Abundance patterns of subtidal solitary ascidians in the San Juan Islands,Washington, as influenced by food preferences of the predatory snail Fusitriton oregonensis

The assemblage of solitary ascidians in rocky subtidal habitats of the San Juan Islands, Washington, USA, is dominated numerically by a few species of stolidobranchs: Pyura haustor, Halocynthia igaboja, Cnemidocarpa finmarkiensis and Boltenia villosa. Several phlebobranch species occur abundantly only on floating docks and/or muddy bottoms, where they co-occur with P. haustor and other stolidobranchs. These patterns of abundance correlate with feeding preferences and distributional patterns of the predatory snail Fusitriton oregonensis, which occurs only in rocky subtidal sites, prefers ascidians over other invertebrates as prey and prefers phlebobranchs over stolidobranchs. With the exception of B. villosa, stolidobranchs commonly co-occuring with the snail in the rocky subtidal are not eaten. Transplants of phlebobranchs from docks to rocky subtidal sites and transplants of F. oregonensis into established dock communities suggest that predation by the snail may largely explain the observed patterns of absolute and relative abundance.     

Sediment disturbance and loss of beta diversity on subtidal rocky reefs

How changes in environmental complexity and heterogeneity affect beta diversity is poorly known. We investigated patterns of beta diversity in subtidal assemblages of algae and invertebrates in the northwest Mediterranean in relation to inclination of the substratum and sedimentation. Vertical and horizontal substrata supported distinct assemblages under low, but not under heavy, ambient loads of sediment. To test the hypothesis that sediment reduced the dissimilarity between assemblages, sedimentation was increased experimentally in plots established on vertical and horizontal surfaces at sites experiencing low ambient levels of sedimentation. Patterns were compared to those occurring at unmanipulated sites and at sites exposed to heavy loads of sediment about 2 km apart. After one year, assemblages on vertical substrata were indistinguishable from those occurring on flat surfaces at manipulated sites and both converged toward those occurring at sites exposed to heavy loads of sediment. Control sites still supported distinct assemblages on vertical and horizontal substrata by the end of the experiment. Similar effects of sediment were observed on recovering assemblages in experimental clearings. These results show that sediment increased similarity in assemblages overriding the influence of habitat complexity on beta diversity at small and large spatial scales.     

Limited change in the diversity and structure of subtidal communities over four decades

A unique archive of photographs from 1969 to 1974 permitted a test of the hypothesis that the diversity and composition of contemporary epilithic communities on subtidal rock walls in the San Juan Islands, WA, USA, has changed over 40 years. Notably, the richness and diversity of sessile taxa was significantly higher in 2008–2011. Furthermore, the multivariate community structure of sessile and mobile taxa differed between the historic and modern eras. Historic communities were characterized by a high percent cover of bare rock and non-calcified algal crusts, consistent with the effects of grazing by chitons and urchins. The rate of sessile community turnover, an index less susceptible to spatial sampling artifacts than richness or diversity, was not significantly different between the two eras. Together with the naturally high spatial variability in these epilithic communities, and the limited replication of historic quadrats, we interpret cautiously the data as evidence of limited change despite a clear shift in temperature and predator (fish) guild composition. The lack of substantial change in rock wall communities may be due in part to their vertical topography, which limits physical disturbance and the preemption of space by weedy algae, two processes that are often associated with “phase-shifts” in other marine ecosystems.     

Trophic importance of subtidal metazoan meiofauna: evidence from in situ exclusion experiments on soft and rocky substrates

In coastal marine ecosystems, predation might affect spatial distribution and population dynamics of benthic assemblages. Here, by means of experimental exclusion of potential predators, we compared the effects of epibenthic predation on metazoan meiofaunal assemblages on soft and rocky substrates. Different patterns of abundance were observed in uncaged versus caged plots, across habitats. In caged soft substrates, the abundance of Nematodes, Copepods and Polychaetes increased by 56, 45, 57%, respectively, in the first 3 months. An increase in the number of meiofaunal taxa was also observed. The exclusion of predators from rocky substrates showed less clear patterns. It did not affect the number of taxa while a decrease in meiofaunal abundance was observed. Our results suggest that the exclusion of epibenthic predators had clear effect on total metazoan meiofaunal abundance and on the number of taxa, only in soft bottoms. The different impact of predation across habitats can be potentially explained by differences in terms of spatial variability and substrate complexity. We estimated that, coarsely, more than 75% of total metazoan meiofaunal production can be channeled to higher trophic levels through predation on soft-bottoms. Among meiofaunal taxa, Polychaetes and Nematodes provided the major contribution to benthic energy transfers. These results suggest the trophic relevance of metazoan meiofauna in coastal food webs and claim for the refinement of further experiments for the quantification of its role in different ecological systems.     

Two contrasting effects of predation on species richness in coral reef habitats

The species richness of sessile organisms on settlement panels on a coral reef was measured by the slope of a regression of log_e number of species against log_e area of sample. At a well illuminated site where panels were colonised by algae, the species richness of algae was 19% smaller on surfaces grazed by fishes than on protected surfaces. At a second site in a cave, the species richness of animals on grazed surfaces was 20% greater than on protected surfaces. These results are discussed in the light of differences between the sites. The contrasting effects of predation at the two sites are probably the result of more selective predation at the cave site than at the other site.     

Regulation of benthic algal and animal communities by salt marsh plants: impact of shading

Plant cover is a fundamental feature of many coastal marine and terrestrial systems and controls the structure of associated animal communities. Both natural and human-mediated changes in plant cover influence abiotic sediment properties and thus have cascading impacts on the biotic community. Using clipping (structural) and light (shading) manipulations in two salt marsh vegetation zones (one dominated by Spartina foliosa and one by Salicornia virginica), we tested whether these plant species exert influence on abiotic environmental factors and examined the mechanisms by which these changes regulate the biotic community. In an unshaded (plant and shade removal) treatment, marsh soils exhibited harsher physical properties, a microalgal community composition shift toward increased diatom dominance, and altered macrofaunal community composition with lower species richness, a larger proportion of insect larvae, and a smaller proportion of annelids, crustaceans, and oligochaetes compared to shaded (plant removal, shade mimic) and control treatment plots. Overall, the shaded treatment plots were similar to the controls. Plant cover removal also resulted in parallel shifts in microalgal and macrofaunal isotopic signatures of the most dynamic species. This suggests that animal responses are seen mainly among microalgae grazers and may be mediated by plant modification of microalgae. Results of these experiments demonstrate how light reduction by the vascular plant canopy can control salt marsh sediment communities in an arid climate. This research facilitates understanding of sequential consequences of changing salt marsh plant cover associated with climate or sea level change, habitat degradation, marsh restoration, or plant invasion.     

Factors controlling spatial distribution of the sea cucumber Psolus chitonoides: Settling and post-settling behavior

Substratum selection experiments were carried out in situ with larvae of the holothurian Psolus chitonoides Clark in the rocky subtidal of the San Juan Islands, Washington. The experiments indicated, in agreement with laboratory findings, that pentactula larvae settle gregariously, either on the adult, or (when the bivium is obscured by epizooites) by clustering around the margin of the adult sole. Late larvae and early juveniles are strongly photonegative. Following metamorphosis, juveniles migrate from the adults into nearby shaded habitats, where they may take up residence indefinitely. Although adults retain the capacity to move, they relocate only when their living conditions become unsuitable; in most circumstances they are effectively sessile. Small-scale spatial pattern of the adults, which was documented quantitatively on subtidal rock walls, boulders and cobbles, correlates well with microtopographical features. The aggregation observed in the field is primarily due to post-settling migration rather than larval substratum choice at settlement.     

Rocky substrate communities of the infralittoral fringe of the Boulonnais coast,NW France: a quantitative survey

Infralittoral fringe communities were investigated at 26 stations, in all four seasons (summer 1982, spring, autumn, winter 1983), in different habitats. The investigated habitats can be divided into undersurfaces of boulders, cavelike habitats, crevices between rocks, horizontal and vertical surfaces on rocks. The stations were distributed over four locations, all in the district Pas de Calais, arranged in a sequence of increasing exposure to wave action: Boulogne, Audresselles, Ambleteuse and Cap Gris Nez. Data on quantitative distribution of the biota were collected using quadrat sampling techniques; the data were processed by using cluster analysis. Several communities could be distinguished as separate units, viz. those of undersurfaces of boulders, caves, medium-exposed/exposed vertical/horizontal surfaces. A comparison with previous, preliminary quantitative investigations carried out in 1980 revealed that communities from shaded habitats are apparently stable enough to yield completely reproduceable results. When all habitats are compared, 78% of the samples of 1980 were clustered together with corresponding samples of 1982/1983.     

Regulation of intertidal food webs by avian predators on New England rocky shores

Although there is a large body of research on food webs in rocky intertidal communities, most of the emphasis has been on the marine benthic components. Effects of avian predation on highly mobile predators such as crabs, remains practically unstudied in rocky shore ecosystems. The crab, Cancer borealis, is an important component of the diet of gulls (Larus marinus, L. argentatus) at the Isles of Shoals, Maine, USA. C. borealis prey include the predatory gastropod Nucella lapillus L., the herbivore Littorina littorea, and mussels Mytilus edulis L. We hypothesized that gulls reduce abundance of C. borealis in the low intertidal and shallow subtidal, thereby allowing C. borealis prey to persist in high numbers. A study of crab tidal migration showed that C. borealis density nearly doubled at high tide compared to low tide; thus, crabs from a large subtidal source population migrate into the intertidal zone during high tides and either emigrate or are removed by gulls during low tides. Results from a small-scale (1 m2) predator caging experiment in the low intertidal zone indicated that enclosed crabs significantly reduced L. littorea abundance when protected from gull predation. In a much larger-scale gull exclusion experiment, densities of C. borealis increased significantly during low and high tides in exclosures relative to the controls. C. borealis density was inversely correlated with changes in the abundance of two mesopredators Carcinus maenas and Nucella lapillus, and with the space-occupier M. edulis. There was a similar negative correlation between abundance of C. borealis and the change in abundance of the herbivore L. littorea, but the trend was not significant. Mortality of tethered L. littorea was associated with C. borealis density across sites. However, preferred algae did not change in response to L. littorea density during the experiment. Thus, we found suggestive, but not conclusive, evidence for a three-level cascade involving gulls, crabs, and L. littorea. Our studies strongly suggest that gulls, as apex predators, generate three-level trophic cascades in rocky intertidal food webs by preventing the highly mobile subtidal predator, C. borealis, from establishing substantial populations in the low-mid intertidal zone thereby indirectly enhancing densities of two key mesopredators (N. lapillus, Carcinus) and blue mussels (M. edulis).     

Meiofauna on the seagrass Thalassia testudinum: population characteristics of harpacticoid copepods and associations with algal epiphytes

The composition and abundance of bladedwelling meiofauna was determined over a 15 mo period (1983–1984) from a Thalassia testudinum Banks ex König meadow near Egmont Key, Florida, USA. Harpacticoid copepods, copepod nauplii, and nematodes were the most abundant meiofaunal taxa on T. testudinum blades. Temporal patterns in species composition and population life-history stages were determined for harpacticoid copepods, the numerically predominant taxon. Sixteen species or species complexes of harpacticoid copepods were identified. Harpacticus sp., the most abundant harpacticoid, comprised 47.8% of the total copepods collected, and was present throughout the study. Copepodites dominated the population structures of the blade-dwelling harpacticoid species on most collection dates. Ovigerous females and/or copepodites were always present, indicating continuous reproductive activity. Results suggest that epiphytic algae influence meiofaunal abundance on seagrass blades, as densities of most meiofaunal taxa at Egmont Key were positively associated with percent cover of epiphytic algae throughout the study. The majority of significant correlations between meiofaunal density and cover of epiphytic algae involved filamentous algae, although encrusting algae dominated the epiphytic community. It appears that resources provided by epiphytic algae to seagrass meiofauna (additional food, habitat, and/or shelter from predation) may be associated with algal morphology.     

Does spawning behavior minimize exposure to environmental stressors for encapsulated gastropod embryos on rocky shores?

Adults of motile intertidal invertebrates are able to seek shelter to avoid environmental stress associated with low tides, but embryos within egg masses are effectively sessile for the duration of their encapsulation. Gastropod egg masses from 34 taxa on two rocky shores in SE Australia (34°37′08″S, 150°92′03″E and 34°35′45″S, 150°53′20″E) were surveyed over 2 years (June 2002–May 2004) to test the hypothesis that eggs are deposited in patterns that minimize exposure to environmental stress. Egg masses were expected to be predominantly deposited in shaded habitats not prone to environmental extremes. It was also anticipated that the deposition of egg masses in habitats exposed to UVR, desiccation, and/or extremes in temperature would occur when exposure to these abiotic factors was minimized. Among the taxa investigated, only four species spawned in full sun (Bembicium nanum, Nerita morio, Siphonaria zelandica and S. denticulata). Summer had the highest UVR index, water temperature, and air temperature as well as the lowest daytime tides. Univariate and multivariate analyses confirmed that egg mass abundance was highest during summer, with no change in egg mass size. This study shows that those species depositing egg masses on the surfaces of rock platforms do not adjust the seasonal timing or macrohabitat location of their spawning to avoid physiologically stressful conditions, particularly UVR. Alternate reasons for the evolution of egg mass deposition behavior in apparently sub-optimal habitats are discussed, and it is almost certainly the complex interplay of a variety of highly species-specific factors that is responsible for the patterns observed.     

Regulation of algal community development in a Macrocystis pyrifera forest

The effects of small and large-scale roughness, overstory development, competition for space with sessile animals, and grazing on algal community development in a subtidal Macrocystis pyrifera forest were examined using specially prepared concrete blocks as substrata. Variation in small-scale roughness (crevices and grooves in the order of 0.1 to 3 mm width and depth) had no significant effects on community composition. However, M. pyrifera colonization, algal diversity, and sessile animal biomass were higher near the upper horizontal edges of blocks and concrete prisms. This “edge” effect may result from a combination of increased spore and larval settlement and enhanced growth of plants and animals associated with the turbulent eddies formed around these obstructions. Natural and experimentally produced variations in the algal overstory demonstrated that the presence of an overstory can reduce algal diversity and cover beneath. Caging experiments suggest that predatory fishes and sea-stars indirectly affect the algal community by removing sessile animals (primarily bryozoans) which compete with the algae for space. The exclusion of grazers resulted in increased growth of Gigartina spp. Selective grazing on this genus may account for its reduced abundance in the study area.     

Whole-community mutualism: associated invertebrates facilitate a dominant habitat-forming seaweed

Many habitat-forming, or foundation, species harbor diverse assemblages of associated taxa that benefit from the refuges from predators or harsh physical conditions that foundation species provide. Growing numbers of studies show how specific taxa associated with foundation species can benefit their hosts, but the aggregate effects of the entire community of associated species remain poorly understood. Here, we evaluate the role that a diverse assemblage of invertebrates plays in mediating the dominance of a foundation species, the green filamentous seaweed Cladophora columbiana Collins, in rocky intertidal habitats. Cladophora is a fast-growing seaweed with a high nitrogen demand, and we suggest that it persists in nutrient-limited high-intertidal pools because of local-scale nitrogen excretion by the invertebrate taxa living within its filaments. Removal of associated invertebrates resulted in a fourfold increase in the rate of water-column nitrogen depletion by Cladophora, and ammonium concentrations inside Cladophora turfs with invertebrates present were seven times higher than in the adjacent tide-pool water. The ammonium excreted by invertebrate meiofauna far surpassed the nitrogen used by Cladophora, suggesting that all of Cladophora's nitrogen requirements could be met by the invertebrates associated with it. This study links host performance to the total aggregate biomass of mutualists rather than the particular traits of any one species, suggesting the potential for important feedbacks between individual hosts and the communities of associated species that they support.     

Stalked tunicates Boltenia ovifera form biogenic habitat in the rocky subtidal zone of Nova Scotia

The stalked tunicate Boltenia ovifera is widely distributed in the Arctic and North Atlantic oceans on rocky substrata at 10–300 m depth, although its ecological role in benthic communities is poorly understood. The distribution and abundance of B. ovifera were recorded at 10–100 m depth in towed-video transects in November 2011 and in June, July, and November 2012 off a wave-exposed headland near Halifax, Nova Scotia. Specimens also were collected at 30 m depth using SCUBA (44°26.88′N, 63°31.59′W) in September 2012. Areas dominated by B. ovifera had densities of 10–100 individuals m^?2 on rocky substrata at 30–60 m depth. These tunicate beds often were associated with sparse kelp (Agarum clathratum) and turf-forming red algae. A generalized additive model indicated that depth, substrate type, and benthic algal type were strong predictors of tunicate abundance. Twenty-two epibiotic species were found on specimens of B. ovifera, including juvenile conspecifics. Filter-feeding macroinvertebrates, including anemones and soft corals, were more abundant in areas with B. ovifera than in areas without these tunicates. Our findings suggest that beds of B. ovifera can act as biogenic habitat to enhance local species richness in the rocky subtidal zone of Nova Scotia.     

Diversity and cover of a sessile animal assemblage does not predict its associated mobile fauna

Habitat-forming organisms often determine the structural properties and food resources available to a wide diversity of associated mobile species. Sessile invertebrate assemblages on marine hard substrates support an abundant fauna of mobile invertebrates whose associations with traits of their host assemblages are poorly known. To assess how changes to habitat-forming species are likely to affect their associated mobile fauna, the relationships between abundance, diversity and composition of mobile invertebrates and the diversity, cover and composition of the sessile assemblages they use as habitat were quantified in Sydney Harbour, Australia (33°50′S, 151°16′E). Similar compositions of sessile species were more likely to share a similar composition of mobile species, but univariate measures of the habitat (percent cover, species and functional diversity, prevalence of non-indigenous species) did not predict variation in associated mobile assemblages. These results demonstrate that in this habitat it is difficult to predict the diversity of marine assemblages based on common surrogate measures of biodiversity.     

Harbours as marine habitats: hydroid assemblages on sea-walls compared with natural habitats

Sessile hydrozoans constitute a common component of marine rocky communities. We compared the hydrozoan assemblages occurring on sea-walls of commercial harbours with those on natural rocky cliffs along the southern Iberian Peninsula, to identify differences in the multivariate structure of the assemblages and species richness. Harbour hydroid assemblages significantly differed from natural ones mainly due to their qualitative composition. Medusa-less taxa, optimized for low dispersal and long-term persistence on the substratum, are barely represented in harbours, but abundant at natural sites. “Port species” assemblages were composed of (1) small, short-living species with typical opportunistic characteristics; (2) cosmopolitan large-size taxa, significantly represented both in harbours and in natural habitats; (3) non-indigenous species. Contrarily to the expected lower richness of communities in confined areas, our results demonstrate that richness of hydroid assemblages in harbours is comparable to that of natural habitats.     

Distribution and morphological variation of low-shore algal turfs

The distribution of three functional groups of algae (filamentous, corticated terete and calcareous articulated) was investigated in low-shore, turf-forming assemblages from rocky shores in the western Mediterranean Sea (Italy). Algae were sampled along shores from three different stations. Shores were either exposed to wave action or sheltered and were characterised by various inclinations of the rocky substratum (horizontal, sloping and vertical). The hypotheses tested were: that the relative abundance of the three functional groups of algae (1) varies between sheltered and exposed shores, (2) is influenced by substratum inclination and (3) that these distributions are consistent across stations. There was no clear relationship between the morphology of turf-forming algae and shore characteristics. Covers of filamentous, corticated terete and calcareous articulated algae differed among shores and stations, but patterns were not consistent across shores with similar exposure to waves and substratum inclination. Turfs often comprised algae with different morphologies, in a combination of anchor and epiphytic species. It is suggested that functional groups of algae, as currently identified, are not effective in describing changes in distribution of algae between sheltered and exposed shores, when turf morphologies are considered. Future research on relationships between morphology and ecology of algae should consider the potential importance of positive interactions among associated species, which could override responses of individual functional groups.     

Interactions among sea otters, sea stars, and suspension-feeding invertebrates in the western Aleutian archipelago

The structuring and organizing effects of apex predators on ecosystems are becoming increasingly well documented. The enhancement of kelp forests via sea otter predation on herbivorous sea urchins is among the earliest and best known examples. This study provides evidence for direct and indirect trophic interactions among sea otters, predatory sea stars, and filter-feeding mussels (Mytilus trossulus) and barnacles (Semibalanus cariosis). In western Massacre Bay at Attu Island (173°E, 53°N), subtidal transects showed sea star body size and biomass density declined markedly between 1983 and 1994 as sea otters reinhabited this area. Mussels and barnacles translocated from the rocky intertidal zone to shallow subtidal habitats to assess loss rates from sea star predation showed lower mortality rates after the arrival of sea otters. Prey mortality rates in subtidal caged controls were consistently low and similar to those of intertidal controls in both years. These findings elucidate a trophic pathway by which sea otters can influence ecosystems separate from the well-known sea otter/sea urchin/macroalgae cascade.     

Seascape-dependent subtidal-intertidal trophic linkages

In this study, we test in southern New Zealand a conceptual model of food web linkage that is seascape dependent, which can explain some of the variability in rocky shore community structure among sites and coasts. Using a comparative-experimental approach at local and distant sites we demonstrate that mobile subtidal predators (fish and crabs) can exert strong predation pressure on small mussels in the low tidal zone, but only in sites where the seascape includes subtidal reefs. On intertidal benches with adjacent subtidal reefs (+SR), 60-100% of small (5-15 mm) transplanted mussels were removed within a day from experimental tiles on the low shore when unprotected from predation, compared to fully caged controls that had approximately 100% survival over several months. In partial cages that exclude fish but not crabs, survivorship was intermediate. In contrast, on benches without subtidal reefs (-SR) 40-100% of mussels survived for months, even if unprotected. This difference is expressed in lower cover (0-60%) of mussels on rocks at +SR benches compared to -SR benches (70-99%). The central to northern west coast of the South Island is composed mostly of -SR benches, and predation on small mussels there was low and similar to the -SR benches on the east coast, whereas the +SR benches on the east coast had much greater predation. This contrasts to other studies in New Zealand that examined only predation on larger mussels by seastars and concluded that predation is strong on the west coast and weak on the east coast. Excluding large predators from low-shore areas with new recruits for a year in one +SR site showed longer-term predation effects on their abundance and cover. Short-term sampling at the east coast sites showed that mussel settlement was greater in -SR compared to +SR sites, providing some evidence that seascapes may also affect settlement. Overall, predation depended on the local seascape and ultimately affected community structure via suppression of effective recruitment rates. This study emphasizes the importance of predation on early life stages of basal species and the influence of seascapes on top-down interactions between subtidal predators and their intertidal prey.     

Community development on subtidal temperate reefs: the influences of wave energy and the stochastic recruitment of a dominant kelp

Patterns of community development on subtidal rocky reefs in Marmion Lagoon, southwest Australia, were investigated with a settlement panel experiment. We tested the hypothesis that community development would differ between outer and inner reefs lines, because exposure to swell and wave energy was significantly greater on outer reefs. Following a 14-month deployment, we recorded pronounced variability between panels and sites, but did not detect any effect of wave exposure on the structure of panel assemblages. Subsequent data exploration suggested the importance of the presence of kelp recruits (Ecklonia radiata) in structuring the overall assemblage. Panel assemblages with kelp recruits were significantly different in structure to those without, principally because of greater space coverage of encrusting coralline algae and less coverage of red turfing algae, spirorbids, and bryozoans. Mature E. radiata act as ecosystem engineers in subtidal rocky reefs in southwest Australia. Our results suggested the importance of young, recruiting kelps in determining patterns of early community development on newly available hard substrata.