

and the mean nearest-neighbour distance expected on a random distribution was calculated from

in which d is treated as a normal variable with zero mean and unit standard deviation (Bailey, 1959, p. 34) and its appropriate probability obtained from Fisher & Yates (1963, table 1). In equation (2) the standard error of the expected mean, cr^ , is given by

where n = number of measurements (Clark & Evans, 1954).

The results of these calculations are given in Table 1. After 10-5 hr. the distribution was still highly aggregated, after 72 hr. it was random and after 95-5 hr. it was regular. The distribution of the larvae in the stock sink was highly regular.

During the experiment, larvae moved out from the central area in which they were originally placed, and also became more regularly distributed. It is of interest to consider the rate at which dispersion occurred by plotting the ratio fjfe ( = R, Clark &

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25°

W. D. EDGAR AND P. S. MEADOWS

Table i. Nearest-neighbour analysis of spacing out

Time from beginning of experi-ment (hr.)

io-S 7 2 0

9 5 5 Field population

Population density (no. larvae/cm.1)

0-1414 O-I493 0-1572

0-6355

Mean distance to nearest neighbour (cm.)

L

observed expected

O-5778 I-33O 1-279 1294 1-490 1-261 9-178 0-6272

d

7 3 2 3 0-09665

1-553 247-4

P

< o-ooi

< 0-93 > 0-92 < O-I3 > O-I2

< o-ooi

Aggregated random, or

regular distribution

Highly aggregated

Random Regular Highly regular

10

0-5

50 Time (hr.)

100

Fig. 2. Rate of spacing out of larvae. f0 = observed mean nearest-neighbour distance, and

f. = expected mean nearest-neighbour distance in a randomly distributed population at the same population density. When fop, < 1, the population is aggregated; when fjf, = 1, the

population is randomly distributed; when fjf, > 1, the population is distributed regularly (spaced out).

AGE OF LARVAE

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SUBSTRATE SELECTION

From the experiments on the suitability of various materials for case building (p. 248), it was suggested that larvae were testing the substratum. The following experiments were therefore carried out to determine whether larvae could select suitable substrates.

Table 2. Larval age and the proportion of larvae that move

Proportion of larvae that move out from the original area

0/ /o moving

out 61 26 2 0

0/ /o remaining

in 39 74

7 1

No. of

larvae 18 19 28 Young larvae

Larvae about to pupate Pupae and empty cases

X1 on a 3 x 2 table of larval age against numbers moving out or remaining in the original area = 6-274 with 2 degrees of freedom, P < 0-05, > 0-02 (Fisher, 1958, p. 91; Moroney, 1958, p. 258).

Table 3. Larval age and rate of movement

Larvae

Young about to Empty larvae pupate Pupae

Mean (mm./hr.) 73-227 2-992 i'33i 1-009 8.D. (mm./hr.) 7-116 5-855 2-598 1-008

Table 4. Relationship between larval age and distance moved; analysis of variance

Probability that Sum Degrees the variation is Source of of of Variance not different variation squares freedom estimate from the residual

Between ages 5,912-1 3 1970-7!

Residual 23^795 62 3 7 8 9 } K °"OI> > ° '0 0 1 Total 29,391-6 65 — —

Table 5. Substrate selection

% on % on % on % on

algal sand/gravel No. of substrate substrate larvae Young larvae 95 5 40 Larvae about to 78 22 46

pupate

For statistical analysis see text.

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252 W. D. EDGAR AND P. S. MEADOWS

larvae between the two substrates recorded. The experiment was repeated eight times, and the results are given in Table 5. Both young larvae and larvae about to pupate showed a marked preference for the algal substrate, although this was more marked in young larvae. The significance of the latter difference was assessed by the exact 2 x 2 table

X2 test to give P = 0-02079 (Bailey, (1958, p. 61) recommends this method when the expected value in one of the cells is less than 5). There is therefore a probability of under 3 % that the ratios of numbers on algal substrate to numbers on sand/gravel sub-strate are the same for young larvae and for larvae about to pupate.

DISCUSSION

In the substrate-selection experiments a smaller proportion of older larvae were found on the algal (favourable) substrate than young larvae (Table 5). Similar studies on substrate selection by marine larvae have given contradictory results. Larvae of

Spirorbis boreaHs become less discriminating as they age (Knight- Jones, 1953), although

this conclusion has been queried by work of Williams (1964). Cypris larvae of barnacles, on the other hand, show no loss in discriminatory abilities (Crisp & Meadows, 1963). However, the ratios obtained in choice experiments depend both on discriminatory ability and on the rate of movement of animals; more slowly moving animals will be apparently less discriminating during the earlier parts of a choice experiment, because the population will distribute itself more slowly between the two substrates. Neither ourselves nor the authors referred to above distinguish between these alternatives, and Meadows (1967) has overlooked the possibility of rate of movement affecting choice ratios when defining less discriminating animals. Tables 2-4 show that larvae move more slowly as they age. It follows, then, that the smaller proportion of older larvae in the favourable substrate (Table 5) is the result of either (a) reduced move-ment, or (b) reduced movement and reduced discrimination acting together.

The advantages of nearest-neighbour methods in the experimental analysis of spacing out and gregariousness have not been fully appreciated. They allow accurate quantification of the clumped or regular patterns of dispersion that often occur in experiments on substrate selection (Wisely, i960; Meadows, 1964). Furthermore, unlike the use of x* with the Poisson distribution, they do not depend on choosing a suitable quadrat size (Crisp, 1961; Macfadyen, 1963). Statistical comparison can be made between the observed distribution of a population and its distribution when randomly dispersed at the same population density. The degree of clumping or regularity between different populations can also be compared. Clark & Evans (1954) and Morisita (1954) give details of the statistics.

We have attempted to assess the rate at which spacing out occurs in an experimental environment by plotting fo/fe against time (Fig. 2). If other larvae give similar results,

it should be possible to use the slope of the line, d{rojf^)jdt, as an estimate of the

relationship between spacing out and other variables. The most obvious one to consider would be population density, and in fact Bovbjerg (1964) has discussed the influence of population density on dispersion. He suggests that dispersion increases with popula-tion density in those species which are aggressive. The crayfish Cambarus attend, an aggressive species, disperses more rapidly at high population densities (Bovbjerg,

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Gam-marus pseudolimnaeus (Clampitt, unpublished work reported in Bovbjerg, 1964),

neither of which are aggressive, disperse at the same rate irrespective of the population density.

SUMMARY

1. Larvae of Chironomus riparius construct their cases from algae in a stereotyped manner. Mud and a mixture of mud and algae can be used instead of algae, although not so successfully.

2. Larvae cannot build cases from sand, and when presented with this substrate they alternate periods of active swimming with resting on the substrate. This is reminiscent of the behaviour of marine larvae as they test substrates before settling. 3. Larvae space out on an algal substrate. Nearest-neighbour analysis has been used to quantify spacing out, and has shown that under experimental conditions initially clumped animals become regularly distributed over a period of time.

4. As larvae age, they move more slowly and are also less likely to move.

5. In substrate-choice experiments larvae prefer an algal to a sand/gravel substrate. 6. Substrate selection is less marked in old larvae. This could be due either to a loss of ability to discriminate between substrates with age, or to a reduction in the rate of movement with age.

We should like to thank Dr H. D. Slack, F.R.S.E., for identifying the Chironomus species.

REFERENCES

BAILEY, N. T. J. (1959). Statistical Methods in Biology. London: English Universities Press.

BOVBJERG, R. V. (1952). Ecological aspects of dispersal of the snail Campeloma decisum. Ecology 33, 160-76.

BOVBJERG, R. V. (1959). Density and dispersal in laboratory crayfish populations. Ecology 40, 504-6. BOVBJERG, R. V. (1964). Dispersal of aquatic animals relative to density. Verh. int. Verein. theor. angew.

Limnol. 15, 870-84.

CLARK, P. J. & EVANS, F. C. (1954). Distance to nearest neighbor as a measure of spatial relationships in populations. Ecology 35, 445-53.

CRISP, D. J. (1961). Territorial behaviour in barnacle settlement. J. exp. Biol. 38, 420-46.

CRISP, D. J. & MEADOWS, P. S. (1963). Adsorbed layers: the stimulus to settlement in barnacles. Proc. Roy. Soc. B 158, 364-87.

FISHER, R. A. (1958). Statistical Methods for Research Workers, 13th edn. London: Oliver and Boyd. FISHER, R. A. & YATES, F. (1963). Statistical Tables for Biological, Agricultural and Medical Research,

6th edn. London: Oliver and Boyd.

KNIGHT-JONES, E. W. (1951). Gregariousness and some other aspects of the setting behaviour of Spirorbis. J. mar. biol. Ass. U.K. 30, 201-22.

KNIGHT-JONES, E. W. (1953). Decreased discrimination during setting after prolonged planktonic life in larvae of Spirorbis borealis (Serpulidae). J. mar. biol. Ass. U.K. 32, 337-45.

MACAN, T. T. (1963). Freshwater Ecology. London: Longmans, Green. MACFADYEN, A. (1963). Animal Ecology. London: Pitman.

MEADOWS, P. S. (1964). Experiments on substrate selection by Corophium volutator (Pallas): depth selection and population density. J. exp. Biol. 41, 677—87.

MEADOWS, P. S. (1967). Discrimination, previous experience and substrate selection by the amphipod Corophium. J. exp. Biol. 47, 553-9.

MORISITA, M. (1954). Estimation of population density by spacing method. Mem. Fac. Sci. Kyushu Univ. (Series E). Biology 1, 187-97.

MORONEY, M. J. (1958). Facts from Figures, 3rd edn. Middlesex: Penguin books. SouTHWOOD, T. R. E. (1966). Ecological Methods. London: Methuen.

WALSHE, B. M. (1951). The feeding habits of certain chironomid larvae (subfamily Tendipedinae). Proc. zool. Soc. Lond. i a i , 63-79.

WILLIAMS, G. B. (1964). The effect of extracts of Fucus serratus in promoting the settlement of larvae of Spirorbis borealis (Polychaeta). J. mar. biol. Ass. U.K. 44, 397-414.

Figure

Table i. Nearest-neighbour analysis of spacing out

References

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