SPATIAL PATTERNS OF SHELL SHAPE OF THREE SPECIES OF CO-EXISTING LITTORINID SNAILS IN NEW SOUTH WALES, AUSTRALIA

The shape and relative weight of the shell have been shown tovary intraspecifically and interspecifically in a number ofspecies of gastropods, including many different littorinids.These differences give rise to different shell forms in differenthabitats. In those species which have non-planktotrophic development,differences in shell form among shores have been usually explainedin terms of natural selection because exposure to waves supposedlyfavours light shells with large apertures, while predation bycrabs on sheltered shores favours elongated, thick shells withsmaller apertures. Differences in shell shape among speciesfound at different heights on the shore have been explainedin terms of resistance to desiccation and temperature. Suchvariables would tend to act on a relatively broad-scale, i.e.causing differences among heights on a shore or among shores.Rates of growth, which might vary at much smaller scales withina shore, have also been shown to affect the shapes of many shells. In this study, the shape and relative weight of shells of threespecies of co-existing littorinids (Littorina unifasciata, Bembiciumnanum and Nodilittorina pyramidalis) were measured. These speciesall haveplanktotrophic development and they are found on manyshores where there is no evidence that they are preyed uponby crabs. Before explanations of shell shape are proposed, itis necessary that patterns of variation, within different partsof ashore and among different shores are clearly documented.These patterns were measured at a number of different spatialscales within and among replicate shores with different amountsof wave exposure. Large and small specimens were included toallow intraspecific comparisons among snails of different sizesfound at different heights on the shore. The results showedsignificant differences among shores in shape and relative weightof shells, but these differences could not be explained by exposureto waves. In addition, snails of different sizes and differentspecies did not show the same patterns although they were collectedfrom the same sites. Importantly, the shell shape of Liuorinaunifasciata varied significantly among sites at approximatelythe same height within a shore. These differences could notbe clearly correlated with density, mean size nor exposure towaves. The only consistent pattern was a decrease in relativeaperture size in specimens living higher on the shore. Modelsthat have commonly been proposed to explain shape and relativeweight of shells in other species of gastropods are not adequateto explain the small- and large-scale variation of the measurementsdescribed here. It is proposed that any selective advantageof shell morphology and the effects of any variables on thedevelopment of shell morphology in these species can only beidentified after appropriately designed and replicated fieldexperiments. (Received 4 March 1994; accepted 13 September 1994)     

Predation by small mobile aquatic predators regulates populations of the intertidal limpet Patella vulgata (L.)

Highly mobile aquatic predators are known to forage in the intertidal during periods of immersion. There is limited quantitative information, however, on the extent to which these predators influence the abundance of grazing molluscs which are known to have a key role in structuring intertidal assemblages. Our preliminary video observations revealed that crabs and small fish were abundant on shores in southwest England during high-tide. We then used manipulative experiments to quantify the effect of small mobile aquatic predators on the abundance of limpets (Patella vulgata L.). On the lower shore at two moderately sheltered rocky shores three treatments were established: complete cage, partial cage (cage control) and uncaged (natural condition). The complete cages excluded all predators. The partial cage treatment allowed full access to small predators and the uncaged treatment allowed access to all predators. After two months, limpet abundance in uncaged and partial cage treatments had declined by around 50% compared to the complete cage treatment. Population structure also changed with survival of larger individuals being greater than smaller individuals in the open and partial cage treatments compared to the complete cage treatment. The effects of excluding predators were consistent at small (meters) and large spatial scales (kilometres) and hence, it would appear that the outcomes of our research are generally applicable to similar shores in the region.To explore the mechanism behind the differential effects of predators according to prey size, we compared the detachment force required to remove limpets of differing sizes from the shore. This was around four times greater for larger individuals than for smaller ones indicating that smaller limpets were more vulnerable to predation. These effects were also consistent between locations. Subsequent laboratory observations showed that the crabs Carcinus maenas (L.), Necora puber (L.) and Cancer pagurus (L.) which are locally abundant predators of limpets, had differing handling behaviour but were all highly efficient at removing limpets from substratum. Hence, shell width and attachment force appeared to be critical factors influencing the vulnerability of limpets to predation by these crabs. Limpets are known to control the abundance of macroalgae on shores in the North-east Atlantic and so our conclusions about the role of mobile predators in regulating the abundance of these grazers are important to our broader understanding of the ecology of these shores.     

VARIATIONS IN INFESTATIONS OF SIPHONARIA CAPENSIS AND S. CONCINNA(PULMONATA) BY LARVAL TREMATODES IN SOUTHERN AFRICA

The prevalence of digenean trematode parasites in Siphonariacapensis (Pulmonata) was examined in populations from 24 sites,and in S. concinna from 10 sites along the coast of southernAfrica from 1989 to 1991. Sporocysts containing cercariae werefound in the digestive gland of both species. To date, two typesof cercaria have been isolated from S. capensis and one typefrom S. concinna, these types possibly representing three distinctspecies of trematode. In approximately 30% of parasitized S.capensis and 10% of parasitized S. concinna the digestive glandwas completely destroyed. In both species of Siphonaria therewas considerable temporal and spatial variation in parasiteprevalence. At some locations up to 40% of S. capensis wereinfected whereas at others, no parasites were found. Prevalenceof trema-todes in S. concinna rarely exceeded 12%. At severalsites the prevalence of infestation within the limpets increasedwith increasing host size. (Received 8 June 1992; accepted 6 July 1992)     

The influence of simulated exploitation on Patella vulgata populations: protandric sex change is size‐dependent

Grazing mollusks are used as a food resource worldwide, and limpets are harvested commercially for both local consumption and export in several countries. This study describes a field experiment to assess the effects of simulated human exploitation of limpets Patella vulgata on their population ecology in terms of protandry (age‐related sex change from male to female), growth, recruitment, migration, and density regulation. Limpet populations at two locations in southwest England were artificially exploited by systematic removal of the largest individuals for 18 months in plots assigned to three treatments at each site: no (control), low, and high exploitation. The shell size at sex change (L₅₀: the size at which there is a 50:50 sex ratio) decreased in response to the exploitation treatments, as did the mean shell size of sexual stages. Size‐dependent sex change was indicated by L₅₀ occurring at smaller sizes in treatments than controls, suggesting an earlier switch to females. Mean shell size of P. vulgata neuters changed little under different levels of exploitation, while males and females both decreased markedly in size with exploitation. No differences were detected in the relative abundances of sexual stages, indicating some compensation for the removal of the bigger individuals via recruitment and sex change as no migratory patterns were detected between treatments. At the end of the experiment, 0–15 mm recruits were more abundant at one of the locations but no differences were detected between treatments. We conclude that sex change in P. vulgata can be induced at smaller sizes by reductions in density of the largest individuals reducing interage class competition. Knowledge of sex‐change adaptation in exploited limpet populations should underpin strategies to counteract population decline and improve rocky shore conservation and resource management.     

RECRUITMENT AND GROWTH OF THE BLUE-RAYED LIMPET, HELCION PELLUCIDUM (L.), IN SOUTH EAST IRELAND

This account reports on recruitment, growth and population structureof the macroherbivorous gastropod Helcion pellucidum (L.) onan exposed rocky shore in S.E. Ireland, with particular referenceto the pellucidum form. Recently settled Helcion spat were foundinitially on living crustose lithothamnia in pools and on openrock, on the lower shore, during February-April and subsequentlyrecruited to macro-algal fronds. By May they were found in abundanceon Mastocarpus stellatus and Laminaria spp. Helcion was markedlyseasonal in its occurrence on Mastocarpus, being most abundantduring May-August. A complementary pattern was evident on growingHimanthalia receptacles, with limpets abundant from July onwards.It is suggested that Helcion may migrate from Mastocarpus toHimanthalia as they grow. Data on growth rates are presentedfor Helcion in the various habitats studied. Growth rates werehighest on the Laminaria spp. (Received 21 February 1992; accepted 8 June 1992)     

SPATIAL VARIATION IN DENSITY AND BIOMASS OF PATELLID LIMPETS INSIDE AND OUTSIDE A MARINE PROTECTED AREA

Although differences in the mean density, biomass and size ofrocky intertidal biota inside and outside MPAs have been welldocumented, little is known about the variability in these attributesat different spatial scales. This topic merits investigationbecause it can influence the outcome of reproduction, competitionand predation, and thereby affect the viability of populations.In this study, a nested sampling design was used to assess differencesin density and biomass of seven species of patellid limpetsat the scales of plots, sites and shores inside and outsidea marine protected area (MPA) on the southeast coast of SouthAfrica. At the scale of plots, significant variation was morecommon inside than outside the MPA. This probably reflects differencesin the general pattern of space occupancy and quality of habitatsavailable to limpets inside and outside the MPA. Significantvariation at the scale of sites was rare. This suggests thateither the processes contributing to variability at this scalecounteract each other or that the sites were similar in termsof habitats and ecological processes. At the scale of shores,significant variation was more common in densities than in biomass,but both occurred with equal frequency inside and outside theMPA. Variation at this scale is probably driven by a combinationof recruitment and/or mortality. Five species exhibited greaterspatial variability inside than outside the MPA. The spatialpatterns observed did not appear to be linked with differencesin the mobility, habitat requirements or susceptibility of thesespecies to exploitation. The lack of consistent patterns suggeststhat each species probably responds in a different way to theecological processes operating at these spatial scales. (Received 1 February 2005; accepted 29 July 2005)     

MORPHOLOGICAL AND GENETIC VARIATION IN THE ANTARCTIC LIMPET NACELLA CONCINNA (STREBEL, 1908)

Samples of the Antarctic limpet, Nacella concinna (Strebel,1908), were collected from four sites in the South Orkney Islandsand from Stromness in South Georgia during 1988. At three SouthOrkney sites, both littoral and sub-littoral samples of limpetswere taken. Shell dimensions were measured and littoral andsub-littoral limpets were shown to have significantly differentshell shapes. Foot and digestive gland tissues were subjectedto electrophoresis and five polymorphic loci {Es-1, lcd-1, lcd-2,Gpi, Got-1 and Pgm-1) and two monomorphic loci (Es-2 and Got-2)were scored. Genetic identities between littoral and sub-littoralforms and also between sites were calculated. At the loci scored,the littoral and sub-littoral forms were virtually identicaland this suggests that the shell shape differences between theforms are the result of environmentally induced phenotypic plasticity.However, significant genetic differences and some morphologicaldifferences were apparent between the sample of limpets fromSouth Georgia and all other sites. Genetic identity values suggestedthat the South Georgia and South Orkney N. concinna are geographicallyseparated populations of a single species rather than distinctsub-species. (Received 21 November 1990; accepted 17 April 1991)     

Grazing dynamics in intertidal rockpools: Connectivity of microhabitats

Differences between rockpool and emergent rock communities are often attributed to their contrasting physical conditions. However, differences in grazing pressure between rockpools and open rock could also exert an important structuring role. Greater densities and/or the lack of tidal constraints on foraging may allow grazing intensity to be greater in rockpools. Here, wax discs were deployed to compare grazing intensity between rockpool and emergent rock habitats at each of three tidal heights on a moderately exposed shore in SW England. Grazing intensity was then examined in relation to herbivore density. Grazing intensity in pools was twice that on emergent rock, despite a lower density of herbivores in the rockpools. Of these herbivores, patellid limpets are the dominant grazers on rocky shores throughout the NE Atlantic and are recognised to have a major role in structuring intertidal communities. Thus, subsequent experiments focussed on the influence of limpets in determining the differences in consumer pressure between rockpools and emergent rock. Three alternative explanations were considered: (1) the effect of continuous immersion on grazing intensity in rockpools; (2) differences in limpet species abundance between the two habitats; (3) movement of limpets from emergent rock into pools to feed. The level of grazing pressure exerted by Patella ulyssiponensis (Gmelin), the predominant species living constantly immersed in rockpools, was similar to that of P. vulgata (Linnaeus) which is predominantly found on emergent rock. P. vulgata were observed moving from emergent rock into rockpools during high tide. Manipulative experiments confirmed that these foraging excursions resulted in a 2-fold increase in grazing intensity in the pools. Grazing activity of P. vulgata in rockpools was not consistent between sites and may be influenced by differences in wave exposure and/or the abundance of microbial resources. Elevated consumer pressure in rockpools may be an important factor influencing algal assemblages and probably explains the predominance of grazer resistant-species in these pools.     

Feast or famine for intertidal grazing molluscs: a mis-match between seasonal variations in grazing intensity and the abundance of microbial resources

Distinct seasonal variations in the abundance of photosynthetic microbiota and limpet grazing intensity were recorded at Port St Mary, Isle of Man between January 1994 and June 1996. Microbial abundance was negatively correlated with insolation stress, while grazing intensity was positively correlated with sea and air temperature. These patterns result in a mis-match between the supply of and the demand for microbial resources with maximal grazing intensity during the summer and autumn, but maximal microbial standing stock during the winter and early spring. The importance of top-down control of microbial assemblages by grazing was demonstrated by experimental exclusion of limpets during autumn 1993. This resulted in a four-fold increase in the abundance of cyanobacteria within 6 days, followed by a more gradual proliferation of ephemeral algae during the next 4 weeks. The abundance of diatoms remained relatively constant and was not influenced by the removal of grazers at this time of year. The influence of microbial resource availability on the growth and mortality of limpets was examined using experimental enclosures of differing densities of either Patella vulgata or P. depressa. After 6 months, there were significant relationships between grazer density and both mortality and growth with increased mortality and reduced growth for P. vulgata at increased densities, and reduced growth for P. depressa at increased densities. Hence, the availability of microbial resources may also influence the biomass of grazers on rocky shores from the bottom upwards. A conceptual model is presented which describes seasonal and annual variations in microbial resources and grazing intensity and their potential consequences for other shore dwellers.     

Gender Expression in Abies pinsapo Boiss., a Mediterranean Fir

Gender expression was recorded for three geographically distinctpopulations and two stand densities of pinsapo fir (Abies pinsapo) in southern Spain, during 4 consecutive years (1990–1993).During this period the trees only flowered in 1991 and 1992,and thus only two cone crops could be quantified. No significantgeographical variation in flowering intensity or in cone cropproduction was found. The most extreme variations were correlatedwith stand density: flowering intensity and cone crop were alwaysgreater in low density trees.Abies pinsapo is a monoecious speciesbut all the populations studied showed a predominance of femaleplants, with a few male and monoecious individuals, thus indicatinga functionally subdioecious breeding system. No differencesin gender expression between populations were recorded. Differencesin sex expression of individuals at two different stand densitieswere found, but these were only significant in 1991. In densestands of pinsapo firs branches were limited to the top of treesand thus, they produced mainly female cones, whereas isolatedtrees had branches from ground level and as a consequence theywere predominantly male, with a higher total reproductive effort.The results indicate that relative investment in male and femalereproductive structures byA. pinsapo individuals is a responseto plant architecture, plant resource status and environmentalvariation. Abies pinsapo ; plant density; gender variation; reproductive biology; monoecy; Mediterranean fir     

SHELL SCARRING IN CALLIOSTOMA ZIZYPHINUM (PROSOBRANCHIA: TROCHIDAE) FROM STRANGFORD LOUGH, NORTHERN IRELAND

Pink and white shells of Calliostoma zizyphinum show both undamagedand damaged scars. White shelled individuals predominate inStrangford Lough, Northern Ireland where Calliostoma densitiesand scarring levels are high. No white shells were found intertidally,outside the entrance channel to the Lough. Significant differencesoccur in the size of Calliostoma shells between sampling sitesbut not between different colour morphs at each site. Howeverin pooled samples, pink individuals are significantly largerthan white individuals. When sites and morphs are consideredseparately, pink Calliostoma density is negatively correlatedwith water movement. At sites where pink Calliostoma occur,the percentage of pink shelled individuals is negatively correlatedwith total Calliostoma density. Damaged and undamaged scarring values per unit area of shell,show highly significant differences between sites and damagedscars are significantly higher in white individuals. Undamagedscars are not correlated with any of the environmental parametersrecorded, but are positively correlated with damaged scars suggestinga common causative factor. The level of damaged scarring ispositively correlated with crab/total Calliostoma ratio at allsites and where each colour morph was considered separately.Multiple regression analyses reveal that crab/Calliostoma ratiosaccount for 42% of the between site variation in damaged scars.Significantly higher levels of damaged scars are found at siteswith high crab densities and significantly larger individualsare found at sites where crab densities are intermediate invalue. The largest and most highly scarred individuals occur at siteswith most coarse substrata where Calliostoma are present intheir lowest densities. The higher scarring levels and smallersize of white individuals reflect either higher mortality orreduced growth in white shelled Calliostoma. (Received 25 November 1991; accepted 5 November 1992)     

RELATIONSHIPS BETWEEN SHELL SHAPE, WATER RESERVES, SURVIVAL AND GROWTH OF HIGHSHORE LITTORINIDS UNDER EXPERIMENTAL CONDITIONS IN NEW SOUTH WALES, AUSTRALIA

The shell morphologies of the highshore littorinids, Littorinaunifasciata Gray and Nodilittorina pyramidalis (Quoy & Gaimard)have previously been shown to vary at a variety of spatial scales,including among replicate sites at the same height, from heightto height and from shore to shore. In this study, the relationshipsbetween morphology of the shell, the reserves of water heldwithin the shell, the size of the foot and survival on differentshores and rates of growth in different habitats were examinedfor L. unifasciata and, to a lesser extent, N. pyramidalis.Reserves of water were not consistently related to size or shapeof the shell, but did increase as relative weight of shell increased.This may be due to the relatively smaller body providing moreinternal volume for extra-corporeal water. Water reserves andthe amount of free water held in the shell were also not relatedto loss of water or survival during extended periods of emersion.Although the shape of the shell on sheltered and exposed shoreswas correlated with size of the foot, with the snails on anexposed shore having larger apertures and feet than those ona sheltered shore, transplant experiments did not show differentialmortality between morphs from the different shores. All translocatedand transplanted snails disappeared from the exposed shore ata greater rate than from the sheltered shore, but this was probablydue to the snails dispersing out of the experimental areas ratherthan due to mortality. Therefore, many of the large-scale modelsthat have previously been used to describe patterns of shellshape in intertidal gastropods do not appear to be importantin these highshore littorinids. Finally, field experiments ongrowth of juvenile L. unifasciata indicated that rate of growth,largely governed by opportunity to feed rather than type andquantity of food, is the most likely explanation for the small-and large-scale patterns of shell shape that have been previouslydescribed in this species. (Received 22 October 1996; accepted 19 February 1997)     

FAECAL PRODUCTION AND AN ESTIMATE OF FOOD INTAKE IN THE WILD OF THE ANTARCTIC LIMPET NACELLA CONCINNA (STREBEL)

The Antarctic limpet Nacella concinna is common in shallow watersaround the Antarctic Peninsula, and has a mean density of 125individuals/m² at Signy Island (Picken, 1980). In the australsummer 1986/87 ammonia excretion and faecal egestion were measuredas part of a programme to develop a detailed individual energybudget for this species. Ammonia production was measured in 102 individuals ranging from3 to 720 mg dry weight. For a standard limpet of 200 mg dryweight, ammonia excretion was 0.13 µg-at/hr. Comparedwith previous measures of oxygen uptake these data suggest anO:N atomic ratio of between 15 and 25. The relationship betweenammonia excretion and dry weight could be expressed by a powercurve with a weight exponent of 0.82 (SE 0.042). Faecal production was measured in limpets freshly sampled fromthe field, and placed in dean seawater for 4 days. During thistime faecal production decreased (since the limpets were notfeeding), although only data from the first 24 hours were used.Again the relationship between faecal egestion and dry weightcould be expressed by a power curve, although this time theweight exponent was 0.94 (SE 0.101). The ash content of thefaecal strings increased significantly with the size of thelimpet, possibly because larger limpets were ingesting a greaterproportion of substrate during feeding. If this is so, thenthis would also explain the weight exponent dose to 1.0, ratherthan the value of about 0.8 to be expected from metabolic measures(since larger limpets would be passing relatively more faecalmaterial). If assimilation efficiency is known, then food intake in thewild may be estimated from a measurement of faecal egestionand faecal organic content (Clarke et at., 1988). Applying typicalvalues of assimilation efficiency for limpets (40 to 60%) suggestsa daily food intake in Nacella of 2 to 3% body weight per day.This is a quite typical figure for a grazing limpet, and suggeststhere is no resource limitation for Nacella at Signy in summer.     

CORRECTING AERIAL SURVEY COUNTS OF HARBOR SEALS (PHOCA VITULINA RICHARDSI) IN WASHINGTON AND OREGON

Aerial surveys of harbor seals on land produce only a minimum assessment of the population; a correction factor to account for the missing animals is necessary to estimate total abundance. In 1991 and 1992, VHF radio tags were deployed on harbor seals ( n = 124) at six sites in Washington and Oregon. During aerial surveys a correction factor to account for seals in the water was determined from the proportion of radio-tagged seals on shore during the pupping season. This proportion ranged from 0.54 to 0.74. Among the six sites there was no significant difference in the proportion of animals on shore nor was there a difference in age/sex categories of seals on shore between sites. The pooled correction factor for determining total population abundance was 1.53. An additional 32 seals were radio tagged in 1993 at one of the sites used in 1991. Comparing data from the two years, we found no interannual variation. Aerial surveys of all known harbor seal haul-out sites in Washington ( n = 319) and Oregon ( n = 68) were flown during the peak of the pupping season, 1991–1993. The Washington and Oregon harbor seal population was divided into two stocks based on pupping phenology, morphometics, and genetics. Mean counts for the Washington inland stock were 8,710 in 1991, 9,018 in 1992, and 10,092 in 1993. Oregon and Washington coastal stock mean counts were 18,363 in 1991, 18,556 in 1992, and 17,762 in 1993. Multiplying the annual count by the correction factor yielded estimates of harbor seal abundance in the Washington inland stock of 13,326 (95% CI = 11,637–15,259) for 1991, 13,798 (95% CI = 11,980–15,890) for 1992, and 15,440 (95% CI = 13,382–17,814) for 1993. In the Oregon and Washington coastal stock the corrected estimate of harbor seal abundance was 28,094 (95% CI = 24,697–31,960) in 1991, 28,391 (95% CI = 24,847–32,440) for 1992, and 27,175 (95% CI = 23,879–30,926) for 1993.     

ASYMMETRICAL DEVELOPMENTAL PLASTICITY IN AN INTERTIDAL SNAIL

Field and laboratory experiments were used to investigate the variation and phenotypic plasticity in the adhesive abilities of the intertidal snail Nucella lapillus between high- and low-wave-energy environments. Whelks from an exposed coast produced a larger pedal surface area and were more resistant to dislodgement than were similar-sized individuals from a protected shore. Tenacity (g/cm²) was similar between individuals from exposed and protected shores, indicating that variation in resisting dislodgement was solely a function of pedal surface area. Whelks from exposed and protected shores did not differ in pedal surface area as they emerged from egg capsules or when reared in the laboratory under uniform conditions, suggesting that variation between populations does not represent genetic differentiation. Individuals from high-and low-wave-energy environments reared intertidally produced a larger pedal surface area than did those reared in the laboratory. The extent to which pedal surface area increased corresponded to the intensity of wave action. These findings suggest that pedal surface area is a highly plastic character modulated or induced by the water turbulence accompanying breaking waves. A reciprocal-transplant experiment confirmed this notion but revealed an asymmetry in the plasticity. Snails from the protected site transplanted to an exposed shore formed a much larger pedal surface area than did controls reared on the protected shore. In contrast, whelks from a wave-swept shore transplanted to a protected shore differed little from their controls reared on the exposed shore. The asymmetric response parallels a possible asymmetry in the risks of acclimating to a temporally unpredictable environmental cue, such as wave action.     

Density and shell-size variation of Nodilittorina lineolata (Orbigny, 1840) in the intertidal region in southeastern Brazil

Nodilittorina lineolata is a small littorinid characteristic of the supralittoral of rocky shores but extends its vertical range to the lower midlittoral. Shell-size and density of Nodilittorina were measured at two different levels, the supralittoral and barnacle zones, in six sites along the south-eastern Brazilian coast. The sites studied varied in the degree of wave exposure. Densities were lower in the supralittoral, and higher overall values occurred on the most exposed shores. Shell size increased significantly in an upshore direction at five of the six sites. However, mean-size and the magnitude of the difference between levels varied according to the site studied. The shell-size gradient can be explained by larval recruitment in the barnacle zone and subsequent ascending migration to the supralittoral zone, where the larger individuals were found. Reciprocal transfer experiments revealed that differential mortality of large individuals in the lower levels, possibly due to lack of refuges, may also influence the size-distribution. It was also shown that some individuals can be passively transported to upper levels of the shore by wave action during storms.     

TEMPORAL VARIATION IN FORAGING BEHAVIOUR OF PATELLA GRANULARIS (PATELLOGASTROPODA) AND SIPHONARIA CONCINNA (BASOMMATOPHORA) ON A SOUTH AFRICAN SHORE

Foraging activity of two mid- to low- shore species of limpet,Patella granulans (Prosobranchia) and Siphonaria concinna (Pulmonata)from an exposed shore on the Eastern Cape coast of South Africawas monitored. In both species, activity was compared duringspring and neap tides and, in P. granularis between summer andwinter. Rhythms of activity of the two species were similar,with foraging excursions being mainly associated with nocturnallow tide times, although some P. granularis foraged during daytimelow tides. It is suggested that foraging excursions in P. granularisare triggered by wave action. Both species foraged further onspring tides than on neap tides and this is suggested to bea result of the limited time limpets have to forage. P. granulariswas also found to forage further during summer when comparedto winter and the possibility that seasonal micro-algal productivityinfluences foraging distances in limpets is discussed. The foraging activity of both species could be divided into3 distinct phases, a relatively rapid outward phase, a muchslower foraging phase and a rapid homeward phase. Whether ornot these limpets graze throughout an excursion is not known.S. concinna was found to home to a fixed scar, although duringthe experiment some scar-swapping occurred. P. granularis didnot home to a fixed scar but possessed a ‘home range’(approx. 5 cm²) to which it returned after each excursion. Patella granularis was found to move randomly during foraging,whilst S. concinna foraged in a non-random direction -whichtook individuals upshore. No tidal-influence is thought to bepresent in this case and the possibility of a learning componentin the foraging behaviour of certain limpet species in relationto the return to optimal feeding patches is discussed. (Received 18 May 1996; accepted 2 September 1996)     

Quantification of radular marks as a method for estimating grazing of intertidal gastropods on rocky shores

Wax discs have been used previously on intertidal rocky shores to record the grazing activity of gastropods. This study has evaluated this methodology for recording grazing of four common intertidal microalgal grazers on intertidal shores in New South Wales, Australia. In the laboratory, the four species examined-the patellid limpet, Cellana tramoserica (Sowerby), the trochid, Austrocochlea porcata (A. Adams), the neritid, Nerita atramentosa Reeve and the littorinid, Bembicium nanum (Lamarck)-made distinctive marks in the wax. These allowed identification of each species or combinations of species grazing over the different discs. Field experiments showed that the intensity of grazing, as indicated by the mean number of scratches per disc, was positively related to the number of gastropods in the surrounding area during low tide for C. tramoserica. The number of scratches per disc in any area was correlated with the percentage of discs scratched. The relationship for C. tramoserica was found at two scales-in sites (approximately 3x3 m) and also in plots (50x50 cm) within sites. Therefore, densities that were measured when these limpets were inactive during low tide provided good estimates of grazing activity during high tide. This is largely because these limpets do not move far between where they rest and where they feed. The amount of microalgal food in the vicinity was not correlated with density, nor with grazing intensity. No relationship between density and grazing intensity was found for N. atramentosa, although experiments were only done in the field at one spatial scale (in sites, 3x3 m). Results obtained in the laboratory and in the field show that wax discs are useful to distinguish grazing by different species of gastropods on Australian rocky shores and allow tests of hypotheses about grazing activity at different spatial scales.     

Food supply, grazing activity and growth rate in the limpet Patella vulgata L.: a comparison between exposed and sheltered shores

The limpet Patella vulgata L. is an important microphagous grazer on intertidal rocky shores of north-west Europe, occurring across the wave exposure gradient. Groups of P. vulgata were selected at mid-tide level of two exposed shores and two sheltered, fucoid dominated shores on the Isle of Man, British Isles, and manipulated to equivalent densities and population structure. The level of grazing activity and growth rate were determined over a 1-year period. At the same time, the abundance of epilithic microalgae, measured as the concentration of chlorophyll a, was determined as an estimate of food supply. Microagal abundance showed a seasonal pattern in both exposed and sheltered conditions, with higher levels in winter compared to summer. In both seasons, the microalgal resource was more abundant on the sheltered shore studied. The level of grazing activity in P. vulgata showed a seasonal pattern on the exposed but not the sheltered shores. Averaged over the year, grazing activity on the exposed shores was over double that on sheltered shores. Thus, in sheltered conditions, food supply for limpets was high and grazing activity low; in exposed conditions, food supply was low and grazing activity high. The growth rate of P. vulgata, measured as increase in shell length, showed no significant difference between exposed and sheltered shores. Growth rate was also determined in P. vulgata at natural densities. Although the overall density declined with decreasing exposure to wave action, the density per unit area of grazeable substance was higher in shelter. In these populations, the mean growth rate was over twice as high on exposed compared to sheltered shores.     

An analysis of distribution and abundance of populations of the high-shore limpet,Notoacmea petterdi (tenison-woods)

Summary Notoacmea petterdi occurs only on vertical surfaces at the highest levels on the shore. On a given surface, density of limpets decreases with increasing height. Mean and maximum sizes of limpets increase with increasing height, due to faster rates of growth at higher levels where densities are lower. Rates of mortality are also negatively correlated with height on the shore, and are therefore generally less at higher levels. Both adult and juvenile limpets exhibit well-developed homing behaviour. There is no evidence of seasonal migrations, density-dependent migrations, nor gradual upward migrations as limpets grow older.Hypotheses are formed on the basis of these preliminary observations and measurements. Results of subsequent experimental manipulations of densities support the hypothesis that density-dependent mortality, due to intraspecific competition, plays a major role in regulating population densities of this limpet. It is also suggested that the pattern of settlement of juvenile limpets is the major factor that determines the limits to the vertical distribution of this species.