Effects of prey density,prey size and predator size on rates of feeding by an intertidal predatory gastropod Morula marginalba Blainville (Muricidae), on several species of prey

As a prerequisite for models of foraging behaviour of the whelk, Morula marginalba Blainville (Muricidae), the effects of variation in density of prey on the rate of feeding of the predator were examined in field conditions for three coexisting species of prey. Densities of prey used were those at which the prey, two limpets and a barnacle, occurred naturally in the rocky intertidal habitat.Large limpets, Cellana tramoserica (Sowerby) can resist attacks by predatory gastropods by raising the mantle over the outside of the shell. These experiments showed that no C. tramoserica were killed by Morula marginalba even at very great densities and with no alternative prey present. For the small limpet Patelloida latistrigata (Angas), one of the whelk's most highly preferred prey, juveniles were eaten 1.4 times as fast as adults. Fitting the random predator equation gave greater attack coefficients and shorter handling times for juvenile than adult limpets.Sizes of both predator and prey affected rates of eating barnacles, Tesseropora rosea (Krauss), but not in a simple way. Whelks of 15-mm aperture length ate adult barnacles 4.2 times faster than did 12-mm whelks, but there was no significant difference in the rates at which the two sizes of snail ate juvenile barnacles.Rates of feeding on T. rosea and Patelloida latistrigata increased significantly with prey density. These results form a basis for including the density of prey in models of spatial dispersion of the predatory gastropod Morula marginalba.     

Experiments on factors influencing settlement,survival, and growth of two species of barnacles in new south wales

Adult Tesseropora rosea (Krauss) and Tetraclitella purpurascens (Wood) are mostly found in the eulittoral (barnacle) zone of rocky seashores in New South Wales. Below this zone most space is occupied by the tube-worm Galeolaria caespitosa (Lamarck) or by various species of macroalgae. Within the eulittoral zone, T. rosea are mostly on sunny areas of rock exposed to relatively strong wave-action. T. purpurascens are present mainly in crevices, caves, and under ledges where there is considerable shade.Cyprids of both species settled on sandstone plates and on experimentally cleared areas in the barnacle and Galeolaria zones. Neither species settled where the substratum was already covered by algae or Galeolaria. No spat of T. purpurascens were found in sunny areas of the barnacle zone. T. rosea, however, settled in cleared substrata in sunny and shaded areas. Neither species settled in the littoral fringe above the upper limit of distribution of adults. On boulders transferred to high levels of the shore during a storm, small T. purpurascens died within a few weeks.Barnacles of both species which had settled in experimentally cleared areas in the Galeolaria zone survived and grew. In these areas some T. purpurascens were killed by being smothered by tube-worms which settled after the barnacles. This probably happens to T. rosea, but was not demonstrated experimentally. In the Galeolaria zone, both species of barnacles were very quickly smothered and killed by macroalgae growing over them, except where these were experimentally removed.Within the barnacle zone, all newly-settled spat of T. purpurascens transferred to sunny sites died within two months, whilst many of those in shaded sites survived. In areas where wave-action was strong, spat of T. rosea survived and grew well in sunny areas, but survived better in the shade. Under a ledge, however, where wave-action was reduced, all the T. rosea in sunny sites, and most of those in shaded sites died within two months; many newly-settled T. purpurascens survived in the shade in this area.The grazing limpet Cellana tramoserica (Sowerby) dislodged and crushed some newly-settled T. rosea and reduced survival in some sunny areas. T. rosea settled preferentially on bare rock and were rarely found on the shells of adult barnacles. Thus, the density of spat was greater where adult barnacles were absent. In contrast, many newly-settled T. purpurascens were found on the shells of adults of their own species in shaded areas; they also settled on cleared rock. Because T. purpurascens tended to settle amongst and on adults, and in crevices and confined areas, they were not much affected by limpets. When newly-settled T. purpurascens were in high densities, they had lower survival than in areas with reduced densities, because of squashing and smothering by each other.The upper and lower limits of vertical distribution (zonation) of these two species of barnacles are determined primarily by the settlement of cyprids. Neither species settled at the highest levels on the shore. Whether this was due to the decreasing time of submersion during high tide towards the top of the shore, or a result of preferences for settlement site is unknown. Even if cyprids were to settle in the littoral fringe, the spat would die very quickly probably as a result of desiccation. Below the barnacle zone, the entire substratum is usually occupied by other sessile species, particularly macroalgae, on which the barnacles do not settle. In experimentally cleared areas below the barnacle zone, or in any naturally cleared areas both species settled, and could survive the physical conditions. Newly-settled spat were, however, overgrown and killed by algae and Galeolaria.Within the barnacle zone, T. purpurascens is restricted to shaded areas because of the inability of newly-settled spat to survive the physical stresses of high temperature and desiccation in sunny habitats. T. rosea appears to be excluded from shaded areas by a combination of the lack of suitable substrata on which to settle, and the effects of reduced water-flow in many crevices and under ledges. T. rosea survived better in areas with strong wave-action and can survive in shaded areas where water-flow is not reduced by the topography of the substratum.     

Analysis of inter- and intra-specific competition amongst intertidal limpets with different methods of feeding

Summary The prosobranch limpet Cellana tramoserica grazes on microalgae, including the spores of macroalgae, and coexists with the pulmonate limpets Siphonaria denticulata and S. virgulata at mid-tidal levels on sandstone shores in New South Wales. These siphonarians graze on macroalgae, leaving the basal parts of the thalli intact. Where Cellana graze, they are capable of removing all algae; where Siphonaria graze, they leave at least a thin film of alga on the rocks, which is available to Cellana. S. denticulata normally show invariant homing behaviour, whereas S. virgulata tend to move around at random when Cellana are present, but apparently home when Cellana are absent. Both siphonarians have been observed to show opportunistic behavioural responses by moving towards patches of macroalgal foods when they become available.Experimental ecclosures of limpets at different densities and in different combinations revealed that Cellana tramoserica suffered increased mortality and reduced growth due to intraspecific competition when at increased densities. There was no effect on Cellana of increased densities of either species of Siphonaria. Nor was there any interspecific interaction between the siphonarians. Both species of Siphonaria showed some reduction of growth at increased intraspecific density. More importantly, both showed increased mortality when enclosed with low densities of Cellana. Larger densities of Cellana had no effect; the numbers of Cellana could not be maintained because of the reductions caused by intraspecific competition. Even after 27 weeks in enclosures with Cellana, the numbers of Siphonaria never declined to zero in any experimental enclosure. Thus, Cellana has a competitive effect on the survival of siphonarian limpets, but is unable to exclude them from an area of the shore. Siphonaria spp., in contrast, have no effect on Cellana.The nature of the competitive interactions between these types of limpets is explaied in terms of their methods of feeding; Cellana can exploit the food-resource before it reaches a suitable size for Siphonaria. The coexistence of Siphonaria spp. with Cellana is discussed with respect to the behaviour of the pulmonates. Intraspecific competition leading to reduced densitities of Cellana, however, will ensure that Cellana cannot exploit all the food resources, and some will be available to Siphonaria. The consequences of inter- and intra-specific competition among grazing gastropods are discussed with reference to the structure of intertidal communities, and it appears that competition for food is fundamentally different from competitive interactions for space in the organization of such communities.     

Trying to fit in: are patterns of orientation of a keystone grazer set by behavioural responses to ecosystem engineers or wave action?

The distribution of animals varies at different temporal and spatial scales. At the smallest scale, distribution may be orientated in regard to particular environmental variables or habitat features. For animals on the rocky intertidal, the processes which set and maintain patterns of distribution and abundance in wave-exposed areas are well studied, with explanatory models focused on wave action and, more recently, the role of biogenic habitats. In contrast, patterns of orientation by intertidal animals have received less attention, although having ecological and fitness consequences. Here, we report tests of competing models to explain the observation that limpets on steeply sloped surfaces orientate downwards. A greater proportion of downwards-facing limpets was found in sheltered sites and areas without barnacles and this pattern was consistent across many shores and sampling occasions. Additionally, the frequency at which limpets were dislodged after a storm was independent of orientation. To test whether orientation is a behavioural response to habitat-forming barnacles, barnacles were removed and/or killed from patches of substrata and the change in proportion of downwards-facing limpets measured. The proportion increased with barnacle removal and this behaviour was a response to the structure of the barnacles, not a biotic effect associated with the living organism. Our study suggests that biogenic habitat not wave action sets patterns of limpet orientation and barnacle shells, regardless of whether the barnacle is alive or not, limit the ability of limpets to adopt a downward orientation.     

The impact of predation by Kelp GullsLarus dominicanus on the sub-Antarctic limpetNacella delesserti

Summary The Southern Kelp GullLarus dominicanus derives about 45% of the mass of its marine diet from the limpetNacella delesserti at sub-Antarctic Marion Island. It selectively feeds on the largest limpets (35–65 mm) —a pattern most obvious in adult gulls — and eliminates about 19.6% of these limpets per year in the intertidal zone and in shallow waters, thus accounting for about 50% of the known annual mortality of limpets of this size. The gulls cannot forage for limpets in water more than 40 cm deep, and in the intertidal zone boulder-fields provide the limpets a refuge against the gulls. The gulls are also selective in feeding most often on limpets with pale shells. This alters the colour composition of limpet populations in habitats where predation is most intense.Larus dominicanus seldom feeds on limpets on the mainland of South Africa, and only on small limpets, probably because a greater range of food is available, but possibly also because the limpet species that occur on South African shores attach with forces that are three to six times those ofN. delesserti.     

Non-linear density-dependent effects of an intertidal ecosystem engineer

Ecosystem engineering is an important process in a variety of ecosystems. However, the relationship between engineer density and engineering impact remains poorly understood. We used experiments and a mathematical model to examine the role of engineer density in a rocky intertidal community in northern California. In this system, the whelk Nucella ostrina preys on barnacles (Balanus glandula and Chthamalus dalli), leaving empty barnacle tests as a resource (favorable microhabitat) for other species. Field experiments demonstrated that N. ostrina predation increased the availability of empty tests of both barnacle species, reduced the density of the competitively dominant B. glandula, and indirectly increased the density of the competitively inferior C. dalli. Empty barnacle tests altered microhabitat humidity, but not temperature, and presumably provided a refuge from wave action. The herbivorous snail Littorina plena was positively associated with empty test availability in both observational comparisons and experimental manipulations of empty test availability, and L. plena density was elevated in areas with foraging N. ostrina. To explore the effects of variation in N. ostrina predation, we constructed a demographic matrix model for barnacles in which we varied predation intensity. The model predicted that number of available empty tests increases with predation intensity to a point, but declines when predation pressure was strong enough to severely reduce adult barnacle densities. The modeled number of available empty tests therefore peaked at an intermediate level of N. ostrina predation. Non-linear relationships between engineer density and engineer impact may be a generally important attribute of systems in which engineers influence the population dynamics of the species that they manipulate.     

Population regulation in the intertidal limpet Patelloida alticostata (Angas, 1865)

Summary Subpopulations of the limpet, Patelloida alticostata, converged slowly toward a density of about 75 per linear meter of vertical rocky shore two years after experimental alteration of densities of adult animals. The changes in number of limpets in subpopulations occurred because large limpets suffered considerable mortality, while recruits experienced almost none. Neither migration, cannibalism, selection of settlement site by the limpets, nor predation by a whelk, acted in a density-dependent manner.Growth rates were density-dependent, and juveniles and recruits reached a much larger size in the absence than in the presence of adult limpets. Increasing the density of adult limpets did not increase the extent of grazing areas and subpopulations of different size compositions utilized similar grazing areas. Food may have been in short supply.These limpets have a great deal of population inertia, accommodating sporadic good recruitment by compensations in growth rates.     

Desiccation,predation, and mussel-barnacle interactions in the northern Gulf of California

Summary Field experiments were conducted in order to determine the potential for desiccation and predation to mediate the effect of mussels (Brachidontes semilaevis) on barnacles (Chthamalus anisopoma) in the highly seasonal northern Gulf of California. We did this by removing both mussels and a common mussel predator (Morula ferruginosa: Gastropoda) and by spraying selected sites with sea water during summertime spring low tides. We also determined the effect of crowding on resistance to desiccation in barnacles, and the effect of barnacles on colonization by mussels. The mussel-barnacle community was not affected by keeping experimental quadrats damp during daytime low tides throughout the summer. Exposure to summertime low tides, however, did affect the survivorship of isolated, but not crowded, barnacles; and barnacle clumps enhanced the recruitment of mussels. Hence crowding in barnacles had a positive effect on both barnacle survivorship and mussel recruitment. Morula had a negative effect on mussel density, and mussels had a negative effect on barnacle density. The effect of Morula on barnacle density was positive, presumably due to its selective removal of mussels. These results suggest an indirect mutualism between barnacles and the gastropod predator, because barnacles attract settlement or enhance the survival of mussels, and the predator reduces the competitive effect of mussels on barnacles.     

The vertical distribution of Chthamalus montagui and Chthamalus stellatus (Crustacea,Cirripedia) in two areas of the NW Mediterranean Sea

The Mediterranean Sea is characterised by a small tidal range (0.3–1 m). Despite this, intertidal communities are well established and their upper limits often extend above mean high water level. Organisms living in the intertidal region and in the supralittoral zone rely on both tides and wave action to perform their biological functions. Lack of food, desiccation and predation are common stresses in such a harsh environment. The present study deals with the vertical distribution of two species of intertidal barnacles, Chthamalus montagui Southward and Chthamalus stellatus (Poli), which are the main constituents of the barnacle belt along Mediterranean rocky shores. Previous work, carried out in the Atlantic, showed that where the distribution ranges of the two Chthamalus species overlap, C. montagui is more common in the upper barnacle zone while C. stellatus is dominant lower down. The main aims of our study are: (1) to establish if there is a relationship between position and extension of the barnacle belt on the shore and tidal range and/or wave exposure, (2) to test the hypothesis that in the study areas C. montagui is more abundant than C. stellatus high on the shore, and that the pattern is reversed lower down. Barnacle populations were monitored in summer 1998 in the Gulf of Genoa (Ligurian Sea) and in the Gulf of Trieste (North-Adriatic Sea). The two areas differ in tidal range and hydrodynamism, the former presenting quite strong wave action and a tidal range of 30 cm, the latter having limited wave action and 1 m tidal range. Three shores were randomly selected in each gulf and two transects on each shore. Counts of barnacles in 10 ^* 10 cm quadrats were done at different shore heights along each transect. The data was subjected to analysis of variance. Results showed that a more pronounced hydrodynamic regime corresponded to a shift of the barnacle belt towards the higher shore (Gulf of Genoa), while in more sheltered areas (Gulf of Trieste), the barnacle distribution was confined to the intertidal region. The relative spatial distribution of C. montagui and C. stellatus within the barnacle belt varied locally, even between transects on the same shore, and this obscured the distribution pattern along the vertical gradient. Nevertheless, it was still possible to conclude that at mid and high shore in Genoa, C. stellatus was more abundant than C. montagui, while in Trieste the pattern was reversed.     

Vertical and seasonal patterns in competition for microalgae between intertidal gastropods

Grazing by the snail Nerita atramentosa and the limpet Cellana tramoserica caused similarly great reductions in abundance of microalgae — measured by direct counts and by estimation of chlorophyll analyses. A smaller snail, Bembicium nanum, caused smaller reductions of microalgal resources, compared with ungrazed areas. These results were consistent with the competitive abilities of these three species. Chlorophyll concentrations in samples of grazed rocks were reliable estimates of the nature and abundance of food available to the grazers.Inter- and intra-specific competition amongst Nerita and Cellana were investigated at various densities in experimental cages. To examine the effects of different availability of food resources, the experiments were repeated at three heights on the shore (abundance of food decreases with height) and during autumn/winter and spring/summer periods of the year (less food is available during summer).Density-dependent mortality of Cellana was caused by the presence of other limpets, or of Nerita. Mortality was greatest at higher levels and during the spring/summer and was significantly, negatively correlated with mean chlorophyll concentration in the experimental cages. The only exception was that all limpets suddenly died in cages at the highest level during the summer period, which cannot be explained solely by competitive interactions. Nerita showed no density-dependent mortality during the short periods of these experiments. Tissue-weights of Nerita declined with increased density, but the effect of Cellana was not as great as the intraspecific effect of Nerita. Snails retained weight better at lower than at higher levels, and during the autumn/winter which is consistent with the availability of food. Tissue weights of both Nerita and Cellana were positively correlated with chlorophyll concentrations inside the cages in both seasons investigated.These experimental results demonstrate that intensity of competition will vary from place to place and time to time according to the densities and mixtures of the grazers, and according to the availability of microalgal food.     

The effect of surface colour on the adhesion strength of Elminius modestus Darwin on a commercial non-biocidal antifouling coating at two locations in the UK

A number of factors affect the adhesion strength of organisms to fouling-release coatings, and except for a few studies focussing on black or white surfaces none have dealt specifically with the effect of coating colour. The aim was to test the effect of colour on the adhesion strength of the barnacle Elminius modestus. Panels coated in six commercial colours of Intersleek 700® were submerged at two field sites and barnacles were pushed-off using a standard assay procedure. The strength of adhesion (SOA) varied between and within sites for colour and by barnacle basal area, SOA per unit area being higher for smaller barnacles. Higher SOA with a small basal area may be because of size-specific predation, differential hydrodynamic effects or adhesive failure with age. The complex effect of colour on barnacle adhesion may be because of physico-chemical surface characteristics varying with pigments, and their interactions with local environmental conditions, as well as interactions with the settling barnacle larvae.     

Effects of interactions between algae and grazing gastropods on the structure of a low-shore intertidal algal community

Summary At low levels on shores in New South Wales, foliose algae are abundant and often occupy all substrata; microalgal grazing gastropods are rare or absent. At higher levels, foliose algae are sparse or absent and grazing gastropods are abundant. Hypotheses for the causes of the lower vertical limits of distribution of these grazers include the effects of increased predation or the deleterious physiological effects of increased period of submergence at lower levels on the shore. Alternatively, the presence of the algae, because they occupy space and deprive the grazers of substratum for feeding, may prevent the downward movement, or survival of the grazers at low levels. Under the first two of these hypotheses, algae are able to colonize and grow in low-shore areas as an indirect result of factors which remove grazers. Under the third hypothesis, the algae are directly responsible for the lack of grazers.Experimental clearings of the low-shore algae and introductions of the mid-shore limpets Cellana tramoserica and Siphonaria denticulata were used to test these hypotheses. C. tramoserica grazes microalgae and removes them from the substratum, preventing colonization. S. denticulata, in contrust, crops the algae, leaving a visible cover of algae on the substratum, which can grow rapidly. Because of its method of feeding, S. denticulata had no measurable impact on the rates of colonization, nor on the dry weights of algae, compared with those of ungrazed areas. C. tramoserica could keep cleared areas tree from foliose algae, but only when the limpets were mainfained in great density (10 per 900 cm²). They were less effective where wave-action was greater.Neither species of limpets could survive when placed onto beds of mature algae, because they had no substratum on which to cling and were swept away by the waves. C. tramoserica did not invade clearings below their lower limit of distribution where they had to move over a bed of foliose algae. Few C. tramoserica moved directly downshore into cleared areas. When placed on bare rock within low-shore beds of algae of different ages, S. denticulata remained amongst the algae and maintained their tissue-weights. Few C. tramoserica remained in areas with well-developed algae, compared with areas having sparse algal growth. Those Cellana which remained amongst well-developed algae lost weight, whereas limpets in areas with less algal growth mammtained their weights. In experimental cages in low-shore beds of algae, where the limpets were inaccessible to potential predators, C. tramoserica lost weight and died. On cleared areas they survived for many weeks, but lost weight and died as algae grew and covered the substratum. In the absence of predation, the micro-algal grazer C. tramoserica could not survive in lowshore areas because algae grew too fast and occupied the substratum, making it inaccessible for the limpets to graze; the algae, once grown beyond small sporelings, are not a suitable food-source for C. tramoserica, and the loss of weight and death of these limpets is attributable to starvation.The lower limit of distribution of C. tramoserica is not due to the direct effects of physical factors associated with prolonged submersion, nor to the impact of predators, but is apparently determined by the presence of rapidly growing, extensive beds of foliose algae at low levels on the shore. The cause of the limit of distribution of S. denticulata is not yet known and predation may prove to be important. Removal of S. denticulata from low-shore algal beds would not, however, affect the domination of substrata by algae. Grazing by S. denticulata at very great density had no effect on algal cover nor weight. In the intertidal community studied, the persistence of a low-shore algal zone, bounded above by abundant grazers is not influenced by the activities of predators, but is a direct result of interactions between the grazers and the algae.     

An experimental evaluation of the direct and indirect effects of endemic seaweeds,barnacles, and invertebrate predators on the abundance of the introduced rocky intertidal barnacle Balanus glandula

The barnacle, Balanus glandula has recently invaded along the Pacific coast of eastern Hokkaido, Japan. To evaluate the direct and indirect effects of endemic seaweeds, barnacles, and invertebrate predators on the abundance of B. glandula on the rocky intertidal coast of eastern Hokkaido, we conducted a field experiment from June 2011 to October 2012 in which we manipulated the presence or absence of these factors. Seaweeds showed no significant effect on the abundance of B. glandula. The endemic barnacle Chthamalus dalli and the invertebrate predator Nucella lima reduced the abundance of B. glandula. However, the simultaneous influence of N. lima and C. dalli was compensative rather than additive, probably due to keystone predation. These findings suggest that competition by the endemic barnacle C. dalli and predation by the invertebrate predator N. lima decreased the abundance of B. glandula, but that N. lima predation on C. dalli weakened the negative influence of C. dalli on B. glandula. The implications of these findings are twofold: the endemic competitor and invertebrate predator may have played important roles in decreasing the abundance of B. glandula in natural habitats, and conservation of endemic invertebrate predators may be crucial to impede the establishment and survival of introduced barnacles in rocky intertidal habitats.     

Community stability: effects of limpet removal and reintroduction in a rocky intertidal community

Summary The stability of a high rocky intertidal community was assessed in a controlled field experiment in which the most common consumers, limpets, were temporarily removed. Compared to the unmanipulated plots, the exclusion plots developed greater algal abundance and altered species composition of both algae and barnacles. The community was not perturbed beyond its capacity to recover, since the community structure of the limpet-removal plots converged on the structure of the unmanipulated plots following limpet reintroduction. Different components of the community recovered at different rates, depending on whether or not the species had a size-related escape from the limpets. Algae had no size-related escape from limpets. The difference in algal abundance between manipulated and unmanipulated plots lasted less than six months after limpet reintroduction. Barnacles, however, had a size-related escape from limpets and therefore recovered more slowly. The difference in barnacle species composition between the perturbed and unperturbed plots lasted for 17 months after limpet reintroduction. The length of the limpet removal period (16 or 28 months) did not appear to affect the rate of community recovery.     

Intertidal and Subtidal Fouling Assemblages in a Patagonian Harbour (Argentina, Southwest Atlantic)

The composition of the early stages of intertidal and subtidal fouling assemblages in Comodoro Rivadavia harbour (Argentina, 45°52′ S, 67°28′ W) and the influence of shore level and season on their structure were analysed. At the beginning of each season, stones were glued to the substratum with epoxy putty and distributed along 4 vertical transects at intervals of 20 m, at 3 levels: upper intertidal, middle intertidal, and subtidal. Substrata remained in the field for 84–100 days. A total of 48 samples (4 seasons × 3 levels × 4 replicates) were analysed. Species richness increased with depth, with 6 taxa in the upper intertidal, 23 in the middle intertidal and 31 in the subtidal. Seasonal differences in richness were less distinct. Green, red and brown algae were the dominant groups. Invertebrates were mainly represented by filter-feeding, sessile organisms, such as cheilostome bryozoans, spirorbid polychaetes and acorn barnacles. The barnacle Balanus glandula and the bryozoan Cryptosula pallasiana were the only non-indigenous species found in this study. Highly significant differences in structure among shore levels and seasons were evidenced by a two-way ANOSIM test. The upper intertidal is characterized by the filamentous green algae Urospora penicilliformis and Ulothrix flacca. The barnacle Balanus glandula is the most abundant species in the middle intertidal. The subtidal is defined mainly by the presence of the spirorbid polychaetes Paralaeospira levinseni and Romanchella perrieri, and the keyhole limpet Fissurella radiosa. Ordination of samples by season was less clear than by shore level.     

Demography, competitive interactions and grazing effects of intertidal limpets in southern New Zealand

Population dynamics and the effects of intraspecific competition on limpet growth and maintenance of bare patches were investigated for the intertidal limpet Cellana ornata (Dillwyn) at a boulder-dominated site and on a rocky platform near Kaikoura (South Island), New Zealand. Distribution and abundance patterns of C. ornata were described in relation to other biota and tidal height. C. ornata occurred almost exclusively in patches devoid of macroalgae, particularly in the mid-tidal zone. Both adult and juvenile limpets were most abundant on the tops of boulders, where their numbers were positively correlated with barnacle cover, which averaged 77%. The size structure and growth patterns of C. ornata were different between populations. Mark-recapture studies showed that the slopes of annual growth increments regressed on initial sizes were similar at both sites but that the annual increments on the platform were about 6 mm greater than on boulders. Growth virtually ceased at 27 mm for limpets at the barnacle-dominated boulder site and at 40 mm at the platform site. Recruiting cohorts had 20% survival on boulders and 37% on the platform during their first year. The largest size classes at both sites had around 57% annual survival. To test the effects of varying limpet densities on the growth and mortality of limpets and the maintenance of bare patches, densities of C. ornata were experimentally increased at both sites. Beyond a density of 4 per 0.25 m², sizes and survival of limpets were reduced at both sites, but the effect was more pronounced at the boulder site. Limpets at the boulder site were more effective at maintaining bare space than those on the reef platform. Enclosing limpets in plots with and without barnacles showed that C. ornata and a co-occurring species (Cellana denticulata (Martyn)) grazed more effectively and had greater growth in cleared plots. Overall, there was considerable variation in the demographics of C. ornata between populations driven by site-specific characteristics.     

Floating castles in the ocean: the barnacles of two giant fish aggregating devices from Okinawa, Japan

Barnacles are often considered invasive species because they spread to non-native waters via shipping and the movement of floating objects. In this study, we surveyed barnacles attached to decrepit, unloaded fish aggregating devices. These objects act like stepping stones for sessile organisms in the ocean. Barnacles were sampled and identified to the lowest taxonomic level possible. Eight barnacle species were recorded, three of which are new records for Japanese waters (Newmanella radiata, Tesseropora sp., Balanus sp.).     

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.     

Fluctuating asymmetry of <Emphasis Type="Italic">Amphibalanus</Emphasis> (<Emphasis Type="Italic">Balanus</Emphasis>) <Emphasis Type="Italic">amphitrite</Emphasis> (Cirripedia: Thoracica) in association with shore height and metal pollution

The level of fluctuating asymmetry (FA), which is defined as random deviations from perfect bilateral symmetry in the morphological traits of an organism, increases with increasing developmental instability, and it may be used as an indicator of environmental and/or genetic stresses. This study attempted to relate FA levels in the opercular plates of the barnacle Amphibalanus (Balanus) amphitrite with shore height and body trace metal concentrations. Barnacles were collected from both low and mid shores at six coastal locations with various degrees of marine pollution in Hong Kong. Four opercular traits, namely scutum length, tergum length, scutum width, tergum width were measured in the specimen while concentrations of five common trace metals (Cd, Cu, Cr, Mn and Zn) were determined in their body tissues using inductively coupled plasma-atomic emission spectrophotometry. Among the four traits, only tergum length and scutum width fulfilled the assumption of FA while their measurement errors were low. Across all sites, mid-shore A. amphitrite consistently exhibited a significantly higher FA level in scutum width than that in low-shore barnacles, but FA levels in tergum length were similar between the two shore heights. These results suggested that FA was trait-dependent, and mid-shore barnacles were possibly under high physical stress, such as desiccation and high temperature, leading to high developmental instability. Although no positive association was observed between FA and metal contamination in low-shore A. amphitrite, FA level in tergum length significantly increased with decreasing body concentration of manganese (Mn) in these barnacles. Such a negative relationship may be explained by the mechanism of bioaccumulation and physiological role of Mn in A. amphitrite with respect to the formation of barnacle shell plates. Handling editor: T. P. Crowe     

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.