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J. Moll Stud. (1997), 63,121-130 © The Malacological Society of London 1997 TEMPORAL VARIATION IN FORAGING BEHAVIOUR OF PATELLA GRANULARIS (PATELLOGASTROPODA) AND SIPHONARIA CONCINNA (BASOMMATOPHORA) ON A SOUTH AFRICAN SHORE D.R. GRAY and A.N. HODGSON Department of Zoology and Entomology, Rhodes University, P.O. Box 94, Grahamstown, 6140, South Africa (Received 18 May 1996, accepted 2 September 1996) ABSTRACT the structuring of intertidal communities through their grazing activities and corresForaging activity of two mid- to low- shore species of ponding dislodgment of other settling species limpet, Patella granulans (Prosobranchia) and Siphonaria concinna (Pulmonata) from an exposed whilst also adding a valuable energy source to the community in the form of mucus (Branch, shore on the Eastern Cape coast of South Africa was 1985; Branch & Barkai, 1988). The importance monitored. In both species, activity was compared of limpets in the ecology of rocky shores has during spring and neap tides and, in P. granulans between summer and winter. Rhythms of activity of prompted numerous studies on their activity the two species were similar, with foraging excurand foraging behaviour, although the majority sions being mainly associated with nocturnal low tide of such studies have centred on northern times, although some P. granularis foraged during hemisphere species (see Hawkins & Hartnoll, daytime low tides. It is suggested that foraging 1983; Little, 1989 for reviews of literature). excursions in P. granularis are triggered by wave Many species of limpet home to a fixed scar action. Both species foraged further on spring tides (Underwood, 1979; Branch, 1981) when not than on neap tides and this is suggested to be a result of the limited time limpets have to forage. P. granu- active. Their activity patterns are also believed laris was also found to forage further during summer to be governed by endogenous rhythms when compared to winter and the possibility that (Funke, 1968) and this has recently been seasonal micro-algal productivity influences foraging proven to be the case for Patella vulgata (Delia distances in limpets is discussed. Santina & Naylor, 1993). A limpet's foraging The foraging activity of both species could be movements are therefore limited to an area divided into 3 distinct phases, a relatively rapid outthat allows a return trip within one activity ward phase, a much slower foraging phase and a period (Little, Williams, Morritt, Perrins & rapid homeward phase. Whether or not these Stirling, 1988). This activity period may occur limpets graze throughout an excursion is not known. whilst the limpets are emersed, submersed or S. concinna was found to home to a fixed scar, when being splashed by rising and falling tides, although during the experiment some scar-swapping occurred. P. granularis did not home to a fixed scar depending on the species (Branch, 1981). but possessed a 'home range' (approx. 5 cm2) to Patella vulgata for example has been shown to which it returned after each excursion. be active under all of these states of the tide at Patella granularis was found to move randomly different localities around the British Isles during foraging, whilst S. concinna foraged in a non- (Hawkins & Hartnoll, 1982; Hartnoll, 1986; random direction .which took individuals upshore. Gray & Naylor, 1996). No tidal-influence is thought to be present in this Much of the work on limpet activity has case and the possibility of a learning component in involved limpet populations from sheltered the foraging behaviour of certain limpet species in relation to the return to optimal feeding patches is shores, such as Menai Bridge, U.K. (Chelazzi, discussed. Santini, Parpagnoli & Delia Santina, 1994) and Port Erin, Isle of Man (Hartnoll & Wright, 1977; Hawkins & Hartnoll, 1982). Lough Hyne, southern Ireland, which has been the INTRODUCTION location of many limpet behavioral studies Prosobranch and pulmonate limpets are often (Little & Stirling, 1985; Little et al., 1988; the most dominant organisms on exposed Little, Morritt, Paterson, Stirling & Williams, 1990) not only is a sheltered site, but also rocky shores and therefore play a major role in 122 D.R. GRAY & A.N. HODGSON exhibits an abnormal tidal regime (Little, Partridge & Teagle, 1991). A study to compare limpet activity inside and outside of the Lough (Little el al., 1991) found that foraging intensity differed significantly. The rocky shores of South Africa are, on the whole, a highly exposed and extreme environment with a particularly diverse limpet fauna, with some species occurring in large densities (Branch, 1971). Except for the work of Branch & Cherry (1985) on the pulmonate limpet Siphonaria capensis, very few quantitative studies have been carried out on the foraging behaviour of South African limpets, although results from numerous qualitative observations (i.e. documenting when animals are active in relation to the tide) of west coast limpets and those inhabiting False Bay (Cape Town) have been published (Branch, 1971; Branch, 1981). These observations were made during only one season and so do not reveal anything about long term (inter-seasonal) or short term (e.g. effects of spring or neap tides) variation in foraging behaviour. The aim of this work was to examine and quantify the foraging activity of two midto low-shore species, Siphonaria concinna Sowerby, 1824 (Pulmonata) and Patella granularis Linnaeus, 1758 (Patellogastropoda) from an exposed shore on the Eastern Cape coast of South Africa. In both species, activity was compared during spring and neap tides and, in P. granularis, in both summer and winter. MATERIALS & METHODS Limpets were studied at Cannon Rocks in the Eastern Cape (33° 44' S; 26° 35' E), an exposed boulder beach composed of quartzitic sandstone and experiencing semi-diurnal tides. The tidal range at Cannon Rocks is 1.9 m above Chart Datum on mean spring tides, and 0.9 m on mean neap tides with highest spring tides phased around 0400 and 1600 hrs (S.A. Navy tide tables, 1995). During day-time low tides, Siphonaria concinna form clusters of between 3 and 35 individuals, aggregating inCTevicesand/or indentations on the surfaces of boulders. The group chosen for this study (mean shell length = 17.96 mm ± 1.9 mm) was resident on a large (about 1 m2) horizontal boulder which enabled easy identification of individuals and accurate measurements. The Patella granularis chosen were located on a vertical surface of a west facing rock (mean shell length = 19.87 mm ± 2.4 mm). Neither set of limpets had any protection from wave action and both occurred at a similar tidal level within the lower balanoid zone (1.1 ± 0.2 m above Chart Datum). Twelve hours prior to recordings, 20 limpets of each species were marked with small plastic numbers (Dymo tape) attached to the shell with epoxy resin (Cook, Bamford, Freeman & Teideman, 1969) after removing all encrusting material from the surface of the shell and blotting off excess water. In order to determine whether limpets had a consistent orientation on a home scar, individuals were marked with a line of cellulose paint from the apex to the margin of the shell (Little & Stirling, 1985). The line was extended on to the adjacent rock face so that when the limpet was 'home', the line was continuous. The limpet's number was painted on the rock next to each limpet. The position of any individual at any given time was determined by triangulation (Cook et al., 1969). Three crosses equidistant from each other (50 cm for S. concinna, 100 cm for P. granularis) were painted on the rock. Using these crosses as reference points, the path of each limpet could be plotted to an accuracy of ± 4.6 mm. Activity (= limpet movement) of S. concinna was recorded on a spring full moon, two spring new moons and a neap quarter moon during winter (Table 1). For P. granularis activity was recorded on spring and neap tides in both Summer and Winter. Previous observations on short term behavioural patterns of Helcion pectunculus within tides and seasons have shown very little variability (Gray & Hodgson, unpublished) and so replications were not carried out during this study. Measurements were carried out at hourly intervals from "when the limpets were uncovered by the tide to re-submergence. Measurements were not taken during high tide due to intense wave activity in the intertidal zone making observations impossible. It was, however, assumed that the limpets remained inactive during immersion due to the fact that they returned to a home scar or site before being covered by water. It has also previously been observed that P. granularis remain inactive during high tide (Thorpe, 1962) as do many species of Siphonaria (Branch, 1981; Branch & Table 1. Dates of observation periods for both Siphonaria concinna and Patella granularis showing the phase of the tide and the season. Tidal phase Siphonaria concinna 26.04.94 Spring Full Moon 10.05.94 Spring New Moon 17.05.94 Neap Quarter Moon 11.08.94 Spring New Moon Patella granularis 14.03.95 Spring Full Moon 25.03.95 Neap Quarter Moon 02.03.05 Spring New Moon 14.07.95 Spring Full Moon 20.07.95 Neap Quarter Moon 26.07.95 Spring New Moon Season Winter Winter Winter Winter Summer Summer Summer Winter Winter Winter FORAGING BEHAVIOUR OF P. GRANULARIS & 5. CONCINNA Cherry, 1985) presumably due to their extremely low tenacity. Limpets were recorded as 'at home' when they were on their home scar and as 'active' when away from their home scar. At night, observations were made using only red light since previous studies have shown that white light causes limpets to clamp down and cease foraging (Cook et al., 1969; Little & Stirling, 1985; Gray & Naylor, 19%). During the observations on P. granularis, physical variables were measured hourly whilst limpets were emersed. Measurements included rock and air temperature (Hanna instruments HI 9040 microcomputer thermometer), relative humidity (Hygrocheck relative humidity probe) and light intensity in n E m ' V (Integrating Quantum/ radiometer/photometer, model LI-188B by Licor inc.). Weather conditions were noted on an hourly basis. A 'foraging angle' was calculated for each excursion for both S. concinna and P. granularis by plotting a line through the home scar and the furthest point reached by the limpet during that 123 excursion and measuring the angle in a clock-wise direction from magnetic North (5. concinna) and the vertical (P. granularis). The mean vector (r) of foraging directions of the two samples, irrespective of the maximum distance travelled, were calculated (Mardia, 1972; Batschelet, 1981). RESULTS Activity Rhythms In both species, limpets foraged primarily during low tide which occurred at night or around dusk and dawn (Figs 1, 2 & 3). P. granularis was, however, found to forage during day time low tides in summer, although such movement mainly took place in the shade (248-890 H-Etrr^"1). Foraging in P. granularis began as the limpets were being washed by waves on the ebbing tide. Individuals were still active, although almost home, as they were covered 100 - 12 14 16 18 20 22 24 Time in hours 8 10 12 12 14 16 18 20 22 24 2 4 8 10 12 Time in hours Figure L The percentage of Patella granularis active Figure 2. The percentage of Patella granularis active recorded every hour for 24 hrs during 3 summer recorded every hour for 24 hrs during 3 winter observation periods a) 2.3.95, b) 253.95 & c) 17.3.95. observation periods a) 26.7.95, b) 20.7.95 & c) Arrows indicate times of low tide. Shaded bar 14.7.95. Arrows indicate times of low tide. Shaded indicates period of darkness. bar indicates period of darkness. 124 D.R. GRAY & A.N. HODGSON New Mooo t • T • t * t 12 ' f • (b) Neap Quarter Moon (C) Spring Full Moon 14 16 18 20 22 24 2 4 8 10 12 Time in hours Figure 3. The percentage of Siphonaria concinna active recorded every hour for 24 hrs during 3 winter observation periods a) 11.8.94, b) 17.5.94, c) 26.4.94. Arrows indicate times of low tide. Shaded bar indicates period of darkness. with water on the flowing tide. On 5 of the 6 observation periods > 80% of the limpets were active. The exception was 14/07/95 when only 50% of the limpets were active. During the 48 hours prior to this observation period, 24 mm of rainfall was recorded in the Cannon Rocks area (Met. Office, Port Elizabeth, pers. comm.). Siphonaria concinna commenced foraging soon after emersion and returned to their home scar prior to immersion on the next flood tide. On the 10/05/94 (SNM), after it had rained heavily for the previous 24 h, no movement of the 20 labelled S. concinna was observed. During the study 100% activity was never recorded with only 50-72% of individuals being active at any one timd Homing behaviour behaviour (Chelazzi, 1990), as only a small proportion (10%) of the limpets returned to a home scar. Limpets did, however, return to the same area on the rock (approx. 5 cm2) i.e. a home site. In S. concinna homing was 100% successful. During the course of the study 16 limpets (80%) exchanged home scars during the spring-neap-spring cycle (9 during the spring-neap period, 7 during the neap-spring period). New positions were considered to be the home scar for further observations. When returning to their home scar, S. concinna rotated their shell until the shell margin fitted the rock surface exactly. The shell seldom touched the rock until the correct orientation was achieved. Both P. granularis and S. concinna never returned to their home scar via their outward path. Trail crossing did occur in both species but no attempt by individuals to change direction and follow the previous trail was observed throughout the study. Distance moved and speed of movement Both P. granularis and S. concinna travelled significantly greater distances on spring tides than on neap tides (Table 2). S. concinna travelled about twice as far on the spring full moon compared to either the spring new moon or the neap quarter moon (p = 0.043; ANOVA, Table 3). Patella granularis travelled nearly three times as far on springs compared to neaps during the summer period of observation (Table 4). An analysis was not carried out on the winter data due to the lack of information for the neap tide. For P. granularis, limpets travelled significantly further (up to 5 times as far; p < 0.001 t-test) in Summer than in Winter (Table 5). On all occasions, both P. granularis and S. concinna moved rapidly away from their home scar as activity commenced (Table 2). Individuals then slowed down upon reaching a particular area, where they remained for some time (1-3 hours). Movement back to the home scar or site was also rapid (figure 4), and after the correct orientation on the scar was reached (S. concinna), activity ceased. The speed of movement of P. granularis was significantly slower in winter (p < 0.05 Mann-Whitney U test). Orientation offoraging movements The direction of outward movement of P. Patella granularis exhibited 'collective' homing granularis did not differ significantly from a FORAGING BEHAVIOUR OF P. GRANULARIS & S. CONC1NNA 125 Table 2. Mean foraging distances, speed of movement (x ± S.D.) for Siphonaria concinna, Patella granularis&L Siphonaria capensis during outward (initial 33% of excursion), foraging (second 33% of excursion) & homeward (final 33% of excursion) phases of foraging excursions (data for S. capensis taken from Branch & Cherry, 1985). Mean displacement Outward Foraging Homeward Duration of ± S.D. (cm) speed (cm/h) speed (cm/h) speed (cm/h) activity (min) Siphonaria concinna SFM (26.04.94) 179.0 ± 31.9 79.0 ± 6.2 9.0 + 2.1 53.0 ± 4.5 540 SNM (10.05.94) No movement recorded NQM (17.05.94) 81.0 ± 29.4 40.0 ± 2.3 5;3 ± 1.6 14.0 ± 2.3 480 31.2 ± 2.9 480 SNM (11.08.94) 106.0 ± 29.1 53.0 ± 4.5 4.6 ± 2.0 Siphonaria capensis Neap tide 1.1 ± 0.8 Spring tide 6.2 ± 3.9 Patella granu'laris Summer 10.6 ±3.6 5.7 ±2.2 11.1 ± 4.1 300 SFM (17.03.95) 47.64 ± 4.5 NQM (25.03.95) 13.5 ± 2.3 No distinct outward and homeward sections SNM (02.03.95) 30.5 ± 5.2 9.6 ± 2.9 6.1 ± 1.8 9.9 ± 3.2 420 Winter SFM (14.07.95) 7.65 ± 1.89 1.9 ± 0.7 0.9 ± 0.02 4.1 ± 1.2 420 NQM (20.07.95) No measurements taken due to rough sea 1.1 ±0.07 5.1 ± 2.1 240 SNM (26.07.95) 13.75 ±1.63 Table 3. Siphonaria concinna: Results of a multiple range analysis (Newman-Keuls) between tidal phases i.e. Spring full moon. Neap quarter moon and Spring new moon. Spring full moon > Spring new moon = Neap quarter moon 179.0 cm > 106.0 cm = 81.0 cm Table 4. Analysis of variance of distances travelled by Patella granularis on the different phases of the tide during the summer period of observations. Source of variation Phase of tide df SS MS 11662.3 5831.1 19.36 0.0001 Results of a multiple range analysis (Newman-Keuls) between phase of the tides Spring full moon > Spring new moon = Neap quarter moon 47.65 cm > 30.5 cm = 13.5 cm random model (p > 0.05; Rayleigh test). Siphonaria concinna, however, showed directional movement upshore (Figure 5). Physical variables During summer low tides, daytime air and rock temperatures reached 24.2 ± 2.9 and 27.1 ± 5.2°C respectively (Table 6). The temperature of the rock surface was generally 1-3°C warmer than the air temperature during the day. At night the air and rock temperatures were similar. Relative humidity of the air was found to be 20-40% higher at night than 126 D.R. GRAY & AN. HODGSON Table 5. Analysis of variance of distances travelled by Patella granularis between seasons. P Source of variation df MS F Season 1 95355 24.361 0.001 Summer > Winter 33.65 cm > 11.2 cm 0 10 20 during the day. In general, temperatures (both air and rock surface) were lower during the winter periods of observation than those recorded during the summer. 30 40 » 60 90 100 % of cicnnkw DISCUSSION In the present study, quantitative data on the foraging activity of two mid-shore South African limpets was obtained. The intention was to establish whether these two limpets, which live at a similar height on the shore in differing habitats, showed similar behaviour to each other and to species studied oh more sheltered shores. The timing of activity of P. granularis on the east coast of South Africa agTees with the observations of Branch (1971) for west coast populations. Movement Figure 4. Speed of limpets (n = 20) plotted against percentage of excursion period, a) S. concinna, b) P. granularis. Solid symbols and continuous line indicate mean speeds. Table 6. Air and rock surface temperature ranges (°C) and the range of relative humidity measurements recorded during activity observations of Patella granularis during both summer and winter. P. granularis Summer 17.03.95 Day Night 25.03.95 Day Night 02.03.95 Day Night Winter 14.07.95 20.07.95 26.07.95 Day Night Day Night Day Night Range of air temp. CO Range of rock temp. (°C) Range of rel. humidity (%) 21.1-27.1 16.0-19.7 18.9-21.9 17.1-18.1 21.2-24.2 19.1-20.5 19.6-32.6 17.2-18.2 20.2-25.6 17.7-17.8 18.3-29.2 19.3-21.0 54.8-70.3 80.4-91.0 68.5-89.6 90.3-93.3 42.8-73.2 92.2-99.7 18.5-20.4 12.2-17.3 16.4-17.6 16.0-17.3 13.2-18.6 13.5-16.8 19.8-26.2 13.1-15.3 18.3-22.6 14.0-14.6 13.4-20.3 14.4-17.4 52.1-56.2 70.8-82.7 29.6-58.6 71.6-86.7 40.6-61.8 69.1-75.3 FORAGING BEHAVIOUR OF P. GRANULARIS & S. CONCINNA occurred at low tide during both day and night and appeared to be initiated by spray from wave action on the ebbing tide. Branch (1971) also observed that P. granularis in the lower balanoid zone often lacked a fixed scar and exhibited random movement, which was also true for east coast limpets. Siphonaria concinna exhibited very similar behaviour to 5. capensis which inhabit exposed rock surfaces (Branch & Cherry, 1985). Both species were active at low tide at night, and both homed to a fixed scar. The ability to home rigidly seems the norm for most species of Siphonaria (Branch, 1981; Creese & Underwood, 1982; Garrity, 1984) although 5. virgulata tends to move randomly and usually lacks a scar (Creese & Underwood, 1982). The activity rhythms of S. concinna may have several driving factors. Garrity & Levings (1983) suggested that limpets remain inactive at high tide to avoid marine predators. This is unlikely in the case of S. concinna, for like many other siphonariids it secretes mucus containing the defensive polypropionate Pectinatone (DaviesColeman, pers.comm.). A more plausible explanation is that by being active at low tide, 5. concinna reduces the chance of being swept away by wave action as they have a low tenacity (Branch & Marsh, 1978). By contrast, P. granularis are active under strong wave action and are known to be highly tenacious (Branch, 1981). Finally, occupying a fixed scar during the day would reduce desiccation, as has been shown in other limpets (Verdeber, Cook & Cook, 1983; Branch & Cherry, 1985; Kunz & Conner, 1986). Day-time air and rock temperatures were higher than those at night. In addition, the relative humidity at night was always greater than 70%, thus by being active during the cooler, more humid conditions, 5. concinna presumably reduces water loss. Neither P. granularis or S. concinna followed outward paths back to their home scar or site. Cook (1971) showed that the limpet 5. alternata, can home without using either distant cues, reverse-displacement or topographic memory, and indicated that limpets were capable of following mucus trails. This does not, however, seem to be the case for either P. granularis or 5. concinna and raises the much debated subject of how limpets home to a fixed scar? Although further work is required it is possible that individuals follow previously laid trails, or that different species of limpet have evolved different homing methods depending upon their environment and situation. The fact that the 127 (a) 270° 90° x - 6.4° r-= 0.477 n-14 p < 0.05 (b) 270° 90° x-43.6° r - 0.894 n-12 p < 0.001 (0 270° 90° x - 3.44° r-0.575 n-10 p<0.05 180° Figure 5. Foraging directions shown by Siphonaria concinna on a) 26.04.94 SFM, b) 17.05.94 NQM & c) 11.08.94 SNM. Each dot represents one excursion, n - number of excursions observed and plotted, r - an estimate of the non-uniformity of the circular distributions given as mean vector lengths by the Rayleigh test. 128 D.R. GRAY & A.N. HODGSON limpets cross their own trails without actually amount of food available, thus forcing the following them may be sufficient to give limpets into a second excursion. Little (1989), the limpet enough information for correct however, suggests that competition between limpets of the same species for food would also orientation. The foraging activity of P. granularis and S. cause the individuals concerned to forage concinna could be divided into 3 distinct during the day as well as at night. It has been phases, a relatively rapid outward phase travel- suggested that barnacles do have an adverse ling away from the home scar/site, a slower effect on limpets because the rough and irreguforaging phase and a rapid homeward phase. lar topography created by barnacles hinders Such behaviour has been recorded for Patella limpet foraging behaviour (Lewis & Bowman, vulgata (Hartnoll & Wright, 1977; Little et al., 1975; Underwood, 1979; Little et al., 1988). 1988; Chelazzi et al., 1994). Although P. Another possibility is that the filter feeding of vulgata was shown to feed for the entire barnacles reduces available algal spores and activity cycle, grazing was most intense during sporelings (Branch, 1976). Hawkins and Hartthe middle phase of the cycle (Little & Stirling, noll (1982) have shown that the incidence of 1985; Evans & Williams, 1991). Whether or not foraging in Patella vulgata is proportional to P. granularis and 5. concinna feed throughout the density of barnacles. Since barnacles appear to reduce feeding efficiency, increased their activity cycles is not known. The activity of 5. capensis during neap and foraging presumably reflects decreased food spring nocturnal low tides shows that the aver- availability. The presence of barnacles may, age displacement of limpets from their scars however, result in a damper surface, enabling and the time spent foraging are both greater limpets to forage without fear of desiccation. during spring tides than during neap tides This therefore raises the question of whether (Branch & Cherry, 1985). This also proved to limpets forage at every available opportunity be the case for 5. concinna. The distance or whether they only forage when they need travelled by a limpet during an excursion food? maybe a function of the time exposed to air i.e. Many species of limpet appear highly opporthe time available for foraging. Siphonariid tunistic. Cellana grata, for example, moves limpets are limited to foraging whilst exposed, whilst awash, but during typhoons individuals presumably due to low tenacity, and so move constantly and when artificially sprayed foraging excursions will be both shorter in time with water, even at low water, will become and distance on neap tides, thus enabling active (Williams & Morritt, 1995; Williams, the limpet to return home before being pers. comm.). C. toreuma has been reported to covered by the incoming tide (Branch & remain active for up to 18 h whilst awash Cherry, 1985; Branch, 1988). However, the (Hirano, 1979). There is evidence that certain average displacement of 5. capensis from their limpet species that undergo long feeding scars is considerably less than that of S. excursions one night are less likely to feed the concinna (Table 2). A possible reason for following night (Branch & Cherry, 1985) but this is the large difference in micro-algal more detailed observations are required to productivity of the east and west coasts of determine whether this is so for all limpets. Southern Africa (Bustamante, Branch, Different environments may well call for Eekhout, Robertson, Zoutendyk, Schleyer, different feeding strategies of limpets. Dye, Hanekom, Keats, Jurd & McQuaid, Patella granularis showed little seasonal 1995). We tentatively suggest that the west difference in the number of limpets active coast, with its higher algal productivity, allows during possible activity periods with between the limpets to forage far shorter distances to 80-100% of the sample being active at any one obtain enough food for sustenance, whilst 5. time on most occasions. The suppression of concinna on the east coast has to travel further activity on the evening of the 14/07/95 may be in order to obtain enough food, although attributed to the heavy rainfall 48 hrs prior to further work is required to substantiate this. observation. Fresh water has been shown to Some P. granularis foraged during daytime suppress limpet activity (Arnold, 1957; Little low tides as well as at night, c.f. low water P. & Stirling, 1985; Little et al., 1990). This may vulgata in Lough Hyne (Little et al., 1990; also explain the lack of any movement by S. Williams & Morritt, 1991), although this only concinna on the 10/05/94 after it had rained ever took place in the shade (248-890 heavily for the previous 24 hrs. jiEm~V). It is possible that the presence of There was a significant difference in the disbarnacles on the rock surface reduces the tance travelled by P. granularis during summer FORAGING BEHAVIOUR OF P. GRANULARIS & S. CONCINNA and winter, limpets travelling nearly three times as far in summer than in winter (Table 1). This could be due to a number of reasons, firstly epilithic algal production (in the form of chlorophyll a) per month peaks during the winter months along the south coast of South Africa (Bustamante et al., 1995) and so it is possible that the limpets need not travel as far to obtain their quota of algae. Cubit (1984) found this to be true for Collisella digitalis occurring high up on the shore (Oregon, U.S.A.). Another possibility is a reduction in activity due to the cold temperatures in winter compared with those of the summer period (see Table 3) the limpets metabolism is much lower and so they do not need as much food or cannot physically move as fast due to a reduction in basic bodily functions (Marshall, 1991). Patella granularis moved randomly during foraging, a behaviour previously observed in lowshore individuals of the species (Branch, 1971). In S. concinna, however, foraging excursions were non-random in direction, with a mean vector upshore in each case. This vector actually took the limpets into an area of rock which had a smooth flat surface compared to the rough, pitted area within which their home scars were situated. P. vulgata has been shown to exhibit directionality during foraging excursions (Little et al., 1988; Gray & Naylor, 1996) and several other patellids show vertical movements in relation to tidal rise and fall (Hirano, 1979; Williams & Morritt, 1995). However, in this study the dominant foraging directions were not in the vertical plane but horizontally across a rock surface. 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