An examination of species dispersion patterns along the intertidal gradient on Macquarie Island (sub-Antarctica) using a restricted occupancy model

It has been widely held that intertidal zonation boundaries and the tidal emersion levels are causally related: the critical tidal level hypothesis. In order to evaluate this hypothesis, the dispersion patterns of species boundaries on the intertidal rocky shores of Macquarie Island (sub-Antarctica) were examined using a restricted occupancy model to test the null hypothesis that the observed species were randomly dispersed along the tidal emersion gradient. Subsequently this investigation demonstrated that the intertidal species boundaries on Macquarie Island shores were randomly dispersed with respect to the tidal emersion gradient. Hence no prima facie evidence was found to support the critical tidal level hypothesis. This suggests that tidal emersion is not a significant factor structuring intertidal communities on Macquarie Island.     

Intertidal assemblages on seawalls and vertical rocky shores in Sydney Harbour,Australia

Abstract Understanding the ecological role of artificial structures, such as seawalls, in shallow coastal waters is necessary in order to plan sound strategies of conservation and management of natural habitats. In Sydney Harbour (NSW, Australia), about 50% of the foreshore is made of retaining seawalls This study evaluates the changes caused to natural assemblages of organisms by these structures, by comparing intertidal assemblages between seawalls and vertical rocky shores. The following hypotheses were tested: that assemblages on seawalls would differ from those on rocky shores at mid‐, but not at low‐shore levels; where assemblages differ between habitats, there would be differences in cover/abundances of widespread species; patterns would be consistent among locations and through time; the variability of assemblages at the scales of 10s of cm and metres would differ between seawalls and rocky shores at mid‐ and low‐shore levels. To test these hypotheses, assemblages on seawalls and rocky shores were sampled at three locations, at roughly 4‐monthly intervals, over a period of about 18 months. Results indicated that mid‐shore assemblages on seawalls were different from those on rocky shores, but this was not the case at low‐shore levels. Few taxa were unique to either habitat. Cover of common species of algae and sessile animals and abundances of mobile grazers were variable with few consistent patterns. Variability at the scales sampled differed between habitats and heights on the shore. Seawalls and rocky shores, in general, supported a similar suite of species, but patterns of abundance and variation differed among locations and from height to height in each habitat. The implications of these findings for the future management of seawalls are briefly considered.     

The establishment of fucoid zonation on algal-dominated rocky shores: hypotheses derived from a simulation model

1. A model was developed for the growth of intertidal algae with photosynthesis simulated both in air and in a tidal water column. Morphological data on dry mass per unit area and length–area relationships were used to separate the growth of different fucoid species. The relative growth rate of fronds at any height on the shore depended on a trade-off between net photosynthetic performance and tolerance to desiccation.
2. The simulated zonation patterns and growth rates were consistent with those observed previously for Fucus spp. and Pelvetia canaliculata .
3. The simulated growth of Ascophyllum nodosum was always slower than for the other species. This species did not form its characteristic distribution zones in simulations without including further processes in the model. However, Ascophyllum collected from the field could be separated into upper and lower shore morphologies which formed separate zones when they were simulated in competition with each other.
4. Several hypotheses were proposed concerning the relative locations and sharpness of interspecies boundaries on the shore. Zonation patterns were relatively insensitive to changes in most model parameters except the desiccation rate.     

Thermal tolerance,evaporative water loss,air-water oxygen consumption and zonation of intertidal prosobranchs: a new synthesis

Duration of emergence increases with tidal height on rocky shores therefore, emergence adaptations in intertidal species such as littorine and other prosobranch gastropods have been considered correlated with zonation patterns; temperature tolerance, desiccation resistance and aerial respiration rate all commonly assumed to increase progressively with increasing zonation level. Such direct correlations are rarely observed in nature. Maximal aerial gas exchange occurs in mid-shore, not high shore species. Temperature tolerance and desiccation resistance do not increase directly with shore height. Thus, hypotheses regarding physiological correlates of zonation require revaluation. A new hypothesis is presented that the high tide mark presents a single major physiological barrier on rocky shores. Above it, snails experience prolonged emergence and extensive desiccation; below it, predictable submergence and rehydration with each tidal cycle. Thus, desiccation stress is minimal below the high tide mark and maximal above it. Therefore, species restricted below high tide (the eulittoral zone) should display markedly different adaptive strategies to emergence than those above it (the eulittoral fringe). A review of the literature indicated that adaptations in eulittoral species are dominated by those allowing maintenance of activity and foraging in air including: evaporative cooling; low thermal tolerance; elevated aerial O₂ uptake rates; and high capacity for radiant heat absorption. Such adaptations exacerbate evaporative water loss. In contrast, species restricted to the eulittoral fringe display adaptive strategies that minimize desiccation and prolong survival of emergence including: foot withdrawal, preventing heat conduction from the substratum; aestivation in air; elevated thermal tolerance reducing necessity for evaporative cooling; position maintenance by cementation to the substratum and increased capacity for heat dissipation. In order to test of this hypothesis the upper thermal limits, tissue and substratum temperatures on emergence in direct sunlight and evaporative water loss and tissue temperatures on emergence in 40 °C were evaluated for specimens of six species of eulittoral and eulittoral fringe gastropods from a granite shore on Princess Royal Harbour near Albany, Western Australia. The results were consistant with adaptation to the proposed desiccation barrier at high tide. The eulittoral species, Austrocochlea constricta, Austrocochlea concamerata, Nerita atramentosa and Lepsiella vinosa, displayed adaptations dominated by maintenance of activity and foraging during emergence while the eulittoral fringe littorine species, Bembicium vittatum and Nodilittorina unifasciata displayed adaptations dominated by minization of activity and evaporative water loss during emergence. The evolution of adaptations allowing tolerance of prolonged desiccation have allowed littorine species to dominate high intertidal rocky shore gastropod faunas throughout the world's oceans.     

Physical and biological conditions on a steep intertidal gradient at Rottnest Island,Western Australia

Seven species of grazing molluscs, two littorinids, one nerite, three limpets and one chiton, lived on the vertical intertidal rock wall at the landward edge of the coastal limestone platforms at Rottnest Island, Western Australia. On the average, the vertical ranges of these species overlapped broadly, although a consistent zonation pattern was conspicuous from the platform surface upwards for 2 m. On these vertical intertidal shores, physical and biological conditions were predicted quantitatively from easily made measurements of vertical height on the shore; the percentage of time any shore level was immersed in seawater, the percent weight loss of plaster clods, the standing crop of algae, and the growth rate of the limpet, Notoacmea onychitis, all decreased linearly or semi-logarithmically with increasing height on the shore. The standing crop of animals was greatest, largely due to the presence of the chiton, between 40 and 70 cm from the platform surface and decreased rapidly down the shore and more gradually towards higher shore levels. We interpret this information and the positive correlation between algal production rate, and egestion rate of the animal community at various levels of the shore as evidence supporting the idea that food may be in short supply on these shores.     

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.     

Causes and consequences of thermal tolerance limits in rocky intertidal porcelain crabs, genus petrolisthes

Vertical zonation of intertidal organisms, from the shallowsubtidal to the supralittoral zones, is a ubiquitous featureof temperate and tropical rocky shores. Organisms that livehigher on the shore experience larger daily and seasonal fluctuationsin microhabitat conditions, due to their greater exposure toterrestrial conditions during emersion. Comparative analysesof the adaptive linkage between physiological tolerance limitsand vertical distribution are the most powerful when the studyspecies are closely related and occur in discrete vertical zonesthroughout the intertidal range. Here, I summarize work on thephysiological tolerance limits of rocky intertidal zone porcelaincrab species of the genus Petrolisthes to emersion-related heatstress. In the eastern Pacific, Petrolisthes species live throughouttemperate and tropical regions, and are found in discrete verticalintertidal zones in each region. Whole organism thermal tolerancelimits of Petrolisthes species, and thermal limits of heartand nerve function reflect microhabitat conditions. Speciesliving higher in the intertidal zone are more eurythermal thanlow-intertidal congeners, tropical species have the highestthermal limits, and the differences in thermal tolerance betweenlow- and high-intertidal species is greatest for temperate crabs.Acclimation of thermal limits of high-intertidal species isrestricted as compared to low-intertidal species. Thus, becausethermal limits of high-intertidal species are near current habitattemperature maxima, global warming could most strongly impactintertidal species.     

Maintenance of zonation patterns in two species of flat periwinkle,Littorina obtusata and L. mariae

The zonation patterns of Littorina obtusata (L.) and Littorina mariae Sacchi et Rastelli were shown to be quite distinct on a sheltered rocky shore. L. obtusata was found at all the heights sampled; it reached peak numbers at mid shore on the alga Ascophyllum nodosum L. (Le Jol). There was no difference in the tidal height occupied by adults or juveniles; or in the mean size of L. obtusata along the vertical gradient of the shore. In contrast L. mariae occurred exclusively low on the shore, on Fucus serratus L. Translocation of the two species within their respective levels resulted in random movement after 4 days, although initial movements after 1 and 2 days were sometimes directional. Animals transplanted to the normal level of the other species showed directional movement towards their home zone; this was most pronounced after 4 days. There was no difference in the distance moved by the two species, although the distance moved did vary with tidal height, both species moving further at mid shore than low shore. Distances moved by littorinids at replicate areas in the low shore were similar but those at mid shore did vary. There was an interaction between the species and the different tidal heights which revealed that transplanted species moved further than translocated species at the same tidal level. However, this was only significant in the case of L. mariae. It is suggested that the close relationship between the winkles and their host algae may direct the homing behaviour of displaced individuals.     

Intertidal distribution and species composition of brachyuran crabs at two rocky shores in Central Portugal

The objectives of the present study are to describe and compare the brachyuran community of rocky shores within the Central Portuguese coast and to examine the zonation patterns of the most representative species. For this, randomly placed transects were surveyed to obtain crab counts according to microhabitat and intertidal level. Repeated sampling in two different shores during two different seasons provided spatial and temporal replication for zonation analyses. Seven species were registered: Pachygrapsus marmoratus, Eriphia verrucosa, Xantho incisus, Carcinus maenas, Necora puber, Pirimela denticulata and Pilumnus hirtellus. Species density rankings are the same at both localities, but the less exposed shore presents higher diversity. While most species are mainly confined to specific microhabitats in the lower level, P. marmoratus and E. verrucosa can exploit the whole intertidal range. Regardless of shore and season, E. verrucosa is more abundant in the lower intertidal levels, while no such zonation patterns were recorded for P. marmoratus. Initial predictions concerning the effect of wave exposure and temperature on the zonation of those species are not validated after analysing the factorial model proposed. Between-shore contrasts were found instead, with higher densities recorded in the more exposed locality for both species. Possible causes of the observed patterns are discussed.     

Patterns of organizations of intertidal and shallow subtidal vegetation in wave exposed habitats of central Chile

Wave-exposed rocky intertidal habitats of central Chile exhibit zonation of algal morphologies rather than strict patterns of species zonation. In low shore areas, there is a vertical sequence of perennial belts of calcareous crusts, kelp-like forms and expanded cushions or non-calcareous crusts. The calcareous crusts are represented by species of Mesophyllum, the kelp-like forms include Lessonia nigrescens and Durvillaea antarctica, while the cushions are represented by Gelidium chilense and G. lingulatum and the noncalcareous, expanded crusts by Codium dimorphum. Thin and thick blades, represented by Iridaea laminarioides, Ulva rigida and Porphyra columbia and filamentous forms including Ceramium rubrum, Centroceras clavulatum and Polysiphonia spp. are more patchy than the lower, perennial belts. They may, however, form distinct temporal monocultures at upper intertidal levels. Upper and lower limits of the various zones are set by interactions of several factors, the relative importance of which can change seasonally. When some of the factors restricting species distribution are experimentally removed, other interactions among factors become limiting.Within each zone, species are morphologically similar, with the abundance of species being regulated by symmetric competitive interactions. Competition is often asymmetric at the boundaries of zones except when adults of small-sized forms interact with morphologically similar juveniles of larger forms. Irrespective of their extremely different morphologies, the permanent, zone-forming algal species generally combine escape from grazers or defensive adaptations with clear competitive abilities. Nevertheless, there is a clear competitive hierarchy which is expressed in vertical displacements and zonation. The lowershore habitats could potentially be occupied by any of the different types of algae. Fast growth and large size allow the kelps to occupy this zone pushing the calcareous crust dominated-zone down into shallow subtidal areas and displacing the cushions and fleshy crusts into the low and middle intertidal regions. In turn, these last forms can displace thick and thin foliose forms and filaments to upper levels on the shore. Displaced forms may exist as patches at various levels of the shore.     

Evaporative water loss and intertidal vertical distribution in relation to body size and morphology of stichaeoid fishes from California

Five species of amphibious stichaeoid fishes (four stichaeids, one pholidid) with distinct but overlapping distribution patterns in the rocky intertidal zone of central California were studied in the laboratory with respect to body size and shape, survival out of water, evaporative water loss and water resorption upon re-immersion. Members of all species survived periods of emersion of at least 6 h, but maximum survival times varied with species and body size. Surface area to weight ratios in relation to body length were highest in Xiphister atropurpureus (Kittlitz) and X. mucosus (Girard), lowest in Cebidichthys violaceus (Girard) and Anoplarchus purpurescens Gill and most variable in Xererpes fucorum (Jordan & Gilbert) (the pholidid). Weight-specific water loss (mg · g⁻¹ · h⁻¹) was highest in the two species of Xiphister followed in order by Xererpes fucorum, Cebidichthys violaceus and Anoplarchus purpurescens.

In general, adaptive relationships were found between body size, surface area, tolerance of water loss and intertidal distribution patterns. Among juvenile stichaeoids, for which problems of emersion are most critical, those of Cebidichthys violaceus possess the combination of attributes, including small surface area to weight ratio, high water content and long tolerance of emersion, that should give them an adaptive edge in occupying the upper levels of the shore. In accordance with these characteristics, C. violaceus has the highest, most exposed vertical distribution of the five study species.     

Mechanisms of succession along the emersion gradient in intertidal rocky shore assemblages

Theoretical models predict that the relative importance of competition and facilitation vary inversely along gradients of abiotic stress, with facilitation dominating under harsh conditions (the so called "stress-gradient hypothesis"). To date, very few studies have tested this hypothesis in the framework of succession. Moreover, the generality of the hypothesis is currently under debate and the necessity to examine responses at the community level and using different stress levels has been emphasized. We evaluate the mechanisms of succession across the emersion gradient in a rocky shore system. After complete clearing of experimental plots at four shore heights, we removed two separate components of the assemblage: ephemeral green algae, early colonists and canopy-forming species, main space holders in undisturbed assemblages. We aimed to test two hypotheses: (a) the net effect of ephemeral algae on canopy-forming species shift along the emersion gradient according to the predictions of the stress-gradient hypothesis and (b) the net effect of the canopy-forming group on associate species change, from neutral/negative to positive, as succession progresses and plants grow, especially at the highest shore heights. Our results did not support the predictions of the stress gradient hypothesis in the framework of succession. We did detect changes through time in the effect of canopy-forming species, from negative to positive, but this was not dependent on the shore height. The outcome of interactions depended on the identity and life-stage of the species. Moreover, indirect interactions among species could also create a less predictable relationship between successional mechanisms and the environmental gradient.     

Spatial distribution of algae and invertebrates in the rocky intertidal zone of the Strait of Magellan: are patterns general?

Spatial variability in the distribution of macroalgae and invertebrates was examined at a number of sites in the Strait of Magellan. Two main predictions of models of zonation were tested in this study: (1) consistency in patterns of vertical distribution among sites one to tens of kilometers apart, and (2) homogeneity between areas at the same level on the shore, tens to hundreds of meters apart. Two types of habitat were considered: continuous rocky shores and blocks of rock in gravel beaches. In the former habitat, percent cover estimates of sessile organisms (algae and mussels) were obtained from three replicated plots (0.5 × 0.5 m) sampled non-destructively in each of three random areas (stretches of shore about 20 m long) at each of three levels on the shore: high, middle and low intertidal zone. This procedure was repeated at six different sites along the Chilean coast of the Strait of Magellan. Blocks of rock were sampled at two sites about 1 km apart. Estimates of the abundance of sessile (barnacles and mussels) and mobile (limpets) organisms were obtained for six blocks at each of two levels on the shore (high and low intertidal zone) at each site. At each level on the shore, three blocks were sampled on the top and three on the vertical sides. A single plot was sampled on each block. This design allowed a test of the null hypothesis of no interactive effects between position on the blocks and level on the shore. Both the predictions were falsified: (1) there were large between-site differences in the vertical structure of assemblages and (2) variability between areas at the same level on the shore was large for some of the most common algae. In contrast, the null hypothesis of no interactive effects of position on the blocks and level on the shore was retained. The results of this study show that vertical position on the shore alone is not a good predictor of the structure of assemblages of benthic organisms in the rocky intertidal of the Strait of Magellan. Received: 9 December 1996 / Accepted: 2 May 1997     

Intertidal exposure zones: A way to subdivide the shore

A new way of subdividing the shore is described and Doty's concept of critical tide levels (CTLs), on which the scheme is based, is revised. The various cycles of the tide (e.g., daily, monthly, and annual cycles) define several different orders of CTL at which the duration of continuous exposure or submergence increases abruptly by a discrete interval, the magnitude of which is determined by cycle period. Contrary to popular belief, most CTLs are common to all tidal types (i.e., mixed, semi-diurnal, and diurnal) and thus they can be used as reference levels for cross-comparing different intertidal regions, regardless of tidal type. CTLs naturally subdivide the intertidal region into discrete exposure zones, here called the atmozone, amphizone, and aquazone, which have upper and lower subzones. The amphizone, the intertidal core, experiences exposure and submergence extremes in terms of hours (? 1 lunar day). The overlying lower atmozone and underlying upper aquazone experience exposure and submergence extremes, respectively, of the order of days (?2 to ≈10 days), while in the upper atmozone and lower aquazone extremes are measured in months or years (> ≈20 days).In most intertidal environments (tidal flats, saltmarshes, brackish marshes, and sheltered rocky shores) CTLs (including exposure zone boundaries) probably limit some biological zones. For tidal flats experiencing large tidal ranges it is possible that biological zones in the fringes of the intertidal region shift in response to long-term fluctuations in the levels of CTLs, due to the effects of the 18.6 year soli-lunar cycle.     

Rocky shore zonation in the Ranee tidal power basin

Data from a series of rocky shore transects, within the enclosed basin of the Ranee estuary and immediately outside the barrage, describe the distribution and zonation patterns of the intertidal biota, after more than a decade of its regular operation as a tidal power station. In the absence of quantitative pre-barrage information, the distribution of species has been compared with published information on their former extent of penetration into the estuary.     

Spatial analyses of intertidal assemblages on sheltered rocky shores

Abstract Understanding processes in complex assemblages depends on good understanding of spatial and temporal patterns of structure at various spatial scales. There has been little quantitative information about spatial patterns and natural temporal changes in intertidal assemblages on sheltered rocky shores in temperate Australia. Natural changes and responses to anthropogenic disturbances in these habitats cannot be accurately measured and assessed without quantitative data on patterns of natural variability in space and through time. This paper describes some suitable quantitative methods for examining spatial and temporal patterns of diversity and abundances of highshore, midshore and lowshore intertidal assemblages and the important component species for a number of shores in a bay that has not been severely altered by human disturbance. Despite a diverse flora and fauna on these shores, the midshore and lowshore assemblages on sheltered shores were characterized by a few species which were also the most important in discriminating among assemblages on a shore and, for each assemblage, among different shores. The same set of species was also important for measuring small-scale patchiness within each assemblage (i.e. between replicate sites on a shore). Therefore, these data provide a rationale for selecting species that are useful for measuring differences and changes in abundance among places and times at different scales and, hence, can be used in the more complex sampling designs necessary to detect environmental impacts. There was considerable spatial variability in all assemblages and all species (or taxa) examined at scales of metres, tens of metres and kilometres. There were no clear seasonal trends for most measures, with as much or more variability at intervals of 3 months as from year to year. Most interactions between spatial and temporal measures were at the smallest scale, with different sites on the same shore generally showing different changes from time to time. The cause(s) of this apparently idiosyncratic variability1 were not examined, but some potential causes are discussed. These data are appropriate for testing hypotheses about the applicability of these findings to other relatively undisturbed sheltered shores, about effects of different anthropogenic disturbances on sheltered intertidal assemblages and to test hypotheses about differences in intertidal assemblages on sheltered versus wave-exposed shores.     

Spatial pattern of intertidal macroalgal assemblages associated with tidal levels

The zonation pattern of macroalgal assemblages was investigated from December 1995 to October 1996 on a semi-exposed, rocky intertidal shore at Chungdori (southwestern Korea) based on quantitative and qualitative estimates of species occurrences in 31 permanent quadrats. Variation in cover associated with tidal levels was described for 30 species (that could be discerned with the unaided eyes) including three green, five brown and 22 red algae. Macroalgae inhabiting intertidal zone exhibited distinct zonation patterns. The number of species increased with decreasing intertidal height and was independent of season. The community was dominated by five species (Gloiopeltis furcata, Gelidium divaricatum, Ulva pertusa, Sargassum horneri, Hizikia fusiformis). The intertidal assemblage at the study site can be divided into two groups based on the number of species and the population structure with the division occurring at the critical level of 34 cm above MLW (mean low water). Gloiopeltis furcata, Gelidium divaricatum, Sargassum thunbergii, Monostroma grevillei, and Myelophycus simplex were more abundant in the upper shore zone and rapidly declined in abundance with depth, relative to all other species. Gelidium amansii, Pachymeniopsis elliptica, Hizikia fusiformis, Gigartina intermedia, Laurencia sp., Chondrus ocellatus, Corallina spp. and Gigartina tenella became more dominant in the lower shore zone.     

The effects of tidal height,wave-exposure,seasonality and rock-pools on grazing and the distribution of intertidal macroalgae in New South Wales

The effects of grazers (mostly gastropods), height on the shore, wave-exposure, season of the year and the presence of shallow rock-pools on the abundance of intertidal macroalgae were examined on shores at Cape Banks (Botany Bay). At the beginning of each of the four seasons, experimental plots with and without grazers were cleared at four heights on three shores, of increasing exposure to waves, The colonization and growth of algae in these plots were monitored (by measurement of per cent cover and dry wt) for approximately the next 3 months in each season.In grazed plots, foliose algae only grew at the lowest levels on the shores. They were more abundant where wave-action was greater, and during the cooler periods of the year, when growth of the plants was enhanced. Higher on the shore, there was a positive correlation between algal cover in grazed plots and the amount of rainfall during the previous 10 days. In all seasons, there was much greater colonization of foliose algae where grazers were excluded. There was greater algal growth at lower levels on the shore, and where wave-action was stronger. Less algae grew in sheltered areas during warmer times of the year. The major seasonal difference found was the more rapid growth and occupancy of the rock by algae during the cooler seasons of the year.Experimental rock-pools were colonized more rapidly at lower levels on the shore, and during the winter. There was no difference between pools and control (non-pool) areas during winter. During summer, however, there was a greater per cent cover and biomass of algae in pools from which grazers had been excluded than in similar control areas.The results can be interpreted as being due to the greater survival and more rapid growth of algae under conditions of increased moisture, decreased emersion and decreased temperatures and light regimes during low tide (i.e. when physical stresses were reduced). These physical factors were, however, less important to the distribution of the algae than were the effects of grazers. Although few algae were present in any experimental plot, the number of species of algae per plot was considerably reduced in grazed areas.The results confirm that the patterns of vertical distribution and abundance of algae on rock-platforms in New South Wales are primarily the result of the activities of grazers. All algae in the present study were capable of living higher on the shore than they were normally found. Much of the variation along a gradient of wave-exposure, from season to season, and small-scale variation from place to place at any time can be explained by the complex interactions between the activities of grazers, and the effects of variations in the physical environment that influence the recruitment, survival and growth of the algae.     

Why are intertidal snails rare in the subtidal? Predation, growth and the vertical distribution of Littorina littorea (L.) in the Gulf of Maine

The North Atlantic gastropod Littorina littorea exhibits a characteristic “intertidal” distribution: the snail is abundant in the littoral zone but scarce in the shallow subtidal and the relatively few subtidal individuals are larger (in shell size) on average than those in the intertidal zone. For highly mobile species like L. littorea, this vertical distribution is primarily determined by directional movement. Biotic and abiotic factors vary across tidal heights, and natural selection for movement to shore levels where fitness is maximized provides the ultimate (evolutionary) explanation for vertical distribution patterns. In this study, we asked whether variation in growth rate and/or predation pressure among tidal heights provide an ultimate explanation for vertical gradients in L. littorea size and abundance. We used a cage experiment to compare juvenile growth rate among tidal heights and a series of field and laboratory experiments to examine variation in predation pressure among tidal heights and snail size classes. Juvenile growth rates were highest in the low intertidal zone, declining at both higher and lower levels. Predation risk for tethered L. littorea increased with both decreasing tidal height and decreasing body size (shell height). Almost all tethered prey were consumed by shell- breaking predators and a census revealed that the two most abundant such predators were the crabs Carcinus maenas and Cancer borealis. Laboratory feeding experiments were used to compare size-dependent prey vulnerability and prey-size preferences for these two key predators. We found that L. littorea vulnerability decreased with increasing snail size and increased with increasing size of both predator species. However, whereas C. borealis were capable of consuming even the largest L. littorea, most Carcinus were unable to feed on individuals larger than 10 mm in shell height. Additionally, C. borealis preferred larger sizes of L. littorea than did Carcinus. Thus, Carcinus, which co-occurs with L. littorea in the intertidal, is a much less effective predator than C. borealis, which is found primarily in the subtidal. We conclude that predation on L. littorea by C. borealis and other subtidal consumers has resulted in the scarcity of this ecologically important grazer in the subtidal. This effect has been produced both through direct predation and by imposing strong selection for movement of L. littorea to higher tidal zones.     

Thermal constraints on daily patterns of aggregation and density along an intertidal gradient in the periwinkle Echinolittorina peruviana

Variations in environmental temperature have both direct and indirect effects that affect organisms at levels ranging from intra-cellular physiological processes to ecological patterns. These variations are especially important for intertidal marine ectotherms such as littorinids since they alternate between periods of immersion in seawater, and must also experience long periods of emersion. In central Chile, Echinolittorina peruviana is one of the most conspicuous species on rocky intertidal shores, occurring at high tidal levels and in the splash zone. The species is known to resist direct exposure to the sun for long periods, although juveniles tend to be restricted to protected microhabitats. Adults show seasonal variations in abundance and vertical distribution and may form aggregations that have been shown to help reduce water loss and body temperature. In this study, we evaluate the relationship between daily thermal variations throughout the vertical distribution of this species and how these affect the patterns of density and aggregation. Our results suggest that one of the leading determinants of the spatio-temporal variation of density in E. peruviana may be operative temperature (T_O: the amount of stored heat resulting from the balance between heat fluxes into and out of the body, measured with taxidermic mounts mimicking heat transfer properties of the snail). T_O showed a strong negative relationship with density and a strong positive relationship with aggregation in the highest intertidal level monitored. The strength of these relationships decreased in importance at lower levels. While T_O alone cannot explain the abundance of E. peruviana throughout its range of distribution, our results show that it does have a strong influence that should be considered in addition to other ecological factors affecting the density of intertidal littorinids.