Acknowledgements I thank Dr. Don Abbott for seemingly endless patience and encouragement in the preparation of this paper, and for his valuable advice during the course of experiments. I also thank Dr. Robin Burnett for his help with statistical analysis. 8 Introduction The harpacticoid copepod Tigriopus californicus is a major inhabitant of pools in the highest intertidal zone on the rocky coast of western North America. These tide pools experience water exchange with the sea during only the very highest tides or in rough weather, and are otherwise wetted by splash and spray alone. Prelimminary observations showed Tigriopus to exhibit two general types of reactions to light: (1) the tendency to aggregate either toward or away from an illuminating source of relatively stable intensity, and (2) dramatic changes in the activity, swimming speed, and rate of turning in response to sudden changes in light intensity. Early field observations suggested that the distribution of Tigriopus within high tide pools is influenced by light intensity; the densest concentra¬ tions of copepods were seen in the most dimly illuminated areas. Published studies of the light responses in Tigriopus are lacking, but some other species have been examined. The harpacticoid copepod, Harpacticus fulvus, tends to congre- gate away from bright sunlight (Blum 1934). Several copepod species are known to orient to polarized light, including the harpacticoid Tisbe furcata (Umminger 1968). Finally, it is well-established that the vertical distribution of pelagic oceanic copepods fluctuates diurnally, and the animals respond to light intensity cues (Enright 1967, Backus 1965). The environment of Tigriopus californicus is quite different from the open ocean; it is extremely variable with respect to salinity, temperature, pH, dissolved oxygen concentration, and turbulence, and affords little capacity to buffer environmental changes (fgloff 1964). It is consequently easy to envisior adaptive advantages for particular responses to light under particular environmental stimuli, yet to date no study on the phototactic response in Tigriopus or of the effect of light intensity on the distribution of Tigriopus within high tide pools exists. Early observations also revealed that Tigriopus exhibits a specific "dart-and-freeze" response (a sudden increase in forward speed, followed immediately by immobility) when exposed to a sudden decrease in illumination. This behavior has not been reported in other copepods or high tide pool invertebrates, though many animals show a "shadow reflex" of some sort. Early observations also indicated clearly that Tigriopus behaves differently upon first exposure to a given light intensity than it does after a period of adjustment. The aim of the present research was to answer two separate questions: (1) How does Tigriopus californicus respond to sudden changes in light intensity?, and (2) How does a population of Tigriopus californicus distribute itself in a light intensity gradient? Response to Sudden Changes in Intensity of Light General Materials and Methods The response to changes in light intensity was assessed in terms of activity, as measured by (1) the percent of animals moving, and (2) the swimming velocity of moving animals. For the purpose of this research, animals swimming at speeds less than 2 mm/sec were considered stationary, To determine activity, experimental copepods were collected from a single moderate-sized tide pool at a tidal height of about eight feet above mean lower low water near the Hopkins Marine Station on Mussel Point, Pacific Grove, California, from April to June, 1977. During the course of the experiments, the salinity in this pool was observed to vary between 35 and 60 parts per thousand, and the concentration of dissolved oxygen between 7 and 11 ml 09/1. For each experiment, 25 to 40 copepods collected from the tide pool about twenty minutes beforehand were placed in a pyrex petri dish 14 cm in diameter filled with freshly filtered sea water at a depth of 3 mm. The petri dish was evenly illuminated from above at intensities of up to 2.0 watts/m2, as measured by a Lambda Instrument Corporation Model LI-185 photometer held just below the petri dish. The light originated from four 12 volt tungsten filament bulbs at a distance of 15 cm above the dish, and was passed through one sheet of ordinary white paper before entering the petri dish so as to simulate the diffuse overhead lighting of a tidepool on an overcast day (fig. 1). The copepods were left in darkness in this set-up for fifteen minutes before any experimental light stimulus was presented. The movement of the copepods was followed by high speed photographs made at 0.5 second intervals with a Nikon F-2S camera and a MD-2 motor drive attachment mounted 40 cm below the petri dish, and using available light. All copepod velocities were calculated by projecting the sequence of negatives through a photographic enlarger and tracing on paper the image of the copepods shown in each negative. A grid above the petri dish permitted tracing successive images in exact register, thereby indicating the position of each individual copepod at 0.5 second intervals. Prelimminary observations indicated no significant difference in the swimming behavior of males and females. Consequently, the more clearly visible ovigerous females were used in all experiments unless otherwise indicated. Since the entire petri dish was not visible in the photographs, the swimming speeds of copepods very near the edges or otherwise out of the field of view were not measured. Swimming velocity, in each case, was calculated from measure¬ ments made directly from tracings of the successive images of the same animal; all measurements were made from the anterior end. When motion was too slight to permit measurement every half second, measurements were made as often as possible. All experiments were performed at 2111 C and in a salinity of 35 ppt. Experiments How Does Tigriopus Respond to the Onset of Illumination? Before determining the effect of any light stimulus on Tigriopus, it was first necessary to distinguish the effect of turning on the light from the effect of the light itself. An experiment was performed, employing the procedure described in "General Materials and Methods," and using an intensity around 1.1 watts/m. The light stimulus was presented, and activity was measured immediately and at intervals for nearly two hours thereafter. The results are shown in figure 2. When Tigriopus is exposed to light after a period of darkness, a marked decrease in activity occurs immediately, reaching a minimum within two minutes, from which the animals recover to previous activity levels. Abrupt illumination has been found to arrest the swimming movement of other copepod species as well (Rose 1925). fifteen Levels of activity do not change rapidly after minutes, and this may represent short (5min.) the state of activity characteristic of a given intensity after a period of adjustment. Does the Response to Illumination Depend on the Past Light History of Tigriopus? Before comparing the activity of Tigriopus at various light intensities presented in sequence, it was necessary to establish that the light response does not depend on thehistory of illumination of the experimental animals. The experiment illustrated in figure 3 compares the response of a group of copepods after two identical exposures of light separated by fifteen minutes of darkness, using the basic procedure given in "General Materials and Methods." The response was essentially similar in each instance; the animals started at the same level of activity, displayed a sudden decrease in activity of the same magnitude followed by a gradual increase, and reached the same level after fifteen minutes of illumination. The response to light considered here, then, does not depend significantly on the past light history of Tigriopus, at least beyond the last fifteen minutes. Does the Response to Illumination Depend on Intensity? In order the role of intensity in the light response, a group of randomly sexed copepods were exposed sequentially to four light intensities from 1.95 to 0.05 watts/m', in order of decreasing magnitude, preceded in each case by fifteen minutes of darkness, following the basic procedure given in "General Materials and Methods." The results are shown in figure 4a and b. The qualitative nature of the response is the same in each case, but there are some important quantitative differences related to intensity. The activity is shown to increase on balance with light intensity, when compared after fifteen minutes exposure to light (figures ba and b). This type of behavior, where a locomotory response increases with light intensity, has been observed in Branchipus (Haidinger 1844), and Daphnia magna (Koller 1928). The variation in swimming velocity is large, and skewed toward low speeds, approximating a Poisson-type distribution, as might be expected by the periodic pauses and bursts of swimming characteristic of Tigriopus. Studies in other invertebrates suggest that responses to change in light intensity may be governed largely by the sensitivity of the eye over various intensity ranges and the capacity of the eye to adapt to intensity (Fraenkel and Gunn 1961). The fraction of Tigriopus affected by the stimulus is indicative of the animal's sensitivity to that stimulus. When the activity immediately following exposure to light (time zero), M, is compared with the activity at maximum response (minimum activity), M, a measure of percent effect is obtained, which corresponds to the absolute sensitivity of Tigriopus to that light intensity, as shown in figure 6. An area of heightened sensitivity to light occurs around 0.35 watts/m, where the product of intensity and relative sensitivity to light (S, the relative response/quanta of light stimulus) reaches maximum. The response/quanta is greatest in dimest light. probably a result of adaption. The sensitivity to light has been shown to exhibit various minimum intensities in other crustaceans with median eyes: 0.4 watts/m' in Daphnia magna (Heberdey and Kupka 1942) and between 1 and 10 watts/m in Balanus improvisus (v. Buddenbrock 1930). The degree to which Tigriopus resumes time zero levels of activity following the maximum response is indicative of the animal's ability to adapt to changes in light intensity. If the magnitude of the response (difference between the mean swim¬ ming velocity at time zero and at maximum response, i.e. V -V is compared with the magnitude of the recovery (difference between the mean swimming velocity at maximum recovery within fifteen minutes and at maximum response, i.e. V... -V ), a measure of the ability to adapt to intensity is obtained (figure 7). There are two salient features to this relation: (1) Tigriopus adapts more readily to dim light than strong light (perhaps explaining the greater relative sensitivity, to dim light), and (2) there is a region of "negative adaption," at higher intensities where the recovery is larger than the response. How does Tigriopus respond to increases in illumination from below? In order to answer this question, the procedure described in "General Materials and Methods" was followed with these exceptions: (1) the petri dish was illuminated from below at intensities from 4.5 to 7.0 watts/m, as measured directly above the petri dish, (2) the light originated from two 12 volt tungsten filament bulbs about 15 cm below the dish, (3) photographs were taken with the camera mounted about 40 cm above the dish, (4) randomly sexed animals were used, and (5) animals were collected and left in darkness for fifteen minutes as before. After this per¬ iod, the copepods were exposed to a light intensity of 4.5 watts/m2. No photographs were taken at this time. After fifteen minutes, ten frames of film were exposed at two frames/sec. After the third frame was exposed, the light intensity was increased to 7.0 watts/m'. The results are presented in figure 8. Upon increased illumination, the swimming velocity of the copepods did not change much; however the amount of turning increased markedly, and then attenuated with time, This response provides suggestive evidence for the involvement of klinokinesis in the orientation of Tigriopus to light, in the scheme of Fraenkel and Gunn (1961), where the rate of change of direction is a function of light intensity. The observations made of Tigriopus are similar to those obtained from the klinokinetically orienting planarian Dendrocoelum (Ullyott 1936), except that the magnitude of the response is up to fourteen times greater and the speed of the res¬ ponse is up to 250 times as rapid; Tigriopus resumes ordinary swimming behavior well within one minute after the increase in intensity. Since the attenuation is in part a function of sensory adaption in the eye, the behavior suggests a particularly rapid ability of Tigriopus to adapt to changes in illumination, even when compared with the adaption times measured for other crustaceans: 6-10 minutes in Palaemon serratus and Praunus flexusus, 3 minutes in Pandalus montaqui (deBruin and Crisp 1957), and 3-5 minutes in Ligia (Ruck and Jahn 1954). How does Tigrionus respond to Sudden Decreases in Light Intensity? Tigriopus californicus in the water column of tide pools dart downwards for a brief period of time when a shadow is cast upon them, become immobile, and then swim up from the bottom in great numbers. In order to study this response in the laboratory, the basic procedure given in "General Materials and Methods" was used. After a fifteen minute exposure to an intensity of 1.5 watts/m, the light intensity was decreased to 0.05 watts/m', photographing the response of Tigriopus continuously at two frames/sec. The response, measured in terms of activity, closely parallels field observations in time course (fig.9). There are three distinct phases to the response: a "darting" response, characterized by a marked increase in activity over 0.5 seconds, a "freeze" response in which a four-fold decrease in activity occurs within 2 seconds, and a gradual increase in activity that immediately follows. It was noticed that Tigrionus must be exposed to some light for a brief period before any response to shadow can be observed, the length of time required being longer at low intensities, emphasizing the role of adaption in responses to light. Extended periods in the dark have been found to eliminate the shadow response in Balanus for some time as well (v. Buddenbrock 1930). The"dart-and-freeze" response observed may be of special ecological significance to Tigriopus, in the form of an escape response. This type of behavior has been observed in some other animals as part of a predator-escape mechanism, although it is usually associated with camuflaging and is not specifi¬ cally a response to shadow. It is possible, however, that Tigriopus "darts-and¬ freezes" in response to the shadow of a large predator, such as Pachygrapssus. It is also possible that the shadow response is used to avoid being washed to sea during a high tide or rough weather. When the sun is at a small angle above the horizon, waves can cast shadows into tide pools. In fact, any turrbulence in tide pools can produce changes in the pattern of illumination and shadow that may elicit a response from the copepods. Tigriopus is positively geotactic and negatively phototactic in response to turbulence (Foster,and Burnett, personal communication). The quick downward movement characteristic of the shadow response may serve to enhance these taxes and permit Tigriopus to avoid turbulence by aggregating in dark cracks and under rocks on the bottom of tide pools. The value of the subsequent gradual increase in activity after immobility is uncertain. Distribution in a Light Intensity Gradient Materials and Methods In order to determine the distribution of Tigriopus in a constant light gradient, the density of copepods was assessed at various points along a plexiglas trough exposed to horizontally increasing light intensity. A horizontal gradient was chosen in order to clearly discriminate between phototactic and geotactic responses. The illumination was provided by a standard 43cm 15 watt flourescent bulb above and parallel to the trough as shown in figure 10a One end of the light source was covered by several layers of neutral density screen and cardboard tubing; the resultant gradient is shown in figure lOb, as measured with a Lambda Instrument Corporation Model LI-185 photometer held as near to one cm below the surface as possible. For each set of experiments, about 200 randomly sexed Tigriopus were collected as described in earlier experiments. The copepods were placed in the trough, distributed evenly, and left in darkness for fifteen minutes, at which time the light gradient was presented. The distribution of animals was not observed to change appreciably after fifteen minutes exposure to the gradient. Consequently, the distribution recorded between 15 and 45 minutes were taken to be indicative of phototactic behavior. Tigriopus density was determined by looking through the unpapered side of the trough and counting the number of copepods visible in ½x cm quadrats at various positions along the length of the gradient, one cm below the surface. This depth represented the population distribution within the entire trough, since the relative densities of Tigriopus did not appear to vary with depth. Animal density 10 was determined by correcting for the volume counted. For each set of replicate experiments, the same animals were used, collected in a fine screen, and reintro¬ duced to the trough prior to the experiment. In order to correct for variations in the total number of animals used in comparable experiments, the distribution was expressed in some cases as the percent of total animals counted occuring in each of nineteen 2 cm segments along the trough (figure 10a). In order to study the effect of dissolved oxygen concentration on phototaxis, three preparations of filtered sea water were used in the trough: water water water N2-bubbled, aerated, and 0,-bubbled, with respective dissolved oxygen concentra- tions of 0.7, 6.6, and 14+ ml 0,/1 before the experiment, and 4.0, 6.5, and 14+ ml 0,/1 afterwards, as measured with a Yellow Springs Model 54 oxygen electrode. All experiments were performed at 2111 C and in a salinity of 35 ppt. Experiments How do Tigriopus distribute in the trough in the Abs ence of a Light Gradient? When placed in the trough in aerated sea water under even illumination, ed copepods demonstrat no preference for one end of the trough or the other after fifteen minutes (fig 10c). How do Tigriopus distribute in the trough in the Pres ence of a Light Gradient? How does Dissolved oxygen concentration affect this Distribution? In order to answer these questions, the distribution of Tigrionusin the gradient was measures at three different concentrations of dissolved oxygen. For each concentration, several replicate experiments were performed. The results of each experiment are shown in figure 11. A number of observations were made in each case, and the results in figure 11 represent the means for observations recorded between 15 and 45 minutes. 11 The mean values for all observations are shown in figure 12. It is evident from these experiments that (1) Tigriopus is positively phototactic at these intensities over a broad range of dissolved oxygen concentra- tions, and (2) the degree of positive phototaxis is greatest at low oxygen con¬ centrations, where the population distribution is more visibly skewed toward the light. A measure of the phototactic tendend, is given by the slope of the regression line for relative copepod density as a function of light intensity because (fig. 12). For this measure, two cm at each end of the trough were excluded copepods were found to accumulate at both ends simply because of the associated decrease in the number of available swimming vectors. While the data gives a reasonable fit to a straight line, it is possible that a nonlinear function more nearly describes the actual behavior. The slopes of the regression lines are compared with dissolved oxygen concentrations in figure 13, General Discussion From the studies on Tigriopus californicus, it is clear that experimental light conditions are highly artificial. However, laboratory experiments have revealed aspects of the phototactic response that cannot be studied separately in the field. In the laboratory, attempts were made to simulate natural lighting as nearly as possible; however, conditions differed from those in the field in three major respects: (1) the intensity range, (2) the degree of directionality, and (3) the degree to which the light intensity attenuates with distance. Intensities in most experiments ranged from less than 0.04 watts/m? to 2.3 watts/m', as compared with light of 100 times greater magnitude striking the surface of a tide pool on a bright day. The intensities studied, however, are by no means completely unnatural, since: (1) the range observed in the field extends from less than 0.002 watts/m at night to around 200 watts/m on a bright day, and the range of intensities studied is well within this, (2) at least half the time, tide pools are more dimly illuminated in the field than under conditions studied in the laboratory ,(3) at least two times a day, at dawn and dusk, tide pools are exposed to the intensities studied, and (4) the sensitivity to light in crustaceans can extend well below the range of intensities studied, to 10 2 or 10 watts/m (Waterman 1939). On a clear day, parellel light rays from the sun clearly provide a directional source of illumination to tide pool inhabitants. Light from a laboratory source is not composed of parllel rays, and consequently may present a different kind of stimulus to an experimental animal. In order to distinquish a response to a to light intensity gradientfrom a response to a directional light source in Tigriopus, attempts were made to provide as nearly a nondirectional source as possible; however, the at sence of directionality was in no way completely acheived. Since light intensity decreases inversely with the square of the distance from the source, the intensity of light at the surface of a tide pool is very water nearly the same as the intensity reaching the bottom, unless the tide pool is very turbid. In the atsience of vertical intensity gradients, copepods may orient to horizontal ones produced by shadows. In the laboratory, however, light intensity attenuates rapidly through a distance of water. Consequently, a horizontal gradient was used in these studies to more closely approximate what Tigriopus might encounter in nature. From these studies, there is evidence that kinetic and tactic mechanisms are both involved in the response of Tigriopus to light, that Tigriopus can exhibit 13 both positive and negative phototaxis, and that each may be important to its behavior in the field. The increase in swimming speed and activity associated with increases in light intensity, and the apparent dependence of activity on light in general suggest that orthokinesis may be involved in the light response. The increase in turning upon increasing illumination indicates that klinokinesis may be involved as well. At least, there is the suggestion of a kinetic component to the light response. If the light response was exclusively kinetic, then the evidence collected would predict Tigriopus to show negative phototaxis up to at least 2.0 watts/m’, The copepods would cluster in dimly illuminated areas where they were least active and where an approach of the margins stimulated turning. In the trough, however, Tigriopus is attracted to the brightest end, making the inclusion of a directed taxis in the mechanism of the light response unavoidable. The frequent observation of Tigriopus in aquaria congregating away from bright sunlight in the aks ence of an intensity gradient further establishes the necessity for a directed taxis in the light response mechanism. In fact, light-directed swimming can clearly be observed in the phototactic response of Tigriopus to high salinities (positive phototaxis), turbulence (negative phototaxis), and environmental anoxia (positive phototaxis). It is quite possible that both kinetic and tactic mechanisms are involved in Tigriopus. A kinetic component to a phototactic response has been studied before in the mussel crab Pinnotheres maculatus say (Welsh 1932). While the kinetic and tactic components of a locomotory response are ordinarily inseparable, there is some indication that they act independently to some extent in Tigriopus. The "dart-and-freeze" response to shadow in Tigriopus 14 is clearly directional in the field, but in experiments using diffuse light, swim¬ ming is undirected; the response observed is purely kinetic, and yet in that aspect it follows closely the time course of the field response. Tigriopus also appears to be both positively phototactic at low intensities and negatively phototactic at high intensities. This behavior has been reported in another copepod species, Labidocera (Herter, 1927). Other investigators have found a progressively stronger positive phototaxis in Tigriopus to overhead lighting as intensity is increased to 16 watts/m that diminishes at higher intensities (H. Townsend, pers. comm.). The significance of this trend may lie in the control of diurnal migration, or in the capacity of Tigriopus to seek a preferred isolume that is optimal in some respect to its life history. Observations of the copepod aggregated along a shadow interface suggest that this possibly occurs (R. Burnett, pers. comm.). Summary Tigriopus californicus, a high tide pool copepod, shows several separate responses to light. Dark-adapted T. californicus demonstrate a marked decrease in activity immediately following the onset of illumination, reaching a minimum after 1-2 minutes. Quantitative analysis of the light response indicates: (1) T. californicus exhibits a heightened sensitivity to light at an intensity of 0.35 watts/m'. (2) dark-adapted T. californicus more readily adaptsto dim light than strong light, (3) the swimming velocity and activity of T. californicus increases with the intensity of light exposed to. I. californicus exhibits a specific "dart-and-freeze" response to shadow which may involve independentkinetic and tactic reactions, and which may be an 15 escape mechanism. T. californicus responds to an increase in illumination from below by a marked increase in the rate of turning which attenuates with time. T. californicus exposed to a horizontal gradient of overhead lighting extending from 0.008 to 2.3 watts/m exhibit positive phototaxis in still water over a broad range of oxygen concentrations. The magnitude of this taxis increases as the level of dissolve oxygen decreases. Comparison of the intensity-dependent activity of T. californicus and its distribution within a horizontal light gradient indicates that the mechanism of the light response involves both perception of light intensity and direction. T. californicus demonstrate both positive phototaxis at low light intensities and negative phototaxis at high intensities. References Backus, R.H., R.C. Clark, and A.S. Wing. 1965. "Behavior of certain marine organisms during the solar eclipse of July 20, 1963." Nature. 205:989-991 Blum, H.F. "L'Orientation du Copepode Harpacticus fulvus sous l'influence de la lumiere." Arch Int Pysiol. 38:1-8. 1934 deBruin, G.H.P., and D.J. Crisp. 1957. "The influence of pigment migration on vision in higher Crustacea." J. Exptl Biol. 34:447-463, cited in Waterman, T.H., The Physiology of the Crustacea, Academic Press, New York (1961) Egloff, D.A. "Biological aspects of sex ratio in experimental and field populations of the marine copepod Tigriopus californicus." 1967. PhD dissertation. Stanford University Enright, J.T., and W.H. Hamner. 1967. "Vertical diurnal and endogenous rhythmicity.' Science. 157:937-941 Fraenkel, G.S., and D.L. Gunn. The Orientation of Animals. Dover Publications, Inc. New York. 1961. Haidinger, W. 1844."Ueber das direkte Erkennen des polarisierten Lichts und der Lage der Polarisationsebene." Ann Pysik Chemie. 63:29-39, cited in Waterman, T.H., The Physiology of the Crustacea, Academic Press, New York (1961) Heberdey, R.F., and Kupka, E. 1942."Helligkeitsunterscheidensvermoegen von Daphnia pulex? Z. vergleich Physiol. 29:541-582, cited in Waterman, T.H., The Physiology of the Crustacea, Academic Press, New York (1961) Herter, K. 1927. Taxien und Tropismen der Tiere? Tabulae Biol. 4:348-381., cited in Waterman, T,H., The Physiology of the Crustacea, Academic Press, New York (1961) Koller, G. 1928. Versuche über den Farbensinn der Eupaguriden. Z vergleich Physiol, 8:337-353, cited in Waterman, T.H., The Physiology of the Crustacea. Academic Press, New York (1961) Rose, M. 1925. Contribution a l'etude de la biologie du plankton? Arch zool exptl et gen. 64:387-542, cited in Waterman, T.H., The Physiology of the Crustacea, Academic Press, New York (1961) Ruck, P., and Jahn, T,L. 1954. Electrical studies on the compound eye of Ligia occidentalis Dana (Crustace: Isopoda). J Gen Physiol. 37:825-849, cited in Waterman, T.H., The Physiology of the Crustacea, Academic Press, New York, (1961) Umminger, Bruce L., 1968. "Polarotaxis in Copepods II. The Ultrastructural Basis and Ecological Significance of polarized light sensitivity in Copepods.' Biol Bull, 135: 252-261 The behavior of Dendrocoelum lacteum I—Responses Ullyott, P. 1936. J Exp Biol, at light-and-dark boundaries. II—Responses in Nondirectional gradients." 253-278, cited in Fraenkel, G.S., and D.L. Gunn, The Orientation of Animals, Dover Publications, Inc., New York, 1961. v. Buddenbrock. 1930. Untersuchungen über den Schattenreflex.“ Z vergeigh Physiol. 13: 164-213, cited in Wateman, T.H., The Physiology of the Crustacea, Academic Press, New York, (1961) Waterman, T.H., Nunnemacher, R.F., Grace, F.A., Jr., and Clarke,G.L. 1939. "Diurnal migrations of deepwater plankton." Biol Bull. 76:256-279, cited in Waterman, T.H, The Physiology of the Crustacea, Academic Press, New York, 1961 Welsh, J.H. "Temperature and Light as factors influencing the rate of swimming of larvae of the mussel crab, Pinnotheres maculatus say." Biol Bull 63:310-326, 1932. Figure 1: Apparatus for measuring Tigriopus activity, as viewed from the side. 18 Ove + ——4— — + 1— S 8 8 8 — + — — — — — — 8 8 2 E 8 —+ — —+ 8 L —— — — X — + HE —L 1 — H — + + E — —— l —— — t — — Figure 2: Response of Tigriopus to the onset of illumination, as measured in the percent of the population moving. Animals are kept in dark for fifteen minutes (dashed line), and then the light is turned on at an intensity of 1.l to 1.3 watts per meter“. 20 2 — -+—-— — —— — -— - â X +t 2 X 2 — S 3 —1— — Ho S 8 — t + — 8 — L o 5 0 2 + 1 — — + 8 HOIENO 4 — —+ t â ++ Ht ttit Figure 3: Response of Tigriopus to two sequential light stimuli, preceded each time by fifteen minutes of darkness (dashed lines). The response is measured in terms of percent of population moving. 22 5. 2 — X 8 — H t + + S S s P 3 2 J 8 + + ++ 8 S — + S ++ 8 E EHA — S Le + — + — Figure 4: a. Response of Tigriopus to four sequential light stimuli of decreasing intensity, preceded each time with fifteen minutes of darkness: A--1.95 watts/m, B--1.05 watts/m, C—-0.58 watts/m, and D—0.05 watts/m. An illustra¬ tion of response magnitude is shown by the double-headed arrow in response A. The percent population moving is given by My at at maximum re- the onset of illumination (time zero), and by M sponse. b. The same experiment as in Figure 4a, but with results given as mean swimming velocity for moving Tigriopus (O) and mean swimming velocity for all Tigriopus observed (0). The response magnitude and recovery magnitude are illustrated by double-headed arrows in response B. The mean swimming velocity of the population is given at time zero by V, at maximum and at maimum recovery within fifteen minutes response by V by V, as shown in A. CCNTIMETCR — — E — — — + — t t + 8 X + + + — 8 1 X ut S — 8 2 S 8 8 8 8 S 8 ++ t 8 S — S —1 — Ro — — -0 o 0 8 4 20 O 5 8 S — — + 21 — E — — — 8 H S 2 t — — a 1 8 — S S 4 —++ S tttittt O S 8 2 3 + 90 A V 2 +++ —— — 2 Figure 5: a. Histogram showing the distribution of the swimming velocities of Tigriopus fifteen minutes after the onset of illumination. The percent of all copepods swimming within 6 mm/sec class intervals is shown for the four intensities studied. b. Activity given as a function of light intensity in terms of percent of population moving () and mean swimming velocity of moving Tigriopus (0). 28 2 OCIA MMNEVE S - 1 qt WA -1 S0 7 —— — — S — S D — S — — t — . + — ( E X +++ 8 — 7 S + — + + 8 S S + — +— + t —— — — — — — + — 2 — —3 — t + 2 + + O LIGNT INTENSITY (WRTTSN m 23 Figure 6: Sensitivity of Tigriopus to light at various intensities, as measured by the response to the onset of illumination. Absolute sensitivity is given by the magnitude of the response, as measured by percent effect, E, where: (see Fia ta, page 24 E S x 100. Relative sensitivity to light, S, is given by absolute sensitiv¬ ity corrected for intensity, I, in terms of relative percent effect: S-E/I. This is a measure of the relative magnitude of response per quanta of light stimulus. This adjusts for the fact that at higher intensities, there is more light to produce an effect. t 1 — — — + E —1 — --- * + — — — — 140 — X X 1 + — 1 — —— — — O — — — + — 8 509 +: 17 . + 0 1.0 — 2 1 GHEITERST A â — Figure 7: Tendency to adapt to light at various intensities as measured by the ratio of the response upon illumination to the recovery. Absolute tendency to adapt, A, is given by: (xsee Ha 4b, page 241 A = ( Vmr X 100. rec mr The line where Recovery equals Response is shown with a dashed line (A=100). Relative tendency to adapt is given by (A-100)XI where I is the intensity of light. This corrects for the fact that low intensities represent a less radical shift from darkness than higher ones and adaption occurs more readily. a — —— 1 - pf . 9 O + X S — + — —— — — 2 X —1 t —— Se REO 0 — â — — —1 110 1 + — — S t + — + — 11 20 8 — LGAE INIENS + WE 32 Figure 8: The response of a typical copepod to increased illumination from below. The intensity was increased from 4.5 to 7.0 watts/m. The trace of the swimming path during this time is shown, withthe interval between successive views of the same copepod being 0.5 sec. The angle of turning is indicated. 34 T + — -3— S X + — 7 2 — — 4 21 — i 41 1 X — — + — —+ 5 S S 3 A NTENSH — — Si 4 + — +— t 1 7 - t Figure 9: a. Response of Tigriopus to a sudden decrease in light intensity. After fifteen minutes of darkness (dashed line), the light is turned on at an intensity of 1.50 watts/m for fifteen more minutes. The intensity is then decreased to 0.05 watts/m. with time scale b. The same response as in 9a, only, magnified 12 times, in order to show critiçal areas of response. —withhe se c. The same response as in 9a,(magnified,72 times. t t + —+ 0 — — 40 1 S — —+ 8 + + — + — a — — — 0 4 a S — 8 8 2 + 8 +0 — — + — S E 5 0 0 DER S Ss — t 36 0 + â — — — — + — 2 t 8 + —+ 8 S +++ + 2 t ++ t — + + — + 4 + —+ X — + — — 0 + 4 8 — 1 0 tt 10 3 + ++ 38 — — . — - + S — 8 po 6 + SHWIF —+ — + + X — 8 3 ++ 1 — — S 0 -+ P + ++ — Figure 10: a. Trough used in distribution experiments. b. Light gradient produced in the trough. c. Distribution of Tigriopus in the trough under even illumination. 40 38 2 8 — - + + — 0 S 5 — + 8 E S + H tttft ftttt — — + H — + 8 S — S — E 8 X —4/ + — +++ Dg — + — ARE — — ENR Figure 11: Results of trough studies. The Tigriopus distribution is indicated by the mean copepod density at various points along the trough. A graph is given for each trial, with the indicated number of observations made The points represent the mean for all observations within a trial, and the standard deviations are shown with vertical bars. a. N,-bubbled water b. Aerated water c. O,-bubbled water 42 e a + + 1 ++ I 7 Be - . 2 S S Mabs 8 S -1— + — . + X 8 2 + 8 X tt Il 2 8 t — +1— ++ S + 8 t S X — + 4 + 4 — S E + E —+ t 5 + . + + e ++ 1 + — — tt S +t 3 + 2 + S —E 8 8 — HEEN O 42 + 8 + + t X et 4 Beteton — 8 E 8 + E 4 44 r — Eepal JE 8 8 + + — S + 8 8 + 8 OE 24 + DISTENE E — — — kakaaa- + — + + Figure 12: The mean values for all observations as a function of distance from the brightest end of the trough and light intensity. In the top graph, the range and standard deviations are indicated. A regression line is drawn for the lower graph, excluding the observations made within two cm of each end, which are indicated by (0). The Pearson correlation coefficient is given for each regression with the probability, p. that there is no correlation. 46 — N 4 — 2 — — ——— —+ — + — 8 — t S 8 + S — — — —— + L — + S a 7 t 4 E 91 E D —— —: Dr —1 Fp E —++ E S — 8 + — — H — — — X + tt + — 9 + 2 S - + 1—1— O — — Ht 8 8 t S - 2 S + 8 ++ 8 8 E 4 + + 8 S 8 — t — + + + 9 —1 + 8 + 8 8 0 +- — 2 4 2 17+ — + — p — 4 Figure 13: Concentration of dissolve oxygen in the trough water ys. the percent population in the brightest two cm of trough (O). Concentration of issolved oxygen in the trough water vs. the slope of the regression lines calculated for figure 12 (). S o — — 8 + — — + 2 S — — S 5 S S — + X S 2 t — 50 5