The effects of grazing by gastropods and physical factors on the upper limits of distribution of intertidal macroalgae

Summary The cover of foliose algae is sparse to non-existent above a low-level algal zone on many shores in N.S.W., except in rock-pools. Above this algal zone, encrusting algae, mostly Hildenbrandia prototypus, occupy most of the primary substratum on sheltered shores. Experimental manipulations at midtidal levels were used to test hypotheses about the effects of grazing by molluses and of physical factors during low tide on this pattern of algal community structure.Fences and cages were used to exclude grazers: molluscs grazed under roofs and in open areas. Cages and roofs provided shade, and decreased the harshness of the environment during low tide: fences and open areas had the normal environmental regime.In the absence of grazers, rapid colonization of Ulva and slower colonization by other foliose algae occurred in all experimental areas. The rate of colonization by Ulva sporelings was initially retarded on existing encrusting algae, but after a few months, cover of Ulva equalled that on cleared rock.Most species of algae only grew to maturity inside cages, and remained as a turf of sporelings inside fences. No foliose algae grew to a visible size in open, grazed areas. Grazing thus prevents the establishment of foliose algae above their normal upper limit on the shore, but the effects of physical factors during low tide prevent the growth of algae which become established when grazers are removed. Physical factors thus limit the abundance of foliose algae at mid-tidal levels.The recolonization of cleared areas by Hildenbrandia was not affected by the presence of a turf of sporelings, nor by the shade cast by roofs, but was retarded in cages where mature algae formed a canopy. Even under such a canopy, Hildenbrandia eventually covered as much primary substratum as in open, grazed areas. This encrusting alga is able to escape from the effects of grazing by having a tough thallus, and by its vegetative growth which allows individual plants to cover a lot of substratum, and by the tendency for new individuals to start growing from small cracks and pits in the rock, which are apparently inaccessible to the grazers.Mature foliose algae are removed from the substratum by waves, and many individual plants died during periods of hot weather. Sporelings in a turf were eliminated, after experimental fences were removed, by the combined effects of macroalgal grazers, which invaded the areas, and microalgal grarers which ate the turt from the edges inwards.The results obtained here are discussed with respect to other studies on limits to distribution of intertidal macroalgae, and the role of grazing in the diversity and structure of intertidal algal communities. Some problems of these experimental treatments are also discussed.     

Molluscan grazing and macroalgal zonation on a rocky intertidal platform at Perth,Western Australia

Abstract Intertidal limestone platforms off Perth show a characteristic pattern of algal zonation, with dense macroalgal beds nearshore bounded by a ‘barren zone’ along the seaward edge. Abalone (Haliotis roei) and several species of limpets and chitons are abundant in the barren zone, which is generally devoid of non-coralline macroalgae. The relative importance of abalone versus limpets and chitons in limiting macroalgal abundance in the barren zone was evaluated by manipulating the presence of each group in a factorial experiment. Percentage algal cover was measured photographically in 0.25m² plots at 1–2 month intervals for 9 months. Mean algal cover (mainly the foliose green alga, Ulva rigida) was highest in plots where all grazers were excluded (77–99%), intermediate where only limpets and chitons were excluded (37–85%), and lowest where only abalone were excluded (4–30%) or where no grazers were excluded (2–19%). The effect of limpets and chitons accounted for 55–89% of the variance in total algal cover, whereas the effect of abalone generally accounted for <10% of the variance. Similar results were obtained in terms of the biomass of Ulva rigida at the end of the experiment. Haliotis roei are relatively large and sedentary herbivores, feeding mainly on drift algae. Their effect on algal abundance was mediated both through pre-emption of space, which might otherwise be colonized by algae, and by grazing around their home scar. Limpets and chitons are smaller than abalone, but were much more abundant. Intensive grazing of the reef surface by limpets and chitons precluded the establishment of non-coralline macroalgae, even where abalone were absent.     

The grazing effect of limpets on the macroalgal community of a rocky intertidal shore

Abstract Abundance of macro-algae in the mid-littoral zone on a Victorian intertidal rocky shore varied seasonally, algae being rare in Summer and common in Winter and Spring. Field experiments demonstrated that, of two species of grazing limpets (Cellana tramoserica and Siphonana diemenensis), only Siphonaria had a major effect on the abundance of foliose algae (e.g. Scytopsiphon lomeniarid) and neither species had a great effect on the encrusting algae (Ralfsia spp.). There was no evidence of competition for food between the two species of limpets, in contrast with results found for similar species in New South Wales.     

The interplay of physical and biological factors in maintaining mid-shore and low-shore assemblages on rocky coasts in the north-west Mediterranean

This study examined the interactive effects of grazing by limpets and inclination of the substratum in maintaining differences between mid-shore and low-shore assemblages of algae in the northwest Mediterranean, at different scales of space and through time. Alternative models leading to different predictions about these effects were proposed and tested. Limpets were excluded by fences from areas of the substratum at mid levels on the shore. The response of algal assemblages to this manipulation was compared with control and enclosure plots at the same level, and with unmanipulated plots in the low shore where limpets are less abundant. The effects of limpets were examined at several replicated sites (0.1–4 km apart) for each slope of the substratum (nearly horizontal vs vertical), at different locations (hundreds of kilometres apart) and at different times. Individual taxa responded differently to limpet exclusion. The percentage cover of the coarsely branched and filamentous algae increased significantly in exclosure plots, in some loser reaching values found on the low shore. These patterns, however, varied greatly from shore to shore and significant effects were found both on horizontal and vertical substrata. Multivariate analyses indicated that grazing by limpets accounted for about 20% of the differences between mid-shore and low-shore assemblages. This effect was independent of substratum inclination and was consistent in space and time, suggesting that physical conditions were not as stressful for macroalgae on vertical substrata as initially supposed. Variable recruitment of algae is proposed as a possible explanation for the lack of consistency in the effects of limpets at the scale of the shore. The results of this study emphasize the need for multiple-scale analyses of the interactive effects of physical and biological factors to understand the organization of natural assemblages. Received: 22 June 1999 / Accepted: 15 November 1999     

Effects of Chiton granosus (Frembly, 1827) and other molluscan grazers on algal succession in wave exposed mid-intertidal rocky shores of central Chile

Molluscan grazers can have important effects on the abundance, colonization rates, and successional pathways of algal assemblages and the entire intertidal community. In general, early successional algae are more readily consumed than corticated algae and kelps, which usually get established later in the community succession. To generalize, however, the effect of different grazers on algal assemblages must be examined on different coasts and under different scenarios. This information could help us understand the mechanisms of ecosystem processes and situations in which general models do not apply. Along the coast of Chile, humans harvest large keyhole limpets, which seem to be the only invertebrate grazers capable of controlling the dominant corticated alga Mazzaella laminarioides, a canopy-forming species that can cover extensive areas of the mid intertidal zone. In this scenario, where large limpets are harvested, the overall effects of the diverse molluscan assemblage of limpets, chitons and snails on algal succession and on corticated algae in particular are not clear. We conducted a 26-month-long experiment to evaluate the effects of molluscan grazers on mid-intertidal algal succession and to isolate the effects of Chiton granosus, the most conspicuous member of the assemblage at these tidal elevations. At sites heavily impacted by humans the molluscan grazer assemblage had strong negative effects on colonization and abundance of green algae such as ulvoids and Blidingia minima. In doing so, the grazer assemblage had a strong negative indirect effect on the establishments of chironomid fly larvae, which were only observed on green algal mats and rarely on bare rock. No significant effects were detected on epilithic microalgae, and effects on sessile invertebrates were highly variable over space and time. C. granosus also had significant negative effects on green algae but did not account for the total grazing pressure exerted by the guild. Limited foraging excursions (ca. 35 cm) from refuges and moderate site (crevice) fidelity in this species may contribute to the patchiness in green algal distribution observed in the field. Nearly 13 months after rock surface were experimentally cleared, M. laminarioides appeared in all experimental plots, but increased over three times faster in enclosures containing C. granosus than in exclosures plots or controls, suggesting that moderate levels of herbivory could actually facilitate the establishment of this alga in the succession and that the green algal cover found in the absence of grazers may delay its establishment.     

The effect of two scales of habitat architecture on benthic grazing in a river

1. This experiment studied the effects of differing levels of the complexity of substratum architecture at two spatial scales on the distribution and abundance of benthic algae and invertebrates, and the strength of the trophic interaction between invertebrate grazers and algae. Some estimates of the effects on invertebrate colonization rates were also made. 2. Four levels of microhabitat architectural complexity were created using artificial substrata (clay tiles) and placed in Mountain River, Tasmania, in two riffle types (bedrock and boulder-cobble) of differing large-scale substratum complexity. After a colonization period, invertebrate grazers were removed from half the tiles to measure the effects of grazing. Invertebrates on the tiles were also counted and identified. At the end of the experiment, algae were removed from the tiles and analysed for chlorophyll a. 3. Invertebrate grazers did not reduce algal biomass during the experiment, and microhabitat-scale architecture influenced algal biomass more strongly than riffle-scale architecture. Highly complex microhabitat architecture increased algal biomass by providing more surface area, but once standardized for surface area, algal biomass decreased as the complexity of microhabitat architecture increased. 4. Microhabitat-scale architecture was also predominant in determining invertebrate density and the identity of the dominant grazer species. In contrast to algal biomass, invertebrate densities and species density increased with the complexity of microhabitat architecture, suggesting that refuges from flow (and possibly predation) were as important to river invertebrates as the distribution of their food source. 5. Riffle-scale architecture had some effect on the colonization of two slow-moving grazer taxa, but, overall, the colonization processes of slow-moving grazers were determined mostly by the complexity of microhabitat-scale architecture.     

Experimental analyses of patch selection by foraging black surfperch (Embiotoca jacksoni Agazzi)

The dynamics of microhabitat use by foraging adult and juvenile black surfperch (Embiotocajacksoni Agazzi) were explored. Detailed observations of black surfperch feeding at Santa Catalina Island, California, revealed that adults and young-of-year juveniles co-occurred in the same habitat but used different algal substrata as foraging sites. Juveniles selected invertebrate prey almost exclusively from the surface of foliose algae. The occurrence of young E. jacksoni was highly correlated with that of foliose algae. Adults tended to bite most frequently from turf, a low-growing matrix of plants, colonial animals, and debris covering the rocky substratum. The abundance of adults was negatively correlated with the occurrence of foliose algae. Adults and juveniles showed marked, but different, preferences in their utilization of taxa of algae as foraging substrata. Certain algae (e.g., Zonaria farlowii Setchell & Gardner) were preferred while other taxa (e.g., Sargassum palmeri Grun) were avoided by both age groups. However, most types of algae were preferred by one group but not the other. To test the hypothesis that knowledge of algal substratum composition allows prediction of fish occurrence and foraging behavior in a patch, algal cover on 2 × 2 m² areas of bottom was manipulated creating plots dominated by turf, Zonaria farlowii, or Sargassum palmeri. Fish occurrence could be accurately predicted on the basis of abundance of foliose algae, but foraging activity of fish was highly dependent on the algal taxon that dominated the patch. Differential prey availabilities among foraging substrata provided some insight into the patterns of foraging patch preferences displayed by adult and juvenile Embiotoca jacksoni.     

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.     

Different Nutritional Histories Affect the Susceptibility of Algae to Grazing

We hypothesize that algae with different cell compositions are differently perceived by their predators and consequently subjected to selective grazing. Five populations of the diatom Phaeodactylum tricornutum that differed in organic and elemental composition, but were otherwise identical, were generated by acclimation to distinct growth regimes. The different populations were then mixed in pairs and subjected to predation by either the rotifer Brachionus plicatilis or the copepod Acartia tonsa. The presence of rotifers had no impact on the ratio between any two algal populations. The presence of copepods, however, affected the ratio between algae previously acclimated to a medium containing 1 mM NH₄⁺ and algae acclimated to 0.5 mM NO₃^?, and to either a lower irradiance or a higher CO₂ concentration. We discuss the possible reason for the influence of different nutritional histories on the vulnerability of algae to predators. The differential impact of grazers on the growth of algae with different nutritional histories may result from direct selective grazing (i.e., grazers can detect algae with the most palatable cell composition), alone or combined to an asymmetric utilization of the nutrients regenerated after predation by co‐existing algal populations. Our results strongly suggest that the nutritional history of algae can influence the relationships between phytoplankton and grazers and hint at the possibility that algal cell composition is potentially subject to natural selection, because it influences the probability that algae survive predation.     

Distribution and abundance of the acmaeid limpet,Patelloida latistrigata,and its interaction with barnacles

Summary At Cape Banks, New South Wales, adults of the small intertidal limpet, Patelloida latistrigata occur exclusively in the barnacle zone, and are primarily associated with the barnacle, Tesseropora rosea. Limpet density increases with barnacle density. Juvenile limpets can be found throughout the barnacle zone, and on patches of bare rock that may be temporarily available at lower levels on the shore. The failure of juveniles to survive and grow in places other than among Tesseropora is due to a combination of factors. These include desication at high levels on the shore, smothering by rapidly growing algae low on the shore, and the grazing activities of the larger limpet, Cellana tramoserica. These latter two factors also reduce the survival of experimentally transplanted adult Patelloida: algae by covering the substratum and smothering the limpets, and Cellana by outcompeting them for food. The density of Cellana is greater on patches of bare rock than among barnacles, and these large limpets may be unable to move and feed effectively over the irregular surface created by Tesseropora. Patelloida, however, is small enough to feed over and among these barnacles, and hence has a refuge from competition with Cellana. Barnacles may also provide shelter from the effects of desiccation and strong wave action, and thus increase the survival of juvenile Patelloida. By being associated with barnacles, however, Patelloida becomes vulnerable to intermittent predation by the whelk, Morula marginalba. This effect may be serious enough to eliminate small local populations of limpets, either by direct predation or by removing the refuge-providing barnacles. The association with barnacles may also limit the maximum size to which Patelloida can grow. Patelloida is not always found with Tesseropora, and adults in different localities can be found in association with other sessile organisms. It may be argued that small species of limpets require a spatial refuge from physical and/or biological pressures. To examine this hypothesis, the relationship between Patelloida and Tesseropora is compared to other published accounts of limpets with specialised modes of life.     

The influence of moss on grazers in high‐altitude streams: food,refuge or both?

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Disturbance and reef topography maintain high local diversity in Ecklonia radiata kelp forests

Disturbance of competitive‐dominant plant and algae canopies often lead to increased diversity of the assemblage. Kelp forests, particularly those of temperate Western Australia, are habitats with high alpha diversity. This study investigated the roles of broad‐scale canopy loss and local scale reef topography on structuring the kelp‐dominated macroalgal forests in Western Australia. Eighteen 314 m² circular areas were cleared of their Ecklonia radiata canopy and eighteen controls were established across three locations. The patterns of macroalgal recolonisation in replicate clearances were observed over a 34 month period. Macroalgal species richness initially increased after canopy removal with a turf of filamentous and foliose macroalgae dominating cleared areas for up to seven months. A dense Sargassum canopy dominated cleared areas from 11 to 22 months. By 34 months, partial recovery of the kelp canopy into cleared areas had occurred. Some cleared areas did not follow this trajectory but remained dominated by turfing, foliose and filamentous algae. As kelp canopies developed, the initial high species diversity declined but still remained elevated relative to undisturbed controls, even after 34 months. More complex reef topography was associated with greater variability in the algal assemblage between replicate quadrats suggesting colonising algae had a greater choice of microhabitats available to them on topographically complex reefs. Shading by canopies of either Sargassum spp. and E. radiata are proposed to highly influence the abundance of algae through competitive exclusion that is relaxed by disturbance of the canopy. Disturbance of the canopy in E. radiata kelp forests created a mosaic of different patch types (turf, Sargassum‐dominated, kelp‐dominated). These patch types were both transient and stable over the 34 months of this study, and are a potential contemporary process that maintains high species diversity in temperate kelp‐dominated reefs.     

Mutualism between the territorial intertidal limpet Patella longicosta and the crustose alga Ralfsia verrucosa

Mutualistic relations between plants and animals are well documented on land but have received less attention in marine systems. This study examined the relationship between the territorial intertidal limpet Patella longicosta and the crustose brown alga Ralfsia verrucosa. Adult Patella are found exclusively in association with Ralfsia, on which they feed, while Ralfsia occurs primarily, but not exclusively, in Patella territories. Ralfsia benefits directly from both the presence and the territorial behaviour of Patella. Algal productivity was assessed by measuring oxygen evolution and utilization in situ and deriving photosynthesis/irradiance curves. Productivity was increased by about 30% by the presence of Patella in both summer (P _max of grazed algae 0.0098; ungrazed algae 0.0063 mg C · cm^-2 · h^-1) and winter (P _max grazed algae 0.0081; ungrazed algae 0.0053 mg^-2 · C · h^-1). Algal growth rates were not significantly increased by the application of limpet mucus in the laboratory. We did not examine nutrient regeneration by the limpet, but the increase in photosynthetic rate may depend on the limpet's grazing pattern which creates secondary sites for growth. Ralfsia also benefited from the territorial behaviour of Patella. The effects of different grazing regimes were investigated in different seasons by removing territorial limpets and either excluding all limpets using copper-based antifouling paint, or allowing access to non-territorial limpets (mostly P. oculus) using partial paint barriers. Exclusion of all limpets resulted in rapid overgrowth of Ralfsia plants by the foliose green alga Ulva sp.. Where non-territorial limpets had access to the plants overgrowth was reduced but Ralfsia plants were entirely removed by destructive grazing. Non-territorial grazers removed 90% of Ralfsia plants within 4 weeks in summer and 60% in winter. In control treatments P. longicosta prevented overgrowth by Ulva and actively excluded vagrant grazers, preventing overgrazing. Based on these findings, the association between the limpet and alga can be regarded as a nonobligate mutualism.     

An experimental approach to the study of factors affecting algal colonization in a sublittoral kelp forest

Factors influencing the early colonization by benthic macroalgae and the early successional sequence in a subtidal Ecklonia radiata (C. Agardh) J. Agardh forest in Port Jackson (Sydney) were investigated using controlled experiments involving the microscopic sampling of settlement plates. The preliminary results indicate that the Tolerance model of succession may be applicable. The effect of grazers, and competition between different algal types were important and could, later in the succession, lead to the operation of the mechanisms suggested by the Facilitation or Inhibition models. Controls for the use of settlement plates showed that thick settlement plates led to an over-estimation of initial algal growth compared with the growth on natural patches of substratum. Other results suggested that thick plates excluded grazers. Borders of plates were investigated, but the establishment of algae was random over the plates′ surfaces and border effects were the result of physical or biotic factors acting on algae after settlement. An experiment was developed for use in subtidal areas to test for artifacts due to cages. It was found that the presence of the cages influences the community by attracting small snails. Fish were excluded from some areas to monitor their effects on initial algal growth. Exclusion of fish was found to cause a reduction in algal growth as well as increases in numbers of small invertebrates such as amphipods and copepods. The results suggested that the amount of algae initially present was affected by invertebrate grazers, the abundances of which were, in turn, affected by predation. The species diversity of the early algal growth did not show any consistent trends between high and low grazing pressure. Grazing was important in determining the temporal variability of early growth of individual species. Siltation was positively correlated with early algal growth; a mechanism to account for this effect is suggested. The application of the methods used in this study to future experimental studies of sublittoral community dynamics is considered.     

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.     

Recruitment and metamorphosis of Haliotis larvae induced by molecules uniquely available at the surfaces of crustose red algae

Crustose red algae induce substratum-specific settlement, attachment and metamorphosis of the planktonic larvae of Haliotis rufescens Swainson (gastropod mollusc), upon direct contact by the larvae with any of a number of algal species tested. Larvae are not induced by contact with intact foliose red, brown or green macroalgae. Geniculate red algae are only slightly active. Larval settlement and metamorphosis are shown to be triggered by a class of chemical inducers associated with macromolecules and found in extracts of all species of crustose, geniculate, and foliose red algae tested; these inducers are not found in extracts of brown or green macroalgae. The substratum specificity of larval settlement and metamorphosis is shown to result from the unique availability of these inducers at the surfaces of the crustose red algae. Using a newly-developed improved method of purification based upon size-separation by gel-filtration, followed by ion-exchange chromatography over a diethylaminoethyl (DEAE)-acrylamide matrix, the principal inducer of Haliotis larval settlement and metamorphosis has been resolved from the red algal phycobiliproteins. Sensitivity of this inducer to reduction in molecular weight by digestion with trypsin demonstrates that this inducer is associated with protein.     

Antarctic patterns of shallow subtidal habitat and inhabitants in Wilke’s Land

Studies of east Antarctic marine assemblages on hard substrata are rare. In relation to sea-ice breakout, we assessed benthic patterns of habitat and inhabitants between islands and bays at each of two depths (6 and 12 m) across the Windmill Islands coast. Island sites experience sea-ice breakout in the austral spring, while bay sites typically retain sea-ice cover into the summer and in some places the cover is virtually permanent. Composition of assemblages differed between sheltered bays and exposed islands. Islands were dominated by macroalgae, which also varied with depth. Immediately below the ice–foot zone at 6 m, substratum space were monopolised by foliose red (Palmaria decipiens) and foliose brown (Desmarestia sp.) algae, whereas at 12 m large canopies of Himantothallus grandifolius was abundant. The understorey consisted of a mixture of turfs and encrusting red algae at 6 m, and coralline algae at 12 m. Sheltered bays had large areas of sediment/algal complex and no canopy-forming macroalgae. We found more sponges and hydroids in bays, and more brittle stars around islands. Experiments testing factors that covary with exposure and depth in Antarctica, such as light, sedimentation and ice scour are necessary to determine processes that maintain these striking patterns.     

Seasonal changes in subarctic sea urchin populations from different habitats

This paper documents seasonal variation in certain sea urchin (Strongylocentrotus polyacanthus) characteristics in habitats of varying environmental conditions. At Shemya Island, Alaska, three habitat types [dense kelp beds, intermediate kelp beds, and algal barrens (low to no foliose algal cover)] were monitored seasonally from September 1995 to August 1996, for live and drift foliose algae. In general, drift algal abundance was greater in areas with more attached kelp, but this varied with season. Along with drift algae, sea urchin density, test size diameter, gonad and nutrition indices, and mobility were seasonally sampled within each habitat. Densities were highest in the algal barrens and lowest in the kelp beds. Seasonally, densities varied between summer/fall, and winter/spring, with lower numbers in the winter/spring. Test size was largest in the kelp habitats when compared to the intermediate or barren sites. Test size was seasonally consistent in the kelp habitats but not in the intermediate or barren sites. Here, test size did vary depending on season (larger urchins were found in winter). The gonad index showed much seasonal variation at the kelp and intermediate kelp sites, but was relatively more stable over time in the barren habitats. Between habitats, gonad and nutrition indices were larger in areas with kelp. Urchin movement varied seasonally between habitats, with more overall movement and variation in barren habitats. These results illustrate the importance of small-scale temporal and spatial variation. Monitoring for 1 year demonstrated that certain parameters varied more in areas of higher foliose algal cover (gonad indices), while other parameters varied more in low kelp areas (test size and movement). These results suggest that studies involving urchins should consider both time of year and overall algal community composition when conducting any type of experimental or monitoring work.     

Diversity,heterogeneity and consumer pressure in a tropical rocky intertidal community

Summary Previous studies indicated that at Taboguilla Island (Gulf of Panama), persistence of many intertidal organisms depended on holes and crevices in the rock as refuges from both vertebrate (fishes) and invertebrate (crabs, gastropods, chitons) consumers. Here, we evaluate the influences of substratum heterogeneity and consumers on patterns of diversity of sessile organisms in this habitat. Local substratum topography is highly variable, ranging from smooth to irregular surfaces. Algal crusts typically dominate all low zone rock surfaces, and most other sessile spegies (invertebrates and foliose algae) occur in holes and crevices. Number (S) and diversity (H) of sessile species is lower on homogeneous surfaces than on heterogeneous surfaces. Rate of increase in S with area sampled is positively correlated with substratum heterogeneity; number of species sampled per transect at a homogeneous site would be about 10 vs 30 to 60 on a heterogeneous site. Large fishes and crabs forage intensively over both substratum types, but cannot enter holes and crevices to eat prey. Gastropods, chitons, limpets, and small crabs feed on both substrata but vary in abundance from hole to hole. Prey mortality is thus intense and constant on open surfaces, but variable in space and time in holes and crevices. When consumers are excluded from the general rock surface, algal crusts are settled upon and overgrown by foliose algae, hydrozoans, and sessile invertebrates, particularly bivalves. Both S and H first increase, as sessile species invade and become more abundant, and then decrease as the rock oyster Chama echinata begins to outcompete other species and dominate primary space. Hence, consumers normally keep local diversity low by removing most sessile prey from open surfaces.In these experiments, a consumer pressure gradient was established by removing 0, 1, 2, 3, and all of 4 distinct groups of consumers. As predicted by the intermediate disturbance hypothesis, lowest diversity occurred at lowest (total exclusion) and highest consumer pressure (normal condition). Highest diversity occurred at intermediate consumer pressure. Unexplained variation in this relationship is probably due to quantitative and qualitative differences in consumer regime, variation among plots in substratum heterogeneity, and insufficient time for competitive dominance by Chama to be fully expressed. On a small (0.25 m²) spatial scale, consumers maintain low diversity by keeping prey scarce and causing local extinctions. On larger spatial scales, they may maintain and even produce high diversity through their interaction with substratum heterogeneity and possibly low dispersal rates of sessile species.     

Experimental studies on homing in the intertidal patellid limpetCellana tramoserica (Sowerby)

Summary A model of two-dimensional random walk was developed to allow statistical tests for the presence of homing behaviour in intertidal populations of the limpetCellana tramoserica. Not all limpets return to a home-site after feeding excursions. Some move around at random. The latter pattern of movement was tested to justify the assumptions of the model. Limpets can stop homing and begin to move at random; moving limpets can become homers. Thus, random movement and consistent homing behaviour are not discrete patterns of behaviour in a population.Controlled field experiments showed that the proportion of animals which home is not affected by the height on the shore, force of wave action, the cover of macroalga, nor the irregularity of the substratum. Small limpets, however, home more than large ones.Close proximity to other homing limpets causes individuals to stop homing and move away. Alterations of density demonstrated thatCellana shows density-dependent dispersal. More limpets emigrate from areas of increased density and more immigrate into areas of decreased density, when compared with control areas. Less animals immigrate into, and remain in, areas from which microalgal food was experimentally removed. More animals emigrate from these areas.These experiments support the hypothesis that homing behaviour is an adaptation which regulates local density and dispersion to maximize utilization of food resources and, thus, to reduce intraspecific competition for food at high densities of limpets.