Overt Responses of Polychoerus carmelensis (Turbellaria: Acoela)

to Abrupt Changes in Ambient Water Temperature

ROBERT G. SCHWABl

KNOWLEDGE of an animal's response to a changein the ambient environment contributes to anunderstanding of its behavior, activity pattern,and methods of survival. Responses to environ ­mental stimuli by triploblastic animals having acomparatively low order of tissue/ organ devel­opment as in the Acoela has special evolutionarysignificance because, according to the hypothesisof Hadzi, they are the stem group of theEumetazoa and were derived from ciliates(de Beer, 1954; H anson, 1958) . Of specialinterest are the flatworms which inhabit tidepools of the midtide horizon, where ambienttemperatures may fluctuate because of shallow,relatively nonturbulent water. Such is the habi­tat of the Acoela flatworm Polycboerns car­melensis in the vicinity of Monterey, California(Ricketts and Calvin, 1952) . During low tidethis species is often abundan tly present on algae­covered rocks. At high tide Polycboerus takesshelter under rocks and gravel, appare ntly inresponse to water turbulence caused by the in­coming tide. Because of potentially pronouncedenvironmental changes within its ecosystem,Polycboerns was selected for study of the overtresponses by an exothermic marine animal tochanges in the ambient water temperature.

Dr. Donald P. Abbott, Assistant Director ofthe Hopkins Marine Station , contributed severalmuch-appr eciated suggestions dur ing this study.

METHODS

Specimens of P. cnrmelensis collected atPoint Pinos (vicinity of the Hopkins MarineStation of Stanford Uni versity, Pacific Grove,Californi a) were transported to the Un iversityof California Animal Physiology Laboratory at

1 Department of Animal Physiology, University ofCaliforn ia, Davis, California. Manuscript receivedDecember 3. 196 5.

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D avis, Califo rnia. One group of animals wasmaintained for 24 hr and another for 48 hr ina darkened chamber at a temperatur e of 13°_l40C. Thi s temperatur e approximates that ofMonterey Bay in late spring. In the followingtests an individual flatworm was removed fromthe chamber, placed on a horizontal plasticgrid, and quickly submerged to a depth of 1 cmin a controlled-temperature sea water bath.During the experiments the water temperaturewas maintained within ± 0.5°C of the desiredtemperature at the upp er surface of the sub­merged plastic grid. The amount of illum inationfrom fluorescent room lights at water level wasconstant at 60 ft -c throughout all tests. This wassufficient to induce a photokinetic response fromthe dark-conditioned animals.

As an animal moved across the plastic grid,its horizontal movements durin g a 30-secondperiod were transcribed by the author onto arecord sheet gr id. Such a record was obtainedfor each individual tested. All animals wereallowed to travel a distance of about 1 cm be­fore the record tracings were initiated. The totaldistance traveled, often in a highly erraticcourse, was measured from the record sheetgrid; the rate of locomotion was computed inmrri/rnin .

Afte r being dark-condit ioned for 24 hr, 10individuals were tested at each of the followingtemperatures : 30 , 50 , 80 , 110 , 14 0

, 170 , 21°,25°, 29°, 33° , 35°, and 38°C. N o furthertests were made on any individual, once its lo­comotion rate at a specific temperature was de­term ined. On the following day, each test wasrepeated using 10 animals dark-cond itioned for48 hr. There was no apparent difference in theaverage rate of locomotion related to durationof conditioning. Therefore, the locomotion ratedata for all 20 individuals tested at eachspecific temperature were consolidated.

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RESPONSES TO AMBIE N T WATER TEMPERAT URE

Reactions to Cold T emp erature

P. carmelensis placed in sea water at a tem­perature of 3°C immediately contracted into aU-shaped posture with normally ventral portionof the animal forming the inside surface of theU. Few animals exposed to this temperaturehad noticeable muscular or ciliary motion whilein this posture. Thus , the well-developed auto­maticity norm ally associated with ciliary motion(Prosser and Brown, 1961 ) apparentl y did notoccur in P. carmelensis exposed to sea waterat 3°C. After several minutes' exposure, disinte­gration of the epidermal cells occurred, andshortl y thereafter a gentle motion of the watercaused by stirr ing with a probe resulted in dis­organization of the body structure.

All potycboerus exposed to an ambient watertemperature of 5°C contr acted into the U­shaped posture and were motionless for severalminutes. Thereafter most were capable of loco­motion (it was necessary to test 24 ind ividualsto obtain locomotion rates for 20 animals) .These animals were motile long enough to mea­sure locomotion rates ranging from 5.0 to 32.5mrn/rnin with an average of 17.4 mrrr /min.This was the lowest average obtained in thesetests. At the same temperature, 16 individualsmoved about while in the U-shaped posture.Apparently th is movement was accompli shedentirely by motion of cilia on the dorsal surf ace,since no muscular contractions were noticed asthe animals glided over the surface of the plas­tic grid. These animals moved about for onlya minute or two and thereafter tissue disintegra­tion took place as described above. The highestrates of locomotion at 5°C were obtained from4 animals that, after a short per iod in the U­shaped posture, moved in the typical flatwormposture. W ithin a few minutes movementceased, whereupon they again contracted intothe U-shaped posture and died.

Reactions to Changes in Ambien t WaterTem peratu re

The locomotion rate of P. carmelensis wasclearly influenced by the temperature of am­bient sea water und er the conditions of theseexperiments. However, there were pronouncedchanges in the manner in which locomotion

PACIFIC SCIENCE, Vol. XXI, January 1967

occurred. Several of the 20 animals exposed towater at 8°C began to move while in the U­shaped posture . Locomotion was accomplishedby action of the dorsal cilia, the only portionof the body in contact with the plastic grid.These animals soon reoriented to the typicalflatworm posture and the rate of locomotion at8°C was measured from this posture only. Atth is temperature Polycboeras traveled at an av­erage rate of 44.9 mm /rnin. Accelerated loco­motion rates associated with increases in am­bient water temperatures were measured at 110 ,

14° , and 17°C with average values of 64.8,83.0, and 90.4 mm /rnin respectively. Thus, theaverage rate of locomotion for P. carm elensisacclimated at 13°-1 4°C increased from 17.4 to90.4 mm /rnin in response to a 12-degree rise(from 5° to l7°C) in temperature. This in­crease in speed of locomotion took place at anearly uniform rate of 6.1 mm/min/ oC in­crease in water temperature (see Table 1).

P. carmelensis specimens respond to tempera­tures higher than 17°C by decreasing their rateof locomotion. A reduction in average loco­motion rate was measured at 21°C ( 78.5mru/min), 25°C (66.2 mm /min ) , 29°C ( 50.2mrn/rnin ), and 33°C ( 32.8 mm/rnin ), Thisresults in a steady decrease in the speed oflocomotion at an approximate rate of 4.4mm/min;C C rise in temperature between 17°and 33°C.

Reactions to Warm T emperattt1'es

At an ambient water temperature of 29°CPolycboerns usually contracted into a curledposition with the posterior portion of the bodydrawn up under the more anterior portion . Lo­comotion in this posture was primarily accom­plished by action of the anterior portion of thebody, since much of the posterior portion wasnot in contact with the plastic grid. The animalshad an average locomotion rate of 50.2 mm /rninwhile in this posture and, although several ani­mals died after 5- 10 minutes' exposure, it isreasonable to assume that they would havefound a more suitable temperature within thislength of time in thei r natural habitat.

Most of the individuals exposed to an am­bient water temperature of 33°C immediatelycontracted into the curled posture mentionedabove and were capable of locomotion for only

Polycboerus carmelensis and Temperature Changes-c-Scrrwxs

TABLE 1

REACTIONS OF P. carmelensis To CHANGES IN AMBIENT WATER TE MPERATURE*

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TEMPERATURE LOCOMOTION RATE (MM/MIN) STANDARD STANDARD ERROR(0C) Av erage Ran ge DEVIATION OF THE MEAN

3':'*5 17 .4 5.0- 32.5 6.8 3 1.5 18 44.9 25 .0- 62.5 9.81 2.19

11 64 .8 40.0- 90 .0 14 .21 3.1814 83 .0 62.5-115 .0 14. 70 3.2517 90 .4 57.5-122.5 17 .06 3.8221 78 .5 45.0- 95 .0 13 .89 3.1025 66.2 45 .0- 87 .5 11 .40 2.5329 50.2 35.0- 62.5 7.41 1.6533 32.8 17 .5- 65 .0 11.49 2.5735':' *38 ':":'

• A total of 20 ind ividu als was tested at each of the temperatu res.•• See text.

1-3 minutes. Thereafter disintegration of thetissues took place. Some of the flatworms testedat this temperature immediately formed the U­shaped posture. After about a minute most indi­viduals reoriented to the curled posture andmoved about for a minute or two. After thisshort period of movement they again formedthe U-shaped posture and all movement ceased.Animals removed after about 3 minutes' expo­sure to 33°C water temperature did not recoverwhen placed in 14°C sea water. An exposure of5-10 minutes at this temperature results in anapparently complete disorganization of the bodystructure.

Exposure to sea water at a temperature of35°C resulted in a very brief but rapid locomo­tion by several of the 20 individuals tested.Generally this occurred while the animal wasin the curled posture. However, most individ­uals remained in the U-shaped posture assumedimmediately upon contact with the 35°C waterand had no measurable amount of locomotion.All animal s showed signs of tissue disintegra­tion within 60 seconds after exposure.

Polycboerus exposed to sea water at 38°C ap­peared to die immediately. Several individualswere dipped into water at this temperature andthen quickly returned to 14°C sea water butthere were no recoveries.

DISCU SSION

The locomotion rate of P. carmelensls was

clearly related to the water temperature underthe conditions of these experiments. The nearlyuniform increase of locomotion rate at 6.1mm/min;oC rise in temperature between 50and 17°C suggests that changes in tide pooltemperatures may have a pronounced effect onthe activity and behavior of th is species. Themechani sm by which temperature induces in­creased locomot ion activity is not known. How­ever, it is likely that this accelerated locomotionis fundamentally similar to the increases ofchemical and physical reactions normally asso­ciated with an increase in temperature. Manybiological processes, including rate of develop­ment, behavioral reactions, speed of locomotion,and metabolism show increases associated withhigher temperatures (Prosser and Brown,1961) .

It is significant that the highest rate of loco­mot ion occurred at a temperature ( l7°C) nearthat measured in these tide pools during latespring. This suggests that the maximum loco­mot ion rate of Polycboerus may be a functionof the most suitable ambient environmentaltempe rature with respect to possible acclimatiza­tion of the animal.

The decrease in locomotion rate measured atambient water tempe ratures above 17°C (4.4mm /rnin / PC rise in temperature between 170

and 33DC) is considerably less than that mea­sured for locomotion increases (6 .1 mm/min/oCrise in temp erature between 50 and l7°C).This suggests a temperature-related differential

SS

response rate as well as a different ial behaviorresponse. The temperature threshold at whichthe type of response, increased or decreasedlocomotion, and the rate at which the responsetakes place is approximately 17°C for P. car­melensis accustomed to an ambient water tem­perature of 13°-14°C. A possible explan ationfor this is that accelerated locomotion resultsfrom a direct influence by the ambient tempera­ture on body processes, and decreased locomo­tion results as a secondary effect of temperature ­related factors, such as the reduced availabilityof oxygen as the ambient water temperature in­creases. Polycboerus obtains oxygen from theaquatic environment by diffusion through epi­dermal tissues. The physical characteristics ofthis species are such that sufficient oxygen formetabolic processes should be available fromthe environment at the h igher temperaturelevels tested if the entire surface of the animalis effectively exposed to the environment. How­ever, it is possible that little oxygen diffusesthrough the ventral epidermal tissues because ofthe close pro ximity of the animal to the surfaceon which it crawls. This would reduce by nearlyone-half the effective diffusion surface and is apossible explanation for the curled position re­sulting from elevated temperatures in that thisposition exposes about one-half of the ventralepidermal tissue to the oxygen-bearing environ­ment.

Locomotion of P. carmelensis at temperaturesfrom So to 25°C inclusive takes place in thetypical flatworm-type posture. The highest in­dividual rate of locomotion obtained duringthese tests (2 .0S) was noted from a wormtested at 17°C. The lowest average locomotionrate in this temperatur e range was 0.75 at SoC,and the high est average of 1.50 occurred at17°C. These locomotion rates are expressed inmm / sec and were measured at a light intensityof 60 ft-e. Th ey compare closely with the loco­motion rates reported by Armitage ( 1961 ) of0.s6 mm/ sec and 1.34 rum/ sec measured atillum ination levels of 6 and 37 ft-c, respec­tively. H owever, he states that the behavior ofPolycboem s was highly erratic dur ing his loco­motion rate tests, in that some individuals spentconsiderable time turn ing the head fro m side toside and others ceased crawling before reachingthe end of a 5-cm course. A possible explana-

PACIFIC SCIENCE, Vol. XXI, January 1967

tion of this erratic activity, based on the tem­perature-l ocomotion rate relationship obtainedin my experiments, is that the water in the petridish used by Armitage in his tests becamewarm during the course of the observations be­cause of warm room temperature and/or heatfrom the light source used to stimulate locomo­tion. In the present tests, none of the 120worms tested between So and 25°C showedsuch behavioral responses. However, several ofthose tested at 29°C and essent ially all at 33°Creacted in the erratic manner described by Armi­tage.

Survival Value of T emperature-LocomationRelationship

Accord ing to Armit age (1961 ) , P. carmelen­sis does not possess a persistent diurnal rhythm.Therefore, th is species must depend on an en­vironmental "cue" or a combination of suchenvironmental stimuli to regulate its daily ac­tivity pattern. Observations by Arm itage indi­cate that light intensity and water turbulenceplay a pronounced role in the regul ation of ac­tivity and behavior of this animal. Because ofthe pronounced influence on the velocity ofmovement resulting from slight changes in theambient water temper ature , it seems reasonablethat temperature and temper ature -related factorsmay also functi on as stimuli regulating activityand behavior.

Arm itage postul ates that on bright days th eabsence of Polycboerus f rom the upp er surfaceof rocks and gravel dur ing low tide and rela­tively calm water is caused by an increasednegative phototropic response to high light in­tensity . H owever, on April 29, 1965, a coolbut very bright day with morn ing sea water atabout 15DC, I observed Polychoerus active onthe upper surfaces of rocks and gravel thro ugh­out the period of low tide. This observation,made at the same location but earlier in theseason than that by Armitage, documents thefact that high light intensity does not alwayscause a negative phototropic response, and sug­gests that there is more than a single factorregulating this behavior. Amb ient water tem­peratures above 17°C cause a reduction in therate of locomotion for animals conditioned to13°- 14°C. It is likely that this reduced rate oflocomotion in response to such temperatures or

Polycboem s carme lensis and Temperature Changes-ScHWAB 89

temperature-related factors corresponds to aless suitable ambient environment, and that suchconditions in their natural environment maycause P. carmelensis to vacate the upper sur­faces of rocks and gravel.

In my experiments Polycboerns specimenswere totally incapacitated soon after exposureto ambient water temperatures above 29°C. Itis reasonable to assume that environmental fac­tors would trigger a behavioral escape mechan­ism should such temperatures occur in theirnatural habitat. The survival value of such anenvironmental stimulus or combination of stim­uli is dependent upon the sensitivity and re­sponse of the animal to this factor or factors.Armitage reports a 55% increase in rate ofcrawling when the light intensity was increased640% , and that in his tests P. carmelensis wasnegatively phototropi c. Although the magnitudeof the light intensity change (in ft-c or in per­cent increase) and the sensitivity of the animalsin terms of response to this factor (rate ofcrawling in mrn/ sec or percent increase) cannotbe directly compared to similar calculationswith respect to temperature change and asso­ciated activity response, it seems certain thatPol vcboerus is at least as sensitive to ambientwater temperature as it is to light intensity.

Costello and Costello (1938 ) ind icate thatP. carmelensis may be positively phototropicin that individuals showed a tendency to groupon the moderately lighted side of an aquarium.Th erefore, there is evidence of both positiveand negative phototropic response for this spe­cies. Armitage suggested that this species mayhave a differential response to low and highlight intensities. Whether such a light sensi­tivity threshold exists, or whether the behavioralevidence supporting this possibil ity results froma water temperature-light intensity relationship,is at present unknown.

Th e physical properti es of water are such thatthe heat energy associated with even relativelyhigh light intensities may have littl e immediateeffect on the temperature of the tide pool. Con­versely, tide pool temperatures may in time be­come elevated on overcast days with relativelylow light intensities. Thus, it is possible thatthe absence of this species from tide pools onbright days as observed by Armitage may be atleast in part the result of an elevated ambient

water temperature associated with high insola­tion, rather than the result of high illum inationlevels as a discrete factor. Regardless of thenature of causative stimuli , the rate of locomo­tion between 8° and 29°C is sufficient forPolycboerus to seek a more desirable situationunder rocks and gravel should the tide poolenvironm ent warrant such behavior.

Environm ent-Induced Posture Responses

The U-shaped posture resultin g from low,high , and often from moderately high ambientwater temperatures appears to be the sameposture described by Armitage for Polycboerusexposed to osmotically unsuitable salinity con­centrations. Apparently this species has only theU-shaped posture response to both hypo- andhypertonic sea water, since Armitage does notmention other postures such as the curled pos­ture observed in my experiments at certainelevated water temperatures.

During my experiments the U-shaped pos­ture often occurred as a response to a gentlemechanical stimulus from contact with a soft­bristled brush used to transfer the animals fromone container to another. The fact that thi sposture occurs as a result of temperature change,salinity change, and mechanical stimulus sug­gests that this posture is a characteristic behav­ior response to many und esirable environmentalcondition s.

Further experimentation may show that thecurled posture also occurs as a response to avariety of stimuli. Howe ver, it is certain th atthis response is clearly associated with ambientwater temper atures near the upper lethal level.It is not known if the curled-posture responseoccurs as a direct result of temperature percep­tion by a discrete thermoreceptive mechanism oras a characteristic response to temperatur e­related environmental factors. It may be that thecurled posture common at 29°C fun ctions toincrease the amount of surface area in directcontact with the aquatic environment, thus facili­tating gaseous exchange, while enabling theanimal to take shelter. The U-shaped postureexposes even more surface area but has no ob­vious survival value at temper atures above29°C, inasmuch as the locomotion rate is lowand the animal soon dies. Howe ver, this con­clusion is hypothetical, since possible advan-

90

tages of either posture to animals exposed tohigh water temperatures are not known.

CONCLUSIONS

1. The ambient water temperature clearlyinfluences both the rate and the sign of Poly­cboerus' locomotion. A rate-directional responsethreshold was measured at 17°C for animalsconditioned to 14°C in that, as temperaturesincreased from 5° to 17°, the speed of locomo­tion increased from an average of 17.4 to 90.4rnm/min, at the rate of 6.1 mm /rnin/ PC. Fur­ther temperature increases from 17° to 33°Ccaused a steady decrease in locomotion speedfrom an average of .90.4 to 32.8 mm /rnin, andthe rate of reduction per degree of temperatureincrease, - 4.4 rom/min, was relatively low.

2. The highest average speed of locomotion(90.4 mm /rnin ) and the greatest ind ividualrate (122.5 mrn/rnin) were measured at 17°C,which is near the temp erature of sea water atthe location and season at which these experi­ments were conducted. This suggests the pos­sibility that th e maximum locomotion rate is afunction of the ambient water temperature withrespect to possible seasonal acclimatization bythe anim al.

3. It is postulated that the increased rate oflocomotion as temp eratures changed from 5°to 17°C corresponds to a general temper ature­related acceleration of th e body processes. It isnot known why temperature increases above17°C cause a reducti on of locomotion speed .Possibly this is a function of decreased amountsof available oxygen due to elevated water tem­peratures and/or crossing a critical temperaturethreshold for enzymatic action.

4. Polycboerus has a differential movementposture with respect to high and low watertemperatures. At 5°C th is species contracts intoa U-shaped position, and movement at an aver­age rate of 17.4 rom/min results from mot ionof the cilia on the dorsal surface of the animal.In ambient sea water temperatures of 29°C and

PACIFIC SCIENCE, Vol. XXI, January 1967

above, movement generally occurs while theanimal is in the usual flatworm position, butwith the posterior portion of the body drawnup under the more anterior portion. In this po­sition the average rate of movement was 50.2rnm/rnin at 29°C and 32.8 rnrn/rnin at 33°C.

5. Sea water temperatures slightly below5°C and above 29°C are not suitable for thesurvival of P. carmelensis conditioned at atemperature of about 14°C. However, the rateof locomotion at these temperatures appe arssufficient to allow this species to avoid suchconditions should they occur in the tide pool.Measurements of temperatures prevailing be­neath the rocks and gravel in these tide poolsis needed.

6. It is probable that the temperature ofthe ambient sea water , as well as the intensityof illumination and the turbulence of water,function as an environmental stimulus regulat­ing the activity and behavior of Polychoerus.

REFERENCES

ARMITAGE, K. B. 1961. Studies of the biologyof Polycboerus carmelensls (Turbellaria :Acoela). Pacific Sci. 15:203-210.

COSTELLO, H. M., and D . P. COSTELLO. 1938 .A new species of Polvcboerus from the Pa­cific Coast. Ann. Mag. N at. Hist., Ser. 11,1 :148-155.

D E BEER, G. R. 1954. The evolut ion of themetazoa. In: Juli an Huxley et aI., eds., Evo­lution as a Process. George Allen and Unwin,London. 367 pp.

HANSON, E. D . 1958. On the origin of theEumetazoa. Systematic Zool. 7 :16- 47 .

PROSSER, C. 1. , and F. A. BROWN, JR. 1961.Comp arative Animal Physiology. 2nd ed. W.B. Saunders Co., Philadelphia. 688 pp .

RICKETTS, E. F., and J. CALVIN. 1952. BetweenPacific T ides. 3rd ed. Rev. by J. W. Hedg­peth . Stanford Uni versity Press, California .502 pp.