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.     

Ecology of the intertidal snail Littorina acutispira Smith

Littorina acutispira Smith, a minute gastropod of < 3 mm shell height, lives at great densities in pools and on rock-surfaces at the highest levels of sea-shores in New South Wales. Populations from pools and dry areas were sampled on two shores for 18 months to investigate seasonal changes in density, size-structure, rates of growth and reproductive biology of the snails. Densities of snails increased between February and May, due to an influx of juveniles, and then declined until the following February, when they increased again. The decrease in density was due to the death of the largest snails at the end of summer, and the mortality of medium-sized snails between June and January. Longevity was estimated as 1–2 yr, but most individuals died by ≈ 16 months from settlement on the shore. Newly-settled snails grew to merge in size with those of the previous year's population by winter. During the summer months, the rate of growth of snails from a sheltered shore was greater than that of snails on a shore exposed to wave-action. Laboratory experiments revealed that this could be attributed to the presence of better quality food, or food in greater abundance on the sheltered shore, compared with the exposed shore.During winter months, but not during the summer, snails from dry areas grew more slowly than those from pools. An experiment demonstrated that some snails from dry areas might be able to compensate for reduced periods of feeding by being able to feed faster when submersed. This could not explain the differences in natural rates of growth.L. acutispira bred from October–November to March–April. Spawning in the laboratory was greatest during late summer (January to March). The percentage of mature oocytes in the gonads was small in winter and increased in early summer. Among the largest-sized snails, females outnumbered males. Two experiments, on unsexed and pre-sexed snails, demonstrated that the biased sex-ratio of the largest snails was due to faster growth by females.There was a greater density of snails on the exposed shore, which was correlated with the presence of barnacles. When barnacles were removed from experimental areas, the density of the snails declined within 24 h. This suggested that barnacles provided a refuge from wave-shock, rather than shelter from desiccation or high temperature. In laboratory experiments, snails were exposed to higher temperatures and less humidity than they would normally encounter on the shore. There was negligible mortality of small or large snails after 24 h of these conditions.This minute species grows quickly, recruits annually and has a short life-span. This type of life-history is discussed in comparison with similar small species from other habitats.     

Are the opposing selection pressures on exposed and protected shores sufficient to maintain genetic differentiation between gastropod populations with high intermigration rates?

Throughout the world intertidal gastropods living on exposed rocky shores differ strikingly in a number of morphological and life history traits from those on protected shores. Where surf is heavy gastropods tend to be smaller and to have thinner and smoother shells with larger apertures than do those from sheltered areas where crab predation is more intense. These morphological differences can occur within a species and there is evidence that they can be partially genetic and partially environmental. In addition the convergence of shell features in each habitat suggests that there are consistent differences between the selective pressures on exposed shores and the selective pressures on protected shores. I constructed a simulation model for a polygenic trait that experiences different selective pressures on exposed and sheltered shores. The results show that genetic differences can be maintained between the two populations despite high intermigration rates. Replacement of a portion of the random environmental variance with adaptive environmental variance reduces the effect of selection and thus the size of the difference maintained between the two populations. Genetic differentiation between exposed and protected populations can persist for significant periods of time and may have sometimes been the first step in speciation.     

The effects of exposure and predation on the shell of two British winkles

Changes in shell size and shell shape of the two British winkles Littorina nigrolineata and L. rudis were studied in relation to exposure and to crab-size. In both species, shells from exposed shores are smaller and more globose than those from sheltered shores. Also, in rudis of exposed shores the mouth is relatively wider. In shores of equally sheltered conditions, shells are bigger at those localities where crabs are large than at those localities where they are small. The largest shells are found in those localities where it is extremely sheltered, and the crabs are very large.
It is argued that on exposed shores, small shells are favoured because they have more possibilities than large ones to shelter in crevices and in barnacle interspaces, from the impact of winds and waves. A globose shell could accommodate more foot muscle and thus enable a stronger adherence to the rock; and an increased mouth diameter would increase the area of foot adherence to the rock. On sheltered shores, on the other hand, large, narrow-mouthed shells are favoured because they discourage crab predation, large crabs being abundant mainly on sheltered shores.
The possible significance of shell size and shape in relation to zonation is discussed, in view of the different predatory and physical conditions which prevail in different zones of the shore, and the different shell specializations which these conditions would require.     

Factors organizing rocky intertidal communities of New England: Herbivory and predation in sheltered bays

In New England, U.S.A., shores exposed to severe wave action are dominated by the common blue mussel Mytilus edulis L. while moderately protected areas are covered with perennial algae. It is thought that algae are limited by mussels which are a superior competitor. Because the effectiveness of predators is inhibited by wave activity, it is assumed that the rate of predation, which varies across this environmental gradient, accounts for the observed distribution of mussels and algae.Shores along sheltered bays appear to be an exception to this pattern and this study addresses some of the possible causes. In New England bays, mussels and barnacles Semibalanus balanoides (L.) are the most common organisms on the solid surfaces in the lower intertidal zone. Perennial macroalgae, such as Chondrus crispas Stackhouse and Fucus vesiculosus L., are rare. The distribution and abundance of species differs from that on moderately protected shores and is similar to very exposed shores which are dominated by mussels and barnacles.Herbivory by the common periwinkle Littorina littorea (L.) limits the abundance of F. vesiculosus and indirectly affects the success of mussels. During 4 years of experimental manipulations, F. vesiculosus rarely recruited in the presence of periwinkles but dominated experimental surfaces if periwinkles were excluded. When experimental surfaces with F. vesiculosus, which had been protected from herbivory for > 1 year, were exposed to natural conditions, herbivores cleared most of the surfaces within several months. Recruitment by barnacles and mussels was higher when periwinkles were excluded. However, the effect of periwinkles on mussels was indirect; the snails reduced barnacle success and thus reduced mussel recruitment which was enhanced by the surface irregularities provided by barnacles.The occurrence of mussels in sheltered bays is not due to a lack of predators. Predators were commonly seen at all sites. Most mussels on experimental surfaces were removed <4 wk when surfaces were exposed to natural levels of predation. Experiments do not provide an explanation for the occurrence of mussels, although the enhancement of mussel recruitment by barnacles suggests that the availability of settlement sites may be important.     

Some observations on shell shape variation in Pacific Nucella

This paper considers the patterns of shell shape variation shown by Nucella canalicuata, N. emarginata and N. lamellosa from two areas of the Pacific Northwest: the shores near Friday Harbour on San Juan Island and near Bamfield on the west coast of Vancouver Island. No clear pattern of variation in association with changes in exposure was seen in either N. canaliculata or N. lamellosa . It appears that genetic influences are more important controls of shell shape than environmental selection in both these species. Nucella emarginata shows the nearest approximation to the pattern shown by the Atlantic species, N. lapillus , but only at the exposed end of the wave-action gradient. On those shores, enclaves from the most surf-washed open coast headlands have shells with proportionally larger apertures (and thus a shorter, squatter form) than their equivalents in local shelter. But, unlike in N. lapillus , the trend does not continue onto genuinely sheltered shores. Under these circumstances the species is generally rare and, where enclaves do occur, their shells are of much the same shape (although of a much larger size) as in more exposed situations.     

Visual selection of shell colour in two littoral prosobranchs

The rough periwinkle Littorina “saxatilis” exhibits a wide range of shell colours. In current literature it is claimed that these colours are not related to the environment, that they arise randomly as genetic accidents and that they have little positive survival value. In a previous paper it has been shown that, in Wales, “saxatilis” consists of four separate, fully sympatric species. This paper reports an investigation as to whether the colour of two of these species, nigrolineata and rudis, is related to the colour of the background. Because of difficulties in quantitatively describing the colour of the background it was decided to concentrate mainly upon one aspect: whether or not the frequency of red shells is larger upon shores consisting of red sandstone than elsewhere in Wales. In both species the association between red shells and red sandstone is highly significant. All nigrolineata samples in which red was found at a frequency above 15% are from red sandstone. On red sandstone, red shells of this species were found mainly upon sheltered shores, their frequency decreasing and that of white shells increasing with exposure. This may be because barnacles, which occupy the same vertical zone as nigrolineata, are more abundant upon exposed shores. Partly covering the red rock, barnacles create a white background upon which white shells, rather than red, are cryptic. Yellow shells are found mainly on sheltered shores, where the brown alga Fucus is abundant. It was observed that when the tide is in and the algae spread out, a yellow shell situated beneath them is well concealed. Yellow is also found upon barnacles which because of fungal and lichen infections, are dirty yellow. It is suggested that a striped (“nigrolineated”) pattern breaks up the shape of the shell. It also resembles the colour of the dark rock and the dark sutures between barnacle plates. In rudis 80% of the samples containing over 25% red are from red sandstone. Contrast to nigrolineata, in this species the relative frequency of red decreases on sheltered shores. This could be because the brown alga Pelvetia, which occupies the same vertical zone as rudis, is more abundant in sheltered conditions and its colour partly covers over that of the rock beneath. The fact that in both species red shells are more frequent upon red sandstone than elsewhere in the study area, suggests that visual selection is restricting their distribution to the background that they match most. Rock pipits and shore-crabs prey upon winkles. They have colour perception and could be partly responsible for this selection.     

Barnacle larval supply to sheltered rocky shores: a limiting factor?

In northwest Europe, sheltered rocky shores are dominated by fucoid canopy algae and barnacles are rare, although the latter are extremely abundant on exposed shores. The supply of the intertidal barnacle Semibalanus balanoides (L.) to sheltered, fucoid dominated rocky shores was investigated to determine the importance of larval supply in limiting the abundance of adults in shelter. Larval supply was measured at two spatial scales, at the scale of shore (100s of metres), by comparing larval concentrations at exposed and sheltered sites, and at a smaller spatial scale (m), by examining the role of fucoid canopies in limiting supply to the substratum. Replicate plankton trawls were carried out above the intertidal zone at high water at two sheltered sites and nearby exposed headlands. The concentration of S. balanoides cyprid larvae was significantly higher at the sheltered sites on two out of three sampling occasions with up to 14 times greater larvae on one occasion than the nearby exposed site. The effect of the macroalgal canopy on supply to the substratum was assessed in two ways: directly, by pumping water from the substratum in areas with and without Ascophyllum nodosum (L.) Le Jolis, and indirectly by measuring cyprid settlement in a canopy-manipulation experiment. Pumped plankton samples from mid tide level showed that the A. nodosum canopy did not form a barrier to larval supply and may have had a positive effect on larval concentrations at the substratum. Cyprid settlement was assessed in the mid shore A. nodosum and low shore Fucus serratus L. zones to areas with canopy algae (but protected from the sweeping effects of macroalgal fronds) and without canopy. Settlement over three consecutive 24-h periods showed a consistent pattern; settlement was consistently lower beneath the F. serratus canopy than in cleared areas, suggesting that this algal species forms a barrier, limiting supply of cyprid larvae to the substratum.     

Consistent spatial patterns of arrival of larvae of the honeycomb barnacle Chamaesipho tasmanica Foster and Anderson in New South Wales

The small honeycomb barnacle Chamaesipho tasmanica occurs in patches at high levels on exposed rocky shores, but often carpets the substratum at mid-shore levels of sheltered shores in south-eastern Australia. Studies of larval supply from 1990 to 1993 and concurrent monitoring of settlement from 1991 to 1993 revealed that larval arrival and settlement were typified by trickles of larvae from late July to December (although some were observed in January and February). Major pulses of arriving cyprids were also recorded once or twice each year. While local patterns of water-flow had no impact on numbers of larvae arriving, major peaks of larval arrival were always associated with strong southerly winds during new and full moons. There was a consistent spatial pattern of larval supply; more larvae were always caught in one area low on the shore. Numbers of larvae caught were, however, very sporadic within a given year and very variable from one year to the next. While the different numbers of cyprids in different places cannot be explained by cyprids arriving first on lower parts of the shore, longer periods of submersion nor aggregations of larvae in the plankton, recurrent patterns of arrival of larvae suggest that local site-specific characteristics have an influence on the demography of populations of this species. Variations in numbers of larvae arriving were responsible for the variations in distributions of juveniles on the substratum.     

Large scale patterns and trophic structure of southern African rocky shores: the roles of geographic variation and wave exposure

In this study we revise the biogeographic delimitation, and large-scale patterns of community structure of the intertidal rocky shores of southern Africa. We use binary (presence/absence) and per-species biomass data collected at fifteen localities and thirty-seven different rocky sites, encompassing the shores of southern Namibia, South Africa and southern Mozambique. Multivariate analyses revealed that the shores of southern Africa (south of 25°) can be divided into three main biogeographic provinces: the west coast or Namaqua province, the south coast or Agulhas province and the east coast or Natal province. The biomass structure of the intertidal rocky shores communities of southern Africa varied at a large scale, corresponding to biogeographic differences, while local-scale variation accorded with the intensity of local wave action. The average biomass of west coast communities was on average significantly greater than that of the south and east provinces. At a local scale, the community biomass on exposed shores was an order of magnitude greater than on sheltered shores, within all biogeographic provinces. Semi-exposed shores exhibited intermediate average biomass. The trophic structure of these communities varied significantly with wave action: autotrophs, filter-feeders and invertebrate predators were more prevalent on wave exposed than sheltered shores, whereas grazers were more abundant on sheltered and semi-exposed shores. Exposed shores were consistently dominated by far fewer species than semi-exposed and sheltered shores, independently of biogeographic differences. Within all biogeographic provinces semi-exposed and sheltered shores were more diverse than exposed shores. West coast intertidal communities therefore had high levels of biomass, but were consistently species-poor. Several working hypotheses that could explain these large and small-scale patterns are presented.     

Refuge function of marine algae complicates selection in an intertidal snail

Species with restricted gene flow often show trait-shifts from one type of environment to another. In those rock-dwelling marine gastropods that lack larval dispersal, size generally decreases in wave-exposed habitats reducing risk of dislodgement, while increases in less exposed habitats to resist crab-crushing. In Littorina fabalis, however, snails of moderately exposed shores are generally much larger (11–14 mm) than snails of sheltered shores (5–8 mm). Observations from the White Sea (where crabs are not present) indicate that in the absence of crabs snails are small (6–7 mm) in both habitats. We assumed that the optimal size for L. fabalis in the absence of crabs is less than 8 mm, and thus that increased size in moderately exposed habitats in areas with crabs might be a response to crab predation. In a crab-rich area (Sweden) we showed that crab predation is an important mortality factor for this snail species in both sheltered and moderately exposed habitats. In sheltered habitats, snails were relatively more protected from crab-predation when dwelling on their habitual substrate, fucoid algae, than if experimentally tethered to rocks below the algae. This showed that algae function as snail refuges. Snail dislodgement increased, however, with wave exposure but tethering snails in moderately exposed habitats showed that large snails survived equally well on rocks under the algae as in the canopy of the algae. Thus in sheltered habitats a small snail size is favored, probably due to life-history reasons, while increased risk of being dislodged from the algae refuges promotes a large size in moderately exposed habitats. This study shows an example of selection of a trait depends on complex interactions of different factors (life-history optimization, crab predation, wave induced dislodgement and algal refuges).     

Exposure of seawalls to waves within an urban estuary: effects on intertidal assemblages

Abstract Despite being largely protected from sea swell by headlands, shores within Sydney Harbour, New South Wales, Australia are subjected to considerable wave action from wind waves and the numerous recreational and commercial boats and ships using the waterways. The aim of this study was to test whether assemblages on artificial shores, that is, seawalls, either exposed to or sheltered from waves would show similar patterns to those published for natural shores exposed to or sheltered from sea swell. Specifically, the hypotheses were tested that sheltered seawalls would have greater cover of algae, whereas exposed seawalls would have greater cover of sessile invertebrates and greater abundance of mobile invertebrates. It was found that encrusting algae had greater cover on sheltered seawalls, whereas cover of turfing algae was greater on exposed walls. Sessile filter‐feeders generally had greater cover on exposed seawalls, although a dominant space occupier, the oyster Saccostrea glomerata, showed either no difference, or greater cover on sheltered seawalls among different locations. The pattern for the dominant grazer on seawalls, the pulmonate limpet Siphonaria denticulata showed the predicted pattern low on the shore, but the opposite pattern at mid‐tidal levels. Despite some inconsistencies the results were similar to those predicted from studies on natural shores, showing that assemblages on urban structures respond to wave action in a way that is predictable from studies on natural shores. Experiments were carried out where assemblages were transplanted to sites with differing exposure to waves to determine whether differences in recruitment or post‐recruitment mortality were responsible for patterns of difference. We found that some sessile invertebrates did not survive when transplanted to sheltered seawalls, suggesting that difference in survival of organisms was responsible for observed patterns. This was not, however, found when the experiment was repeated at other locations. It is likely that processes affecting the assemblages are temporally and spatially very variable, or that more than one process interacts to cause observed patterns. Identifying such complexity requires rigorously structured sampling designs and appropriate manipulative experiments.     

Evidence of a reproductive barrier between two forms of the marine periwinkle Littorina fabalis (Gastropoda)

Studies of allozyme variation may reveal unexpected patterns of genetic variation which challenge earlier conclusions of species delimitations based on morphological data. However, allozyme variation alone may not be sufficient to resolve this kind of problem. For example, populations of the marine intertidal snail Littorina fabalis (=Littorina mariae) from wave exposed parts end from protected parts of the same shores are distinguished by different alleles of arginine kinase (Ark) while indifferent, or very nearly so, in another 29 loci. Intermediate populations have large deficiencies of exposed/sheltered heterozygote classes of Ark and we have earlier suggested habitat-related selection in this locus as the explanation. In this study we estimated growth rate of individual snails of different Ark-genotypes in three different habitats (exposed, sheltered and intermediate). In all habitats the snails homozygous for alleles of ‘exposed’ type grew faster and matured at a larger size than did snails homozygous for alleles of ‘sheltered’ types. This relationship was indirectly confirmed in three additional sites of intermediate exposure where exposed AA-genotypes dominated among large (>8 mm) snails while the sheltered genotypes dominated among small (<5 mm) snails of truly sympatric samples. We furthermore found small differences in allele frequencies of two other loci (Pgi and Pgm-2) and in shell colour frequencies, comparing sympatric snails of exposed and sheltered Ark-homozygotes. Although we found no signs of habitat-related selection among snails of different Ark-genotype, or selection against heterozygotes, we cannot reject selection in Ark, as our experiments only covered one island, one season and grown-up snails. The coupling between allozyme and phenotypic characters in strictly sympatric samples of snails suggests the presence of two gene pools. Perhaps the large and small forms of L. fabalis represent very closely related cryptic taxa. However, introgression between them seems a possible explanation for the striking similarities in the vast majority of morphological and allozyme characters.     

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.     

Precisely proportioned: intertidal barnacles alter penis form to suit coastal wave action

For their size, barnacles possess the longest penis of any animal (up to eight times their body length). However, as one of few sessile animals to copulate, they face a trade-off between reaching more mates and controlling ever-longer penises in turbulent flow. We observed that penises of an intertidal barnacle (Balanus glandula) from wave-exposed shores were shorter than, stouter than, and more than twice as massive for their length as, those from nearby protected bays. In addition, penis shape variation was tightly correlated with maximum velocity of breaking waves, and, on all shores, larger barnacles had disproportionately stouter penises. Finally, field experiments confirmed that most of this variation was due to phenotypic plasticity: barnacles transplanted to a wave-exposed outer coast produced dramatically shorter and wider penises than counterparts moved to a protected harbour. Owing to the probable trade-off between penis length and ability to function in flow, and owing to the ever-changing wave conditions on rocky shores, intertidal barnacles appear to have acquired the capacity to change the size and shape of their penises to suit local hydrodynamic conditions. This dramatic plasticity in genital form is a valuable reminder that factors other than the usual drivers of genital diversification--female choice, sexual conflict and male-male competition--can influence genital form.     

Comparative population biology and reproduction of two sympatric crabs (Grapsidae) on Azores cobble beaches

Population biology of the intertidal crabs Pachygrapsus marmoratus Fabricius, 1787 and P. maurus (Lucas 1846) was investigated at cobble beaches with different levels of wave exposure. Surveys were taken monthly over 1.5 years at three intertidal levels. The growth rate of both species was similar, although P. marmoratus reached larger sizes. This species was more abundant on the mid-littoral zone of sheltered beaches, whereas P. maurus was more abundant on more exposed ones. The sex ratio was male biased in both species, particularly for the smaller and larger size classes. Both species showed an extended reproductive season, coinciding with the rising temperatures of spring and summer, with ovigerous females being found from June to September for P. marmoratus and from May to August for P. maurus. The present study indicates that both could develop adaptive strategies allowing them to coexist in the Azores littoral. The greater size of P. marmoratus seems to be an advantage on the competition for refuges with P. maurus, which results in its higher abundance and general dominance in the higher shore levels and in less exposed shores. However, P. maurus seems to be more fitted to live in the higher hydrodynamic habitats.     

Dislodgement force and shell morphology vary according to wave exposure in a tropical gastropod (Cittarium pica)

Wave exposure has strong influences on population density, morphology and behaviour of intertidal species in temperate zones, but little is known about how intertidal organisms in tropical regions respond to gradients in wave exposure. We tested whether dislodgement force and shell shape of a tropical gastropod, Cittarium pica, differs among shores that vary in wave exposure. After adjusting for body size, we found that C. pica from exposed shores required greater dislodgement force to remove them from the shore, had slightly larger opercula (the closure to the shell aperture), and were slightly squatter in shape (reduced in shell height relative to shell width) than C. pica from sheltered shores. These morphological adjustments are consistent with those observed in temperate gastropods, which are argued to represent adaptive responses to the risk of mortality associated with dislodgement.     

A comparison of reproduction in co-occurring chthamalid barnacles, Chthamalus stellatus (Poli) and Chthamalus montagui Southward

Reproduction in Chthamalus montagui Southward and Chthamalus stellatus (Poli) has been compared along gradients of wave exposure and tidal height. The proportion of barnacles carrying egg masses was greatest at low shore levels in both species, although the difference between levels was more pronounced in C. stellatus. Both species were found to breed between the beginning of May and the end of September. The mean number of broods released per season in both species ranged from 1.0 to 2.1 at high shore levels, from 2.2 to 3.2 at mid shore levels, and from 2.6 to 4.4 at low shore levels. C. stellatus predominated low on the shore at an intermediately exposed site and on the mid and low shore at the most exposed site. At these places it produced more broods per year than C. montagui. Development of the embryos of the two species was complete after 3 wk in vivo at 15°C. Eggs containing embryos of C. stellatus were larger than those of C. montagui, and within each species, larger barnacles produced larger eggs. Egg masses of C. stellanus included more eggs than those of C. montagui. Brood size in both species was greatest in shelter and least in exposure. Egg production was, however, similar in the two species when adjusted for differences in body mass. Egg production per unit body mass was greatest at an intermediately exposed site.     

Feeding in flow extremes: dependence of cirrus form on wave-exposure in four barnacle species

Wave-exposure influences the form of many organisms. Curiously, the impact of flow extremes on feeding structures has received little attention. Barnacles extend feather-like legs to feed, and prior work revealed a highly precise association between leg length and water velocity in one species. To assess the generality of this flow-dependence, we quantified variation in four leg traits (ramus length, ramus diameter, seta length, and intersetal spacing) in four intertidal barnacles (Balanus glandula, Chthamalus dalli, Semibalanus cariosus, Pollicipes polymerus) over a wave-exposure gradient in the North-Eastern Pacific. All species exhibited a negative allometric relation between leg length and body mass. Proportionally longer feeding legs may permit smaller barnacles to avoid lower flow and particle flux associated with boundary layers. Although coefficients of allometry did not vary with wave-exposure, form differences among wave-exposures were substantial. Depending on the species, acorn barnacles of the same size from protected shores had feeding legs that were 37-80% longer and 18-25% thinner, and setae that were 36-50% longer and up to 25% more closely spaced, than those from exposed shores. Differences were less pronounced for the gooseneck barnacle, P. polymerus. Moreover, in situ water velocity explained an impressive percentage of overall leg-length variation: 92% in B. glandula, 67% in C. dalli, 91% in S. cariosus, and 92% in P. polymerus. Clearly, both size and shape of barnacle feeding legs respond to local flow conditions. This response appears widespread--across two orders of thoracican barnacles, Pedunculata and Sessilia, and two superfamilies of acorn barnacles (Balanoidea and Chthamaloidea)--and likely adaptive. Longer rami and setae would yield a larger feeding area in low flow, whereas shorter, stouter rami with shorter setae would be less vulnerable to damage in high flow. Finally, the proportionally most variable species was abundant in the widest range of habitats, suggesting that increased plasticity may permit a wider niche breadth.     

NATURAL FORAGING OF THE DOGWHELK, NUCELLA LAPILLUS (LINNAEUS); THE WEATHER AND WHETHER TO FEED

The natural cycle of foraging and sheltering in dogwhelks isclosely associated with changing weather conditions. The behaviourof individual dogwhelks was quantified over three, approximately40 day, periods on the rocky shores of Llanddwyn Island, Anglesey.Dogwhelks were monitored in a range of habitats, from sheltered,high shore, barnacle covered substrata to wave exposed, lowshore, mussel covered substrata. The proportion of dogwhelkseither foraging or sheltering varied greatly from day to day,indicating similarity of behaviour among individuals withineach population. In sheltered areas, foraging was limited duringperiods of sunny and warm weather. In exposed areas, however,foraging was restricted during periods of strong wave action.Most dogwhelks in exposed areas foraged in periods of calm,warm and sunny weather. Transplanted dogwhelks appeared initially to be more activeforagers than non-transplanted, control, animals in both exposedand sheltered areas. After about 10 days, however, the transplantsbegan to behave in a similar way to the controls.