tt AOOITIONAL INORMATION, IE AN, CONCERNING AUTHORS. ADDRESS, TITLE, OR CITATION DATA Od PLEASE TYPE ABSTRACT DOUBLE SPACED BELOW MILLER, ALAN C. (Hopkins Marine Station of Stanford University at Pacific Grove, Calif., USA.) Orientation and movement of the limpet Acmaea digitalis (Gastropoda: Prosobranchia) on vertical rock surfaces. The Veliger. —-Acmaea digitalis, at low tide, on dry vertical intertidal rocks tend to orient with the head pointed downward and to the right (facing toward 3 to 6 o'clock). When splashed by a rising tide they turn. either right or left, and move about; no marked orientation tendency appears. On receding tides activity decreases and limpets again come to rest and tend to orient toward 3 to 6 o'clock. Changes in orientation reflect changes in direction of movement. Percent of animals moving, and average distance moved increase with increasing duration of the period of wetting. The population shows a net movement upward during higher high water periods, or during storms, and downward during lower high water periods; this tendency is more marked for high tides occurring in daylight than at night. Some animals display homing behavior. ---Author. PLEASE DO NOT TYPE BELOW THIS LINE Orientation and Movement of the Limpet Acmaea digitalis (Gastropoda: Prosobranchia) on Vertical Rock Surfaces by Alan C. Miller Hopkins Marine Station of Stanford University at Pacific Grove, California At Mussel Point, Pacific Grove, California, Acmaea digitalis Eschscholtz, 1833 is abundant on granite rocks above the Endocladia-Balanus zone (see Glynn, 1965). Pre- liminary observations showed that these limpets tend to orient with their heads facing downward on vertical or nearly vertical rocks during periods of low water. No pre¬ vious studies on orientation in Acmaea digitalis have been found. However, in studies on ciliary currents of Lottia gigantea Sowerby, 1843 by Abbott (1956), and the ecology of Acmaea dorsuosa Gould by Abe (1931, both authors found that the majority of these limpets were oriented head down- ward at low tide. Studies of the movements of Acmaea italis, which might help explain this orientating behavior are lacking. Previous observations of movement in this species have been related mainly to such matters as homing (Villee and Groody, 1940; Galbraith, 1965), or to shifts TTE 8 2 Alan C. Miller in the distribution of populations over extended periods of time (Frank, 1965). In the present study an attempt was made to deter- mine the orientation and movements of Acmaea digitalis on vertical rocks at various stages of the tidal cycle. and to gain some insight into the factors influencing them. FIELD STUDIES ON ORIENTATION An initial field study was carried out to determine the orientation tendency of Acmaea digitalis on vertical surfaces at low tide. Acmaea digitalis were observed on one large vertical rock, and the orientation of 136 lim¬ pets was recorded in terms of the position on a clock toward which the head of each was pointing. Animals with their anterior ends straight up were recorded as 12 o'clock, those with heads straight down at 6 o'clock, etc. The re¬ sults, shown in Figure 1, reveal a clear tendency, at low tide, to orient with the head downward from 4 to 7 o'clock. Later observations on other populations, including those in movement studies, confirm this tendency (Table 1). Young Acmaea scabra (Gould, 1846) on vertical surfaces appear to show a similar tendency, though detailed obser- vations are lacking. Upon finding an orientation tendency at low tide, the next step was an investigation of orientation during a tidal cycle. All the following field observations on or¬ ientation were made on one vertical granite rock face (see Table 4, rock 1). On this rock surface 152 Acmaea digital- Te 1 Alan C. Miller is were individually marked. The shell of each limpet was painted with a patch of yellow paint upon which an ident- ifying number was marked in India ink. Each limpet's or- ientation was recorded approximately every 3 hours during lower low water (LLW), lower high water (LHW), higher low water (HLW), and the intervening mid-tides, on 3 successive days. No reading was taken at higher high water (HHW) due to heavy surf. The results, shown in Table 1, clearly indicate that changes in orientation take place during the tidal cycle. The net change in orientation for each individual be- tween successive observations was then computed, assuming that rotation in each case was accomplished by turning the minimum number of clock units necessary to account for the change (Figure 2). During the first tidal cycle the pop- ulation showed more rotation upward with a rising tide and more rotation downward with a receding tide, but this was not repeated in the next two tidal cycles. The general activity of the population, as measured by the sum of the up and down rotation, and by the per cent of the population rotating (Figure 2), shows a peak associated with each high tide. The decrease in the height of these peaks of activity from one recorded high tide to the next is associated with a slight decrease in the heights of successive LHWs, and a resultant decrease in both the number of animals being effectively wetted and the dura¬ tion of the period of splash. Also, during the first re- corded LHW a fog bank blocked the sunlight, while on later 29 V1 F6.2 Alan C. Miller LHWs direct sunlight dried the rock surface sooner. The results shown in Figure 2 suggest that activity of the THE population is proportional tofdegree of wetting of the limpet population and the rock surface at high tide. Since at some LHWs the whole population was effect¬ ively wetted and at others only the lower limpets re¬ ceived significant splash, for subsequent high tide stud- ies the population on the rock face was divided into groups; a high group ranging from about 14 to 18 feet above mean LLW, and a low group located 9 to 12 feet above mean LLW. Hourly observations were made during a HHW and a LHW period. The results are shown in Table 2 and Figure 3. The amount of rotation and the per cent of the popu- lation rotating is clearly related to the amount of splash received. The graphs in Figure 3 perhaps explain the dif¬ ferences in rotational activity shown by the limpets during the three successive tidal cycles portrayed in Figure 2. In Figure 3, in the low group of limpets at the high tide at 2000 hours, about 30 animals were rotating; as a group they rotated a minimum of 105 clock units. Similar high amounts of rotation occurred all during the splash period. These results, combined with the results on direction of rotation presented in Figure 2, show that orientation and direction of rotation tend to be random during the time the rock is thoroughly wetted. This tendency toward random or- ientation was also apparent from watching the limpet move- ments during periods of splash. For the limpet population to shift from a primarily T Alan C. Miller head downward orientation at low tide to a more random, orientation on the rocks during high tide requires that some limpets rotate upward. To check whether there was a tendency for them to turn upward toward the right (counter- clockwise) or upward toward the left (clockwise), the data from Table 2 were used as follows. The limpet popu- lation was divided into left-oriented animals (limpets facing clock directions 7 to 11) and right-oriented an- imals (limpets facing clock directions 1 to 5). The limp- ets facing 6 o'clock and 12 o'clock were divided equally between right and left facing groups. The mean direction of orientation and standard deviation were calculated for both left and right facing groups for each period of ob- servation shown in Table 2. The results are given in Table 3. Some tendency to turn upward at the beginning of the observation period and downward toward the end is indi- cated, but there is no clearcut preference for either a clockwise or counter-clockwise rotation. Standard deviations are largest and show most variation during periods when the rock was splashed, reflecting the rather random or- ientation of the high and low Acmaea digitalis populations during the two tidal cycles. Rotational movement was great- est during HHW, and greatest for the low group of limpets which received the most splash. As the water level dropped and splash declined, activity of the population decreased and the per cent of limpets with heads oriented downward and toward the right, in the directions of 4 to 6 o'clock. increased. 29 Alan C. Miller While it is established that on vertical rock surf- aces Acmaea digitalis populations tend to orient with heads directed downward and to the right during periods of low tide, neither the selective advantages nor the causes of this behavior are er irely clear. The limpet Lot gantea orients much as does A. dig: talis at low tide, and Abbott (1956) has suggested that with the head down, water run-off coming down the rock from waye splash would help clean the nuchal cavity of waste ma- terials. He also points out that water trapped in the mantle groove as the tide recedes would drain toward the anterior end, thus keeping the head and ctenidium wet while the limpet is exposed at low tide. Abbott also suggests that an orientation with the head down and to the right might be slightly more advantageous than with the head down and to the left, since in the former case water running down the rock would tend to reinforce the existing ciliary currents in the nuchal cavity. In Acmaea, as in Lottia, the anal opening lies on the right side of the nuchal cavity, so waterrun-offon the rock could easily help remove the waste materials from a limpet orientated toward 3 to 5 o'clock. Orien- tation toward 7 to 11 o'clock may present more of a problem since the feces would have to travel a greater distance through the mantle cavity to get out. However. 29 Alan C. Miller according to Yonge (1962), and my personal observations, the ctenidium is flexible enough to be moved from one side of the mantle cavity to the other, carrying with it fecal material. This could transfer the fecal material to the opposite side where it could be washed out if the limpet were oriented to the left on the rock face. With reference to Abbott's suggestion about retaining water around the anterior parts at low tide, it was noticed that when the first splash covered A. digitalis facing downward on a dry, vertical surface, most animals raised their shells posteriorly and extended the mantle for less than a minute, permitting water running down the rock to flow under the shell toward the head. This activity occurred on successive splashes and slowly decreased; then the limpet usually moved. Animals facing upward on the rock sometimes raised the fronts of their shells at the first signs of water run-off, permitting water to run about the head. Almost nothing is known of the factors acting as immediate causes of particular responses and behavior patterns in Acmaea gitalis. Although limpets clamp down more tightly on rock surfaces in reaction to light rom a flashlight at night and to a shadow cast in the day, the present studies, showing that downward orien- tation can occur at night as well as during the day, suggest light is not a primary factor here. Laboratory experiments involving geotropism have not been carried out. Hewatt (1940), in tests with Acmaea scabra, found that geotropism did not seem to be a factor in homing 29 Alan C. Miller ability. There is certainly a good correlation between an increasing exposure to splash and an increase in activity, the latter resulting in a more random orientation on the rocks. Likewise, the decline of splash during a receding tide is correlated with increasing downward orientation. FIELD STUDIES ON MOVEMENT Observations made during the orientation studies suggested that changes in orientation during the period of wetting might be associated mainly with vertical movements. To determine whether this was the case, 25 Acmaea digitalis on vertical granite rocks were watched during night high tides and 25 during day high tides. Rocks 1, 2, 3, and 4 were used in these observations (see Table 4). A spot of paint was placed on the rock surface in front of each limpet's head and also on the peak of its shell; both spots of paint were marked with matching numbers in India ink. During the watches the vertical and horizontal distancesbetween the anterior portion of each limpet and its spot of paint on the rock were recorded every ten minutes. Two of the fifty individual movement paths are shown in Figure 5. The movement paths showed that moving animals were almost always oriented with the anterior end of the shell pointed in the direction of movement, but a change in orientation sometimes occurred with relatively little forward movement (Figure 6). Data on movement and rotation 29 F1G S 10 Alan C. Miller of limpets, collected on nine different watches, are presented in Figures 7 and 8; data are grouped for each thirty minute interval for each watch. The complex relationships between the various var¬ iables shown in Figures 7 and 8 are best considered in relation to specific questions. These are considered be¬ low, and pertinent data are summarized in Figures 9 th ough 14. First, is there a direct relationship between amount of movement and amount of rotation? A scatter diagram (Figure 9) of the data presented in Figures 7 and 8 in¬ dicates that there is a general relationship, but the cor¬ relation between movement and rotation is not a strong one. Second, is there initial movement upward (i.e., move¬ ment in the direction of 10, 11, 12, 1, or 2 o'clock with¬ in the first 30 minutes of motion) by the limpet population in either day or night? Such an upward movement might help explain a tendency to rotate upward as movement commences on a rising tide. In order to supplement the data from Figures 7 and 8, another similar watch -thogePen andeed was undertaken on rock 3 (Table 4), except the movements of the population were recorded for only the first hour of the wetting period during a LHW and the following HHW. The data from these ob- servations, presented in Figure 10, show that 50% or more of the limpets do show an initial upward movement. No sig- nificant difference between day and night is demonstrated. 29 6. 11 13 10 Alan C. Miller Third, is there a final movement downward, corres¬ ponding with the tendency to orient head down at low tide? The final individual movements and orientations for the 50 limpets depicted in Figures 7 and 8 are presented in Fig¬ ure 11. Most limpets do exhibit a final movement downward associated with a final head down orientation. Of those whose final movement was upward, some only changed direc¬ tion from, say, 6 to 4 o'clock. While this is an upward movement, the final orientation is still down, explaining why fewer limpets had a terminal head up orientation than exhibited final movement upward. Fourth, does the percent of limpets which moved during a given high tide increase with increasing dura- tion of the period of wetting? Data from the 9 watches in Figures 7 and 8 are plotted in Figure 12; they show that the per cent of animals moving does tend to increase as the period of wetting gets longer. The longer wetting periods occurred at night, so one can't adequately compare the results obtained during night and day. Fifth, are there relationships between the amount of time spent moving, the distance moved, the duration of wetting, and the time of day? Data for individual limpet movements appear in Figure 13. The results show that the distances moved and time spent moving tend to increase as the wetting periods get longer. Distance moved is roughly proportional to time spent moving. The animals were generally wetted for longer periods at night, so it is difficult to contrast the day and night results. 290 1 11 Alan C. Miller Six, do limpets tend to move more rapidly, on the average, during high tides at night than during high tides in daylight hours? The average rates of movement for all of the individual limpets used to obtain data for Figures 7 and 8 are summarized in Figure 14. The average rates for day and night differ by only 0.1 mm/min. In a "t test" applied to these results, t=1.31 with a probability of .10-.20 that the null hypothesis that there is no difference between day and night rate of loco¬ motion should be rejected. Therefore, the difference is not statistically significant. The higest rate encountered TIME at anyAin these studiés occurred during an uncompleted day¬ time watch when one limpet moved 13 mm/min during a 5 minute period. However, raves of locomotion well above average (i.e., 5-8mm/min) for short poriods on the part of individual limpets were more commonly observed at night than during the day. It was noticed in some watches that a majority of the population of limpets moving showed a net vertical displace¬ Frank (1164 Lund vnat verleir, manernit ment either upward or downward.. Two hypotheses which might account for this behavior were proposed: (1) local limpet populations may show a net upward displacement during HHW, and a net downward displacement during LHW; (2) the net dis¬ placement of the population upward or downward during a high tide may differ according to whether it occurs during the day or night. Sixty A. digitalis on vertical rocks 1,6,7,8,9, and 10 were individually marked and their positions recorded before . c.. F16 Alan C. Miller 12 and after each period of high water for several successive days. The per cent known to have moved to a new position (homing limpets and non-moving limpets had the same posit¬ ions) and the per cent showing a net upward displacement at each LHW and HHW period are recorded in Figures 15 and 16. The results clearly show that the per cent of limpets moving and showing an upward displacement consistently increases during HHW and decreases during LHW. Two factors which might be responsible for this behavior, duration of the period of wetting and time of the high tide, are con- sidered below. Is there a correlation between duration of the per- iod of wetting and the net upward (or downward) displace¬ ment of the population at high tide? Actual duration of II splash was not measured, but the periods of HHW and LHW were arranged in order of decreasing height of the tide (as actually measured on the tide guage operated by the U.S. Naval Postgraduate School, about one mile from the observation site). The per cent of the population showing net vertical displacement upward and downward (Figure 17, top), and the amount of vertical displacement upward and downward (Figure 17, bottom) were calculated for each high tide. While the method used to establish relative duration of wetting periods did not take into account variations due to differences in turbulence, the results show that, in general, the higher the height of high water, the larger the per cent of the population exhibiting a net upward dis¬ 300 13 Alan C. Miller placement. The amount of vertical displacement upward also tends to be greater during the longer periods of wetting and lower during the more brief wetting periods. The tendency of the population to respond as it does, moving up or down depending on the height of the tide, appears to provide a mechanism by which the population is maintained at a favor- able level on the rocks. Conspicuous upward vertical displacements may occur during periods of unusually heavy surf. One such instance occurred on rock 9 (see Table 4) between LHW on Sept. 18 and 71 LHW on Sept. 19; during a storm, 9 of 10 limpets observed moved up on the rock an average distance of 33cm (range - 4-78cm). These observations are similar to those of Frank (1965) who found that a population of Acmaea digitalis on the Oregon coast moved higher on the rocks during the winter an action apparently related to heavy surf caused by winter storms. However, in the present case the marked upward move- ment was observed to occur in less than 24 hours. Is there any difference in vertical displacement tendencies during high tides at night as opposed to those during theday? Pertinent data from the population shown in Figures 15 and 16 have been summarized in Figure 18. The tendency to move upward during HHW and downward at LHW is much more marked when the high water occurs during the day. The differences in behavior at LHW and HHW would probably have been more marked had it not been for two circumstances. First, rough surf occurred during the last two watches in Figure 16, for both HHW and LHW, causing 30 Alan C. Miller 14 longer than usual periods of wetting and also causing the population to move upward on the rock. Second, the height of LHW in the last watch was about 5 feet, and that of HHW only about 6 feet, so the poriods of wetting were of nearly the same duration for both high tides. It was only during LHW periods with a maximum height of 4.4 to 3.0 feet (Figure 17) that tendency toward a downward displacement with little upward displacement became marked. Approximately 25% of the limpets observed in the course of the present study showed a tendency to home, i.e., re- turn to a specific position and orientation on the rock. Galbraith (1965) reported finding homing in A. digitalis on relatively smooth rocks bearing ridges and numerous small depressions. Millard (1966) likewise found some A. digitalis homing on granite rocks at Pacific Grove. Villee and Groody (1940) also noted this tendency but preferred not to call it homing behavior. Frank (1965), in Oregon, reported no evidences of homing. CKNOWLEDGEMEN I would like to express my thanks to Dr. Donald P. Abbott who helped and advised me throughout this study. Thanks are due also to Galen Hilgard and Raymond Markel for advice and help. The research was supported by Grant GY8O6 from the Undergraduate Research Participation Program of the National Science Foundation. 30 Alan C. Miller 15 SUMMARY 1. At low tide Acmaea digitalis on dry vertical rock sur- faces remain in place and tend to orient with the head pointed downward and to the right (facing toward 3 to 6 o'clock). 2. When wetted by a rising tide, many limpets start to move; they tend to turn upward, either to the right or lef orientation of the population becomes more random as movements increase,Movement either up or down on the rock face may occur. 3. On a receding tide, as splash lessens, limpets on vert¬ ical surfaces show decreased activity and again tend to orient facing downward and to the right. 4. In general, the longer the period of wetting at high water, the larger the percentage of the population which moves, the longer the period of movement, the greater the amount of body rotation, and the greater the distance moved by the population. 5. The limpet population tends to move upward on the rocks at periods of higher high water, and downward at periods of lower high water. The tendency is more marked for high water periods occurring during daylight hours. With rough surf, the limpet population as a whole moves upward on vertical rock faces, and comes back down during ensuing periods of calm weather. 6. Approximately 25% of the A. digitalis observed showed homing behavior. 30 O Alan C. Miller 16 7 TTT DITERATURE CITED Abe, Noboru 1931. Ecological Observations on Acmaea dorsuosa Gould. The Science Reports of the Tohoku Imperial University. Series 4, 6: 403-427. Abbott, Donald P. 1956. Water Circulation in the Mantle Cavity of the Owl Limpet Lottia gigantea Gray. The Nautilus, 69(3): 79-87. Robert T. Galbraith, 1965. Homing Behavior in the Limpets Acmaea dig- gigantea. Amer. Midland itails and Lottia ge Nat. 74(1): 245-246. Frank, P. W. 1965. The Biodemography of an Intertidal Snail Population. Ecology, 46(6): 831-844. Hewatt, Willis G. 1940. Observations on the Homing Limpet, Acmaea scabra Gould. Amer. Midland Nat. 24(1): 205- 208. Glynn, Peter W. 1965. Community Composition, Structure, and Inter- relationships in Marine Intertidal Endocladia muricata-Balanus glandula Association in Monteray Bay. Beaufortia, Amsterdam, 12(148): 1-198. 30 Alan C. Miller 17 Millard, Carol S. 1966. The Clustering Behavior of Acmaea di talis. The Veliger........ Villee, C. A. and Groody, T. C. h Reference to 1940. The Behavior of Limpets wi Their Homing Instinct. Amer. Midland Nat. 4(1): 190-204. Yonge, C. M. ts in the Mantle Cavity of 1962. Ciliary Curren Species of Acmaea. The Veliger, 2(3): 119-123. 308 18 FOOTNOTES (page 1) Permanent address: Alan C. Miller 306 19 Alan C. Miller TABLE CAPTIONS TABLE 1. Orientation shown by a limpet population during the period from lower low water to higher low water on three successive days, May 3-5, 1966. - TABLE 2. Percent of limpets oriented in each clock direc- tion, during higher high water and lower high water, May 16-17, 1966. Condition on the rock surface: S-splash, W= damp, D-dry. TABLE 3. Mean orientations (clock directions) and stan- dard deviations for left and right-facing lim- pets during HHW and LHW, May 16-17, 1966, com¬ puted from Table 2. P7 TABLE 4. Description of rocks used in orientation and movement studies. Widths and heights give the size of the vertical surface available for lim- pet movement. Secondary waves occurred after the main force of the waves was broken by outer rocks. The height of the lowest portion of each rock above tidal datum is given in column 6. 30 C Alan C. Miller 20 FIGURE CAE TIOI Figure 1. Orientation of 136 Acmaea dig talis at low tide on a vertical rock surface. The per cent of the population with heads pointed in each clock direction, from 1 to 12, are: 3, 6, 9, 15, 20, 13, 15, 2, 2, 4, 2, and 9. Figure 2. Change in orientation in the limpet population shown in Table 1, during the period from lower low water to higher low water on three successive days, May 3-5, 1966. Figure Quantitative measures of changes in orientation of limpets high (14 to 18 feet) and low (9 to 12 feet) on a vertical rock face, May 16-17, 1966, based on data in Table 2. Duration of the period of splash groups of limpets high and low on the rock face for is shown just above the scale showing time of day. Time and height of high water are indicated by the black bars on the splash diagram and the numbers just above them. Cheareunderthelineonthe raph of splash-hi tingtherockshows whichgroups. (higher loware ming splashed.henthein thgroups arebeingetted. reassmamn, WHighlandowaaysftthehi ghgroupof limpets-andtothe lo groupespectively. 39 Figure 4. Figure 5. Figure 6. 21 Alan C. Miller Tendency of limpets to orient with heads downward and to the right during HHW and LHW, May 16-17, 1966. Dotted lines show percent of limpet population or- iented with the head downward (4 to 8 o'clock, plus half the individuals headed toward 3 o'clock and 9 o'clock). Solid lines show percent of limpet pop¬ ulation oriented with the head to the right (1 to 9 o'clock, plus half the individuals headed toward 12 o'clock and 6 o'clock). Time and splash diagrams are the same as those on Figure 3. Typical movement paths for Acmaea digitalis during high tide. The clockorier ation of the limpet (underlined figures) and the time at each partic- ular position are given. The starting position is indicated by the double circle. The limpet whose path is shown on the left returned to its original home site. Time lapse diagram of a moving A. dig talis showing the relationship between movement and change in orientation. Outlines show displacement at intervals of 3.33 minutes. Time and orientation are given as in Figure 5. The dotted line shows the actual move- ment of the head of the limpet (the apex of the trianglo). The solid line shows the movement path and the orientation that would have been recorded had the observations been made every 10 minutes as in Figure 5. 30 Kian 0. Milier 22 Figures 7 and 8. The relation between the per cent of the pop¬ ulation which changedtheir positions, the number of clock units rotated by all individuals, the per cent of the population which rotated, and the distance moved by all individuals together.in successive 30 minute periods, duringdifferont high tides, Results obtained during the day appear in Figure 7; those obtained at night are shown in Figure 8. Height of the population boxes (N) shows the total number of limpets watched in each tidal period; the darkened area in each box shows a cumulative total of the num- ber of these limpets which have moved at all at each successive per iod during the watch. The triangular tidal diagrams show only the time and height of the tidal peak and the points of initial and final splash. For each tide all limpets were at approximately the same level, and were subjected to the same conditions of splash. Figure 9. Scatter diagram of the clock units rotated and mm acu moved ing30 minute period for the limpets shown in Figure. 7 (circles) and Figure 8 (solid dots). 3/0 ALAN C. MILLER 23 Figure 10 The per cent of the limpets which showed an initial upward rotation during two high tide studies. N=25 for group B (calculated from data used in figures 7 and 8). In group A (supplementary dataj see xt), N=21 for the night and N=13 for the day, xsd os i Termisal Figure 11 rinel movement and orientation of Acmaea digitalis ferthehightide tides hes. N=25 for both day and night observations. Figure 12 The per cent of the population that moved during a high tide versus the duration of wetting. Circles give results for daytime high tides, solid dots give those for high tides at night. Figure 13 Scatter diagrams showing distance moved and time spent moving versus duration of wetting, and time spent moving versus distance moyed. Circles give daytime rates, dots give rates at night. Figure 1 Rates of movement of Acmaea digitalis during day and night high tides. N=25 for the day; N=22 for the night. Average rate for the day is 0.9 mm/min.; average rate for the night is 1.0 mm/min. Figuregl. Vertical displacement observations on a population of 10-60 limpets, and 16 for the periods: July 30-31, August 1-5, 9-10, 23-26, (figure 15): 70 broken September 5-8, 17-19; October 13-17, (figure 16). The detted line in both figures show the per cent of the population known to have moved at each high tide. The solid line indicates the per cent of the population which showed a net vertical displacement upward, (i.e. the per cent of the moving limpets whose terminal positions were higher than their starting positions). Hrepresents HHW; represents LHW; N is night; D is day; D and N represent dawn and 3 Alan C. Miller Figures 17 and 18. Net displacement upward or downward on the rock face during high tide, for the limpets whose movements are recorded in Figures 15 and 16. Fig¬ ure 17 shows per cent of population moving up or down (top), and total net displacement up or down (bottom), in relation to height of tide. Figure 18 shows per cent of population moving up or down (top), and total net displacement up or down (bottom), in relation to time of day for all LHW and HHW periods. The data from the last watch on Figure 16 Fioure were notused in as the correct tidal heights are not known. 95 DRY DAMP PLASI DAMP DRY DRY DRY SPLA DAMP DRY DAME DAMP DAMP SPLA DAMP DRY Perce Oriented 14 14 10 Limpe in Each Clock Direction Time 0430 0730 1030 1330 1600 0530 0830 1130 1445 1630 0300 0545 0830 1730 1900 Wlle 3/8 Hn O CLOCK DIR ECTION 7 Hlan C. Aller HIGH LIMPET PERCEI S OF LIMPETS ORIEN ED IN POPULATION =55 EACH CLOCK DIRECTION TIME 8 8 8 CONDITION OF S SSSSWW ROCK SURFACE 5 4 2 3 11 13 5 9 13 13 4 11 5 5 5 4 4 20 16 16 25 25 33 31 16 16 14 5 7 13.14 4O5555 9 5 4 9 11 14 13 5 4 25 4 4 11 9 9. 13 4 13 5 11 11 74O50OO .-- LOW LIME POPULATION N=50 CONDITION OE S SSSSSW ROCK SURFACE 8 2 2 4 6 6 12 18 8 10 6 10 10 6O4224 4 10 10 12 14 14 12 12 8 6 14 14 22 30 28 6 20 6 44 6 4 18 20 18 8 8 8 6 10 12 6 8 10 10 10 4614 6 4 4 10 14 8 4 12 6 6 8 8 8 6 6 2 2 326 244 4 HIGHER HIGH WATER 8 7 I LIMPET hiG. POPULATION N=56 oooo g — o DDWSSS 20 O O 2 12 11 11 11 12 7 11 5 7 4 12 11 12 11 9 5 25 30 30 27 2 14 14 18 18 14 9 1111 99 9 12 9 9 9 9 11 25 224 4 4 4 220OOO 4 4 4 5 4 4 OOOOO0 - TMT LOW LIMP POPULI TION N=48 WSSSS 4 2 4 4 4 12 12 12 8 8 6 4 2 6 4 20 8 10 10 15 17 12 35 35 33 31 23 27 6 8 4 12 8 8 20 2 12 12 8 8 8 10 4 6 15 4 4 4 O 86 10 10 8 6 8 8 4 4 2 2 0 OO1222 LOWER HIGH WATER 2 31 O HIGH LIMI POPULATION N=55 20. 22. 330 0030 0130 OW LIME JLATION N=50 1930 2030 2130 230 2330 0030 — — --— HIGH LIMPET P0P ULATION V=56 0830 0930 1030 1130 1230 1330 - TMT LOW LIME CION POI N=48 0830 0930 1030 1130 1230 1330 -T ED LEI LIMPETS ORIEN STANDARD y TTON 9.05 1.82 9.50 1.59 8.88 1.46 9.19 1.72 9.04 1.56 8.90 1.63 1.63 8.90 9.52 1.68 8.82 1.64 9.18 2.81 8.50 1.60 9.00 1.63 8.53 1.62 8.53 1.62 7.74 1.56 7.74 1.56 1.54 7.63 1.67 7.80 7.74 1.49 1.56 7.76 9.00 1.46 1.58 8.93 8.71 1.. 1.59 7.80 2.05 8.10 8.29 1.38 LIMPETS ORIENTED RIGHT STANDARD VIATION 4.06 2.41 1.93 3.73 1.36 2.04 3.82 1.12 3.86 1.12 3.89 1.14 4.18 3.20 1.60 3.41 4.45 2.74 3.75 1.50 4.07 2.18 4.12 2.03 4.15 2.03 4.11 1.31 4.11 1.31 4.16 1.06 4.17 1.40 4.24 1.28 4.08 1.40 4.03 1.47 4.06 1.46 4.1 1.97 4.48 1.85 4.28 2.03 4.48 2.05 Tie 3 3/8 (lan C Hlleg C ROCK 10 ROCI DIRI NIV NNE ACE AE -. WID 305cm 9lcm 122cm 213cm 430m 274cm 274cm 122cm 610cm 152cm rar HEIGF 671cm 9lcm 152cm 69cm 81cm 91cm 122cm 122cm 305cm 137cm WAVE m EXPOSUF direct sondary soondary sondary secondary secondar direct direct direct direct HTTGH HEIC AB TIDAL DAT 3. 4. 4. 3ft . 9 Han d. Plle 316 61 12 12 10 C F 1 314 PERCENT OE POPULATON ROTATINC 75 50 MINIMON CESCEOSITS LIMPETS ROTATE) 200 00 A 80 Sowden ofpers r Ae 40 0 0 5 44 2 2 20 40 60 - DAY DAA 9413 1045 930 0000 | 1130 | 0245 | 0545 | 230 | 1710 Fic. 2 o TIIIIE TIT LILTL 1 E- — F 320 Mte C 0 PERCENT OF POPUCATOS ROTATINC P 2 . 55 o 8 25 MINIMOM oce ONTTS COTATEO 100 50 00 OOMGC OE UMPETS RA O 70 0 20 . + 30 5. -20 4.2 54 Hu L HHW Lutt 4 DAv T 930 1030 1130 93 1130 2330 0130 35 32 C FIG3 L LAAAE LA LALLAA — I — — S2. FIG 2 y 50 .. 4.2 Low AICA L HAW LRWO TIME DAY o30 1030 1130 1930 1130 2320 0130 7 X ) — — — - --- — F4 3. 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