- Email: [email protected]
The response of the planarian Dendrocoelum lacteum to an increase in light intensity
Anita. Behav., 1971, 19, 269-276
THE RESPONSE OF THE PLANARIAN DENDROCOELUM LACTEUM TO AN INCREASE IN LIGHT INTENSITY BY F. R. HARDEN JONES Fisheries Laboratory, Lowestoft, Suffolk Abstract. Dendrocoelurn is said to respond to an increase in light intensity by an increase in its rate of change of direction (r.c.d.). An infrared viewing technique was used to observe and record, on 16-mm film, the response of dark adapted Dendrocoelurn to sudden increase in light intensity of 1 lux. The planarians moved faster, turned through bigger angles, and turned more frequently: the response has both ortho-and klino-kinetic components. The increase in the speed of gliding was maintained after the r.c.d, had fallen back to its original level. Dendrocoelum reacted to intensities as low as 10-5 lux, and at these intensities they moved faster and turned more frequently but the mean angle of turn decreased: there was a fall in r.c.d. The results are discussed in relation to Ullyott's earlier work and shock reactions. Ullyott (1936) described the response of the planarian Dendrocoelum lacteum Oersted to a sudden increase in light intensity as an increase in the rate-of-change-of-direction (r.c.d.), measured as the sum of the angular degrees of turn per minute. The response was subsequently classified as a klino-kinesis by Fraenkel & Gunn (1940) and the phenomenon of r.c.d, with adaptation was believed to account for the collection of Dendrocoelum in the dark end of a 'nondirectional' gradient of light intensity. Ullyott's (1936) account implies that a change in r.c.d. was accompanied by a change in the frequency of turning, and in this he was followed by Fraenkel & Gunn (1940) and others (for example, Carthy 1958, pp. 39--40). Patlak (1953) analysed the one relevant Dendrocoelurn track published by Ullyott. During the period of adaptation following an increase in light intensity, the frequency of turning (turns per minute) was more or less constant, but the angle of turn (degrees per turn) decreased, suggesting that the klino-kinetic mechanism involved a change in the angle of turn rather than a change in the frequency of turning. Trials of an infrared viewing technique developed for fish behaviour studies provided an opportunity to make a limited number of observations on the light responses of Dendrocoelum. The results showed that both the frequency of turning and the mean angle of turn changed when Dendrocoelum was subjected to a sudden increase in light intensity. The speed of movement also increased: the response has both ortho- and klino-kinetic components.
Methods Dendrocodum, 7 to 15 mm long, were supplied by the Freshwater Biological Association, Windermere, and kept at the Fisheries Laboratory, Lowestoft, in aerated and aged tap water at 10~ to 12~ The planarians were kept without food for a week or more before experiments were carried out. The stock was replenished as required. The Dendrocoelum were kept under conditions which allowed their movements to be observed and photographed by infrared radiation both in the dark and during illumination by visible light of known intensity. The experimental arrangement is shown in Fig. 1. Experimental Tray A Dendrocoelum for observation was kept in a shallow rectangular Perspex tray, 32 • 20 • 4 cm deep, filled with clean water from the stock aquarium to a depth of 1.5 cm. The outer surface of the bottom of this tray was scribed with grid lines filled with an opaque compound to provide a scale and reference. As shown in Fig. 1 the sides were extended below to support the tray in a water-bath provided by a second and larger rectangular Perspex tank. The waterbath rested on a dexion frame above the infrared source. Infrared Apparatus Observations were made with an infrared monocular comprising a diode image converter tube (Mullard 6929), power pack, and optical assembly. The visible screen was viewed through 269
ANIMAL
•
270
dispersing lens~
/
/
0• filter
BEHAVIOUR,
2
Photography
rubber bettows infrered
~
---monocular
75cm
[
"-.. experimentat
"-.~ray ",<
water bath
--ai~' out
>
air--
in
19,
t'~uta?"tungsten fitament tnmps Fig. 1. Diagram of experimental arrangements used to observe planarians by infrared radiation. The components are not drawn to scale and in practice the dispersing lens and monocular-camera assembly were positioned 75 cm above the experimental tray. the viewfinder of a Bolex H16 Reflex cine camera fitted with a Rank Taylor Hobson television lens (focal length 3.0 cm). Camera and monocular were mounted together rigidly on a special frame. The lens-monocular coupling was covered with a rubber bellows to prevent the escape of light from the visible screen of the monocular, Trials showed that the planarians were best viewed in silhouette against an infrared background. The infrared source was therefore placed below the dexion frame supporting the waterbath and experimental tray. Six 60-W tubular pearl tungsten filament lamps were housed in a wooden box fitted with a cooling fan and light-proof vents. The box was roofed with a strip of unpolished Chance-Pilkington OX5 filter glass, 45 x 23 • 0.5 cm thick. According to the manufacturers the transmission of this glass rises from 2,5 and 10 per cent at 750 nm and 800 nm respectively to a peak of 91 per cent between 1500 and 2500 nm. No light was visible through the filters used in these experiments:
Photographs of the monocular screen were taken on panchromatic film by single shot at 5-s intervals. This repetition rate was sufficient to allow a reconstruction of a planarian's track. The best compromise between resolution and field of view was obtained when the monocular was about 75 cm above the experimental tray. In this position the effective field of view was 10 x 12 cm. The monocular-camera assembly was therefore mounted so that it could be moved freely to cover the whole area of the tray.
Visible Light The source was a 100 W, 12 V, tungsten iodine lamp under-run at 10 V (colour temperature 3050~ A heat-absorbing filter (ChancePilkington HA 1) and neutral density filters (Wratten A) were placed in the optical system before the formation of an enclosed parallel beam. This was directed towards the midline of the experimental tray and reflected down over it by a prism and a concave dispersing lens set at a slight angle to the vertical as shown in Fig. I. This arrangement gave an almost uniform field of illumination over the tray. When tracking a planarium the monocular-camera assembly was carefully positioned to avoid shading the water within the field of view. The intensity of illumination close to the water surface was measured with an Eel selenium cell (response type S). Walter (1907) showed that Planaria gonocephala moved faster in the light than in the dark over a range of intensities (0.94 to 431 lux) but Ullyott (1936, p. 268) suggested that the increases 'were at least partly due to temperature effects'. Most of the experiments described here were carried out with water surface intensifies of 1.0 lux and others were made at intensities of 10-2, 10-4 and 10-5 lux. Preliminary trials showed that the water temperature did not increase under the conditions of these experiments.
Experimental Procedure The experiments were carried out in a basement darkroom housed in an old gunsite. This room was relatively free from the vibrations and disturbances of the main laboratory. A planarian was allowed to settle down for 48 hr after being placed in the tray. The monocular and fan in the infrared source box were left on during the day but the tubular lamps below the OX5 filter were only switched on for observation. Some film sequences of 30 to 100 frames (2.5 to 8 rain)
HARDEN JONES: PLANARIAN RESPONSE TO INCREASE IN LIGHT INTENSITY were taken during the settling down period. In an experiment the visible light was switched on after the moving flatworm had been photographed in the dark near the centre of the tray. Photography was continued until the flatworm reached the side of the tank when the experiment was concluded unless the worm moved away from the side again within 20 to 30 s. The water temperature was then recorded and the flatworm removed, or the visible light was switched off and another experiment attempted later. So for each flatworm photographic records were obtained of its movements in the dark, and, for a successful experiment, a second record which enabled a direct comparison to be made between its movement in the dark and in the light. Treatment of Data The tracks were reconstructed by plotting the positions of the middle of the tail from a frame-by-frame analysis of the film with a Specto Motion-Analysis projector. From these plots a scale expander was used to make a working drawing twenty-five times natural size from which the following measurements were made. (1) Distance moved in 5 s, i.e. between frames. From this measurement the speed of gliding (when the planarian was moving) was calculated in centimetres per minute. (2) Angle of turn, as degrees per turn. Differences of less than 2 ~ between one line of movement and the next were not measured and such deviations were not counted as turns. (3) Number of turns per minute. As positions were recorded at 5-s intervals, the frequency of turning could not exceed 12 per rain and was averaged over the period during which the planarian was moving. Tracks within 1 cm of the side were not used in making these measurements. Response of Dendrocoelum to Infrared Radiation Dendrocoelum observed by visible light did not respond to infrared radiation, neither when still nor when moving. Similarly, in the dark, Dendrocoelum known to be at rest in a particular position in the experimental tray, did not move when infrared radiation was switched on. It was concluded that the planarians did not respond to the levels of infrared radiation to which they were exposed during the experiments.
271
Results Observations by infrared radiation showed that a planarian roamed freely about the experimental tray for the first 2 to 3 hr in the dark but thereafter both the frequency and speed of movement usually declined. The planarians all showed the normal locomotory activities described by Pearl (1903). Individual tracks varied from a steady or irregular course across the tray to tracks inclined to the left or right indicating a bias in turning. There were no obvious environmental factors to account for the different tracks. Exceptionally the activity of some animals increased after several days in the dark, but others remained still for many days and observations were made hourly during the working day for up to 2 to 3 weeks before there was an opportunity to complete an experiment. The work was therefore tedious and prolonged; between November 1967 and November 1968 numerous trials yielded only sixteen experiments which merited detailed quantitative analysis and these involved eight individuals. Response to a Sudden Increase in Intensity of I lux The response of a gliding planarian was usually immediate. A typical track is shown in Fig. 2 and the relevant statistical data (experiment 3, D 3) are summarized in Table I. For the 5 min in the dark (up to frame 59) the planarian followed a more or less steady course, with a mean angle of turn of 17.9 ~, turning frequency 9.0 per min, mean speed 4.1 cm per min. The light was switched on immediately after frame 59. The planarian continued a short distance and stopped (60 to 61); moved forward and stopped again (63 to 66). After some 15 s, gliding continued intermittently with other stops (70 to 71; 78 to 80; 90 to 91; 104 to 105; 108 to 109; and 115 to 116). During these checks, the head was raised and made vigorous lateral sweeping movements before gliding continued. The tortuous nature of the track after frame 59 is clearly shown in Fig. 2. After frame 120 (5 min after switching on the light) the track became less twisted. The worm approached (frame 143) and then moved away from the side of the tray (frame 152). Filming stopped soon after frame 165 when the planarian started to glide along the side wall. In the light the mean angle of turn was 26.5 ~, the frequency of turning 10.4 per min and the speed of gliding 5.5 cm per min; all three increased in the light. The r.c.d.'s (mean angle of turn • frequency of turning) in the dark and light were 161 ~ and
272
ANIMAL
BEHAVIOUR,
19,
2 ]~4Z
".... 157
2Z, ...,/'/Direction " of gtiding
Of-
Fig. 2. Reconstruction of the track of Dendroeoelum3. The points record the mid-tail positions at 5-s intervals, the numerals frame numbers, and the vertical bars normal to the track minutes. The planarian was exposed to an intensity of 1 lux immediately after frame 59. The notation 60-1, 63-6, ere, indicates that the planarian did not change position between frames 60 and 61, 63 and 66, etc. For further description see text. 276 ~ respectively. Fig. 2 also shows that the increase in speed was maintained up to the end of the experiment (frames 152 to 165) although the angle of turn then decreased. Details of the experiments which were subjected to quantitative analysis are summarized in Table I. All showed an increase in the mean angle of turn. Inspection of the raw angle-ofturn data suggested that many of the distributions were skewed, particularly in those experiments where the planarians responded by large turns. This was confirmed by the appropriate test. A square root transformation normalized most, but not all, the distributions and the results of the t-tests given under the column 'angle of turn' in Tables I and II relate to the normalized and not to the raw data. Table I shows significant increases in the mean angle of turn in experiments 2, 3 and 7. Five experiments showed an increase in th~ frequency of turning (but not experiments 2 and 6), and all showed an increase in r.c.d. Speed of gliding increased in six experiments (but not in 2) and
the increase was significant in five experiments (not in 6). Experiment 2 showed a significant decrease in speed (from 8.4 to 6.8 cm per rain). An explanation of this anomolous result was suggested by the track of this particular worm which moved rapidly along a straight course in the dark. In the light it made many turns of more than 45 ~. The mean angle of turn more than doubled and the track became very twisted. Gliding slowed up, suggesting that the planarian could not move quickly along a convoluted track. Ullyott (1936, p. 276) makes the same point. The response of Dendroeoelum to a sudden increase in light intensity of 1 lux may be summarized under seven headings as follows: (1) Arrest of gliding: this may occur several times and there may be a few seconds' delay between illumination and the first check. (2) Sweeping movements: when gliding stops the head is raised high and makes lateral sweeping movements which may be continued for 10 to 30 s.
H A R D E N JONES: PLANARIAN RESPONSE TO INCREASE IN LIGHT INTENSITY §
I
+
§
§
§
§
§ §
+ §
§
§
§
',D
§ §
~ §
]
§
§
§
+
I~
§
"O
~00 ,.~0
.~'~ I~ ~ . ~
c~
,r.B
273
274
ANIMAL
~D
I
i' ?
@
BEHAVIOUR,
19,
2
§ §
§ §
§ §
§ §
§ §
§ §
§ §
§
§
§
§
I
I
§
§
§
§
§
I
§
I
§
§
§
L
o
§
§
§
~
o
§
o
o
o
§
~
o
O
o
o
o
§
o
o
§
9
§
§ §
§ I
§ §
I L
§ §
L
§
O
O
§
O
~-.o.
~
,..~ r
&
tqc'q
t'-- r
9,~,. r r r
t.-q r ,...~ c q
r
,...~ r
eh~b~
~t'-t'q'r
t'q
0
' ~~, o~., "~_~
g O ~
v',l r
D
l
i
G%
tr
xO
HARDEN JONES: PLANAR!AN RESPONSE TO INCREASE IN LIGHT INTENSITY (3) Large turns: when gliding is resumed there may be one or more large changes of course involving turns between 70 ~ to 180 ~. Turns of this magnitude are rarely made by planarians gliding in the dark. (4) Mean angle of turn: even in the absence of large turns there may be an increase in the mean angle of turn which is sometimes significant when averaged over the period of observation in the light. (5) Frequency of turning: increases. (6) Rate-of-change-of-direction: increases. (7) Speed of gliding: increases, and is usually significant when averaged over the period of observation in the light. The response shown by the planarians is summarized under these headings in Table I.
Response to Lower Light Intensities (10 -2, 10-3 and 10-5 lux) Some experiments were carried out at lower light intensities and the results are summarized in Table II. Several experiments were carried out with Dendrocoelum No. 7. This planarian was put in the tray on 11 June but was not seen moving in a suitable position for photography until 27 June. Thereafter several independent observations were made o f its movements in the dark and five of these were followed by sudden increases in light intensity. Some indication of the variability of movement in the dark is given by the range of mean angles of turn (11-9 ~ to 29-7~ frequency of turning (7.4 to 10.3 turns per rain) and mean speeds (4.8 to 8-5 cm per rain). This range of gliding speeds is very similar to the normal speed of locomotion for Dendrocoelum given by Lehnert (1891). In experiments 12 to 15, Dendrocoelum 7 showed a significant increase in speed. The mean angle of turn (and r.c.d.) decreased in experiments 12, 13 and 15. The increase in speed at 10-5 lux in experiment 15 was unexpected, but Dendrocoelum 8 (experiment 16) also showed a clear, but different, response to the same intensity. Dendrocoelurn 7 did not usually respond to the lower intensities with an arrest of gliding and when this did occur (experiment 14) the reaction was delayed for 40 s and was not followed by sweeping movements. Two other planarians (Dendrocoelum 6 and 8) showed similar delays of 20 and 15 s respectively.
275
Discussion Dendrocoelum kept in the dark and suddenly subjected to a light intensity of 1 lux usually moved faster, and turned more frequently with an increased angle of turn: the response has both ortho- and klino-kinetic components. Increases in speed were also observed in lower intensities and, while the limited results with Dendrocoelum 7 suggest a trend towards faster speeds at higher intensities, the results as a whole are consistent with the hypothesis that the speed of movement in the light is partly dependent on the speed in the dark: there is an increase in speed but the planarians moving fastest in the dark also moved fastest in the light. Ullyott (1936) found no difference in the speed at which Dendrocoelum moved under different intensities and Walter's (1907) results with P. gonocephala were similar. Walter (1907, p. 57) concluded that 'the rate of locomotion depends not so much upon the intensity of light as upon other factors which tend to produce individual behaviour upon the part of each particular worm'. With the exception of experiment 4, intensities of 1 lux led to an increase both in the frequency of turning and in the mean angle of turn and thus to an increase in r.c.d. Similar changes were observed at lower light intensities, but experiments 12, 13 and 15 with Dendrocoelum 7 showed a decrease in the mean angle of turn sufficient to produce a decrease in r.c.d. The results recall Waiter's (1907) conclusions that planarians turn more in the dark than in the light. Walter (1907, p. 51) made a distinction between 'indefinite' and 'definite' changes of course, the difference between the two being in the magnitude of the angle o f turn. Walter noted that the smaller 'indefinite' turns were more frequent in the dark and the larger 'definite' turns are more frequent in the light. The present results with Dendrocoelum suggest that it is the mean angle of the 'indefinite' turns which is greater in the dark rather than the frequency of turning: in experiments 12, 13 and 15 the mean angle of turn fell, but the frequency of turning rose following a sudden increase in light intensity. It seems probable that Dendrocoelum has a basal mean angle of turn and frequency of turning in the dark, the product of the two being equal to Ullyott's basal r.c.d. Exposure to relatively low light intensities leads to an increase in the speed of gliding and in the frequency of turning, and to a decrease in the
276
ANIMAL
BEHAVIOUR,
mean angle of turn: the r.c.d, falls. At higher intensities, however, both the angle of turn and frequency of turning increases and there is, therefore, an increase in r.c.d. Still higher intensities have a more marked effect on locomotion, the planarian making a limited number of turns through angles of 70 ~ or more and so effecting sudden changes of course. Higher intensities may also lead to an arrest of gliding and sweeping movements of the head. These three responses, an arrest of gliding, sweeping movements of the head, and large turns, would appear to be the components of a shock-reaction which, following Ullyott's (1936) line of argument, could be regarded as an extreme form of klino-kinesis. The positive ortho- and ldinokinesis shown by Dendroeoelum to sudden increases in light intensity are clearly related to the avoidance of bright light with the suggestion of an upper level to an optimum zone. The significance of the negative klino-kineses observed at lower intensities is not immediately obvious
19,
2
and requires further study. Clearly the behaviour of Dendrocoelum to sudden increases of light intensity cannot adequately be described in terms of r.c.d, alone. REFERENCES
Carthy, J. D. (1958). An Introduction to the Behaviour of Invertebrates. London: Allen & Unwin. Fraenkel, G. S. & Gunn, D. L. (1940). The Orientation of Animals. Oxford: Clarendon Press. Lehnert, G. H. (1891). Beobachtungen an landplanarien. Arch. Naturf., 57, 306--50. Patlak, C. S. (1953). A mathematical contribution to the study of orientation of animals. Bull. Math. Biophys., 152, 431-76. Pearl, R. (1903). The movements and reactions of freshwater planarians: a study in animal behaviour. Q, Jl microsep. Sci., 46, 509-714. Ullyott, P. (1936). The behaviour of Dendroeoelum laeteum. II. Responses in non-directional gradients J. exp. Biol., 13, 265-78. Walter, H. E. (1907). The reactions of planarians to light. or. exp. Zool., 5, 35-162. (Received 23 July 1970; revised 4 November 1970; MS. number: 995)
THE RESPONSE OF THE PLANARIAN DENDROCOELUM LACTEUM TO AN INCREASE IN LIGHT INTENSITY BY F. R. HARDEN JONES Fisheries Laboratory, Lowestoft, Suffolk Abstract. Dendrocoelurn is said to respond to an increase in light intensity by an increase in its rate of change of direction (r.c.d.). An infrared viewing technique was used to observe and record, on 16-mm film, the response of dark adapted Dendrocoelurn to sudden increase in light intensity of 1 lux. The planarians moved faster, turned through bigger angles, and turned more frequently: the response has both ortho-and klino-kinetic components. The increase in the speed of gliding was maintained after the r.c.d, had fallen back to its original level. Dendrocoelum reacted to intensities as low as 10-5 lux, and at these intensities they moved faster and turned more frequently but the mean angle of turn decreased: there was a fall in r.c.d. The results are discussed in relation to Ullyott's earlier work and shock reactions. Ullyott (1936) described the response of the planarian Dendrocoelum lacteum Oersted to a sudden increase in light intensity as an increase in the rate-of-change-of-direction (r.c.d.), measured as the sum of the angular degrees of turn per minute. The response was subsequently classified as a klino-kinesis by Fraenkel & Gunn (1940) and the phenomenon of r.c.d, with adaptation was believed to account for the collection of Dendrocoelum in the dark end of a 'nondirectional' gradient of light intensity. Ullyott's (1936) account implies that a change in r.c.d. was accompanied by a change in the frequency of turning, and in this he was followed by Fraenkel & Gunn (1940) and others (for example, Carthy 1958, pp. 39--40). Patlak (1953) analysed the one relevant Dendrocoelurn track published by Ullyott. During the period of adaptation following an increase in light intensity, the frequency of turning (turns per minute) was more or less constant, but the angle of turn (degrees per turn) decreased, suggesting that the klino-kinetic mechanism involved a change in the angle of turn rather than a change in the frequency of turning. Trials of an infrared viewing technique developed for fish behaviour studies provided an opportunity to make a limited number of observations on the light responses of Dendrocoelum. The results showed that both the frequency of turning and the mean angle of turn changed when Dendrocoelum was subjected to a sudden increase in light intensity. The speed of movement also increased: the response has both ortho- and klino-kinetic components.
Methods Dendrocodum, 7 to 15 mm long, were supplied by the Freshwater Biological Association, Windermere, and kept at the Fisheries Laboratory, Lowestoft, in aerated and aged tap water at 10~ to 12~ The planarians were kept without food for a week or more before experiments were carried out. The stock was replenished as required. The Dendrocoelum were kept under conditions which allowed their movements to be observed and photographed by infrared radiation both in the dark and during illumination by visible light of known intensity. The experimental arrangement is shown in Fig. 1. Experimental Tray A Dendrocoelum for observation was kept in a shallow rectangular Perspex tray, 32 • 20 • 4 cm deep, filled with clean water from the stock aquarium to a depth of 1.5 cm. The outer surface of the bottom of this tray was scribed with grid lines filled with an opaque compound to provide a scale and reference. As shown in Fig. 1 the sides were extended below to support the tray in a water-bath provided by a second and larger rectangular Perspex tank. The waterbath rested on a dexion frame above the infrared source. Infrared Apparatus Observations were made with an infrared monocular comprising a diode image converter tube (Mullard 6929), power pack, and optical assembly. The visible screen was viewed through 269
ANIMAL
•
270
dispersing lens~
/
/
0• filter
BEHAVIOUR,
2
Photography
rubber bettows infrered
~
---monocular
75cm
[
"-.. experimentat
"-.~ray ",<
water bath
--ai~' out
>
air--
in
19,
t'~uta?"tungsten fitament tnmps Fig. 1. Diagram of experimental arrangements used to observe planarians by infrared radiation. The components are not drawn to scale and in practice the dispersing lens and monocular-camera assembly were positioned 75 cm above the experimental tray. the viewfinder of a Bolex H16 Reflex cine camera fitted with a Rank Taylor Hobson television lens (focal length 3.0 cm). Camera and monocular were mounted together rigidly on a special frame. The lens-monocular coupling was covered with a rubber bellows to prevent the escape of light from the visible screen of the monocular, Trials showed that the planarians were best viewed in silhouette against an infrared background. The infrared source was therefore placed below the dexion frame supporting the waterbath and experimental tray. Six 60-W tubular pearl tungsten filament lamps were housed in a wooden box fitted with a cooling fan and light-proof vents. The box was roofed with a strip of unpolished Chance-Pilkington OX5 filter glass, 45 x 23 • 0.5 cm thick. According to the manufacturers the transmission of this glass rises from 2,5 and 10 per cent at 750 nm and 800 nm respectively to a peak of 91 per cent between 1500 and 2500 nm. No light was visible through the filters used in these experiments:
Photographs of the monocular screen were taken on panchromatic film by single shot at 5-s intervals. This repetition rate was sufficient to allow a reconstruction of a planarian's track. The best compromise between resolution and field of view was obtained when the monocular was about 75 cm above the experimental tray. In this position the effective field of view was 10 x 12 cm. The monocular-camera assembly was therefore mounted so that it could be moved freely to cover the whole area of the tray.
Visible Light The source was a 100 W, 12 V, tungsten iodine lamp under-run at 10 V (colour temperature 3050~ A heat-absorbing filter (ChancePilkington HA 1) and neutral density filters (Wratten A) were placed in the optical system before the formation of an enclosed parallel beam. This was directed towards the midline of the experimental tray and reflected down over it by a prism and a concave dispersing lens set at a slight angle to the vertical as shown in Fig. I. This arrangement gave an almost uniform field of illumination over the tray. When tracking a planarium the monocular-camera assembly was carefully positioned to avoid shading the water within the field of view. The intensity of illumination close to the water surface was measured with an Eel selenium cell (response type S). Walter (1907) showed that Planaria gonocephala moved faster in the light than in the dark over a range of intensities (0.94 to 431 lux) but Ullyott (1936, p. 268) suggested that the increases 'were at least partly due to temperature effects'. Most of the experiments described here were carried out with water surface intensifies of 1.0 lux and others were made at intensities of 10-2, 10-4 and 10-5 lux. Preliminary trials showed that the water temperature did not increase under the conditions of these experiments.
Experimental Procedure The experiments were carried out in a basement darkroom housed in an old gunsite. This room was relatively free from the vibrations and disturbances of the main laboratory. A planarian was allowed to settle down for 48 hr after being placed in the tray. The monocular and fan in the infrared source box were left on during the day but the tubular lamps below the OX5 filter were only switched on for observation. Some film sequences of 30 to 100 frames (2.5 to 8 rain)
HARDEN JONES: PLANARIAN RESPONSE TO INCREASE IN LIGHT INTENSITY were taken during the settling down period. In an experiment the visible light was switched on after the moving flatworm had been photographed in the dark near the centre of the tray. Photography was continued until the flatworm reached the side of the tank when the experiment was concluded unless the worm moved away from the side again within 20 to 30 s. The water temperature was then recorded and the flatworm removed, or the visible light was switched off and another experiment attempted later. So for each flatworm photographic records were obtained of its movements in the dark, and, for a successful experiment, a second record which enabled a direct comparison to be made between its movement in the dark and in the light. Treatment of Data The tracks were reconstructed by plotting the positions of the middle of the tail from a frame-by-frame analysis of the film with a Specto Motion-Analysis projector. From these plots a scale expander was used to make a working drawing twenty-five times natural size from which the following measurements were made. (1) Distance moved in 5 s, i.e. between frames. From this measurement the speed of gliding (when the planarian was moving) was calculated in centimetres per minute. (2) Angle of turn, as degrees per turn. Differences of less than 2 ~ between one line of movement and the next were not measured and such deviations were not counted as turns. (3) Number of turns per minute. As positions were recorded at 5-s intervals, the frequency of turning could not exceed 12 per rain and was averaged over the period during which the planarian was moving. Tracks within 1 cm of the side were not used in making these measurements. Response of Dendrocoelum to Infrared Radiation Dendrocoelum observed by visible light did not respond to infrared radiation, neither when still nor when moving. Similarly, in the dark, Dendrocoelum known to be at rest in a particular position in the experimental tray, did not move when infrared radiation was switched on. It was concluded that the planarians did not respond to the levels of infrared radiation to which they were exposed during the experiments.
271
Results Observations by infrared radiation showed that a planarian roamed freely about the experimental tray for the first 2 to 3 hr in the dark but thereafter both the frequency and speed of movement usually declined. The planarians all showed the normal locomotory activities described by Pearl (1903). Individual tracks varied from a steady or irregular course across the tray to tracks inclined to the left or right indicating a bias in turning. There were no obvious environmental factors to account for the different tracks. Exceptionally the activity of some animals increased after several days in the dark, but others remained still for many days and observations were made hourly during the working day for up to 2 to 3 weeks before there was an opportunity to complete an experiment. The work was therefore tedious and prolonged; between November 1967 and November 1968 numerous trials yielded only sixteen experiments which merited detailed quantitative analysis and these involved eight individuals. Response to a Sudden Increase in Intensity of I lux The response of a gliding planarian was usually immediate. A typical track is shown in Fig. 2 and the relevant statistical data (experiment 3, D 3) are summarized in Table I. For the 5 min in the dark (up to frame 59) the planarian followed a more or less steady course, with a mean angle of turn of 17.9 ~, turning frequency 9.0 per min, mean speed 4.1 cm per min. The light was switched on immediately after frame 59. The planarian continued a short distance and stopped (60 to 61); moved forward and stopped again (63 to 66). After some 15 s, gliding continued intermittently with other stops (70 to 71; 78 to 80; 90 to 91; 104 to 105; 108 to 109; and 115 to 116). During these checks, the head was raised and made vigorous lateral sweeping movements before gliding continued. The tortuous nature of the track after frame 59 is clearly shown in Fig. 2. After frame 120 (5 min after switching on the light) the track became less twisted. The worm approached (frame 143) and then moved away from the side of the tray (frame 152). Filming stopped soon after frame 165 when the planarian started to glide along the side wall. In the light the mean angle of turn was 26.5 ~, the frequency of turning 10.4 per min and the speed of gliding 5.5 cm per min; all three increased in the light. The r.c.d.'s (mean angle of turn • frequency of turning) in the dark and light were 161 ~ and
272
ANIMAL
BEHAVIOUR,
19,
2 ]~4Z
".... 157
2Z, ...,/'/Direction " of gtiding
Of-
Fig. 2. Reconstruction of the track of Dendroeoelum3. The points record the mid-tail positions at 5-s intervals, the numerals frame numbers, and the vertical bars normal to the track minutes. The planarian was exposed to an intensity of 1 lux immediately after frame 59. The notation 60-1, 63-6, ere, indicates that the planarian did not change position between frames 60 and 61, 63 and 66, etc. For further description see text. 276 ~ respectively. Fig. 2 also shows that the increase in speed was maintained up to the end of the experiment (frames 152 to 165) although the angle of turn then decreased. Details of the experiments which were subjected to quantitative analysis are summarized in Table I. All showed an increase in the mean angle of turn. Inspection of the raw angle-ofturn data suggested that many of the distributions were skewed, particularly in those experiments where the planarians responded by large turns. This was confirmed by the appropriate test. A square root transformation normalized most, but not all, the distributions and the results of the t-tests given under the column 'angle of turn' in Tables I and II relate to the normalized and not to the raw data. Table I shows significant increases in the mean angle of turn in experiments 2, 3 and 7. Five experiments showed an increase in th~ frequency of turning (but not experiments 2 and 6), and all showed an increase in r.c.d. Speed of gliding increased in six experiments (but not in 2) and
the increase was significant in five experiments (not in 6). Experiment 2 showed a significant decrease in speed (from 8.4 to 6.8 cm per rain). An explanation of this anomolous result was suggested by the track of this particular worm which moved rapidly along a straight course in the dark. In the light it made many turns of more than 45 ~. The mean angle of turn more than doubled and the track became very twisted. Gliding slowed up, suggesting that the planarian could not move quickly along a convoluted track. Ullyott (1936, p. 276) makes the same point. The response of Dendroeoelum to a sudden increase in light intensity of 1 lux may be summarized under seven headings as follows: (1) Arrest of gliding: this may occur several times and there may be a few seconds' delay between illumination and the first check. (2) Sweeping movements: when gliding stops the head is raised high and makes lateral sweeping movements which may be continued for 10 to 30 s.
H A R D E N JONES: PLANARIAN RESPONSE TO INCREASE IN LIGHT INTENSITY §
I
+
§
§
§
§
§ §
+ §
§
§
§
',D
§ §
~ §
]
§
§
§
+
I~
§
"O
~00 ,.~0
.~'~ I~ ~ . ~
c~
,r.B
273
274
ANIMAL
~D
I
i' ?
@
BEHAVIOUR,
19,
2
§ §
§ §
§ §
§ §
§ §
§ §
§ §
§
§
§
§
I
I
§
§
§
§
§
I
§
I
§
§
§
L
o
§
§
§
~
o
§
o
o
o
§
~
o
O
o
o
o
§
o
o
§
9
§
§ §
§ I
§ §
I L
§ §
L
§
O
O
§
O
~-.o.
~
,..~ r
&
tqc'q
t'-- r
9,~,. r r r
t.-q r ,...~ c q
r
,...~ r
eh~b~
~t'-t'q'r
t'q
0
' ~~, o~., "~_~
g O ~
v',l r
D
l
i
G%
tr
xO
HARDEN JONES: PLANAR!AN RESPONSE TO INCREASE IN LIGHT INTENSITY (3) Large turns: when gliding is resumed there may be one or more large changes of course involving turns between 70 ~ to 180 ~. Turns of this magnitude are rarely made by planarians gliding in the dark. (4) Mean angle of turn: even in the absence of large turns there may be an increase in the mean angle of turn which is sometimes significant when averaged over the period of observation in the light. (5) Frequency of turning: increases. (6) Rate-of-change-of-direction: increases. (7) Speed of gliding: increases, and is usually significant when averaged over the period of observation in the light. The response shown by the planarians is summarized under these headings in Table I.
Response to Lower Light Intensities (10 -2, 10-3 and 10-5 lux) Some experiments were carried out at lower light intensities and the results are summarized in Table II. Several experiments were carried out with Dendrocoelum No. 7. This planarian was put in the tray on 11 June but was not seen moving in a suitable position for photography until 27 June. Thereafter several independent observations were made o f its movements in the dark and five of these were followed by sudden increases in light intensity. Some indication of the variability of movement in the dark is given by the range of mean angles of turn (11-9 ~ to 29-7~ frequency of turning (7.4 to 10.3 turns per rain) and mean speeds (4.8 to 8-5 cm per rain). This range of gliding speeds is very similar to the normal speed of locomotion for Dendrocoelum given by Lehnert (1891). In experiments 12 to 15, Dendrocoelum 7 showed a significant increase in speed. The mean angle of turn (and r.c.d.) decreased in experiments 12, 13 and 15. The increase in speed at 10-5 lux in experiment 15 was unexpected, but Dendrocoelum 8 (experiment 16) also showed a clear, but different, response to the same intensity. Dendrocoelurn 7 did not usually respond to the lower intensities with an arrest of gliding and when this did occur (experiment 14) the reaction was delayed for 40 s and was not followed by sweeping movements. Two other planarians (Dendrocoelum 6 and 8) showed similar delays of 20 and 15 s respectively.
275
Discussion Dendrocoelum kept in the dark and suddenly subjected to a light intensity of 1 lux usually moved faster, and turned more frequently with an increased angle of turn: the response has both ortho- and klino-kinetic components. Increases in speed were also observed in lower intensities and, while the limited results with Dendrocoelum 7 suggest a trend towards faster speeds at higher intensities, the results as a whole are consistent with the hypothesis that the speed of movement in the light is partly dependent on the speed in the dark: there is an increase in speed but the planarians moving fastest in the dark also moved fastest in the light. Ullyott (1936) found no difference in the speed at which Dendrocoelum moved under different intensities and Walter's (1907) results with P. gonocephala were similar. Walter (1907, p. 57) concluded that 'the rate of locomotion depends not so much upon the intensity of light as upon other factors which tend to produce individual behaviour upon the part of each particular worm'. With the exception of experiment 4, intensities of 1 lux led to an increase both in the frequency of turning and in the mean angle of turn and thus to an increase in r.c.d. Similar changes were observed at lower light intensities, but experiments 12, 13 and 15 with Dendrocoelum 7 showed a decrease in the mean angle of turn sufficient to produce a decrease in r.c.d. The results recall Waiter's (1907) conclusions that planarians turn more in the dark than in the light. Walter (1907, p. 51) made a distinction between 'indefinite' and 'definite' changes of course, the difference between the two being in the magnitude of the angle o f turn. Walter noted that the smaller 'indefinite' turns were more frequent in the dark and the larger 'definite' turns are more frequent in the light. The present results with Dendrocoelum suggest that it is the mean angle of the 'indefinite' turns which is greater in the dark rather than the frequency of turning: in experiments 12, 13 and 15 the mean angle of turn fell, but the frequency of turning rose following a sudden increase in light intensity. It seems probable that Dendrocoelum has a basal mean angle of turn and frequency of turning in the dark, the product of the two being equal to Ullyott's basal r.c.d. Exposure to relatively low light intensities leads to an increase in the speed of gliding and in the frequency of turning, and to a decrease in the
276
ANIMAL
BEHAVIOUR,
mean angle of turn: the r.c.d, falls. At higher intensities, however, both the angle of turn and frequency of turning increases and there is, therefore, an increase in r.c.d. Still higher intensities have a more marked effect on locomotion, the planarian making a limited number of turns through angles of 70 ~ or more and so effecting sudden changes of course. Higher intensities may also lead to an arrest of gliding and sweeping movements of the head. These three responses, an arrest of gliding, sweeping movements of the head, and large turns, would appear to be the components of a shock-reaction which, following Ullyott's (1936) line of argument, could be regarded as an extreme form of klino-kinesis. The positive ortho- and ldinokinesis shown by Dendroeoelum to sudden increases in light intensity are clearly related to the avoidance of bright light with the suggestion of an upper level to an optimum zone. The significance of the negative klino-kineses observed at lower intensities is not immediately obvious
19,
2
and requires further study. Clearly the behaviour of Dendrocoelum to sudden increases of light intensity cannot adequately be described in terms of r.c.d, alone. REFERENCES
Carthy, J. D. (1958). An Introduction to the Behaviour of Invertebrates. London: Allen & Unwin. Fraenkel, G. S. & Gunn, D. L. (1940). The Orientation of Animals. Oxford: Clarendon Press. Lehnert, G. H. (1891). Beobachtungen an landplanarien. Arch. Naturf., 57, 306--50. Patlak, C. S. (1953). A mathematical contribution to the study of orientation of animals. Bull. Math. Biophys., 152, 431-76. Pearl, R. (1903). The movements and reactions of freshwater planarians: a study in animal behaviour. Q, Jl microsep. Sci., 46, 509-714. Ullyott, P. (1936). The behaviour of Dendroeoelum laeteum. II. Responses in non-directional gradients J. exp. Biol., 13, 265-78. Walter, H. E. (1907). The reactions of planarians to light. or. exp. Zool., 5, 35-162. (Received 23 July 1970; revised 4 November 1970; MS. number: 995)