ON THE TROPISMS OF SOME FRESHWATERPLANARIANS

BY KATHLEEN E. CARPENTER, PH.D., M.SC.

{Received ist August 1927.)

(With Two Text-figures.)

THE behaviour of Planarians belonging to a variety of species has been shown bya number of workers to depend largely upon the action of external stimuli—•mechanical, chemical, light, heat, contact, etc.—inducing responses usually de-scribed as "tropisms," which, while subject to some slight individual variation,remain on the whole constant in character for stimuli of the given degree of intensity.Observations have been made on reactions of this type in certain species occurringwithin the Aberystwyth district of Cardiganshire (Carpenter, 1926, 1927), withthe special object of determining how far, if at all, such responses may assist indetermining the peculiar ecological distribution of the species in question; theresults seem worthy of a brief separate consideration, as they show that there maybe considerable diversity of reaction between even closely allied species.

The three species used in these experiments were:(a) Planaria albissima Vejdovsky, common in all streams of the district, and

particularly in their slower and more muddy reaches.(b) Planaria alpina Dana, common in clear brooks and springs.(c) Poly celts cornuta Johnson, found in great numbers in stony lower reaches

of the alpina brooks.The two last-named are regarded by continental workers as glacial relicts in

cold mountain brooks and springs whose temperature varies only within narrowlimits: they do not appear to penetrate into the slower eurythermous waters ofthe plains. The current theory is that these species, generally distributed overEurope during the Ice Age, have since died off in most places, either owing tothe directly deleterious action of changes in temperature (Steinmann 1907, Thiene-mann 1912), or under stress of competition with incoming eurythermous types(Voigt 1904); it has, however, been suggested (Wilhelmi 1904) that alpina andcornuta may actively wander up and down the streams in search of the zones ofoptimum temperature: observations on their reactions to variations in temperatureand to currents of water of different intensities, especially, are of peculiar interestin this connection. PL albissima, which appears to be a eurythermous type tolerantof widely varying external conditions (Carpenter 1926, 1927, 1928), has been usedin this study by way of contrast.

It is necessary to give some account of the ordinary reactions of Planarians asestablished by previous workers. These may be classed according to the natureof the evoking stimulus,

On the Tropisms of some Freshwater Planarians 197

A. REACTION TO LIGHT.The early observations of Duges (1828) and Kennel (1888) showed that free-

living Planarians of several species are markedly sensitive to light, which, as opposedto darkness, tends to Induce motion which on the whole Is directed away from thesource of the stimulus. Loeb (1893,1894), working with Planaria torvaf has pointedout that the reaction Is not a simple troplsm, but that there Is a differentia! sensibilitywhich results In the animals ultimately coming to rest In an area of least Intenseillumination rather than at a spot furthest removed from the source of the stimulus.The detailed studies undertaken by Parker and Burnett (1900) and by Walter(1907) with PL gonocephala, PL maculata, Dendrocoeium lacleum and Phagocatagracilis, have confirmed this view, and also Loeb's assertion that the function oflight perception Is not confined to the eyes or even to the anterior part of the body.

The behaviour of all three of the local species Investigated with regard to lightis in conformity with the general types although there Is some evidence of Inter-specific differences as regards the speed of the reaction: on the whole, PL albissimaappears to react most promptly, Pol. cornuta most slowly, to the light stimulus—apoint of some little interest, as the former Is milk-white In colour, whereas both.Pol. cornuta and PL alpina have a good deal of the dark pigment which has beensupposed to assist In the perception of the stimulus.

B. REACTIONS TO CONTACT STIMULI.Pearl (1903) has shown that, In PL maculata, PL gonocephala and PL doroto-

cephala, the application of a weak mechanical stimulus elicits a positive reaction,whilst a stronger stimulus of the same nature produces a negative; he has alsoconfirmed the suggestion made by Loeb (1893, J^9A) ^na* the negative and positive" stereotroplsmSJ of dorsal and ventral surfaces respectively play an Important partIn determining both the maintenance of the normal posture in locomotion and thetendency to settle In crannies. No doubts remain as to the function of reactions ofthe type described as "thigmotactic," In combination with the characteristicreaction to the light stimulus, In- determining the habit of resting on the ender-surfaces of stones which Is so common among Planarians in their natural habitat.In both these cases the reactions of the three species under consideration are ofthe normal type.

C. REACTIONS TO A CURRENT OF WATER.Pearl, In the work above mentioned, observed In three species of Planaria a

"very precise rheotactic reaction," positively directed to the weak current producedby allowing water to trickle by its own weight out of a narrow tube in which theanimals had been placed. Later, Allen (1915) described the reactions of PL d&roto-cephala to a circular current produced by 'stirring the water in a shallow pan:the rheotropism was positive to a strong current and negative to a weak one? andboth positive and negative types could be seen at the same time, the current nearthe circumference being a good deal stronger than that near the centre. Allen

198 KATHLEEN E. CARPENTER

claims that the biological importance of the positive reaction was to some extentconfirmed by field observations on PL dorotocephala in a muddy ditch: on oneoccasion, 60 individuals were counted travelling up-stream and 5 down-stream, atanother time the counts were 40 up to 4 down. Allen also found that the characterof the reaction was to some extent modified by alterations in temperature andchemical character of the water: lowering the temperature by 4°-io° C. produceda reversal of reaction from positive to negative in 17 cases, from negative to positivein 2, out of 32 experiments: raising it by 4°-io° C. gave reversals from + to — in4 cases, from — to 4- in 1 case, out of a total of 13. The use of weak chemicalstimuli, such as 1 per cent, cane-sugar solution, had similar effects. In all casesthe original type of reaction was restored so soon as the animals had becomeaccustomed to the new conditions.

Working with PL albissima, I have been able to confirm a rheotropic reactionsimilar in type to that exhibited by dorotocephala. To ensure greater accuracy,however, a " rheotactometer" of simple type has been devised, which possessesthe advantage of providing a steady and uniform current of water whose forcecan be regulated at will. The whole apparatus, as shown in Fig. 1, being filledwith water and the screw clip closed, the animals are introduced at B and allowedto find their way to a temporarily shaded area mid-way between 4̂ and B and tosettle there before the commencement of each experiment. The temporary shadeis then removed, and the long axis AB kept horizontal, evenly illuminated andperpendicular to the main light rays. The current passing through AB can beregulated by turning the screw clip: I define a "weak" current as one producinga slow succession of drops from the end B, while a "strong" one (which yet mustnot be so strong as to sweep away the Planarians) produces a continuous streamin parabolic curve. (This apparatus may be safely recommended for demonstrationpurposes.)

The reactions of PL alhissima to currents of water in the rheotactometer maybe tabulated as follows:

Temp.

CurrentTotal of casesReaction +Reaction —Indefinite (gripping glass orunorientated wandering)

15° C

Weak1 0 0

5 +8 3 -12

Strong1 0 0

61 +2 9 -1 0

Raising the temperature by 50 C. gave rise to the following variations:

A.Temp. 150 C.

B.Temp. 200 C.

CurrentTotal of casesReaction +Reaction —Indefinite

Weak2 0

5 +14-

1

Strong2 0

16 +4 -0

Weak2 0

11 +8 -1

Strong2 0

6 +14 ~

0

On the Tropistns of some Freshwater Pkmarians

Introducing a i per cent, cane-sugar solution also produced variations:

Temp. 130 C.

199

CurrentTotal of casesReaction +Reaction —Indefinite

A. Culture water

Weak206 +

1 4 -0

Strong2019 +

T

O

B. 1 % sugar solution

Weak2010 +5 ~~5

Strong20

13 +7~0

,Y Z B

Fig. 1. The Rheotactometer.

X, Aspirator-jar. Y, Rubber-bung. Z, Screw-clip on rubber tubing connectingshort glass-tube K with long glass tube AB. Length of AB—2 feet. Internaldiameter—o-8 cm.

These figures are in fair agreement with those quoted by Allen for PL doroto-cephala: with the two glacial relict species, however, very different results wereobtained. The rheotaxy in both was found to be strongly negative, whatever theintensity of the current: this may be easily seen without special apparatus, as if thedish containing specimens be tilted so that the water runs out of it, the Planarianswill all glide in the direction of the outgoing current. Specimens placed on a glassplate and allowed to get a good grip of its surface, then subjected to a stream, ofwater from a tube at either end, may be made to turn about like a regiment ofsoldiers at drill, in accordance with the slight tilt which causes the stream to flowalternately in reverse directions over the plate, and the reaction is so strong thatit overrules entirely the normal negative reaction to the light stimulus. In therheotactometer, the following figures were established, at a temperature of n ° C :

CurrentTotal of casesReaction +Reaction —Indefinite

Polycelis cornuta

Weak50

2 +4 6 -

2

Strong50

05 0 -

0

Planaria alpina

Weak508 +

4 0 -2

Strong5°9 +

4 1 -0

Raising the temperature has little or no effect on the reaction: at 20° C. 20 indi-viduals of Pol. cornuta gave reaction to both weak and strong currents in every case5while, of 20 PL alpina at this temperature, 17 gave — to a strong current, 13 — toa weak.

200 KATHLEEN E. CARPENTER

Reactions to a strong current, in i per cent, cane-sugar solution, were asfollows:

Total of casesReaction +Reaction —Indefinite

Polycelis cornuta

20nil1 8 -

2

Planaria alpina

20

1 +1 7 -

2

After being kept for 30 minutes in this solution, all of the original 40 specimensgave the negative reaction.

This strong negative rheotropism is particularly interesting in relation to theconstant habitat of the two species, i.e. swift, clear-running streams, or springs:not only is it strongly at variance with the usual positive rheotropism of mostanimals of swift streams (Allee 1912), but also it gives rise to speculation as to thefactors which keep the two species under consideration to the upper courses of thestreams which they inhabit, contrary to the working of this negative rheotropism

Fig. 2. The Thermotactometer.AB is a glass tube of internal diameter 1-5 cm. The length of the horizontal portion is2 feet, a, h are thermometers inserted in the open ends. The temperature at intermediatepoints is approximately estimated by the use of thermometers placed in contact with theglass. (An obvious improvement would be to bore holes for their insertion.)

which might be expected to bring them down-stream. This point will receivefurther discussion below (p. 202. See also Carpenter 1928).

It should be mentioned that only perfectly fresh specimens were used in the experi-ments reported above. Allee (1912) has drawn attention to the loss of positive rheotaxyon the part of animals kept for several days in still water, and, in the writer's experience,there is, at any rate, relatively slow response to stimuli of all kinds on the part of Planarianswhich have been in the laboratory for more than a day or two.

D. REACTIONS TO CHANGES IN TEMPERATURE PER SE.

No previous records of observations on thermotropism have been discovered,although Pearl (1903) draws attention to the probable interest of such work.Mendelssohn's account (1902) of a thermotropic reaction on the part of Paramoeciumhas been criticised by Loeb (1918), who considers that this is probably not trulytropistic in character, but is due to a reversal of ciliary action induced by changesin temperature. For the study of the reactions of Planarians to changes in tem-perature, I have employed the simple apparatus shown in Fig. 2.

Water is poured into the tube AB, leaving a clear passage for air throughout,and the temperature at either end can be adjusted by contact with a metal rod,

On the Tropisms of some Freshwater Pianarians 201

heated by a Bunsen flame, or with a freezing mixture of ice and salt: thermo-meters are inserted into the open ends.

In the simplest type of experiment, the Pianarians are allowed to settle at theend A and the temperature noted. The whole apparatus being kept shaded,the Pianarians usually remain at rest so long as they are undisturbed. The reactionsunder differential heating or cooling are then noted.

In the case of PL albissima there appears to be very little sensitivity to gradationsin temperature between 20 C. and 20° C : placed in the tube at 120 C. the animalsremain fairly quiescent between these limits; more extreme temperatures, however,induce activity, and in an experiment where the temperature at A was raised to21-5° C. and at B depressed to o°C. the 10 individuals employed confined theirwanderings throughout the period of observation to the middle third of the tube*(The temperature as read through the glass at the mid-point was 17-5° C.) A riseof temperature to 320 C. is sufficient to kill albissima, even if the heating be gradual:relative to this fact, the sensitivity of the species to heat must be designated as low.The observation is in agreement with an unpublished note by Miss Blodwen Fox,M.Sc. (of U.C.W. Department of Zoology, Aberystwyth), on the behaviour ofPlanaria atnara n.sp.: Miss Fox finds that, whereas a temperature of 34°-39° C.is fatal to PL amara in the laboratory, the animal exhibits no reaction to a rise intemperature culminating below 25° C. PL alpina and Pol. cornuta are far moresensitive to temperature variations: with alpina, A being shaded and B exposedto light at an even initial temperature of 140 C , the animals clustered at A; thetemperature at A was then raised to iy° C , and all the individuals promptlymoved out towards B and remained within 2 inches of this end (where the tem-perature was now 14-6° C.) within 15 minutes of observation. A was then allowedto cool, and, the temperature being once more even throughout the tube, theanimals clustered once more at Ay the shaded end. The reaction to heat was thusshown to be so strong as to overrule the operation of the light stimulus. Similarexperiments established a thermotactic reaction positive to the lower temperatureat all temperatures down to about 50 C : below this, the action is reversed, andextreme cooling produces a negative reaction.

In the thermotactometer, PL alpina undoubtedly seeks its optimum temperatureof 5°-y° C.—a fact which is in striking agreement with observations on the distri-bution and physiology of the species made by continental biologists (Steinmann1907, Thienemann 1912). The precise nature of the reaction is difficult to determine sbut there appears to be a real repulsion from the zones of extreme temperatures,evidenced by the "tasting" movements (described by Pearl in another connection)which usually mark the reception of a fresh stimulus.

Polycelis cornuta, though more sensitive than PL albissima, to the differentialtemperature stimulus, does not equal alpina in accuracy of response: raising A to200 C. and cooling 5 to o° C , I find that cornuta tends to wander between B anda point Y, where the temperature on the outside of the glass is about 15-5° C ;between this region and A, however, very definite repulsion is exhibited. Innature, cornuta habitually occupies waters of a wider range of temperature than

202 K A T H L E E N E. C A R P E N T E R

the alpitia reaches, and the death-point for cornuta In the laboratory is about300 C , for alpina 23°-25° C.

The thermotroplc reactions are undoubtedly significant and seem likely toafford a valuable clue to the association of the two last-named species with thestenothermous waters of the upper reaches of streams: this sensibility, coupledwith a positive reaction to food-stimuli acting by down-stream diffusion, may wellbe the means of confining them to such reaches in spite of the down-stream driveof negative rheotropism.

E. REACTIONS TO CHEMICAL STIMULI.

In testing reactions of this class Pearl (1903) has employed three main methods:the application of localised stimuli, by dropping the solution from a capillarypipette; the method of diffusion, by allowing the solution to pass slowly througha fine aperture in a paraffin trough into the culture water; and the "two-drop"method of Massart. By these methods, he establishes a general positive reactiontowards chemical solutions of low concentration, whatever their composition (afactor of prime importance in the make-up of the food reaction), and a generalnegative reaction towards higher concentrations of the same chemical composition,with one exception, that of cane-sugar solution, which produces a positive reactionat all concentrations above 2 per cent.

Using mainly the pipette method, which I find open to fewest objections, I havebeen able to establish a similar series of reactions on the part of the three localspecies under observation; there are some inter-specific differences in behaviour,manifested in the shifting of the "critical point" of concentration at which thereaction changes from positive to negative, but the general principle holds good,and has been extended to cover a slightly wider range of chemical solutions thanthat of Pearl.

A particularly Interesting reaction, In view of ecological relationships, is thatInduced by "bumlc acid." Using extracts derived by stirring fresh peat in glass-distilled water at ordinary laboratory temperature, I have found in all cases astrong negative reaction on the part of alpina and cornuta; albissima, on the otherhand, appears indifferent to extracts of pH 5-0 and below, although the negativereaction is given on the acid side of this line. In the Aberystwyth district albissimaoccurs quite commonly In moorland brooks of pH 5-6 and below, from which theother two species are markedly absent (Carpenter 1928).

SUMMARY.The tropistlc reactions of three Planarian species occurring in Cardiganshire

streams, Planaria albissima, a common and generally distributed type, and PL alpinaand Polycelis cornuta^ two Ice Age relicts whose occurrence is almost entirelylimited to the neighbourhood of springs, have been studied with a view to deter-mining their ecological significance (if any).

All three species give the responses characteristic of the whole group of

On the Tropisms of some Freshwater Planarians 203

Triciadida (as established by previous workers) to light-, contact-, and chemical-stimuli: they differ to a considerable extent in their reactions to a current of.water,pi. alhissima giving the characteristic positive reaction to a strong current, negativeto a weak one, while the two glacial relicts exhibit a strong and constant negativerheotropism. These two species also show far greater sensibility to extremes oftemperature than does PI. albissimaf as well as a more delicate perception of certainchemical stimuli. Their tropisms, as a whole, are In logical agreement with thefacts of their occurrence only In environments of a very definite type, and witha partial seasonal migration (down-stream In winter, up-stream In summer) whichhas been found to occur in the streams of this district (Carpenter 1928).

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