ABSTRACT
A quantitative study of the responses of Daphnia magna to light was made with the use of an experimental trough illuminated horizontally through one end by a uniform beam of light. The intensity of light was changed by shifting the position of a neutral glass “wedge” interposed in the beam of light.
The difference in the position of Daphnia when positively phototropic and when negatively phototropic is a difference in the postural angle at which the antennae are held, and not a difference in the direction of orientation of the whole organism—the animal’s back being toward the light under all circumstances.
The primary sign of phototropism is not altered according to the absolute intensity of the light, but is affected by (1) the age of the individual, (2) the temperature of the water, and (3) the condition of the culture medium. Sometimes a “spontaneous” change in the primary sign of phototropism occurs.
The occasional movements observed to occur in the direction opposite to that of the primary sign of phototropism appear to be essentially periodic in respect to their times of inception and their duration. These periodic movements of Daphnia are not due to recurring periods of increased or of decreased activity, but probably represent periodic changes in the underlying photic mechanism.
“Variability” of the responses of Daphnia to successive identical tests gives evidence of being fundamentally periodic. A system of experimentation was devised to eliminate the error due to this variability, in so far as this was possible.
It was found that the rate and the magnitude of the change of illumination must rise above a certain threshold to be effective in causing a reversal of phototropic sign. A minimum length of exposure to bright light before the test is made is also necessary.
The relations of (1) length of the latent period, (2) speed of response, (3) magnitude of response, and (4) duration of response to (a) amount of reduction of light intensity, (b) duration of previous exposure to light, (c) duration of previous sojourn in dark, and (d) temperature of the water, were investigated, and the results have been summarised in Table XIII.
My observations are consistent with Ewald’s conclusions that orientation of Daphnia is based on a mechanism which is entirely distinct from that responsible for the other three aspects of phototropism, namely (1) persistent phototropic swimming under any constant illumination, (2) periodic changes of phototropic sign under constant low illumination, and (3) reversal of phototropism produced by changes of light intensity.
It is shown that these other aspects of phototropism of Daphnia could be accounted for by one mechanism of excitation, if it were photoreversible and properly controlled. A theory is proposed that this mechanism is a reversible photochemical system such as that used by Hecht. The theoretical requirements of the mechanism would be fulfilled on the assumption (1) that equilibrium in the system would result in the maintenance of the persistent primary sign of phototropism, and (2) that the upsetting of this equilibrium would result in the production of the secondary signs. Upsetting of the equilibrium by some internal rhythmic process and by changes of illumination would account for periodic phototropic movements and for induced reversals of phototropic sign, respectively.
The results of the experiments on the photic responses have been reviewed in the terms of the proposed theory, and it is found that the evidence strongly supports the hypothesis that a reversible photochemical system is the basis for these aspects of the phototropism of Daphnia.
The experiments and theoretical considerations dealt with in this paper are a continuation of the work described previously (Clarke, 1930).
For an account of the life-history and feeding habits of Daphnia see Naumann (1926), Banta (1921 b) and Brown (1926-7). Stuart, McPherson, and Cooper (1931) have succeeded in raising Moina on pure cultures of living bacteria.
Animals should not be kept in continual darkness since such a procedure results in Daphnia pulex in the production of animals with degenerate eyes after 6–12 days (Kapterew, 1912).
The wedges (Eastman Kodak Co.) consist of neutral glass plates, or filters, the density of which is progressively greater from one end to the other.
These observations conform to the general theory of phototropic orientation. According to this theory it is the difference in intensity reaching the two sides of the eye (one towards the light, the other away from it) which is effective. Orientation is brought about by the differential stimulation by the light of the bilaterally symmetrical organ of light reception. The consequence is that differential changes in the posture of the two swimming appendages on the two sides of the body are produced. As a result the animal is turned until equal amounts of light are received by both sides of the eye and orientation is complete. Experiments using two sources of light confirm this theory. (Cf. Crozier, 1928 b; and Mitchell and Crozier, 1927–9.)
A similar type of orientation was found by Cole (1901) in Pycnogonids.
The primary sign of phototropism was previously defined as that sign which the organism exhibited under constant illumination. Since it is now known that the sign of photo tropism sometimes becomes temporarily reversed (apparently “spontaneously”), the definition should be modified as follows : The primary sign of phototropism is the dominant or most persistent sign exhibited by the individual under constant environmental conditions.
An exhaustive summary of observations of this type on all phototropic animals is given by Rose (1929).
The temporary reversed sign of phototropism has been defined as the secondary sign. Formerly it was stated that the secondary sign was exhibited only following a sudden change of light intensity, but now it appears that this temporary change of sign can occur “spontaneously” under constant environmental conditions.
Other tests made at different times of day revealed no change in phototropic behaviour which could be assigned to a diurnal rhythm.
A similar situation has been found in Chiton (cf. Crozier and Arey, 1918).
This fact explains why Daphnia is not stimulated by a low gradient “place-change”—a question raised previously (Clarke, 1930; §6).
It is interesting in this connection to compare the theory of the excitation-time characteristics (Chronaxie) of irritable tissues, an account of which is given by Lapicque (1926).
For discussion of the special meaning of the term as used here, see Clarke (1930).
It was stated previously that when once the stimulus had taken effect, the response generally proceeded at a maximum rate and continued to a maximum magnitude. Further investigation has shown, however, that the speed and the magnitude of the response vary according to the nature of the stimulation.
For other investigations of the relation between the stimulating efficiencies of intermittent and continuous bght see Parker and Patten (1912-13), and Mast and Dolley (1925).
A diagram is presented (p. 318) to show hypothetical paths of the different impulses from the two parts of the eye to produce their characteristic effects in the musculature. The scheme includes a “Schaltapparat” consisting of various cross connections which, if it existed, might explain the occurrence of periodic changes of sign and other phases of behaviour.
The possibility must be borne in mind that all the relevant processes may not occur in the receptors.
Cf. the phototropic mechanism in Ranatra described by Crozier and Federighi (1924 a).
Although there seems to be no serious objection to this hypothesis that the “sign” of the posture of the antennae is controlled by reciprocal innervation of their muscles, it should be tested in some way. If a “reversal of inhibition” could be demonstrated following subjection to strychnine, the evidence would be considerably strengthened (cf. Crozier and Federighi, 1924ft; and Crozier, 1927). Moore (1913) has already shown that strychnine produces a permanent negative phototropism in Diaptomus, but the effect of a reduction of light intensity under these conditions has not been tested.
A similar hypothesis was suggested by Crozier (1915) for the reaction to shading in Holothuria.
A similar situation in the case of the orientation of Dineutes has been reported by L. B. Clark (1931).
Hecht (1927) showed that in Mya “dark adaptation is a process in the course of which a sensitive material accumulates in the sense cell as the result of a chemical reaction…. The chemical nature of the process is further borne out by the fact that the speed of dark adaptation is affected by the temperature.”
It seems likely that orientation depends upon other reversible photochemical reactions distinct from, but similar to, the system which has been suggested for the other aspects of phototropism in Daphnia Such photochemical reactions, taking place in various parts of the periphery of the eye, would be controlled by the varying amount of light reaching each, according to whether the part of the eye concerned was turned away from or toward the source of light. The resulting responses would produce orientation.
