Shabb INTRODUCTION Collisella paradigitalis (Fritchman, 1960) is a common limpet of the mid to hugh rocky intertidal occuring along the Bacific coast from Southwestern Alaska to Northern Baja California. Previously confused with a South American species, C. strigatella, it is now recognized as a separate species (Lindberg, 1981). In contrast to many other species of intertidal Acmaeids, little work has been done on the biological aspects of this species. The purpose of this study is to characterize the habitat niche and activity patterns of C. paradigitalis. To define its habitat niche, the following aspects were examined: 1) vertical distribution 2) wave exposure 3) orientation of substrate with respect to the sun 4) orientation with respect to direction of wave action 5) slope of substrate 6) presence in crevices. To determine activity patterns, the following questions were posed: 1) how far do they move? 2) what percent moves per high tide? 3) is there a home range or restricted area over which these limpets graze? 4) is there correlation between the size of an animal and the distance it travels? 5) is there any evidence of homing? Many of these questions have been studied in other limpet species, but almost none have been adequately addressed for C. paradigitalis. Shabb DISTRIBUTION METHODS All distribution data were collected during morning low tides during April and May of 1982. All study areas were located on shoreline behind Hopkins Marine Station in Pacific Grove, California, The limpets had a patchy distribution in the study area, and to maximize data collection, sites were selected which had sig- nificant populations of C. paradigitalis. These were distributed along a gradient of wave exposure classified by the average wave size washing each area. An exposure scale of 4 levels was devised. and study sites were assigned to these, as follows; 1) Very protected - waves gently lapping on shore. Outer rocks and islands of rocks blocking main force of waves. Little or no spray. 2) Moderately protected - waves causing some splash or spray, Outer rocks blocking most of the waves. 3) Moderately exposed - waves causing quite a lot of splash with spray. Outer rocks partly blocking waves. 4) Very exposed - waves violently crashing on rocks with much spray. No outer rocks, waves come in full force. For each level of exposure, approximately equal areas were surveyed for limpets. Intertidal height was determined by surveying points from a benchmark of known elevation, and then recording the relative heights for each limpet. In the very exposed study area, there were no Shabb benchmarks nearby, so elevations were determined by establishing tidal level at the time of low low water on a very calm day, and then measuring intertidal heights relative to this value. Orientations of the substratum faces upon which limpets were found were measured by compass and then corrected according to the declination of the Monterey area. The direction from which waves were coming relative to each study area was also recorded in this manner. The slope of substrate, measured with an inclinometer, was categorized as being: 1) horizontal, 0 - 30° 2) intermediate, 31 - 59° 3) vertical, 60 - 90° 4) overhanging, greater than 90°. Presence in crevices was also recorded. RESULT Out of 720 limpets observed, the vertical range extends from f.7 to t3.2 meters over all exposure conditions (Figure 1). The average density occurs at .27 meters below the Endocladia zone, which gives a comparative physiological level between sites. Small standard deviations indicate that the greatest limpet density actually occurs over a much narrower range. Greater densities of C. paradigitalis were also found in the more protected areas. Substrate orientation data indicates a preference for NW and SE facing rocks (Figure 2), while a closer look suggests that limpets may orient themselves according to the degree of wave action they are receiving. Figure 3 compares the number of limpets facing into the waves with the number facing away from the waves (i.e., limpets on the front vs. back sides of rocks). Rocks facing intod Shabb the waves were defined as those facing no more than 45° away from the direction of oncoming waves, while those facing away wer defined as being 180° opposite in orientation from those facing into the waves. The results indicate that more limpets are found facing toward the waves in protected areas than in exposed areas. Table 1 gives more information on microhabitat preference, most notably, that over 902 of the population was found on vertical or slightly overhanging surfaces, and also that over 502 were found in crevices. Large numbers were also found totally submerged in tadepools during low tides, also reported by Fritchman, 1960, but no data was collected on these animals. ACTIVITY PATTERNS METHODS All data was collected from observations made over a 2 week period during consecutive low tides from May 11-25, 1982. The study was conducted with a population of 18 C. paradigitalis in 2 sites located in a very protected area on the Eastern end of Agassiz Beach behind Hopkins Marine Station. The 2 sites where the study was conducted were chosen because they were typical C. paradigitalis habitats, they had relatively flat rock surfaces so that measurements could easily be made, and easily accessible. The position of each limpet was marked with nail polish and numbered during low tide on the morning of May II. At each following low tide until May 25, the location of each limpet was recorded by using a system of polar coordinates with the origin Shabb at the original site of the limpet. Elevations of the study site ranged from 1.0 to 1.5 meters above mean low low water. RESULTS Table 1 summarizes the significant behavioral findings. Movement between low tides ranged from O to 50 cm, averaging 13.4 cm if periods of inactivity are discounted. 932 of the population moved during high tide with the percentage dropping sharply if only those moving greater than a certain distance are considered. C. paradigitalis occupies a more restricted range than would be expected, given the average displacement at each high tide. The ranges were found to have radii from 8 to 80 cm, typically 20 to 25 cm. Figure 5 shows a plot of all the points at which an individual limpet was found during low tides over 2 weeks. A circle with a radius of 19 cm encloses all the points. Only 12 of 18 limpets returned to a specific site over 2 weeks. Only 3 individuals were observed to return to a specific site more than once, 1 returning 5 times, and 2 returning twice. The individual that returned 5 times always returned to the same site with its head in the same orientation, but orientations in other homers were not always so precise. The same individual exhibited an activity pattern, spending 3 consecutive high tides foraging, then 3 at home. 3 cases were observed where a single site was occupied at different times by 2 different limpets, not necessarily of the same Shabb species (Table 2, bottom). Figure 7 is a graph which correlates limpet size with average displacement at high tide. Limpet shell sizes ranged from 8./ to 16.0 mm with average displacement ranging from 3.2 to 22.9 cm per high tide. The point with the hatched circle is believed to be an error. No significant trends were observed when displacements resulting from night high tides were compared with day highs, of when high high tides were compared with low high tides. DISCUSSION The distribution of C. paradigitalis suggests that the species is not as resistant to desiccation stress as the higher intertidal limpets such as C.scabra and C. digitalis whose niches were described by Stoner B. Haven, 1973, and physiological tolerances determined for them by Walcott, 1973. This assumption is reinforced by its tendency to occupy vertical faces, crevices, tidepools, and areas which face toward the waves in protected areas, all positions which would reduce desiccation stresses and maximize water retention. C. paradigitalis can commonly be found low on the rocks buried in the sand after receding tides, a very protected environment which is moist, dark, and safe from possible predation by starfish or birds (Hahn, 1982). Physiological factors are not the sole determinants of the distribution of C. paradigitalis. The decrease in density of C. paradigitalis in areas exposed to heavy surf suggests that it may Shabb suffer in competition from C. digitalis which is most similar to it in habitat niche, and flourishes in wave swept areas. Competition may also be very significant in determining orientations either facing toward or away from waves in varying exposure conditions, which would further take it out of competition with C. digitalis. Substrate orientation data (Figure 2), may best be explained by relating the directions which study sites faced with the direction of oncoming waves. All of the study areas faced toward the ocean in a range from NW to NE, the density of organisms facing NW is a result of the great number of limpets facing into the waves in the protected areas, while the density facing SE is a result of those facing away from the waves in the more exposed areas. Activity patterns are also strongly influenced by physiological factors and competition. If the distance travelled by a limpet is directly proportional to its foraging area, Figure 5 would suggest that larger limpets need to forage over greater distances than smaller limpets. A foraging range or territory (example, Figure 4) would be important in determining spatial distribution so that competition could be minimized and grazing areas could be most efficiently utilized at high limpet densities. If a home range exists it is possible that limpets occasionally return to sites offering physiological advantages which may explain why some C. paradigitalis exhibit occasional homing behavior, a phenomenon which requires further study. Shabb SUMMARY Distribution 1) Vertical distribution: .7 - 3.2 meters above MLLW with average density .27 meters below Endocladia zone. 2) Wave exposure: greatest densities occur in protected areas where more face into the waves than in exposed areas. 3) Orientation of substrate: main density of organisms facing toward NW or SE. NW density faces into the waves in protected areas. SE density faces away from waves in exposed areas. 4) Slope of substrate: 912 of sample population found on vertical or slightly overhanging surfaces. 5) Crevices: 552 of population found in crevices. Activity patterns 1) Average distance moved during high tide = 13.4 cm. 2) Approximately 772 of the population moves greater than 5 om during a high tide. 3) Homing ranges with radii from 8 to 80 cm have been determined with the average between 20 and 25 cm. 4) A correlation exists between the size of a limpet and its average distance moved per high tide. The larger the limpet, the greater the travelling distance. 5) Very infrequent homing in 672 of the population. C Shabb ACKNOWLEDGMENT: I would like to thank Charles Baxter for his patience and advice throughout the quarter. Shabb LITERATURE CITED Fritchman,Harry K., 1960. Acmaea paradigitalis. The Veliger 3(2): 57 Hahn, Thomas, 1982. Personal communication Haven, Stoner B., 1973. Competition for food between the intertidal gastropods Acmaea scabra and A. digitalis. Ecology 54: 143-151 Wolcott, Thomas G., 1973. Physiological ecology and intertidal zonation in limpets (Acmaea) Bio. Bull. 145:389-422 12 Shabb LEGEND OF FIGURES Figure 1. Vertical distribution of C. paradigitalis. Thin vertical bar denotes range, wide hatched bar shows standard deviation, black horizontal bar indicates average density. Dashed lines represent Endocladia level. Orientations of substrate upon which limpets were found. Figure 2. Radial axes show the number of limpets. Relative percent of limpets facing into vs. away from Figure 3. waves over an exposure gradient. Diagram on right shows direction of wave action upon substrate. Hatched areas represent orientations of substrate which were classified as either facing toward or away from waves. Diagram of the homing range of a typical C. paradigitalis. Figure 4. Points indicate positions where the limpet was found during low tides over 2 weeks. Radial axes in cm. Orientations correspond to a 12 hour clock. Dashed circle represents homing range with radius of 19 cm. Sizes of 18 limpets plotted against their average distances Figure 5. moved per high tide (periods of inactivity included in the average). Point with hatched circle believed to be an error. LEGEND OF TABLES Microhabitat preferences of C. paradigitalis. Table 1. Activity patterns of C. paradigitalis. Bottom of table Table 2. diagrams 5 cases where different limpets occupied the same site at different times. O TABLE I OCCURRENCE OF C. PARADIGITALIS IN RELATION TO: I. Slope of substrate A. Vertical or slightly overhanging (slope = 60° - 90° or greater) .. ... 917 B. Intermediate (31° - 59°) ... . . 7.57 C. Horizontal (0° - 30°) .. . . . 1.57 II. Crevices .. . . . 557 n = 721 TABLE 2. BEHAVIOR OF C. PARADIGITALIS Data collected at low tides over 2 weeks May 11-25 n-18 max. 50 cm Amount of displacement between consecutive low tides: min. O cm avg. 13.4 cm (excludes instances when no movement occurs) 932 16.8, * Moving per high tide 13.8, beyond 5 cm . .. 557 9.9, beyond 10 cm . .. 327 beyond 15 cm . . 5.7, 187 beyond 20 cm . .. 3.2, max. 80 cm Ranging area - radius of movement: min. 8 cm avg. 20-25 cm Homing: 12/18 returned to a specific site at least once . . . 677 2nd occupier of site head orientation lst occupier of site same 15.5 mm paradigitalis 15.7 mm paradigitalis same 11 mm digitalis 13 mm paradigitalis almost same 8.7mm paradigitalis opposite 13 mm paradigitalis 16 mm digitalis opposite 10 mm paradigitalis 10 mm paradigitalis — — TE O — — O O P G 0 17 5 OA O m 5 OP: 0) 1 FIG.1 ELEVATION ABOVE MLLW (meters) — Z LA IN + W + —— W. —— =720 + — 100 -50 — S - — . ElG.2 —— 100- U. 75 00 O. 50 — 25 0 L 25- O 50+ u 75+ 9 L 100— â - O 1 O90 n= 105 O 11 2 n=59 L L O L: 0 +* D=51 N + I EIG.3 WAVES 1 WARDS AWAVI 90-— — 105 — 9— 8- — 11 ia — 25 -20 e 5 FIG. 4 +. — + —3— )4 — —- 6 FIG.5 AVERAGE DISTANCE MOVED PER INDIVIDUAL PER HIGH TIDE Ccm) 0 — N C . . . — — N o * + ââ O1 9 O L