Overt Responses of Polychoerus carmelensis (Turbellaria: Acoela) 
to Abrupt Changes in Ambient Water Temperature 
Robert G. Schwab 1 
Knowledge of an animal’s response to a change 
in the ambient environment contributes to an 
understanding of its behavior, activity pattern, 
and methods of survival. Responses to environ- 
mental stimuli by triploblastic animals having a 
comparatively low order of tissue/organ devel- 
opment as in the Acoela has special evolutionary 
significance because, according to the hypothesis 
of Hadzi, they are the stem group of the 
Eumetazoa and were derived from ciliates 
(de Beer, 1954; Hanson, 1958). Of special 
interest are the flatworms which inhabit tide 
pools of the midtide horizon, where ambient 
temperatures may fluctuate because of shallow, 
relatively nonturbulent water. Such is the habi- 
tat of the Acoela flatworm Polychoerus car- 
melensis in the vicinity of Monterey, California 
(Ricketts and Calvin, 1952). During low tide 
this species is often abundantly present on algae- 
covered rocks. At high tide Polychoerus takes 
shelter under rocks and gravel, apparently in 
response to water turbulence caused by the in- 
coming tide. Because of potentially pronounced 
environmental changes within its ecosystem, 
Polychoerus was selected for study of the overt 
responses by an exothermic marine animal to 
changes in the ambient water temperature. 
Dr. Donald P. Abbott, Assistant Director of 
the Hopkins Marine Station, contributed several 
much-appreciated suggestions during this study. 
METHODS 
Specimens of P. carmelensis collected at 
Point Pinos (vicinity of the Hopkins Marine 
Station of Stanford University, Pacific Grove, 
California) were transported to the University 
of California Animal Physiology Laboratory at 
1 Department of Animal Physiology, University of 
California, Davis, California. Manuscript received 
December 3, 1965. 
Davis, California. One group of animals was 
maintained for 24 hr and another for 48 hr in 
a darkened chamber at a temperature of 13°- 
14°C. This temperature approximates that of 
Monterey Bay in late spring. In the following 
tests an individual flatworm was removed from 
the chamber, placed on a horizontal plastic 
grid, and quickly submerged to a depth of 1 cm 
in a controlled-temperature sea water bath. 
During the experiments the water temperature 
was maintained within ±0.5°C of the desired 
temperature at the upper surface of the sub- 
merged plastic grid. The amount of illumination 
from fluorescent room lights at water level was 
constant at 60 ft-c throughout all tests. This was 
sufficient to induce a photokinetic response from 
the dark-conditioned animals. 
As an animal moved across the plastic grid, 
its horizontal movements during a 30-second 
period were transcribed by the author onto a 
record sheet grid. Such a record was obtained 
for each individual tested. All animals were 
allowed to travel a distance of about 1 cm be- 
fore the record tracings were initiated. The total 
distance traveled, often in a highly erratic 
course, was measured from the record sheet 
grid; the rate of locomotion was computed in 
mm/min. 
After being dark-conditioned for 24 hr, 10 
individuals were tested at each of the following 
temperatures: 3°, 5°, 8°, 11°, 14°, 17°, 21°, 
25°, 29°, 33°, 35°, and 38°C. No further 
tests were made on any individual, once its lo- 
comotion rate at a specific temperature was de- 
termined. On the following day, each test was 
repeated using 10 animals dark-conditioned for 
48 hr. There was no apparent difference in the 
average rate of locomotion related to duration 
of conditioning. Therefore, the locomotion rate 
data for all 20 individuals tested at each 
specific temperature were consolidated. 
85 
