J. Exp. Bwl. (1969), 50, 785-801 -785With 8 text-figura

Printed in Great Britain

THE ORIENTATION OF PLAICE LARVAE (PLEURONECTESPLATESSA L.) IN WATER CURRENTS

BY G. P. ARNOLD

Fisheries Laboratory, Lotoestoft

{Received 18 October 1968)

INTRODUCTION

Bishai (i960) and Ryland (1963) studied the reactions of pelagic marine fish larvaeto water currents and measured swimming speeds. They used glass tubes about 1-5 m.long with internal diameters respectively 1-2 and 175 cm. Neither author obtained aresponse to a striped background moving outside the tube, and Ryland concluded thatplaice larvae did not respond to displacement of the visual field. He suggested thatlarvae close to the tube wall, but not in actual contact with it, could detect the currentby the velocity gradient across the boundary layer. Hoglund (1961) suggested thatsmall roach (Leuciscus rutilus L.) might similarly detect the gradients in his fluvarium.In tubes of such small diameter, however, it is not possible to determine whetherlarvae detect the current by visual clues or by the velocity gradients, because herringlarvae {Clupea harengus L.) have been shown to respond visually to objects up to3 cm. away from them (Blaxter, 1962). Experiments have therefore been carried out ina much larger rectangular tank in which the position and orientation of larvae couldbe accurately recorded, so that the clues to which the fish responded could bedetermined. The results are described in this paper.

MATERIAL

Larvae caught at sea in plankton nets are not suitable for this kind of work, becauseeven if they are alive when caught they are invariably damaged. They can, however, bereared successfully in the laboratory from eggs caught at sea, or obtained by artificialfertilization of gametes from 'running' fish. For these experiments, larvae wereobtained from the laboratory's hatchery; most were reared from eggs of North Seafish, but the later larval stages of the 1966 season were from Irish Sea eggs. Largenumbers of each stage were available and all those used were feeding fish in goodcondition. Stunted larvae were avoided when selecting individuals for experiments.

Five developmental stages have been described for the plaice larva by Shelbourne,Riley & Thacker (1963) and their classification has been followed in this work, althoughmore recently Ryland (1966) has described the stages in greater detail and subdividedthem. Both Shelbourne et al. and Ryland (1963) have illustrated these stages. Theduration of each stage depends on temperature and to a slight extent on the amount ofavailable food (Riley, 1966).

30-2

786 G. P. ARNOLD

METHODS

i. Water tunnel

Plaice larvae were observed in a Perspex water tunnel 169 cm. long and 10 cm.square inside, which was connected between two constant-head reservoirs (Fig. 1).These were 46 cm. square, 51 cm. deep, and made of 12 mm. resin-bonded marineplywood coated with ' Araldite' paint. At each end of an observation chamber, 100 cm.long, in the water tunnel there was an entry port for the introduction of larvae. Thewater level in the reservoirs was maintained by a pump (2730 l./hr.) delivering seawater from a sump tank through two valves (Fig. 1). Turbulence produced by the

Overflow

Inlet hoseObservation chamber

Entry port

Potentiometer boxDexion railway Hose to right

drain valve

Constantheadreservoirbox Drain line

Fig. 1. (a) The water tunnel used to study behaviour of plaice larvae in relation to currents.(6) Circulation system of water tunnel.

inflow pipes was reduced by two vertical baffles in the centre of each reservoir, one ofglass-fibre honeycomb and the second of rubberized horsehair matting; a third screenof fine gauze covered the junction of the reservoir and tunnel. A pipe from eachreservoir to the sump allowed a current to be produced in either direction in the tunnel.

Orientation of plaice larvae in water currents 787

The pipes were of polythene, 2-5 cm. bore, controlled by diaphragm valves (Saunderstype A).

As the apparatus was filled from the sump, large bubbles of air trapped at the top ofthe tunnel were removed with a suction hose. When the reservoirs were overflowing,the pump line valves were adjusted so that there was no flow through the tunnel; thiswas observed by introducing potassium permanganate solution. The desired currentwas produced by opening the appropriate drain valve; the right-hand one was fittedwith a pointer and protractor scale. Water flow in the tunnel was turbulent and thevelocity profiles (Fig. 2) were typical for water flowing in a rectangular channel. Anoptical technique of velocity measurement (Seddon & Anwar, 1963) was used, but itwas not possible to measure closer than 1 cm. to the side.

Illumination was provided by six 60 W. tungsten strip lights (each 24 cm. long)controlled by a rheostat. They were mounted three above and three below the tank inorder to keep larvae that were released there in midwater. To avoid impeding theobserver the lights were not positioned vertically above and below the tunnel butdisplaced to lie on a line drawn diagonally through the corners of the tank, with theupper set behind the tunnel and the lower set in front.

(a) (b)

9 -

I5O

X3

1 -

1

- •

-

- j. I

1

2

>

Ji

3

\

•

/

i i

9

I'I5

3

1

1

-

-

-

— i

2

i i

3

\

j•

/1 1

1 2 3 4 SVelocity (cm./sec.)

1 2 3 4 5Velocity (cm./sec.)

Fig. a. Velocity profiles in the water tunnel, at 10 cm. downstream from the left-hand entryport, at three settings of the right-hand drain valve: (i) quarter-open, (2) half-open, (3) full-open, (a) Vertical profiles. (6) Horizontal profiles.

2. Recording techniques

Records of horizontal orientation and three-dimensional position of larvae in thetunnel were made simultaneously. A grid scribed on the outside of the observationchamber divided it into sections. On the top and bottom were transverse lines at 1 cm.intervals, with those at 10 cm. picked out in black for ease of identification. All fourwalls had longitudinal lines at intervals of i, 3, 5, 7 and 9 cm. from the edge. Thusthe cross-section of the tunnel was divided into six horizontal and six vertical sections(Fig. 3 a). The observer looked vertically down into the observation chamber andsimultaneously into a mirror, set at 450 from the vertical, behind it (Fig. 1). He couldtherefore see both the vertical and horizontal positions of a larva in addition to its

50-3

788 G. P. ARNOLD

horizontal orientation, and recorded all three on a twenty-channel chart recorder bymanipulating three wafer switches. These were mounted on a trolley running along a' Dexion' railway in front of the water tunnel (Fig. i). The first switch had six positionscorresponding to the six horizontal sections of the grid, the second six positions for thesix vertical sections, and the third eight positions for recording orientation. Thedirection in which each larva faced was recorded to the nearest of eight primary com-pass points (Fig. 3 b), each of which was therefore the centre of a 450 sector. Each

(a) (b)

So

!

Side

Mid vater

Is

1 2 3 4 5Horizontal positions

Fig. 3. (a) Cross-section of the water tunnel to show grid of horizontal and vertical positions.The categories midtcater and tide are separated by the heavy line, (ft) The eight primary com-pass points used to score the orientation data.

Horizontal Vertical Orientationposition position

1 1

1 mln.

_ — • —

39

13r0

, —

2,—

*—C

ie3

/

)

_-'

>o°

/

/f M

Ox

i

/dwtt

T1•—L^

1

J

>.r

J1 1

•

'11

1

i1.Start

Co-ordinatingline

10 20 30 40 SO 60 70 80 90Distance along observation chamber (cm )

Potentiometnc recorder chart

100 H3 O 3V3Pens.

20-Channel recorder chart

Fig. 4, Annotated records of a single experiment.

Orientation of plaice larvae in water currents 789

switch position was connected to a static pen (sprung stainless-steel wire) on thetwenty-channel recorder, so that at any instant only three lines were drawn on thechart (Fig. 4). As the larva changed its position or orientation the switches werealtered accordingly, recording these movements against time. With practice it wasquite feasible to cope with the three switches together, although there was an inevitableslight delay between the occurrence of the movement and altering the appropriateswitch; this was regarded as a constant factor.

As each larva moved along the water tunnel the observer followed it with the trolley,keeping a bar projecting under the tank (Fig. 1) vertically below it. The movement ofthe trolley was recorded on a potentiometric chart recorder, so that as the larva wasfollowed along the observation chamber its track was also plotted against time (Fig. 4).A string attached to the bar on the trolley drove a ten-turn potentiometer by a pulleywhose diameter was calculated so that as the bar traversed the chamber from o to 100cm. the potentiometer turned its full ten times.

186

89

83

Midwater

49 31

36

Fig. 5. Polar diagrams of orientation in midwater and at the tide of the tank.

3. Data extraction

Prior to each experiment the two moving charts, whose speeds had been accuratelymatched, were marked simultaneously with a co-ordinating line which allowed themto be matched up for subsequent analysis (Fig. 4).

Preliminary observations indicated that larvae further than 3 cm. from a wall of thetank lost their orientation to the current. The cross-section of the tube was thereforedivided into two categories. The central area of the tunnel (vertical sections 3-4 andhorizontal sections 3-4) was designated midwater and the surrounding area as side(Fig- 3«)-

The records of horizontal and vertical position showed when the larva was inmidwater or at the side of the tank, and the orientation data for these two categorieswere extracted separately. The lengths of trace of the orientation record were measured

G. P. ARNOLD

to the nearest second (paper speed = 0-25 cm./sec), and the time the larva spentfacing in each direction was plotted on a polar diagram (Fig. 4). The polar diagramsfor each larval stage were then added together (Figs. 5 and 7).

The recorded track of each larva along the tunnel described its general movementswell, showing whether it was displaced downstream, maintained station or movedupstream. However, physical limitations of the recording system made it impossibleto record movements of less than 5 cm.; smaller movements were obscured by oscilla-tions of the pen and lag in the pulley system. Longitudinal position was registered to+ 3 cm. only, according to the direction of travel of the trolley.

4. Experimental conditions

The experimental conditions are set out in Table 1. Initially, three settings of theright-hand drain valve were used to create different current speeds and gradients inthe tunnel, but later tests were made only at the full-open setting of the valve. Theapparatus was assembled in a controlled-temperature room and the experiments wereconducted at temperatures close to that of the hatchery.

Table 1. Details of experimental conditions

Larvalstage

I

II

III

rv

Testnos.

i - S6—10

ii-iS16-20

I-IOii-iS16-2021-25

I-IO

I-IO

Daysold

1 - 3

i - 3

1-3a-41 0

1 2

1 2

1 4

—

—

Valvesetting

Full-openFull-openHalf-openHalf-openFull-openHalf-openQuarter-openHalf-openFull-openFull-op en

Light

BrightDimBrightDimBrightBrightBrightDimBrightBright

Date ofpeak

hatching1966

14 March14 March14 March14 March14 March14 March14 March14 March

——

Dates oftests1966

14 March14 March14 March15 March24 March26 March26 March28 March20 May

3 June

Source ofeggs

North Seafish inspawningponds

North Seafish inspawningponds

Irish SeaIrish Sea

Temp.at whichlarvaekeptCO

7-37-37-37"3

7-37'37 37-3

IO'O

io-o

Tempof

experimeritsCO7'57-S7'S7-S

7-S7-S7'57-S

io-o

io-o

Before each experiment the current in the tunnel was adjusted to the required speed.Larvae were then introduced singly through the upstream entry port, usually at thecentre of the tunnel, using a large-bore pipette with a rubber teat. Their orientationand position were recorded until they left the observation chamber. Only currentsflowing from left to right were used because of mechanical limitations of the techniqueused for measuring water velocity; previous observations showed that orientationoccurred to currents flowing in either direction.

For observations in bright light the six tungsten lights were run at 200 V., whichgave intensities ranging from 60 to 200 m.c. above and below the tank, and underneathand between the lamps. For Him light experiments the observer first dark-adaptedhimself for thirty minutes and then operated the lamps at 60 V. The larvae were in theroom during this time and so were also dark-adapted. At 80 V. the maximum lightintensity was 2-5 m.c. but at 60 V. it was not measurable with the Eel LightmasterPhotometer used; this had a filter with transmission characters corresponding to thespectral sensitivity curve of the human eye.

Orientation of plaice larvae in water currents 791

5. Analysis of two-dimensional orientation data

The orientation data were analysed in three ways. First, the number of observationsin the upstream sector (positions 8, 1 and 2; Fig. 3 b) were expressed as a percentage ofthe total. Secondly, a %2 t e s t w a s u s e d t 0 examine the distribution of observationsbetween the upstream and downstream sectors for midwater and side. Thirdly, aresultant vector R was calculated for all the observations in each polar diagram. Thenorth-south and east-west components of each of the eight groups of observationswere computed by multiplying the number of observations by the cosine and sine ofthe azimuth respectively. These components were then summed to give the com-ponents of the resultant vector. The magnitude of R is given by

R = V(W^+ F2), where W = £ sinoj, V = £ cosc^,i-l i-1

a is the azimuth from o° to 3600 of each group of observations and n is the number ineach group (Batschelet, 1965). The direction 6 of R is given by tan# = W/V and is agood measure of the central tendency of the distribution, being independent of anyreference direction. The magnitude of R is a sensitive measure of the degree ofconcentration of the original observations about its direction and is comparable tostandard deviation or variance. An excellent correlation exists between vector magni-tude and the standard deviation calculated around the direction of the resultant vector(Curray, 1956).

Finally, the Rayleigh test was used to determine whether each distribution wassignificantly different from a random one. The statistic used was L = (R/n) 100, andthe value of P was determined from graphs of L against n (Curray, 1956). This testwas not used for the two distributions which were obviously bimodal (Fig. ya).

RESULTS

1. Behaviour pattern

Stage III larvae showed a typical rheotropic response. On release from the pipette,both in the centre and at the side of the tank, there was an initial phase of downstreamdisplacement (Fig. 4). Subsequently, larvae near the side of the tank turned to faceupstream, if they were not already doing so, and swam against the current with shortbursts of activity; in between they were displaced downstream. This upstream-swimming phase lasted for several minutes, and although most larvae were ultimatelydisplaced downstream out of the observation chamber, many made ground upstreamfirst. Larvae in midwater did not respond to the current and were carried rapidlydownstream, some through the whole observation chamber without changing fromthe orientation in which they left the pipette. Others swam up to the top of the tankand towards the lights. When these individuals swam within 3 cm. of the tank wall,as they were being displaced, they immediately orientated head upstream and beganto swim vigorously against the current, making headway for distances of up to 5 cm.at a time.

A striking change of behaviour was observed in dim light. In most of these tests,the upstream-swimming phase disappeared entirely, even when the larva was released

792 G. P. ARNOLD

at the side of the tank. Only in a very few cases did even the slightest upstream swim-ming occur, and that was of very short duration, with very little movement upstream.

2. Ontogeny of behaviour

Newly hatched plaice larvae showed no orientation and lay in all postures, often ontheir sides, but usually with the buoyant yolk-sac uppermost. Spatial orientation wasdeveloped between 24 and 48 hr. after hatching, when larvae also orientated to thecurrent. They swam no more vigorously than in still water at this stage, but laterstage I larvae stemmed the current momentarily.

During stage II, orientation was augmented by active upstream swimming and mostlarvae stemmed the current for several seconds, but very few made any headway andthese progressed only 1-2 cm. at a time; the track of a typical stage II larva is shown inFig. 4. The response of stage III larvae to the current was very strong indeed. Orienta-tion occurred at about 3 cm. from the tank wall and swimming was very vigorous, witha high proportion of larvae swimming out of the observation chamber at the upstreamend. Stage IV larvae also showed orientation and active upstream swimming, but thedistances moved upstream were less than those of stage III individuals. These larvaewere denser than stage III larvae and in between their bursts of swimming sank quiterapidly. The body was becoming laterally flattened and the areas of the dorsal and analfins were increasing relative to body size, so that the surface area presented to thecurrent when the larvae were not facing upstream was increased. These stage IVlarvae were more readily displaced downstream than those of stage III and most weredisplaced from the observation chamber. They also showed the behaviour of meta-morphosed fish to currents; from swimming in midwater they went suddenly to thebottom, lay on either side, faced upstream and 'dug-in'.

The dual response shown by stage IV larvae to the cuirent agrees well with theirchanging behaviour at metamorphosis. At this time, they swim with the dorso-ventral axis tilted at io° to 200 from the vertical, and although the greater part of theirtime is spent swimming pelagically, they will lie on the bottom, on either side andwith one eye underneath the body.

3. Orientation to the current

The experimental data for the orientation of the four pelagic larval stages in mid-water and at the side of the tank are given in Fig. 5. The data for all three settings ofthe right-hand drain valve have been analysed together because there was no greatdifference between the current speeds involved, especially close to the tank wall.

If the polar diagrams for orientation in midwater and at the side of the tank arecompared for each larval stage (Fig. 5), it can be seen that there is a marked change indistribution, which is expressed in the percentage of the observations in the upstreamsector. For all four pelagic larval stages there is a marked increase in this value frommidwater to side, and a x2 test for each stage shows that it is significant (Table 2). Ineleven tests it was possible to make a direct comparison of the orientation of individuallarvae in the two situations, and these are shown in Table 3; tests with less than tenobservations in either situation were excluded. In all but two of these cases there wasa very considerable increase in the percentage upstream orientation when the larvacame close to the wall of the tank. The two exceptions involved stage I larvae, which in

Orientation of plaice larvae in water currents 793

midwater were carried downstream for a considerable distance in the same orientationin which they left the pipette. This occurred also with later stages, and accounts forvariations in the percentage upstream orientation in midwater. For a random dis-

Table 2. Orientation of larvae in midwater and at the side of the tank:

comparison of scores in the upstream and downstream segments of the polar diagrams

Observations (sec.)P, for onedegree ofLarval

stage

I

II

I I I

IV

Position

MidwaterSideTotalsMidwaterSideTotalsMidwaterSideTotalsMidwaterSideTotals

K-

Upstreamsegment

15613081464

227618845

1 2

12731285

28860888

Downstreamsegment

193665858465296761

973694661 2 3

" 92 4 2

,

Totals

34919732322

692

9141606

1 0 9

16421751

1 5 1

9791130

Upstreamorientation

(%)

44-7)66-3

- j32-8167-61

irol

77-5- ji8-5)8781

X'

58-44

190-03

228-17

36923

freedom

Table 3. Change of orientation of individual larvae whichmoved from midwater to the side of the tank

Larvalstage

I

II

III

IV

Test no.

1

3453472

46

1 0

Totalobservationsin midwater

(sec.)

491 1

19

15

279

13

12

1310

19

Totalobservationsat side (sec.)

582671 5 01 2 2

567652

350

i n

9 0

639

Upstreamorientation

in midwater(%)

24-554'547'4

1 0 0

0

0

0

0

I5-40

1 0 5

Upstreamorientationat side (%)

89-71 0 0

46-091-083-968-47 1 2

80-9

96-494-495-5

Change oforientation onmoving to side

(%)

+ 65-2+ 45-5

- 1 4

- 9 0

+ 83-9+ 68-4+ 71-2+ 809

+ 8 1 0+ 94-4+ 85-5

Table 4. Summary of Rayleigh test calculations on orientationdata for midwater and side

Stage

I

II

I I I

IV

II(no mirrors)

Position

MidwaterSide

MidwaterSide

MidwaterSide

MidwaterSide

Midwater

n

3491973692914

1091642

1519792 0 5

W

- 4 0 9-813

+ 349- 4 0 8 4

+ 44O-932-5

+ 285- 6 9 9 1

+ 630

V

-i-33+ 171

+ 2935+ 250-6

+ 500+ 328-3

+ 70-1+ 250-7

+ IS-95

tan a

+ 30-76-4-75+ 0-1189- 1 6 3+ 088- 2 8 4

+ 0-4066- 2 7 9

+ 3'95

e268°282°

6°302°

4i°2OO°

22°29O°

75°

R

4098308

295-5479-1

66-69886

7577427

65-0

L (%)

11 742-1

42-752-461160 2

50175 9

31-9

j

010

1010

IOIO

IOIO

IO

794 G. P. ARNOLD

tribution, 37-5 % of the total observations should occur in the upstream segment.Gregory & Fields (1962) found a comparable departure from a random distribution inthe orientation of salmon fingerlings in still water. They recorded values of 24-0 and56-5 % when, in a segment of 1200, 33*3 % would have been expected by chance.

The calculations of the resultant vectors and the Rayleigh test statistics for the datain Fig. 5 are summarized in Table 4 and the vectors are plotted in Fig. 6. This alsoillustrates the occurrence of upstream orientation at the side of the tank; all fourvectors lie in the upstream sector. In contrast, those for midwater lie out of the sectorwith the exception of that for stage I larvae. It should be noted, however, that the

Midwater Side

Fig. 6. The resultant vectors (L) for larval stages I to IV for midwater and tide.

Table 5. Time taken by larvae in midwater to traverse the100 cm. length of the observation chamber

Larval stage

III

III

rv

Test no.

2

1

2569

1 0

1

51

2

79

Displacement time(sec.)

25 0

22-5'21-o18-022-021 -o28-0,

22-o\23 Oj

220121-ol22-OJ21-oj

Average displacementtune (sec)

25-0

22-1

22-5

31 -5

value of L for this stage (Table 4) is very low, indicating a wide dispersal of observa-tions; this can be seen in Fig. 5. Although larvae in midwater did not orientate to thecurrent, the values of L and P (Table 4) show that orientation was not random. Thegrouping of these vectors to the north of the east-west line suggests that there wasorientation to the greater light intensity at the back of the tank produced by the mirror.Light from above was reflected into the tank, and it was observed that larvae collectedin the top back corner of the tunnel, at the brightest point. During the 1967 larval

Orientation of plaice larvae in water currents 795

season the orientation of stage II larvae was recorded in midwater in bright light,with no mirror. The values of L and P were greatly reduced from those for stage IIlarvae of the previous year, when the mirror was present. The vector angle was muchchanged also, but the distribution was still not random.

(a)Stage! Stage II

Bright

Dim

Bright

15

Midwater

3

21

Side

8

Dim

Fig. 7. Polar diagrams of orientation of plaice larvae in midwater and at the tide of the tank, inbright and dim light: (a) in a reduced current; (6) in the full current.

Further evidence for the absence of rheotropism in midwater is given by the dis-placement rates for larvae which were displaced downstream entirely in midwater.The average displacement times for the ioo cm. observation length are virtually thesame for all four pelagic stages (Table 5). If rheotropism were occurring, then thesetimes would increase with each successive developmental stage, as swimming abilityincreased.

4. The effect of light intensity on orientationOrientation of stages I and II larvae in bright and dim light was compared for mid-

water and side. The comparison was made with the drain valve half open, but data forstage I larvae at the full current are also given. Figure 7 shows the original data, andFig. 8 shows the resultant vectors of these observations; the Rayleigh Test calculationsare summarized in Table 6.

796

Bright

G. P. ARNOLD

Side Dim

Midv/iter

Current Current

Fig. 8. The resultant vectors (L) for larval stages I and II in bright and dim light. The vector forlarvae in the full current is (7).

Table 6. Summary of Rayleigh test calculations onorientation data for bright and dim light

e280°

315°

Bimodal distributions

tan a

-5-84— I-O2

29561849

6 1 7

Stage Position Light n W V tana 6 R L (%) P

I Side Bright 479 —291-4 +499(reduced Side Dim 558 —132-1 +1294current) Midwaterl

MidwaterJ

II Side Bnght 220 —96-5 +56-9 —1-70 301° 112-0(reduced Side Dim 76 —15-5 + 2 9 1 —0-53 332° 33-0current) Midwater Bright 216 —4-2 +79'4 —0-053 357° 79'5

Midwater Dim 164 +16-0 +68-8 + 0 2 3 13° 70-6

I Side Bright 597 —410-3 +49-5 —8-29 277° 413-3(full Side Dim 339 +209 —579 —0-361 161° 6i-6current) Midwater Bright 119 —140 +6-6 —2-12 296° 15-5

Midwater Dim 76 —17-0 +29-2 -0-58 330° 33-8

50943'4368

69-218213044-5

IO"IO"

10-™OOOI0-50

Orienation of plaice larvae in water currents 797For stage I larvae at the side of the tank in the full current the resultant vectors

clearly show that there was orientation to the current in bright but not in dim light.The percentage of observations in the upstream sector support this conclusion and theX2 test shows the distributions to be significantly different (Table 7). The differencein orientation between bright and dim light in the reduced current is not so obvious.However, the x2 tests show the distributions to be significantly different for both stages,and there is also a marked drop in the percentage of observations in the upstreamsector (Table 7). The resultant vectors, too, although still falling in the upstreamsector, show clockwise shifts of 350 and 31°, and the value of L is reduced in both

cases.Table 7. Orientation of larvae in bright and dim light: comparison ofscores in the upstream and downstream segments of the polar diagrams

arval stage

I(reducedcurrent)

II(reducedcurrent)

I(full

current)

Position

Side

Midwater

Side

Midwater

Side

Midwater

Light

BrightD i mTotalsBrightDimTotalsBrightDimTotalsBrightD i m

TotalsBrightDimTotalsBrightDimTotals

Observations

Upstreamsegment

380

294674

392 0

59

15734

191

8547

132

499135634

494897

Down-stream

segment

99264363

514495

6342

105

131117248

98204302

7 02898

(sec)

Totals

479558

10370 0

64154

2 2 0

76296

216164380

59733993611976

195

Upstreamorientation

(%)

793152 7 j-

43-3-13,3}

71-41447 !•

394128-7J

836I39-8J

4i-2i63 21

X1

7925

1-828

1635

4242

187-47

8106

P, for onedegree offreedom

o-ooi

o-io

o-ooi

0-05

o-ooi

o-oi

Light intensity did not affect orientation in midwater. With the exception of thatfor stage I larvae in the full current, the polar diagrams showed low values of per-centage upstream orientation and differences of distribution between the two lightintensities which were not significant. The vectors for stage II larvae fell in the down-stream sector; those for stage I larvae in the reduced current have not been calculatedbecause both distributions were bimodal. Although the vector for stage I larvae in thefull current fell in the upstream sector, the value of L was very low (13%) and thedistribution was not significantly different from a random one. The correspondingvector in dim light appears to be anomalous. In fact it was greatly influenced by onelarva which was displaced passively downstream facing at 3150 (position 2) for 39 outof the 42 sec. in which it was in midwater. Ignoring this larva, the remaining data givea resultant vector at 830 with L = 406%.

Since orientation to the current at the side of the tank disappeared in dim light for

798 G. P. ARNOLD

stage I larvae in the full current, and was much reduced for both stages I and II larvaein the reduced current, it is highly probable that the response in bright light is avisual one.

5. Mechanism of orientation

Direct observations of the behaviour of stage III larvae in the dark confirmed thetentative conclusion reached in the previous section. An infra-red viewer was usedwith a tungsten filament lamp screened by a Chance-Pilkington OX 5 filter as aninfra-red source. All available evidence suggests that fish are unable to detect infra-redradiation (Duncan, 1956). Larvae which showed the normal response in light at theside of the tank were displaced in the dark, unless they were in contact with the tankwall. Those still in contact with it when the lights were switched off continued toshow rheotropism in the dark. Larvae being displaced passively immediately re-orientated when they came into contact with the wall.

It can be concluded that in the light vision is the primary sense in the rheotropicresponse of larval plaice, although tactile clues may be supplementary. In the dark thetactile clues become the primary ones. Since the velocity gradients remained un-changed between light and dark they were either not detected or not used as a clue.

6. Validity of observations

Although a very definite orientating movement by stage III larvae was observed, itcould be argued that the orientation observed in the earlier two stages was not anactive response but a passive one produced by physical forces acting on the larva.However, live larvae were capable of heading in all directions near the tank wall(Figs. 4 and 6), whereas anaesthetized (MS. 222) larvae assumed a different orientationin each half of a parabolic velocity gradient. They behaved in the same way as deadmammalian spermatozoa (Bretherton & Rothschild, 1961), being displaced head firstnear the top of the tank. The predominantly upstream orientation observed in stages1 and II larvae was therefore an active response.

7. Threshold response and the kinetic nature of the response

During preliminary experiments, larvae were observed to orientate at currentspeeds of less than 1 cm./sec. There was no distinct threshold and orientation occurredas soon as the larvae were displaced a few millimetres; they could apparently detectvery low displacement rates. It was also observed, during preliminary experiments inround tubes, that swimming speed increased with current speed over the range 0-20cm./sec. In an attempt to quantify the kinetic nature of the behaviour and to investi-gate further the apparent absence of a threshold response, a small moving backgroundapparatus was constructed. A 7 cm. wide annulus of 22 cm. outer diameter had a24 cm. diameter background rotated round it at speeds from 09 to 125-7 cm./sec.The background consisted of alternating black and white stripes of equal widths;2 and 5 mm. wide stripes were used. No response was obtained from any stages II orIII larva to any speed of the background.

Orientation of plaice larvae in water currents 799

DISCUSSION

The general behaviour pattern of stage I and II plaice larvae in currents agrees wellwith that described by Bishai (i960) for yolk-sac herring larvae. The track of a stageII larva (Fig. 4) is very similar in overall pattern to one given by Bishai (Fig. 2, p. 140),and both show an initial displacement phase. In Fig. 5 this phase occurred in mid-water, but even larvae released at the side of the tunnel were displaced initially beforeorientating. Ryland (1963, p. 290) found that: 'The larvae in general were reluctant toswim, and frequently drifted the entire length of the tube quite passively'. He alsofound that larvae which swam against the current frequently gave up and were rolledpassively down the tube before re-orientating. Vigorous swimming, during which thelarvae surged forwards, was never more than momentarily maintained, and Rylandconcluded that the behaviour was in striking contrast to that reported by Bishai forherring larvae. The present work, however, indicates that the behaviour pattern isvery similar in the two species.

Bishai, Ryland and I have all been unable to obtain a response by herring or plaicelarvae to a moving background outside the wall of the tank. This could be because of itsdistance away from the larvae, and the widths of the stripes used, but the present workshows clearly that the rheotropic response is a visual one. Ryland thought that theabsence of a response to a moving background ruled out the possibility of a visualmechanism, and that the larvae detected the current either by direct contact with theside or by the velocity gradients acting on it. Of Bishai's herring larvae, Ryland(1963, p. 290) said: 'It is difficult to appreciate by what mechanism a herring larvamaintaining position in the centre of a tube could detect the direction and strength ofthe current; for there would be virtually no velocity gradient immediately around thelarva*. However Bishai's tube was only 1-2 cm. in diameter, so that the larvae would bewithin visual distance (Blaxter, 1962) of the whole circumference. The present obser-vations show, too, that plaice larvae do not respond to the velocity gradients in thewater tunnel. An important feature of the larval behaviour is that displacement pastthe wall of the tank always precedes the rheotropic response. This can be seen both inthe initial displacement and also during the upstream swimming phase. There is asimilarity here with fish which fall back for short periods when following a movingbackground and then pick up a new reference point to follow (Shaw & Tucker, 1965).This feature, in addition to the visual and kinetic nature of the response, leads to theconclusion that reaction of plaice larvae to water currents is primarily an optomotorone. The conclusion agrees with those of Lyon (1904, 1909) and Dykgraaf (1933) foradult fish, and that of Pavlov (1966) who studied the response in the larvae of variousfreshwater fish.

The absence of an optomotor response by both plaice and herring larvae in themoving background apparatus raises the question of what reference points were usedin the water tunnel. The lack of response by larvae more than 3 cm. away from the wallsuggests that the clues were on the wall itself rather than outside the tank. The scribedlines, scratches on the surface or bubbles in the cemented joints could all providereference points to larvae being displaced downstream. Hasler (1956) found thatadult Phoxinus, trained to find food at a fixed position in a circular tank, orientatedmore readily to marks and scratches on the wall than to a light bulb outside the tank.

800 G. P. ARNOLD

On the other hand, adult pelagic fish generally respond to a striped backgroundrotating outside a circular tank, so for them this must be a stronger stimulus thanmarks on the tank wall. Shaw & Tucker (1965) showed for various carangid fishes thata completely striped background was a more effective stimulus than a partially stripedone, and that a completely white one was ineffective. Since plaice larvae do not react tofood organisms more than about 3 cm. away from them it is quite probable that this isthe furthest distance away at which they can focus. If this is so, they would not react toobjects more than this distance beyond the inside face of the tank wall. Total internalreflection might also prevent them seeing beyond the wall from certain positions.

Optomotor responses have great biological significance for animals living in flowingwater. Among larval fish, trout fry, for example, maintain station near the stream bedin this way and feed by sampling material brought down by the current (Stuart,1953). For pelagic marine larvae the significance is not immediately obvious. There isno evidence that larvae in the surface waters can detect the large-scale water move-ments responsible for their distribution in the sea, although Bishai (i960) suggeststhey may. It is unlikely that they do so, because of the lack of a reference point and alsothe limitations of their sensory systems. However, the four pelagic larval stages of theplaice show a marked vertical migration, with a large proportion of the populationvery close to the sea-bed at midday (D. Harding, personal communication). Larvaevery close to the bottom and in sight of it would be able to detect the direction of thecurrent, and the rheotropic response could be involved in a dispersal mechanism ofthe type originally suggested by Hardy (1953), involving the vertical migration cycle.Shaw & Tucker (1965) and Shaw (1965) have stressed the similarities of behaviourbetween fish in optomotor apparatus and in schools, and have suggested that theoptomotor response plays an important role in schooling. Maximum recorded densitiesof plaice larvae are of the order of o-6 per m.3 (D. Harding, personal communication),so that encounters between individuals must be rare. It is therefore improbable thatthe ecological significance of the optomotor response to the plaice larva lies inschooling.

SUMMARY

1. An apparatus is described to study the response of fish larvae to water currents.2. Close to the wall of the water tunnel all pelagic larval stages of the plaice showed a

typical rheotropic response.3. Vector analyses of orientation showed that the response occurred only within

3 cm. from the wall and was absent in midwater.4. Although the ability to swim against the current increased with age, significant

upstream movement only appeared in stage III larvae. Laterally flattened stage IVlarvae were less able to swim upstream, but on touching the bottom lay on their sidesand showed the behaviour of fully metamorphosed stage V larvae and older fish.

5. In dim light (< 25 m. candles) orientation to the current at the side of the tankalso disappeared, but observations with an infra-red viewer and source showed thatlarvae which touched the wall in the dark could still orientate.

6. It was concluded that the mechanism of the rheotropic response was primarilyvisual, although it could also be produced by tactile clues. There was no evidencethat larvae could respond to the velocity gradients at the tank wall.

Orientation of plaice larvae in water currents 801

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