Experimental effects of kelp canopies on subtidal coralline algae

Plants are often grouped as canopy species or understorey species because it is thought that that these sets of taxa interact in predictable ways. Mensurative experiments in southern Australia demonstrated that the percentage cover of encrusting coralline algae was greater, and articulated (branching) coralline algae less, on boulders under a canopy of dense kelp (>7 plants per m²), Ecklonia radiata, than on boulders without kelp. Experimental clearances of kelp and reciprocal transplants of boulders between patches of E. radiata and patches without kelp showed that canopies maintained and facilitated the growth of encrusting coralline algae and reduced the cover of articulated coralline algae. Potential artefacts associated with clearing kelp and transplanting boulders were not detected when tested with a series of translocation controls. These results reject the model that the co‐occurrence of E. radiata and encrusting corallines is just an assemblage of plants caused by spatial and temporal coincidence. Instead, they support the model that kelp facilitates the growth and survival of understorey algae.     

Predicting understorey structure from the presence and composition of canopies: an assembly rule for marine algae

Assembly rules provide a useful framework for predicting patterns of community assembly under defined environmental conditions. Habitat created by canopy-forming algae (such as kelps) provides a promising system for identifying assembly rules because canopies typically have a large and predictable influence on understorey communities. Across >1,000 km of subtidal South Australian coastline, we identified natural associations between assemblages of understorey algae and (1) monospecific canopies of Ecklonia radiata, (2) canopies comprised of E. radiata mixed with Fucales (Cystophora spp. and Sargassum spp.), and (3) gaps among canopies of algae. We were able to recreate these associations with experimental tests that quantified the assembly of understorey algae among these three habitat types. We propose the assembly rule that understorey communities on subtidal rocky coast in South Australia will be (1) monopolised by encrusting coralline algae beneath monospecific canopies of E. radiata, (2) comprised of encrusting corallines, encrusting non-corallines, and sparse covers of articulated corallines, beneath mixed E. radiata-Fucales canopies, and (3) comprised of extensive covers of articulated corallines and filamentous turfs, as well as sparse covers of foliose algae and juvenile canopy-formers, within gaps. Consistencies between natural patterns and experimental effects demonstrate how algal canopies can act as a filter to limit the subsets of species from the locally available pool that are able to assemble beneath them. Moreover, the subsets of species that assemble to subtidal rocky substrata in South Australia appear to be predictable, given knowledge of the presence and composition of canopies incorporating E. radiata.     

Responses of crustose corallines to epiphyte and canopy cover

Although a dense cover of epiphytes is generally considered to be harmful for some coralline algae (Corallinaceae, Rhodophyta), crustose corallines in the littoral zone seem to be preserved from bleaching when covered by canopy plants and epiphytes during summer. This study aimed to test the responses of coralline crusts to epiphytes and canopy algae and their interaction with grazing limpets. Growth rates and color changes were followed in two crust species in areas with or without canopy algae in the Isle of Man, British Isles. Limpets were excluded, to allow epiphytes to grow upon crusts. Responses were measured both on pieces of crusts upon acrylic plates and on crusts growing naturally on the shore. Fucus canopy and epiphytic Enteromorpha significantly influenced the crusts' growth, depending on season. Epiphytes reduced the light levels beneath by up to 78%, more than the canopy algae (62%). Crusts exposed outside the canopy bleached in summer, but gradually restored their color once they were covered by epiphytes. The fast-growing Phymatolithon lenormandii (Aresch.) Adey recovered its coloration more quickly than the slow-growing P. purpureum (P. et H. Crouan) Woelkerling et Irvine. However, neither crust species could restore its color when epiphytes were reduced by grazing limpets, Patella vulgata L. Bleaching did not kill the crusts, but seemed to interfere with crusts' growth. Restoration of pigmentation was quantified for the first time on bleached coralline crusts. Epiphyte and canopy algae were experimentally shown to be beneficial, probably by providing shade and also protecting crusts from desiccation.     

Kelp and sea urchin settlement mediated by biotic interactions with benthic coralline algal species

Species interactions can influence key ecological processes that support community assembly and composition. For example, coralline algae encompass extensive diversity and may play a major role in regime shifts from kelp forests to urchin-dominated barrens through their role in inducing invertebrate larval metamorphosis and influencing kelp spore settlement. In a series of laboratory experiments, we tested the hypothesis that different coralline communities facilitate the maintenance of either ecosystem state by either promoting or inhibiting early recruitment of kelps or urchins. Coralline algae significantly increased red urchin metamorphosis compared with a control, while they had varying effects on kelp settlement. Urchin metamorphosis and density of juvenile canopy kelps did not differ significantly across coralline species abundant in both kelp forests and urchin barrens, suggesting that recruitment of urchin and canopy kelps does not depend on specific corallines. Non-calcified fleshy red algal crusts promoted the highest mean urchin metamorphosis percentage and showed some of the lowest canopy kelp settlement. In contrast, settlement of one subcanopy kelp species was reduced on crustose corallines, but elevated on articulated corallines, suggesting that articulated corallines, typically absent in urchin barrens, may need to recover before this subcanopy kelp could return. Coralline species differed in surface bacterial microbiome composition; however, urchin metamorphosis was not significantly different when microbiomes were removed with antibiotics. Our results clarify the role played by coralline algal species in kelp forest community assembly and could have important implications for kelp forest recovery.     

Algal canopy as a proxy for the disturbance history of understorey communities in East Antarctica

Much of the macroalgal zonation on Antarctic coasts is thought to be maintained by ice scour. The frequency and severity of ice scour typically decrease with depth, which is hypothesized to drive the zonation of two canopy-forming macroalgae, Desmarestia menziesii and Himantothallus grandifolius. If true, understorey communities should share the same history of ice scour as their respective canopies, and their composition should vary accordingly. To evaluate this prediction we collected boulders from under each canopy species at two depths, 6 and 12 m, at two sites on the coast of East Antarctica. We examined the hard-substrate communities growing on boulders and tested for differences in community composition with respect to canopy species, surface orientation, and depth. Communities under the different canopies showed some variation consistent with the hypothesized difference in disturbance history. Those under H. grandifolius accommodated a greater abundance and diversity of sponges, which is usually characteristic of older, later successional communities. Differences were subtle, however, suggesting that canopies might be maintained by ice disturbance over large temporal scale relative to those at which understorey communities develop, and/or that canopies themselves influence understorey composition. This study describes patterns associated with one of the most prominent examples of bathymetric zonation in shallow Antarctic benthos, and experimental work is now needed to partition the processes at work.     

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.     

3-D distribution of nongeniculate corallinales: a case study from a reef crest of South Sinai (Red Sea,Egypt)

An innovative technique for the estimation of species and growth-form abundance of coralline algae, including information on their vitality, was adopted on the reef crest of Ras Nosrani and Coral Bay, South Sinai. Data of coralline abundance from visual census and collection of voucher specimens were plotted on a 3-D sketched representation of the horizontal and vertical planes of the reef crest and of its crannies. Coralline dominance at the two investigated sites was not significantly different, with values ranging from 8.55 and 10.06% on the vertical plane and from 5.3 to 7.17% on the horizontal plane. About one-third of total corallines of the South Sinai reef crest was located in crannies, where the algae are completely overlooked by routine field surveys. Pink to violet, healthy corallines with encrusting growth-form, mainly belonging to Hydrolithon onkodes and Neogoniolithon, with subordinate fruticose Lithophyllum kotschyanum dominated the reef crest at both sites. The fruticose growth form, usually associated with L. kotschyanum, was more common in the horizontal than in the vertical plane. Purple, healthy, encrusting Sporolithon uncommonly occurred. Whitish, gray or green unhealthy or dead corallines were more common on the horizontal plane at both sites, possibly resulting from excessive solar radiation.     

Endolithic algae: an alternative source of photoassimilates during coral bleaching

Recent reports of worldwide coral bleaching events leading to devastating coral mortality have caused alarm among scientists and resource managers. Differential survival of coral species through bleaching events has been widely documented. We suggest that among the possible factors contributing to survival of coral species during such events are endolithic algae harboured in their skeleton, providing an alternative source of energy. We studied the dynamics of photosynthetic pigment concentrations and biomass of endoliths in the skeleton of the encrusting coral Oculina patagonica throughout a bleaching event. During repeated summer bleaching events these endolithic algae receive increased photosynthetically active radiation, increase markedly in biomass, and produce increasing amounts of photoassimilates, which are translocated to the coral. Chlorophyll concentrations and biomass of endoliths were 4.6 +/- 1.57 and 1570 +/- 427 microg cm(-2) respectively, in skeletons of relatively healthy colonies (0-40% bleaching) but up to 14.8 +/- 2.5 and 4036 +/- 764 microg cm(-2) endolith chlorophyll and biomass respectively, in skeletons of bleached colonies (greater than 40% bleaching). The translocation dynamics of (14)C-labelled photoassimilates from the endoliths to bleached coral tissue showed significantly higher 14C activity of the endoliths harboured within the skeletons of bleached corals than that of the endoliths in non-bleached corals. This alternative source of energy may be vital for the survivorship of O. patagonica, allowing gradual recruitment of zooxanthellae and subsequent recovery during the following winter.     

Dispersion and phagokinesis in the echinoid Evechinus chloroticus (Val.)

Movement in the echinometrid sea urchin Evechinus chloroticus (Val.) was investigated in relation to the availability of a preferred food species, the laminarian alga Ecklonia radiata (C. Ag.). Patterns of microhabitat occupancy, dispersion, and movement were described in two habitats: Coralline Flats, dominated by encrusting coralline algae; and Sediment Flats, occupied by turfing corallines, Ecklonia and the mussel Modiolus areolatus (Gould). In both habitats, sea urchins were positively associated with encrusting coralline algae, despite the fact that encrusting corallines occupied a minor proportion of the substratum in the Sediment Flat habitat. Sea urchins were significantly clumped in both habitat types, and more so in sites within the Sediment Flats habitat. Movement was not directional in either habitat, but the magnitude of movement was almost twice as high in sites from the Coralline Flats habitat, where kelp was absent, as in the Sediment Flats sites, where kelp was present. Behaviour in response to the presence of Ecklonia in both habitats was investigated experimentally. In both habitats, urchins exposed to a “drift” kelp plant at a distance of 1–2 m exhibited net movement approximately twice as great as that shown by control urchins, not exposed to a kelp plant. Despite the relatively small distances the urchins were from the kelp, there was no evidence of directionality in movement. We conclude from the present study that further understanding of the feeding behaviour of urchins requires that a distinction be made between attached and drift algae, and between directionality and magnitude of movement, and that the interaction of these and other factors are best investigated under field conditions.     

Effects of solar UV-B radiation on canopy structure of Ulva communities from southern Spain

Within the sheltered creeks of Cádiz bay, Ulva thalli form extended mat-like canopies. The effect of solar ultraviolet radiation on photosynthetic activity, the composition of photosynthetic and xanthophyll cycle pigments, and the amount of RubisCO, chaperonin 60 (CPN 60), and the induction of DNA damage in Ulva aff. rotundata Bliding from southern Spain was assessed in the field. Samples collected from the natural community were covered by screening filters, generating different radiation conditions. During daily cycles, individual thalli showed photoinhibitory effects of the natural solar radiation. This inhibition was even more pronounced in samples only exposed to photosynthetically active radiation (PAR). Strongly increased heat dissipation in these samples indicated the activity of regulatory mechanisms involved in dynamic photoinhibition. Adverse effects of UV-B radiation on photosynthesis were only observed in combination with high levels of PAR, indicating the synergistic effects of the two wavelength ranges. In samples exposed either to PAR+UV-A or to UV-B+UV-A without PAR, no inhibition of photosynthetic quantum yield was found in the course of the day. At the natural site, the top layer of the mat-like canopies is generally completely bleached. Artificially designed Ulva canopies exhibited fast bleaching of the top layer under the natural solar radiation conditions, while this was not observed in canopies either shielded from UV or from PAR. The bleached first layer of the canopies acts as a selective UV-B filter, and thus prevents subcanopy thalli from exposure to harmful radiation. This was confirmed by the differences in photosynthetic activity, pigment composition, and the concentration of RubisCO in thalli with different positions within the canopy. In addition, the induction of the stress protein CPN 60 under UV exposure and the low accumulation of DNA damage indicate the presence of physiological protection mechanisms against harmful UV-B. A mechanism of UV-B-induced inhibition of photosynthesis under field conditions is proposed.     

Recruitment of the intertidal kelp Lessonia nigrescens Bory in northern Chile: successional constraints and opportunities

The strong 1982/83 El Niño event caused local extinction in populations of the low intertidal kelp Lessonia nigrescens Bory on northern Chilean coasts. The kelp has partially recolonized, but its recovery has apparently been low. This study documents the effect of biological factors that potentially decrease the velocity of kelp recolonization in northern Chile. A removal experiment showed that encrusting coralline algae, which dominate the lower intertidal, significantly reduce the recruitment of L. nigrescens in the presence of herbivores. Epithallium shedding by encrusting corallines is the most probable cause for this inhibition process. On the other hand, grazing on encrusting corallines by the chiton Enoplochiton niger (Barnes), a large-sized herbivore (10–20 cm length), could also affect kelp recruitment. The intestinal content of E. niger, where encrusting corallines are the main item (84.2%), revealed the presence of L. nigrescens. Among the natural substrata on which kelp recruits, measured at five different localities, the coarsely-branched alga Corallina officinalis (Decaisne) Kützing reached the highest frequency, despite its extremely low cover (< 1%) in the field. This suggests that settlement on turfs of C. officinalis allows the kelp to escape from herbivory, thus facilitating its recruitment. The roles of timing of kelp recruitment and seasonal grazing are also discussed.     

The nepionic stage of <Emphasis Type="Italic">Solenomeris</Emphasis> Douvillé, 1924 (Acervulinidae,Foraminiferida): new observations from the uppermost Maastrichtian-early Danian of Austria (Kambühel Formation,Northern Calcareous Alps)

In the shallow-water limestone succession of the Kambühel Formation (upper Maastrichtian to upper Thanetian, Northern Calcareous Alps), the benthic-vagile to encrusting-sessile acervulinid foraminifer Solenomeris Douvillé was identified in upper Maastrichtian and, more commonly, lower Danian rocks. We report on the free nepionic (juvenile) stage of this foraminifer, its relation to the adult encrusting stage, and competitive interaction with coralline algae. Whereas morphology and chamber arrangement of the free nepionts clearly assign the Alpine specimens to Solenomeris, the chamber shape in axial sections differs from the Eocene type-species S. ogormani, accounting for classification in open nomenclature. Nepiont chamber shape can be used as a further specific criterion besides size of adult chambers and, perhaps, growth habit. A two-stage concept of ontogenetic development is presented. Solenomeris flourished in early Danian high-stress environments (perhaps related to cooler waters and/or elevated nutrient level) under conditions less favorable for encrusting corallines that competed for substrate. Absence of Solenomeris in upper Danian limestones and in upper Thanetian coralgal limestones that form the top of the Kambühel Formation may be related to recovery of coralline algae in shallow-marine environments of low to moderate nutrient level.     

Leaf senescence and decline of end-of-season gas exchange in five temperate deciduous tree species grown in elevated CO2 concentrations

We measured rates of leaf senescence and leaf level gas exchange during autumnal senescence for seedlings of five temperate forest tree species under current and elevated atmospheric CO₂ concentrations and low- and high-nutrient regimes. Relative indices of whole canopy carbon gain, water loss and water use efficiency through the senescent period were calculated based on a simple integrative model combining gas exchange per unit leaf area and standing canopy area per unit time. Seedlings grown under elevated [CO₂] generally had smaller canopies than their current [CO₂]-grown counterparts throughout most of the senescent period. This was a result of smaller pre-senescent canopies or accelerated rates of leaf drop. Leaf-level photosynthetic rates were higher under elevated [CO₂] for grey birch canopies and for low-nutrient red maple and high-nutrient ash canopies, but declined rapidly to values below those of their current [CO₂] counterparts by midway through the senescent period. CO₂ enrichment reduced photosynthetic rates for the remaining species throughout some or all of the senescent period. As a result of smaller canopy sizes and reduced photosynthetic rates, elevated [CO₂]-grown seedlings had lower indices of whole canopy end-of-season carbon gain with few exceptions. Leaf level transpiration rates were highly variable during autumnal senescence and neither [CO₂] nor nutrient regime had consistent effects on water loss per unit leaf area or integrated whole canopy water loss throughout the senescent period. Indices of whole canopy, end-of-season estimates of water use efficiency, however, were consistently lower under CO₂ enrichment, with few exceptions. These results suggest that whole canopy end-of-season gas exchange may be altered significantly in an elevated [CO₂] world, resulting in reduced carbon gain and water use efficiency for many temperate forest tree seedlings. Seedling growth and survivorship, and ultimately temperate forest regeneration, could be reduced in CO₂-enriched forests of the future.     

The influence of canopy algae on vertical patterns of distribution of low-shore assemblages on rocky coasts in the northwest Mediterranean

Canopy-forming algae have been shown to play a fundamental role in the maintenance of understorey assemblages on rocky shores. In the Mediterranean, low-shore habitats are often monopolised by canopy algae of the genus Cystoseira and, in particular, by the species Cystoseira amentacea var. stricta. Alternatively, low-shore habitats are dominated by turf-forming algae and/or mussels. Previous studies showed that loss of Cystoseira, by natural or anthropogenic disturbances, resulted in the increase of turfing algae. Here, we propose that turf-forming algae may act as colonisers from nearby habitats, specifically from lower portions of the shore. The following hypotheses were tested to investigate this proposition: (1) in presence of Cystoseira, the assemblage living under its canopy will be distinct, in terms of composition and structure, from that found lower on the shore, (2) if the canopy of Cystoseira was removed, the differences between the two assemblages would decrease or disappear, (3) the effects of the canopy would be more important than other causes of variability at the spatial scale investigated (hundreds to thousands of metres), and (4) that effects of removal of the canopy would be consistent through time. These hypotheses were tested by means of a canopy removal experiment, involving several spatial and temporal repetitions of the manipulation.The assemblage underneath the canopy of Cystoseira was distinct from that found immediately lower on the shore, but when the canopy was removed there were no differences between the two. The effects of the canopy were consistent at the spatial and temporal scales investigated. Several species living under the canopy disappeared and were replaced by turf-forming species, resulting in a homogenisation of the two habitats and in a drastic loss of habitat diversity. This study shows that Cystoseira can be considered a habitat-forming species, responsible for the maintenance of two distinct low-shore assemblages. In this light, its importance should be taken into account by policies aiming to preserve biodiversity on rocky shores.     

Ecological Shifts in Mediterranean Coralligenous Assemblages Related to Gorgonian Forest Loss

Mediterranean gorgonian forests are threatened by several human activities and are affected by climatic anomalies that have led to mass mortality events in recent decades. The ecological role of these habitats and the possible consequence of their loss are poorly understood. Effects of gorgonians on the recruitment of epibenthic organisms were investigated by manipulating presence of gorgonians on experimental panels at 24 m depth, for Eunicella cavolinii, and at 40 m depth, for Paramuricea clavata, at two sites: Tavolara Island (Tyrrhenian Sea) and Portofino Promontory (Ligurian Sea). After 4 months, the most abundant taxa on the panels were encrusting green algae, erect red algae and crustose coralline algae at 24 m depth and encrusting brown algae and erect red algae at 40 m depth. Assemblages on the panels were significantly affected by the presence of the gorgonians, although effects varied across sites and between gorgonian species. Species diversity and evenness were lower on panels with gorgonian branches. Growth of erect algae and recruitment of serpulid polychaetes were also affected by the presence of the gorgonians, primarily at Tavolara. Crustose coralline algae and erect sponges were more abundant on E. cavolinii panels at 24 m depth, while encrusting bryozoans were more abundant on P. clavata panels at 40 m depth. Effects of gorgonians on recruited assemblages could be due to microscale modification of hydrodynamics and sediment deposition rate, or by a shading effect reducing light intensity. Gorgonians may also intercept settling propagules, compete for food with the filter-feeders and/or for space by producing allelochemicals. Presence of gorgonians mainly limits the growth of erect algae and enhances the abundance of encrusting algae and sessile invertebrates. Therefore, the gorgonian disappearances may cause a shift from assemblages characterised by crustose coralline algae to filamentous algae assemblages, decreasing complexity and resilience of coralligenous bioconstructions.     

Effects of mean intensity and temporal variability of disturbance on the invasion of <Emphasis Type="Italic">Caulerpa racemosa</Emphasis> var. <Emphasis Type="Italic">cylindracea</Emphasis> (Caulerpales) in rock pools

Disturbance is a key factor influencing the invasibility of habitats and assemblages. This relationship was extensively studied in terrestrial systems, but it was scarcely tested in the marine environment. We investigated experimentally the interactive effects of changes in the intensity and temporal variability of mechanical disturbance by boulders on invasion dynamics of the green alga Caulerpa racemosa var. cylindracea in littoral rock pools. We tested the hypothesis that the success of invasion of C. racemosa would be (1) greater under large than under low intensity of disturbance, (2) greater under large than under low temporal variability of disturbance and that (3) interactive effects could also occur, with variability of disturbance magnifying the effects of intensity. C. racemosa was virtually absent in pools maintained under high intensity of disturbance, independently of temporal variability. High intensity of disturbance was also associated with lower density and length of fronds and thinner diameter of the stolons of the alga. The total number of native taxa and the abundance of encrusting coralline algae increased under high intensity of disturbance. Differently, turf-forming algae were positively affected by temporal variability of disturbance, while canopy-forming algae did not respond to experimental treatments. Our results suggest a direct negative effect of the most severe experimental conditions on the spread of C. racemosa in rock pools. This likely overwhelmed likely concomitant positive and negative effects mediated by resident organisms. The results of this study help anticipating invasion dynamics of C. racemosa in rock pools under climate change scenarios, in which both intensity and temporal variability of extreme meteorological events are predicted to increase.     

Dominance by a canopy forming seaweed modifies resource and consumer control of bloom-forming macroalgae

Degradation of ecological resources by large-scale disturbances highlights the need to demonstrate biological properties that increase resistance to change and promote the resilience of ecosystem regimes. Coastal eutrophication is a global-scale disturbance that drives ecosystem change by increasing primary production and favouring ephemeral and bloom-forming life-forms. Recent synthesis indicates that consumption processes increase the resistance of coastal communities to nutrient loading by controlling the responses of ephemeral macroalgae. Here we suggest a similar ecological function for canopy cover by demonstrating that the presence of a canopy species modifies both resource and consumer control of bloom-forming algae associated with nutrient enrichment. We tested effects of canopy presence on the interaction between consumer and resource control, by field-manipulations of a dominant canopy forming seaweed ( Fucus vesiculosus ), grazer presence (dominated by the gastropod Littorina littorea ) and nutrient enrichment (common agricultural NPK fertilizer). Canopy cover and grazers jointly controlled strong increases of ephemeral bloom-forming algae (dominated by Ulva spp) from nutrient enrichment; nutrients increased ephemeral recruitment almost 10-fold, but only in the absence of both grazers and canopy cover. Recruitment success of the canopy-forming seaweed itself decreased additively with 56.1, 71.3 and 50.5% from independent effects of canopy cover, grazers and nutrient enrichment, respectively. A meta-analysis of nine nutrient enrichment experiments including seaweed, seagrass and stream communities, showed that in the presence of canopies average nutrient effects were reduced by more than 90% compared to without canopies. This corroborates the generality of our finding that dominating canopy species are important for aquatic ecosystems by increasing community resistance to the propagation of nutrient effects.     

Articulated coralline algae of the genus Amphiroa are highly effective natural inducers of settlement in the tropical abalone Haliotis asinina

The initiation of metamorphosis in marine invertebrates is strongly linked to the environment. Planktonic larvae typically are induced to settle and metamorphose by external cues such as coralline algae (Corallinaceae, Rhodophyta). Although coralline algae are globally abundant, invertebrate larvae of many taxa settle in response to a very limited suite of species. This specificity impacts population structure, as only locations with the appropriate coralline species can attract new recruits. Abalone (Gastropoda, Haliotidae) are among those taxa in which closely related species are known to respond to different coralline algae. Here we identify highly inductive natural cues of the tropical abalone Haliotis asinina. In contrast to reports for other abalone, the greatest proportion of H. asinina larvae are induced to settle and metamorphose (92.8% to 100% metamorphosis by 48 h postinduction) by articulated corallines of the genus Amphiroa. Comparison with field distribution data for different corallines suggests larvae are likely to be settling on the seaward side of the reef crest. We then compare the response of six different H. asinina larval families to five different coralline species to demonstrate that induction by the best inductive cue (Amphiroa spp.) effectively extinguishes substantial intraspecific variation in the timing of settlement.     

Lack of recovery from disturbance in high-arctic boulder communities

High-arctic boulder communities that are impacted by anthropogenic and natural influences can result in the removal or scouring of sessile organisms leaving either open space or damaged organisms. This project asks how sessile communities recover after disturbances by determining (1) timing of recolonization, (2) grazer effects on recolonization, and (3) vegetative regrowth rates for encrusting sponges and corallines. Cleared boulders were monitored over 7 years to determine recolonization timing and grazer impacts. Vegetative regrowth rates were determined by monitoring partial clearings for 4 years. This study found that recolonization was slow with less than 10 % of the boulder surfaces being colonized after 7 years. Recolonization was so slow that it was difficult to ascertain grazer impacts, although it appeared there were no impacts. Lastly, this study showed that vegetative regrowth of sponges and encrusting coralline algae was fast. Sponges averaged 100 % regrowth after 2 years and corallines averaged 40 % after 4 years. Coralline regrowth was slower when exposed to higher sedimentation. This study showed that community recovery from disturbances is very slow in the Beaufort Sea, especially if entire organisms are removed. However, if removal is partial, recovery can be quicker, particularly in low sediment areas. Complete community recovery in this system after a disturbance may take a decade or more.     

The Role of Encrusting Coralline Algae in the Diets of Selected Intertidal Herbivores

Kalk Bay, South Africa, has a typical south coast zonation pattern with a band of seaweed dominating the mid-eulittoral and between two molluscan-herbivore dominated upper and lower eulittoral zones. Encrusting coralline algae were very obvious features of these zones. The most abundant herbivores in the upper eulittoral were the limpet, Cymbula oculus (10.4 ± 1.6 individuals m⁻²; 201.65 ± 32.68 g.m⁻²) and the false limpet, Siphonaria capensis (97.07± 19.92 individuals m⁻²; 77.93 16.02 g.m⁻²). The territorial gardening limpet, Scutellastra cochlear, dominated the lower eulittoral zone, achieving very high densities (545.27 ± 84.35 m⁻²) and biomass (4630.17 ± 556.13 g.m⁻²), and excluded all other herbivores and most seaweeds, except for its garden alga and the encrusting coralline alga, Spongites yendoi (35.93 ± 2.26% cover). In the upper eulittoral zone, encrusting coralline algae were only present in the guts of the chiton Acanthochiton garnoti (30.5 ± 1.33%) and the limpet C. oculus (2.9 ± 0.34%). The lower eulittoral zone limpet, Scutellastra cochlear also had a large percentage of encrusting coralline algae in its gut with limpets lacking gardens having higher (45.1 ± 1.68%) proportions of coralline algae in their guts than those with gardens (25.6 ± 0.8%). Encrusting coralline algae had high organic contents, similar to those of other encrusting and turf-forming algae, but higher organic contents than foliose algae. Radula structure, grazing frequencies as a percentage of the area grazed (upper eulittoral 73.25 ± 3.60% d⁻¹; lower eulittoral 46.0 ± 3.29% d⁻¹), and algal organic content provided evidence to support the dietary habits of the above herbivores. The data show that many intertidal molluscs are actively consuming encrusting coralline algae and that these seaweeds should be seen as an important food source.