The impact of benthic algae on the settlement of a reef-building coral

The capacity of corals to re-establish in degraded and algal-dominated habitats will depend on the effects of algae on coral settlement and growth. We tested the effect of 11 macroalgal species, of widely different functional-forms, on swimming and settlement by larvae of the coral Platygyra daedalea from the Great Barrier Reef. Algal turfs and the crustose calcareous algae groups had minor effects on coral settlement, while upright calcareous and fleshy macroalgae inhibited settlement. However, the extent of inhibition of larval settlement differed amongst upright macroalgal species, variations that were not well explained by physical differences and probably reflect chemical differences not explained by functional-form. Thus, while algal functional-form is useful in identifying general competition patterns, more detailed taxonomic and chemical approaches may be required to fully understand algal effects on corals. Different macroalgal communities on degraded reefs may have different effects on coral settlement, and hence on coral population resilience.     

Unveiling commonalities in understudied habitats of boulder-reefs: life-history traits of the widespread invertebrate and algal inhabitants

Compared to stable reef habitats, dynamic boulder-reefs (commonly called boulder-fields when intertidal) host many habitat specialist species. Most occur underneath boulders where they are largely hidden from view; only limited research has assessed their life-histories despite their widespread importance for biological diversity. But some abundant under-boulder species likely structuring this system are habitat generalists widely researched elsewhere. Here we review this research, focusing on three widespread under-boulder sessile taxa: spirorbids, serpulids (tubeworms) and nongeniculate coralline algae, and three mobile taxa: sea urchins, chitons and crabs. Spirorbids have extensive reproductive/colonization capabilities but are readily out-competed. We thus characterize spirorbids as mostly early-successional, while serpulids often have greater competitiveness. Nongeniculate corallines occur underneath boulders where light reaches, although they can withstand low levels of that and most other resources. Such traits characterize nongeniculate corallines as late-successional. Thus, succession underneath boulders may shift deterministically from early tubeworms to late nongeniculate corallines. Habitat generalist sea urchin and chiton species often have strong inter-specific interactions in exposed habitats. Future experiments may find that under-boulder aggregations of these taxa, and also crabs, are impacting algal and invertebrate assemblages. These experiments will be required if dynamic boulder-reefs are to be as thoroughly understood as other benthic systems.     

ZONATION OF DEEP WATER BENTHIC ALGAE IN THE GULF OF MAINE1

Algal community structure is described for a deep-water rock pinnacle in the Gulf of Maine. Three depth zones of algal dominance were apparent consisting of 1) leathery macrophytes (to 40 m), 2) foliose red algae (to 50 m) and 3) crustose algae (fleshy crusts to 55 m and coralline crusts to 63 m). Microscopic filamentous and erect calcareous algae were also present but inconspicuous. Upright macroscopic filamentous and thin sheet-like forms were not observed on the pinnacle. Sea anemones (Metridium senile) dominated some vertical faces and abrupt prominences in the shallowest regions of the pinnacle (to 24 m) and locally appeared to set the upper vertical limits of kelp and possibly foliose reds. Laminaria sp. formed an open park-like canopy from 24 to 30 m whereas Agarum cribrosum, the deepest kelp, grew as isolated individuals to 40 m. Peyssonnelia sp. and Leptophytum laeve were the deepest occurring fleshy (to 55 m) and calcareous crusts (to 63 m), respectively. The occurrence of these algae at record depths for the Gulf of Maine and for cold water marine environments may be the result of an absence of large herbivores and the high productivity potential of the benthos in these relatively clear waters. By compiling data on depth distribution patterns world-wide, it is evident that the three zone structure of algal morphologies observed in the Gulf of Maine is a global phenomenon.     

Influence of the coral reef assemblages on the spatial distribution of echinoderms in a gradient of human impacts along the tropical Mexican Pacific

Fourteen species of echinoderms and their relationships to the benthic structure of the coral reefs were assessed at 27 sites—with different levels of human disturbances—along the coast of the Mexican Central Pacific. Diadema mexicanum and Phataria unifascialis were the most abundant species. The spatial variation of the echinoderm assemblages showed that D. mexicanum, Eucidaris thouarsii, P. unifascialis, Centrostephanus coronatus, Toxopneustes roseus, Holothuria fuscocinerea, Cucumaria flamma, and Echinometra vanbrunti accounted for the dissimilarities among the sites. The spatial variation among the sites was mainly explained by the cover of the hard corals (Porites, Pocillopora, Pavona, Psammocora), different macroalgae species (turf, encrusting calcareous algae, articulated calcareous algae, fleshy macroalgae), sponges, bryozoans, rocky, coral rubble, sand, soft corals (hydrocorals and octocorals), Tubastrea coccinea coral, Balanus spp., and water depth. The coverage of Porites, Pavona, and Pocillopora corals, soft coral, rock, and Balanos shows a positive relationship with the sampling sites included within the natural protected area with low human disturbances. Contrary, fleshy macroalgae, sponges, and soft coral show a positive relationship with higher disturbance sites. The results presented here show the importance of protecting the structural heterogeneity of coral reef habitats because it is a significant factor for the distribution of echinoderm species and can contribute to the design of conservation programs for the coral reef ecosystem.     

Landslide-facilitated species diversity in a beech-dominant forest

To evaluate the extent to which landslides affect community dynamics and consequent species diversity in a beech-dominated forest, differences in the composition and size structure of tree species were compared between landslide and adjacent stable (control) stands. Demography and changes in size were compared between the two stands over a 5-year period about 60 years after a landslide. In the control stand, replacement occurred even amongst late-successional species, with beech (Fagus crenata)—the most dominant species—increasing in relative abundance. In the landslide stand, very few large individuals of late-successional species occurred, whereas large individuals of early-successional species occurred only in the landslide stand. The traits indicate that the landslide strongly facilitated species diversity, not only by reducing the dominance of late-successional species, but also by promoting recruitment of early-successional species. However, new recruitment of early-successional species was inhibited in the landslide stand, although we observed succeeding regeneration and subsequent population growth of late-successional species there. As a result, the relative dominance of late-successional species increased with succession after the landslide, thus decreasing future species diversity. In beech-dominant forest landscapes in Japan that include communities with different developmental stages, the mosaic of serial stages may facilitate species diversity after a landslide.     

The larval and juvenile ecology of the temperate octocoral Alcyonium siderium Verrill. I. Substratum selection by benthic Larvae

The Alcyonacean octocoral Alcyonium siderium Verrill forms dense stands on vertical rock walls (6–18 m subtidal) along the coast of Massachusetts and further north into the Gulf of Maine. Larvae are brooded until their release as crawling demersal planulae in August. Each colony produces several hundred larvae; most crawl onto the nearby substratum although others may be carried away by wave surge.Experimental enclosures were used to examine substratum selection in the field. Planulae metamorphosed in greater numbers than expected by percent cover on the fleshy red crustose alga Waernia mirabilis Wilce (proposed). The crustose coralline algae Phymatolithon rugulosum (Foslie) and Lithothamnium glaciale Kjellman were settled on commonly but less often than expected by their percent cover. Bare rock surfaces were also used, but least frequently. The encrusting invertebrate colonies that cover much of the available substratum were never used as sites of settlement and metamorphosis. These results agree with laboratory evidence that settlement and metamorphosis of Alcyonium planulae can be induced by Waernia and by coralline algae.     

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.     

The population and community structure of Hawaiian fringing-reef crustose Corallinaceae (Rhodophyta,Cryptonemiales)

Measurements of cover, relative density, and frequency are given for the major reefbuilders on the Waikiki fringing reef. Crustose coralline algae cover 39% of the reef surface and exceed all other organisms as the major builders and consolidators of reef materials. An unidentified coralline (melobesioid C) covers the greatest area (17 %), but Hydrollthon reinboldii (Weber-van Bosse & Foslie) Foslie (11 % cover) because of its thicker thalli and higher relative density (45 %) and frequency (68 %) values is the primary limestone former. Melobesioid C ranks second and Sporolithon erythraeum (Rothpletz) Kylin (6 % cover) third in relative importance. Porolithon onkodes (Heydrich) Foslie (3 % cover), although shown by its low density (4 %) and frequency (6 %) to have a comparatively restricted distribution, is more important than P. gardineri (Foslie) Foslie (2 % cover). P. onkodes maintains and provides the surf-resistant reef edge and is, therefore, of great ecological importance. Coelenterate corals cover less than 1 % of the total area and are relatively unimportant on the fringing reef. The hypothesis is developed that the high ratio (200 : 1) of crustose corallines to corals at Waikiki may be partly due to increases in eutrophication.Experimental evidence shows that P. onkodes can withstand intense illumination and is thereby unique among Hawaiian crustose Corallinaceae. Sporolithon erythraeum is more typical of other crustose corallines since it is physiologically adapted to low-light habitats.     

Recruitment and Succession in a Tropical Benthic Community in Response to In-Situ Ocean Acidification

Ocean acidification is a pervasive threat to coral reef ecosystems, and our understanding of the ecological processes driving patterns in tropical benthic community development in conditions of acidification is limited. We deployed limestone recruitment tiles in low aragonite saturation (Ω_arag) waters during an in-situ field experiment at Puerto Morelos, Mexico, and compared them to tiles placed in control zones over a 14-month investigation. The early stages of succession showed relatively little difference in coverage of calcifying organisms between the low Ω_arag and control zones. However, after 14 months of development, tiles from the low Ω_arag zones had up to 70% less cover of calcifying organisms coincident with 42% more fleshy algae than the controls. The percent cover of biofilm and turf algae was also significantly greater in the low Ω_arag zones, while the number of key grazing taxa remained constant. We hypothesize that fleshy algae have a competitive edge over the primary calcified space holders, coralline algae, and that acidification leads to altered competitive dynamics between various taxa. We suggest that as acidification impacts reefs in the future, there will be a shift in community assemblages away from upright and crustose coralline algae toward more fleshy algae and turf, established in the early stages of succession.     

Species-rich Nardus stricta grasslands host a higher vascular plant diversity on calcareous than on siliceous bedrock

Background: Species-rich Nardus stricta grasslands are a priority habitat for conservation in Europe. They typically occur on siliceous substrates and less frequently are found on calcareous bedrock.

Aims: The present paper aimed to identify the environmental factors (i.e. bedrock type, topographic, and climatic factors) that are related with community diversity and to assess if differences in plant diversity between N. stricta communities on calcareous and siliceous bedrock occur. We hypothesised that Nardus grasslands on calcareous bedrock hosted a higher vascular plant diversity than those on siliceous bedrock.

Methods: Based on 579 vegetation surveys carried out in the south-western Alps, we assessed vascular plant diversity (species richness, Shannon diversity, and Pielou’s equitability index) of species-rich Nardus grasslands and compared it between N. stricta communities on calcareous and siliceous bedrock.

Results: Elevation was identified as the main factor related to species composition, while species diversity was mostly related to mean annual precipitation and bedrock type. Species richness, Shannon diversity, and Pielou’s equitability index were higher within the communities on calcareous rather than on siliceous bedrock and a total of 89 and 34 indicator species were detected, respectively.

Conclusions: Based on our results, we suggest to protect primarily, as a habitat of priority interest, N. stricta grasslands on calcareous substrates for the higher vascular plant diversity hosted.     

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.     

Diet,food preference,and algal availability for fishes and crabs on intertidal reef communities in southern California

Synopsis Herbivory by wide-ranging fishes is common over tropical reefs, but rare in temperate latitudes where the effects of herbivorous fishes are thought to be minimal. Along the west coast of North America, herbivory by fishes on nearshore reefs is largely restricted to a few members of the Kyphosidae, distributed south of Pt. Conception. This paper presents information on natural diets and results from feeding choice experiments for two abundant kyphosids from intertidal habitats in San Diego, California —Girella nigricans andHermosilla azurea, and similar data for the lined shore crab,Pachygrapsus crassipes, which also forages over intertidal reefs. These results are compared with the availability of algae in intertidal habitats measured during summer and winter, on both disturbed and undisturbed habitats. The diets of juveniles ofG. nigricans andH. azurea collected from nearshore habitats were dominated by animal prey (mainly amphipods), but adults of these fishes, andP. crassipes, consumed algae nearly exclusively, with 26, 10, and 14 taxa of algae identified fromG. nigricans, H. azurea, andP. crassipes, respectively. Algae with sheet-like morphologies (e.g.Ulva sp.,Enteromorpha sp., members of the Delesseriaceae) were the principal algae in the diets of the fishes, and calcareous algae (e.g.Corallina sp.,Lithothrix aspergillum) and sheet-like algae (Enteromorpha sp.) comprised the greatest identifiable portion of the shore crab's diet. Feeding choice experiments indicated that the fishes preferred filamentous algae (e.g.Centroceras clavulatum, Polysiphonia sp.,Chondria californica) and sheet-like algae (e.g.Enteromorpha sp.,Ulva sp.,Cryptopleura crispa) over other algal morphologies, whereas the shore crab chose jointed calcareous algae (e.g.Lithothrix aspergillum, Corallina vancouveriensis, Jania sp.) most frequently. The diets and preferences for algae by the fishes were generally most similar to the assemblage of algae available in early successional (disturbed) habitats during summer when sheet-like and filamentous algae are abundant. The shore crab exhibited the opposite trend with a diet more similar to late successional (undisturbed) habitats.     

Photosynthesis and respiration of a deep-water periphyton community (Macclesfield Bank,South China Sea)

The production parameters of the deep-water periphyton were investigated on the Macclesfield Bank (South China Sea) at depths down to 240 m. At 70–90 m depths the substrate is composed of the remains of dead corals and fragments of porous calcareous material overgrown with red coralline and articulated thalline algae Halimeda sp. At 220–240 m depths the substrate consists of rhodolites, which are individual unattached nodules encrusted mainly with red calcareous coralline algae Lithophyllum sp., Porolithon sp. and red lamellar alga Hypoglossum sp. At all depths the substrate is pierced with green filiform algae Ostreobium sp. and covered with benthic diatom algae. The zooperiphyton in the studied area is comprised of bryozoans, sponges, foraminifers and drilling mollusks. Respiration (R) and the gross (Pg) and net (Pn) primary production of the deepwater periphyton community were determined using the oxygen technique (polarographic electrode) at values of photosynthetically active radiation (PAR) close to natural levels. At 70–90 m depths the Pg of the photoperiphyton was 23.3 mg O₂/(m² h), and the Pn of the community had negative values due to respiration of phytoand mainly zooperiphyton and bacteria. At 220–240 m depths, where less than 0.001% of the surface PAR penetrates, the Pg of the phytoperiphyton decreased to 9.6 mg O₂/(m² h), and the R of the community was 25.9 mg O₂/(m² h). A high efficiency of light energy utilization by the phytoperiphyton was found. The apparent quantum yield (Fa) for algae collected at 70–90 m depths was close to the maximum level of 0.097. The values of Fn that we obtained for 220–240 m depths and calculated from identical data [27], were much higher than theoretical values, which cannot be explained based on modern views on the mechanisms of photosynthesis. Possible sources of errors are discussed. It is proposed that we observed additional evolution of oxygen as the result of H₂O₂ degradation.     

Phylomineralogy of the Coralline red algae: Correlation of skeletal mineralogy with molecular phylogeny

The coralline algae in the orders Corallinales and Sporolithales (subclass Corallinophycidae), with their high degree of mineralogical variability, pose a challenge to projections regarding mineralogy and response to ocean acidification. Here we relate skeletal carbonate mineralogy to a well-established phylogenetic framework and draw inferences about the effects of future changes in sea-water chemistry on these calcified red algae. A collection of 191 coralline algal specimens from New Zealand, representing 13 genera and 28 species, included members of three families: Corallinaceae, Hapalidiaceae, and Sporolithaceae. While most skeletal specimens were entirely calcitic (range: 73–100 wt.% calcite, mean 97 wt.% calcite, std dev = 5, n = 172), a considerable number contained at least some aragonite. Mg in calcite ranged from 10.5 to 16.4 wt.% MgCO₃, with a mean of 13.1 wt.% MgCO₃ (std dev = 1.1, n = 172). The genera Mesophyllum and Lithophyllum were especially variable. Growth habit, too, was related to mineralogy: geniculate coralline algae do not generally contain any aragonite. Mg content varied among coralline families: the Corallinaceae had the highest Mg content, followed by the Sporolithaceae and the Hapalidiaceae. Despite the significant differences among families, variation and overlap prevent the use of carbonate mineralogy as a taxonomic character in the coralline algae. Latitude (as a proxy for water temperature) had only a slight relationship to Mg content in coralline algae, contrary to trends observed in other biomineralising taxa. Temperate magnesium calcites, like those produced by coralline algae, are particularly vulnerable to ocean acidification. Changes in biomineralisation or species distribution may occur over the next few decades, particularly to species producing high-Mg calcite, as pH and CO₂ dynamics change in coastal temperate oceans.     

Effects of canopy gaps on the growth of tree seedlings from subtropical broad-leaved evergreen forests of southern China

Seedling performance, morphology, and leaf characteristics of evergreen tree species from different successional stages of Chinese broadleaved evergreen forests were studied in four simulated gap environments: in 100, 55, 33, and 18% of full sunlight. The first two represent the light regimes of clearings (gaps secondarily become more open due to human activity and natural erosion). The hypothesis was tested that early-successional species achieve their greatest seedling size in clearings and contrast in this respect with late-successional species. Late-successional species were expected to show a stronger morphological response (leaf/root ratio) to clearings than early-successional ones. The results provided some evidence in support of these hypotheses with one exception, namely that late-successional Castanopsis fargesii appeared to be a highly sun-tolerant species. It is suggested that Castanopsis seedlings are competitive in large gaps and clearings.     

Calcification by Reef-Building Sclerobionts

It is widely accepted that deteriorating water quality associated with increased sediment stress has reduced calcification rates on coral reefs. However, there is limited information regarding the growth and development of reef building organisms, aside from the corals themselves. This study investigated encruster calcification on five fore-reefs in Tobago subjected to a range of sedimentation rates (1.2 to 15.9 mg cm⁻² d⁻¹). Experimental substrates were used to assess rates of calcification in sclerobionts (e.g. crustose coralline algae, bryozoans and barnacles) across key reef microhabitats: cryptic (low-light), exposed (open-horizontal) and vertical topographic settings. Sedimentation negatively impacted calcification by photosynthesising crustose coralline algae in exposed microhabitats and encrusting foram cover (%) in exposed and cryptic substrates. Heterotrophs were not affected by sedimentation. Fore-reef, turbid water encruster assemblages calcified at a mean rate of 757 (SD ±317) g m⁻² y⁻¹. Different microhabitats were characterised by distinct calcareous encruster assemblages with different rates of calcification. Taxa with rapid lateral growth dominated areal cover but were not responsible for the majority of CaCO₃ production. Cryptobiont assemblages were composed of a suite of calcifying taxa which included sciaphilic cheilostome bryozoans and suspension feeding barnacles. These calcified at mean rates of 20.1 (SD ±27) and 4.0 (SD ±3.6) g m⁻² y⁻¹ respectively. Encruster cover (%) on exposed and vertical substrates was dominated by crustose coralline algae which calcified at rates of 105.3 (SD ±67.7) g m⁻² y⁻¹ and 56.3 (SD ±8.3) g m⁻² y⁻¹ respectively. Globally, encrusting organisms contribute significant amounts of carbonate to the reef framework. These results provide experimental evidence that calcification rates, and the importance of different encrusting organisms, vary significantly according to topography and sediment impacts. These findings also highlight the need for caution when modelling reef framework accretion and interpreting results which extrapolate information from limited data.     

Halimeda growth and diversity on the deep fore-reef of Enewetak Atoll

The deep fore-reef at Enewetak has been examined from the submersible Makali'i. Green algae grow to about-150 m at photon flux densities of approximately 1 Em^-2s^-1. Halimeda cover is 50% at many sites down to-90 m. Halimeda populations are important within the zone of scleractinian corals down to about-65 m, while a Halimeda zone with low coral cover or lacking corals between-65 m and-150 m probably is an important source of reef carbonate. Halimedas of the deep fore-reef, like those of the lagoon, constitute an important structural component in reef building. Other calcareous green algae such as Tydemania are less important on the deep fore-reef, but growth of coralline red algae continues to over-200m. Halimeda diversity is high down to near the base of the euphotic zone.     

Community structure of rhodolith-forming beds on the central Brazilian continental shelf

The community structure of rhodoliths beds in the central Brazilian continental shelf was studied under the hypothesis that nongeniculate coralline algae are the major contributors of the individual rhodoliths. Samples were collected from five localities within a single area at 17–18 m depth. At each locality, rhodoliths were collected in 10 random quadrat samples along a 20-m transect. Our results show that dead cores of rhodoliths were significantly composed by nongeniculate coralline red algae rather than bryozoans, corals, or inorganic material. The live outer layers of the rhodoliths are composed mainly of 7 species of nongeniculate red coralline algae (Lithophyllum coralline, L. johansenii, L. depressum, L. stictaeformis, Neogoniolithon brassica-florida, Spongites fruticosus, and Lithothamnion muellerii) associated with other encrusting organisms such as bryozoans, sponges, corals, barnacles, and Peyssonnelia red algae. Significant differences were found in the proportion of Lithophyllum species in relation to other red coralline algae found in this study. Our results show that on the Brazilian continental shelf, the rhodolith-forming species are quite higher in size than in any other studied areas in the world. There was no difference in the proportion of live-to-dead rhodolith materials, suggesting an old bed deposit. Also, the amount of calcium carbonate material in the specimens is relevant to take in account in terms of the CO₂ balance worldwide.     

Disturbance and contrasting patterns of population structure in the brachiopod Terebratulina septentrionalis (Couthouy) from two subtidal habitats

The population structures of Terebratulina septentrionalis (Couthouy) from exposed upper rock surface and semi-cryptic rock wall habitats at 33 m depth in the Gulf of Maine differ. Over a 3-yr period, population densities were consistently higher in rock wall habitats. Although both populations were dominated by juveniles (1–4 mm shell length), size-frequency distributions constructed from upper rock surface and rock wall populations were significantly different, as a result of a greater frequency of large brachiopods (> 20 mm shell length) in rock wall populations. Prominent modes occurred at 14–15 mm shell length in upper surface populations and at 19–20 mm length in rock wall populations. Recruitment was higher in rock wall habitats where ambient light intensities were significantly lower than on upper rock surfaces. Differences in recruitment are either the result of larval selection for shaded rock walls or differential juvenile mortality between habitats. The larvae of Terebratulina settle on a diverse array of substrata. These include bedrock, sandy polychaete tubes and algae in upper surface habitats and bedrock, calcareous polychaete tubes, and ascidians in rock wall habitats. Individuals attached to polychaete tubes and algae in upper surface habitats do not attain large body size (> 13 mm shell length). It is suggested that these differences in population structure reflect the greater intensity of disturbance in upper surface habitats. For example, the cod, Gadus morhua (Linnaeus), ingests brachiopods attached to algae and polychaete tubes in this habitat. Gastropod predation affects brachiopods in upper surface habitats but not in rock wall habitats. Predation by gastropods and asteroids is not size-specific. These results are consistent with the hypothesis that predation contributed to the decline in the abundance and diversity of articulate brachiopods since the Mesozoic, and suggest that the restriction of recent populations to semi-cryptic rock wall and crevice habitats is, in part, controlled by disturbance.