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Populations of the green turtle (Chelonia mydas), a mega-herbivore that consumes seagrasses, are recovering worldwide. Understanding green turtle adaptations to herbivory and responses to changes in seagrass availability will be critical to interpreting plant–herbivore interactions as green turtle populations continue to rebound. Ingesta particle size and diet composition of two green turtle foraging aggregations (Bermuda, 32.3° N, 64.8° W; U.S. Virgin Islands [USVI], 17.8° N, 64.6° W) in the Northwest Atlantic (NWA) were evaluated to assess the prevalence of herbivory across foraging sites and life stages, determine if there is an optimum ingesta particle size, and evaluate green turtle responses to changes in seagrass availability. Both aggregations were herbivorous (> 90% seagrass/algae) across size classes (straight carapace length, SCL). Ingesta particle size (mean ± SD) did not differ between Bermuda (2.6 ± 1.4 cm) and the USVI (2.3 ± 1.2 cm). Of seagrass leaves ingested, 20–30% were 1.7 cm in length, indicating a potential optimum for maximizing digestion rates. Turtle size (SCL) had a significant effect on particle size in Bermuda (p = 0.01, R2 = 0.16) (35.1 ± 9.9 cm SCL) but not in the USVI aggregation, which was comprised of larger turtles (49.0 ± 6.1 cm SCL). In Bermuda, there was no apparent response to the declines in seagrass availability. Ingesta particle size and volume of seagrass leaves did not decline from 2015 to 2019, nor was there an increase in volume of seagrass roots and rhizomes. These results indicate herbivory is prevalent across size classes at two NWA foraging sites and ingesta particle size has important implications for optimizing the green turtle grazing strategy and facilitating ontogenetic diet shifts to herbivory in juveniles. Ingesta particle size is a valuable tool for assessing green turtle responses to seagrass declines that should be interpreted within the context of population demographics.

Seagrasses, a polyphyletic group of about 60 marine angiosperm species, are the foundation of diverse and functionally important marine habitats along sheltered sedimentary coasts. As a novel ecological function with high societal relevance, a role of the seagrass leaf canopy for reducing potentially harmful bacteria has recently been hypothesized in tropical regions, but data for temperate regions are lacking. Here, we tested whether or not the abundance of general bacteria and more specifically, those belonging to the genus Vibrio were reduced within temperate Zostera marina (eelgrass) meadows compared to adjacent sand flats and sampled 5 sites in the south-western Baltic Sea using SCUBA. Compared to non-vegetated area, we found an average reduction of 39% for all Vibrio and 63% for the potentially harmful V. vulnificus/cholerae subtype based on robust plate counting data on Vibrio selective agar. The underlying mechanism of the reduction in bacterial load is currently elusive and clearly merits further study. Our results underline the critical importance of seagrasses in maintaining shallow water ecosystem functioning including water quality and provide further motivation for their protection and restoration.

Dietary studies provide key insights into threats and changes within ecosystems and subsequent impacts on focal species. Diet is particularly challenging to study within marine environments and therefore is often poorly understood. Here, we examined the diet of stranded and bycaught loggerhead (Caretta caretta) and green turtles (Chelonia mydas) in North Cyprus (35.33° N, 33.47° E) between 2011 and 2019. A total of 129 taxa were recorded in the diet of loggerhead turtles (n = 45), which were predominantly carnivorous (on average 72.1% of dietary biomass), foraging on a large variety of invertebrates, macroalgae, seagrasses and bony fish in low frequencies. Despite this opportunistic foraging strategy, one species was particularly dominant, the sponge Chondrosia reniformis (21.5%). Consumption of this sponge decreased with increasing turtle size. A greater degree of herbivory was found in green turtles (n = 40) which predominantly consumed seagrasses and macroalgae (88.8%) with a total of 101 taxa recorded. The most dominant species was a Lessepsian invasive seagrass, Halophila stipulacea (31.1%). This is the highest percentage recorded for this species in green turtle diet in the Mediterranean thus far. With increasing turtle size, the percentage of seagrass consumed increased with a concomitant decrease in macroalgae. Seagrass was consumed year-round. Omnivory occurred in all green turtle size classes but reduced in larger turtles (> 75 cm CCL) suggesting a slow ontogenetic dietary shift. Macroplastic ingestion was more common in green (31.6% of individuals) than loggerhead turtles (5.7%). This study provides the most complete dietary list for marine turtles in the eastern Mediterranean.

Relying on a purely taxonomic view of diversity may ignore the fact that ecological communities can be constituted of species having both distinct evolutionary histories and functional characteristics. Thus, considering how the multiple facets of diversity vary along environmental and geographic gradients may provide insights into the role of historic processes and current environmental changes in determining the divergence or convergence of lineages and functions, ultimately influencing the way species assemble across space. However, analyses can be somehow flawed by the choice of traits being analysed, as they should capture the whole functional variability of species in order to assess the relationship between phylogenetic and functional diversity along a gradient. When continuous measures of functional diversity based on a variety of different traits are absent, the use of functional traits known to show strong phylogenetic signal can help elucidating such relationship. By using distributional, traits and taxonomic-distance information, we explored how the taxon, functional and phylogenetic community composition (co)vary along spatial and environmental gradients in seagrass amphipod metacommunity within the Mediterranean Sea. We used beta diversity partitioning and null models to determine the role of deterministic and stochastic processes on the replacement and the net loss/gain of species, lineages and highly conserved {beta}-niche traits. We showed that dispersal-based processes are the main determinants of the high taxonomic and phylogenetic beta diversity, while niche-based processes explain the low functional dissimilarity among assemblages. Moreover, phylogenetic and functional beta diversity showed contrasting patterns when controlling for the underlying taxonomic composition, with the former being not significantly different and the latter significantly lower than expected. Our results suggest the key role of historical and biogeographic processes in determining the present-day patterns of community assembly and species turnover, providing also evidence for parallel assemblage structures in Mediterranean seagrass amphipods.

To understand habitat requirements of green turtles in an important foraging area subject to both anthropogenic and natural perturbations, we examined residency, home range and habitat use of juvenile, sub-adult and adult green turtles (Chelonia mydas) in Port Curtis, Australia, using passive acoustic telemetry. Acoustic tags were attached to turtles at Pelican Banks (n = 33) and Wiggins Island (n = 16). Between May 2013 and September 2014, tagged turtles were detected > 1.3 million times by 46 acoustic receivers. Data demonstrated variability in residence index and home range size between Pelican Banks and Wiggins Island that was largely attributed to larger body size at Pelican Banks and habitat differences. Tide had a significant influence of movement and habitat use. At Pelican Banks turtles moved into intertidal seagrass beds at high tide and retreated to the channel at low tide. At Wiggins Island, high tide 50% KUD area was more than double low tide area and turtles moved into the mangroves at high tide and retreated to channels at low tide. Diet reflected availability of food resources; seagrass dominated diet at Pelican Banks whereas epiphytic red algae dominated diet at Wiggins Island. There was no evidence of a diurnal shift in home range or habitat use. The use of intertidal areas by both turtles and small recreational craft during high tide at Pelican Banks resulted in increased risk of vessel strike. Unusually short residency at Pelican Banks suggested that animals had either relocated to another foraging area or a combination of recent flooding, port development and reduced seagrass coverage during the period of monitoring resulted in higher emigration rates than previously demonstrated for this species. Acoustic telemetry is a valuable tool for obtaining fine-scale, long- term data on movement and residency of green turtles.

Seagrass beds are important habitats in coastal areas but increasingly decline in area and quality, thus conservation measures are urgently needed. Quantitative food webs, describing the biomass distribution and energy fluxes among trophic groups, reveal structural and functional aspects of ecosystems. Their knowledge can improve ecological conservation. For the recently discovered large warm-temperate seagrass (Zostera japonica) habitat in China’s Yellow River Delta wetland, we used δ13C and δ15N measurements and a Bayesian isotope mixing model to construct its food web diagram with quantitative estimations of consumer diet compositions, comprising detritus and 14 living trophic groups from primary producers to fish. We then estimated the quantitative food web fluxes based on biomass measurements and calculated corresponding ecosystem functions. Pelagic producers were significantly 13C-depleted compared to benthic sources. Consumers (except zooplankton) were increasingly 13C-depleted with increasing trophic positions even though the consumed benthic production surpassed the pelagic one. Bivalves dominated consumer biomasses and fluxes and were the first to connect the pelagic and benthic pathways, whereas zooplankton and gastropods were specialized on the two pathways, respectively. We found flat biomass and production pyramids indicating low trophic transfer efficiencies. Generally, the energetic structure of the quantitative food web was consistent with the stable isotope analysis, and the estimated net primary production and most estimated production to biomass ratios of the trophic groups fell within literature ranges. This study provides a systematical understanding of the quantitative trophic ecology of a seagrass bed and facilitates synergistic knowledge on management, conservation, and restoration.

Aegagropiles are round-shaped conglomerations of Posidonia oceanica debris commonly found along the coasts of the Mediterranean Sea. This study presents a detailed description of the composition of aegagropiles in terms of their internal organisation in different layers (and the orientation of the fibres in these layers), the proportion of constituent elements (fibres and minerals) and the histological (by way of microscopic observations) nature of all of these aegagropiles elements. The aim of this work is to take a detailed interest in the structure of the aegagropiles of Posidonia oceanica and to determine the process that forms them. Aegagropiles are an assemblage of two types of debris from the P. oceanica meadow: (1) plant-based: fibres more or less degraded from P. oceanica shoots (leaves and rhizomes) and (2) mineral particles such as silicates and biotic Ca-carbonate debris. On the basis of structural and compositional observations, we proposed an elucidation of the cycle in several phases: initiation of a “roll” by aggregation of litter fibres and sand in the ripple marks, growth, breakdown of the roll into small balls (microbial and mechanical degradation) and export of aegagropiles down (into the abyss) or on to the beaches. Calculations estimate that considering its density of 0.2 g/cm3, an aegagropile represents the accumulation of fibres from approximately 25 shoots of P. oceanica.

Intertidal macrobenthic assemblages associated with monospecific stands of Zostera muelleri, Cymodocea serratula, Halodule uninervis and Halophila ovalis seagrasses are known to display uniform spatial patchiness on the Moreton Bay coast of North Stradbroke Island, Queensland, as do those in Z. capensis in the Knysna estuarine bay, South Africa. Thirty-seven historical datasets of these macrobenthic assemblages were re-analysed to assess variation of local patchiness in each of the 18 most common individual assemblage components at each of these localities in terms of three metrics: overall patchiness (Lloyd's index of patchiness), levels of unoccupancy, and variation in abundance across occupied samples (Lloyd's index of mean crowding). Within-site patchiness was not caused by a restriction of individual species to specific subareas but by variation in their local density, particularly by the extent of unoccupied ‘interstitial’ spaces within patches. Especially in the more uniform Queensland conditions, the more common species occurred relatively widely across the whole locality; individual samples from which a given species was absent never themselves formed patches, the number of such samples conforming to points on truncated normal curves of the frequency of occurrence. Of the 36 species investigated, the two most abundant and widespread both in Queensland and in South Africa displayed significant or near-significant uniformity of levels of local patchiness, whilst five showed significantly uniform mean crowding and ten significantly uniform unoccupancy. This is the first demonstration that some species may display a characteristic level of patchiness in a given habitat type.

In species presenting long intervals between reproductive seasons, atmospheric-oceanic phenomena can affect the yearly number of individuals that become reproductively active. Sea turtles take multiyear intervals between reproductive seasons, which are influenced by environmental processes affecting their nutrition. Atmospheric-oceanic phenomena affect the reproductive output of different sea turtle species and populations. The population of green turtles (Chelonia mydas) nesting at Tortuguero-10.4489° N, 83.5069° W-, Costa Rica, is the largest in the Atlantic Basin and one of the most comprehensively studied. Nevertheless, only one study analyzed the effects of sea surface temperature on the reproductive output of Tortuguero green turtles. We analyzed three decades of data that revealed two likely scenarios: (1) more green turtle clutches were laid at Tortuguero after warm sea surface temperature anomalies during boreal winters and (2) fewer green turtle clutches were laid at Tortuguero 2 years after El Nino and more green turtle clutches were laid 2 years after La Nina events. Atmospheric-oceanic phenomena affect green turtle reproductive output likely through disrupting nutritional pathways, which alters the number of female green turtles that become reproductively active each year. Tortuguero green turtles are herbivorous feeding mostly on seagrass meadows off Nicaragua. As primary consumers, green turtles are tightly constrained by environmental conditions, and if the productivity of seagrass meadows decreased due to elevated sea surface temperature in the Caribbean basin or the altered hydric regime following El Nino events, fewer females would be able to accrue energy for reproduction.

The spread of invasive species is a major component of global ecological change and how and when to manage particular species is a difficult empirical question. Ideally, these decisions should be based on the specific impacts of invading species including both their effects on native competitors and how they may or may not play similar roles in broader ecosystem functioning. Halophila stipulacea is an invasive seagrass currently spreading through the Caribbean, and as seagrasses are foundation species, the effects of invasion have the potential to be particularly far-reaching. To evaluate the impacts of H. stipulacea we quantified spread and potential for displacement of native seagrasses as well as the effects of invasion on multiple ecosystem processes, particularly resource support for higher trophic levels and habitat creation. Long-term monitoring suggested that H. stipulacea likely displaces some native seagrasses (Syringodium filiforme and Halodule wrightii), but not others. Halophila stipulacea had lower N and protein levels and higher C:N ratios than native seagrasses, and as such is a poorer quality resource for consumers. We also observed significantly lower consumption of H. stipulacea than the native S. filiforme but limited differences compared to Thalassia testudinum. We found H. stipulacea created a more nutrient limited environment than T. testudinum and there were significantly distinct invertebrate assemblages in native- and invasive-dominated seagrass beds, but no difference in species richness or invertebrate biomass. These results suggest that the spread of H. stipulacea would impact a variety of ecological processes, potentially restructuring seagrass ecosystems through both direct impacts on environmental conditions (e.g., nutrient availability) and indirect food web interactions.

The presence of a seasonally variable biogenic habitat (eelgrass, Zostera marina) increased the spring–summer variability of associated nekton relative to unvegetated bare tidal flat. This spatio-temporal pattern emerged because most eelgrass-associated taxa tracked the decline in eelgrass biomass from summer to spring, but in one case reached greater density in spring when predation intensity was low. Among 21 taxa (26,884 individuals) captured, a strong correlation arose between structure association and summer dominance, and certain functional traits, in particular morphology and on- vs. off-bottom position, were strong predictors of eelgrass association. Structure-associated taxa were slender-bodied and pelagic schooling fishes, while habitat generalists or bare-associated taxa were more consistent seasonally, primarily benthic, and cryptically colored with sand. Estuarine use (transient, reproducing, or estuarine resident) was not a strong predictor of structure association or seasonality. Because an identical sampling design was used in five regions of Washington State, USA, coarse-scale (> 100 km) differentiation in nekton assemblages was identified, representing less of the total variation than across seasons but more than across different habitats. While regional nekton differences were attributable in part to geographic distance and eelgrass morphotypes, the most seasonally variable nekton were at sites with morphotypes adding the least vertical structure but highest density. These results support two mechanisms that increase seasonal variability of taxa using structured habitats, including both bottom-up provision of habitat and resources, and subsequent possibilities for negative interspecific interactions and top-down control.

Detached and floating macrophytes are commonly found along beaches in Southern Australia, but few studies have investigated their associations with animals along the drift pathway, particularly in the southern hemisphere. This study investigated the colonisation by macroinvertebrates and fish of natural and artificial tagged seagrass within 6 h of release and compared this to fauna found with in situ floating macrophyte clumps netted at different distances from shore in Southern Australia. We found some evidence that more macroinvertebrates may colonise floating macrophytes at distances closer to shore (i.e., 0.25 km) potentially due to shallower depths and increased proximity of source habitats. Particular fauna may stay with macrophytes as they arrive and accumulate as wrack in the surf-zone, while others may leave for more suitable habitats. Colonisation by fish to floating macrophytes was judged more likely to be for habitat and shelter rather than food resources. This study provides some further evidence for rapid colonisation of fauna and hence an important habitat function of detached and floating macrophytes along nearshore coastlines in Southern Australia.

Assessing the movements and habitat use of wide-ranging predators is critical for understanding their role within an ecosystem and vulnerability to anthropogenic threats. The bronze whaler (Carcharhinus brachyurus) is a large-bodied, slow-growing shark species that is widely distributed throughout temperate Australian waters. Currently, habitat use, spatio-temporal presence and fine-scale movement information is unknown for Southern Australian waters. This study investigated the spatio-temporal patterns of occurrence, habitat use, and residency of bronze whalers in Gulf St. Vincent, South Australia. Forty-seven predominately small juvenile bronze whalers (83% of tagged sharks

Understanding drivers behind patterns of functionally important groups of fishes is crucial for successful management and conservation of tropical seascapes. Herbivorous fishes are the most prominent consumers of marine primary production which can have profound effects on reef resilience. We explored environmental variables affecting distribution and foraging patterns of herbivorous and detritivorous fish assemblages (siganids, acanthurids and parrotfish) across distinct shallow-water habitats (coral reefs, macroalgae beds and seagrass meadows) during September-November 2016 at Mafia Island, Tanzania (8 degrees 00S, 39 degrees 41E). We performed underwater visual census to quantify fish assemblages, measured habitat features, deployed macroalgal assays and conducted inventories of grazing scars. Multi-dimensional scaling and mixed-effects linear models were used to evaluate differences in fish assemblages and environmental variables influencing abundance and foraging patterns of fishes. Fish communities of focal functional groups differed among habitats. Abundance of herbivores and detritivores as well as relative browsing and scraping was highest on coral reefs compared to macroalgae and seagrass meadows.Adult fish were more abundant on coral reefs while juveniles were abundant in macroalgal beds. Coral cover and crustose coralline algal cover had a positive effect on the abundance of fish in coral reef areas, while macroalgal cover had a negative effect. Contrastingly, in macroalgae habitats, macroalgal cover had a positive effect on the abundance of parrotfish. These results highlight the importance of considering connectivity between macroalgal beds and coral reefs through ontogenetic shifts in habitat use by primarily microphagous parrotfish and of incorporating a range of habitats within coastal management plans.

Marine conservation aims to maintain ecological functioning through the protection of biodiversity, frequently relying on habitat categories as representative surrogates for biodiversity. Habitat biodiversity surrogates typically represent taxonomic diversity, despite species functional traits being more directly linked to ecosystem functioning. Here, we evaluate habitat biodiversity surrogates based on taxonomic and functional diversity of fish assemblages in seagrass ecosystems of Nova Scotia, Canada. We do so by assessing congruence in habitat differences (i.e., seagrass vs adjacent bare soft sediment) between biodiversity components across varying environmental conditions, and also by identifying relationships of fish biodiversity with environmental, habitat, and landscape variables. We found only partial agreement in habitat differences for each biodiversity type, that is, habitat differences in taxonomic diversity were only sometimes reflected by functional diversity. In fact, taxonomic diversity was consistently higher in seagrass than bare sediment, while this was not always the case for functional diversity. Similar functional diversity between habitat types resulted from redundancy in functional traits and consistent expression of dominant traits. Regression analyses indicated that taxonomic diversity was determined by habitat complexity (canopy height, sediment organic content), wave exposure, and seagrass bed size (R2 ~ 0.60). Functional diversity was determined by habitat complexity (shoot density, canopy height, and sediment organic content) and wave exposure (R2 = 0.475), suggesting the importance of environmental filtering in selecting a certain set of functional traits. Our study suggests that multiple biodiversity components combined with knowledge of the environmental context are necessary for optimal use of habitat biodiversity surrogates in conservation planning.

Intertidal and subtidal zones consist of heterogeneous habitats and dynamic environmental conditions, providing diverse options for fish to take advantage of marine resources. We explored how various environmental factors affected habitat use of an ecologically and economically important tropical marine fish, bonefish (Albula vulpes), on a fringing reef flat in Culebra, Puerto Rico, using a fine-scale acoustic telemetry positioning system. Machine learning algorithms and Bayesian inference via integrated nested Laplace approximation indicated diel period was the most important predictor of bonefish habitat use; bonefish occupied seagrass and mixed bottom (seagrass, macroalgae, sand) habitats most often at night, a deep-water soft sediment lagoon during the day, and infrequently used a shallow coral rubble reef crest. Zero-truncated (presence only) positioning frequency revealed more constrained utilization distributions during daytime and periods of higher water temperatures. Bonefish occupancy was highest in seagrass and mixed bottom habitats at lower water temperatures, and declined rapidly throughout the flat above 30 °C, which is consistent with temperature-mediated physiological constraints on performance (i.e. collapse of aerobic scope). Other factors including lunar phase, tidal state, and tide height had limited influence on bonefish habitat use. Building on a body of research, we propose several drivers of bonefish habitat use patterns amongst the diverse regions and habitats occupied, including predation risk, angling pressure, tidal variations, and temperature-related physiological performance. Our results emphasise the importance of conserving important seagrass foraging habitat through management and restoration.

The sea urchin Lytechinus variegatus inhabits seagrass meadows in the northern Gulf of Mexico where seasonal high temperatures are already capable of inducing physiological stress. Climate change will increasingly expose individuals to longer periods of elevated temperature. Sea urchins (n = 12 per treatment) were chronically exposed for a period of 90 days to ambient early summer temperature (26 °C) and a near-future predicted summer temperature (30 °C). Feeding and absorption efficiencies were measured in sea urchins fed a diet of the seagrass Thalassia testudinum embedded in agar. Test diameter and height, and total wet weight were recorded for each individual every 30 days. Mean spine length and diameter were also recorded for each individual at days 0 and 90. During the first 60 days, there were no significant differences in feeding rate, absorption efficiency, or growth of individuals held in the ambient (26°) or elevated (30 °C) temperature treatment. However, after 90 days, sea urchins exposed to 30 °C exhibited reduced rates of feeding, lower levels of total organic content in the Aristotle’s lantern, and thinner spines than those held at ambient temperature. Accordingly, despite the ability of L. variegatus to feed efficiently and sustain growth at predicted near-future seawater temperature for 60 days, after 90 days, individuals had significantly reduced their allocation of resources to skeletal components (spines and lantern). These findings suggest that in the near-future, chronic exposure to elevated temperature may compromise feeding through inadequate maintenance of the lantern and increase vulnerability to crushing predators through a reduction in spine diameter. Both factors could alter the role of L. variegatus as a key player in seagrass ecosystems.

Green sea turtles (Chelonia mydas) arrive on the geographically isolated Bermuda platform as small juveniles and remain until they are approaching sexual maturity, at which point individuals depart for distant feeding and nesting sites. It has been reported that younger green turtles generally tend to carnivory or omnivory and that seagrasses become a significant food source as the turtles grow. Evidence indicates that grazing by green sea turtles in Bermuda is negatively impacting seagrass beds, thus understanding their diets is important to both conserving the turtles and their food. Stable isotope methods were used to investigate ontogenetic diet shifts of green sea turtles and to determine reliance on seagrass by larger turtles. Skin samples from 157 individual turtles and samples of known turtle foods, plants and animals, were collected for determination of consumer and food δ13C and δ15N values. A Bayesian stable isotope mixing model analysis indicated a wide range among individual turtles’ diets, with the greatest differences occurring between small and large turtles; larger turtles consumed seagrass almost exclusively. We also examined diet changes in 12 turtles captured in two successive years; these recapture data confirmed the changes in diet suggested by the relationship between size of turtles and diet composition. Very limited evidence was found of any diet variation among larger turtles that would indicate a shift away from declining seagrasses as their major food source.

Stable isotopes are widely used to better understand dietary contributions and trophic interactions in benthic ecosystems. However, the traditional δ13C–δ15N bi-plots can only depict food-web structure for a single time period or space location. Various statistical approaches have now been developed to identify food-web changes in temporal or spatial scale. In the present study, three community-level statistical methods, including circular statistics, Layman et al.’s metrics and Bayesian Ellipses, were applied to compare natural abundance stable carbon and nitrogen isotope data from primary producers, meiofauna (nematodes and copepods), and macrofauna (polychaetes) in an intertidal area between summer and winter. Bayesian mixing model was also used to determine the food source contributions of consumers. In summer, data suggested that benthic microalgae were a dominant food source for most consumers, except for nematode Spilophorella sp. which mainly relied on seagrass detritus. While in winter, there was a significant increase in reliance on seagrass-based carbon for the consumers. The trophic positions of polychaete Eunice sp. and nematode Ditlevsenella sp. also differed between summer and winter, suggesting that they could change their feeding habits and behaviour according to time and availability of food. Using community-based statistics to quantitatively analyze stable isotope data can thus elucidate changes in food-web structure. In addition, considerable variations in food source utilization and trophic level for the same genus or species may imply that the feeding ecology of benthic infauna should be interpreted with caution and cannot be relied on buccal morphology alone.

Accurate taxonomy, population demography, and habitat descriptors inform species threat assessments and the design of effective conservation measures. Here we combine published studies with new genetic, morphological and habitat data that were collected from seahorse populations located along the European and North African coastlines to help inform management decisions for European seahorses. This study confirms the presence of only two native seahorse species (Hippocampus guttulatus and H. hippocampus) across Europe, with sporadic occurrence of non-native seahorse species in European waters. For the two native species, our findings demonstrate that highly variable morphological characteristics, such as size and presence or number of cirri, are unreliable for distinguishing species. Both species exhibit sex dimorphism with females being significantly larger. Across its range, H. guttulatus were larger and found at higher densities in cooler waters, and individuals in the Black Sea were significantly smaller than in other populations. H. hippocampus were significantly larger in Senegal. Hippocampus guttulatus tends to have higher density populations than H. hippocampus when they occur sympatrically. Although these species are often associated with seagrass beds, data show both species inhabit a wide variety of shallow habitats and use a mixture of holdfasts. We suggest an international mosaic of protected areas focused on multiple habitat types as the first step to successful assessment, monitoring and conservation management of these Data Deficient species. Natural Environment Research Council Industrial Case studentship [NER/S/C/2005/13461]; Chocolaterie Guylian; European Commission (ASSEMBLE project) [227799] info:eu-repo/semantics/publishedVersion

Food and predation are considered to represent the main factors affecting habitat selection by juvenile reef fish. Generally, coral reefs shelter more invertebrate prey but also harbor more predators compared to vegetated areas. To avoid predation, many juvenile fish may select lower-risk vegetated areas rather than coral reefs. Very little information is known about habitat selection by juvenile fish if coral reefs have similar predation rates as vegetated areas. In this study, we quantified the density, diet, survival rate, and food availability of four reef fish species (Lethrinus harak, L. obsoletus, Lutjanus gibbus, and Parupeneus indicus) as well as predator and competitor density within a low-predation lagoon containing bare sand, coral reefs, and seagrass beds. The results showed that the two lethrinids used seagrass beds as primary habitats as small juveniles. Large juveniles of L. obsoletus (5 cm total length after dietary shift) and most juvenile L. gibbus preferred coral reefs. P. indicus were dietary generalists in most habitats. Overall, these instances of ontogenetic habitat selection may be related to variations in diet in each species. We also found that predation rates were extremely low (

Increased atmospheric carbon dioxide leads to ocean acidification and carbon dioxide (CO2) enrichment of seawater. Given the important ecological functions of seagrass meadows, understanding their responses to CO2 will be critical for the management of coastal ecosystems. This study examined the physiological responses of three tropical seagrasses to a range of seawater pCO2 levels in a laboratory. Cymodocea serrulata, Halodule uninervis and Thalassia hemprichii were exposed to four different pCO2 treatments (442–1204 μatm) for 2 weeks, approximating the range of end-of-century emission scenarios. Photosynthetic responses were quantified using optode-based oxygen flux measurements. Across all three species, net productivity and energetic surplus (P G:R) significantly increased with a rise in pCO2 (linear models, P

To enhance our understanding on the foraging biology of juvenile Chinese horseshoe crab on a seagrass-covered intertidal mudflat, we used dual stable isotopes δ13C and δ15N to determine the diet composition and trophic position of 6th–11th instar juveniles in summer and winter. The δ13C and δ15N values in tissues of juveniles and their potential food sources suggested that the juveniles consumed a mixed diet mainly comprised a variety of polychaetes, crustaceans and bivalves, in which these food groups were abundantly available in the study site and largely supported by seagrass biomass in both summer and winter. While δ15N values in juvenile tissues were statistically similar between the two seasons, the δ13C values in tissues of juveniles had a minimal seasonal difference which was probably due to variations in the source values rather than the changes in diet composition of juveniles. There were no differences in δ13C and δ15N values among the size groups of juveniles through 6th–11th instars. Although juvenile Chinese horseshoe crabs may consume the prey in response to the seasonal availability on the mudflat, a healthy condition of habitat with high water quality is essential to support the juveniles and a wide range of intertidal invertebrates they consume.

An experiment was carried out investigat- ing the colonization ability and specific pattern of copep- ods towards a provisional benthic habitat. Since copepods are known to disperse passively and actively, the experi- ment aimed to investigate the pool of colonizers of mac- rophytodetritus and the species-specific active colonization pathways. The experiment was performed in a Mediterra- nean seagrass Posidonia oceanica meadow on defaunated macrophytodetritus accumulations (mainly dead seagrass leaves) for two time intervals (24 and 96 h). Active coloni- zation by copepods, independently of their adjacent poten- tial source pool habitat (bare sandy sediments, P. oceanica canopy, water column and macrophytodetritus) occurred within 24 h. Natural densities (as in the control treatments) were only reached by active colonization through the water column. Both neither diversities nor species composition of natural macrophytodetritus were ever reached by one single migratory pathway, therefore only a combination of inter- stitial migration and water column migration can explain the species occurrence under natural condition. Moreover, every potential adjacent source pool habitat contributed species to the newly colonized macrophytodetritus. How- ever, the main colonizers were mostly species with good swimming capabilities. The diverse pool of species present in the newly colonized macrophytodetritus underlines the

Shark nursery areas are widely regarded as essential habitats for the growth and survival of young individuals. The effective management and protection of shark nursery habitat are contingent upon a clear understanding of how individual species utilize such habitat both spatially and temporally. Although shark nurseries have been identified in the Caribbean, this information is generally lacking. From 2006 to 2012, we used passive acoustic telemetry to monitor the presence, movements, and habitat use of 65 juvenile blacktip sharks (Carcharhinus limbatus) and 42 juvenile lemon sharks (Negaprion brevirostris) within Fish Bay and Coral Bay, two shark nurseries in St. John, United States Virgin Islands. Both species were present in each bay during all months of the year, but abundance peaked during the summer (June–September). Although telemetry data indicated that blacktip and lemon sharks moved throughout each embayment, each species exhibited strong site fidelity to core areas across all years of the study. Habitat partitioning was observed in both nurseries as blacktip sharks generally occurred in areas characterized by water depths of 1.5–6 m with seagrass and sand/mud substrate, while lemon sharks remained in close proximity to or within shallow (

Seagrass meadows can be assimilated to seascape matrixes encompassing a mosaic of natural and anthropogenic patches. Natural patches within the Mediterranean Posidonia oceanica meadows show a structural particularity which consist in a duality of their edge types. One edge is eroded by bottom currents, while the adjacent meadow colonizes the bare sediments. This study aims to study the dynamics of these two edges through the investigation of the biogeochemistry (pH, total alkalinity, dissolved inorganic carbon, CO2, CH4, N2O, H2S, dissolved inorganic nitrogen, PO4 3−) within vegetated and unvegetated sediments. These observations are compared with the adjacent meadow to have a better understanding of the colonization processes. Our results reveal that the P. oceanica matrix shows differences from the vegetated edges of sand patches, especially with regard to nutrient availability, which is generally more important at the colonized edge (dissolved inorganic nitrogen up to 65.39 μM in June). A clear disparity also occurs between the eroded and colonized edge with both a seasonal and bathymetrical variation of leaf biomass with higher disparities at 10 m in June (colonized edge 1415 gDW m−2; eroded edge 1133 gDW m−2). The most important contrasts during this study were assessed in June, suggesting that the warm period of the year is more suitable for sampling to highlight disparate characteristics in temperate seagrass meadows. These findings put into light the potential importance of biogeochemical processes in the dynamics of natural patch edges. We hypothesize that they may influence the structural dynamics of P. oceanica seascapes.

The seagrass Zostera marina is an important marine ecosystem engineer, greatly influencing oxygen and carbon fluctuations in temperate coastal areas. Although photosynthetically driven gas fluxes a ...

Seagrasses provide an important ecological value as nursery habitats, hosting higher juvenile densities than their surrounding less-structured habitats by offering shelter and food to early stages of fish. However, the quantitative evaluations of this nursery function remain elusive. Surveys assessing abundances of juvenile fish over seagrass meadows have been largely conducted by diving visual censuses, where typically only a shallow depth range is adequately covered. Within this study, we developed a low-cost stereo-baited video camera (SBRUV) with high precision and accuracy able to deliver length frequency distributions, including the smallest juvenile fraction. The capacity of the SBRUV system increased the synopticity with respect to typical surveys. We tested the system over seagrass meadows of Posidonia oceanica in a Mediterranean bay, investigating the relationship of juvenile abundance and size of the sparid Diplodus annularis with depth, time of day and protection status (i.e. inside and outside of a Marine Protected Area, MPA). We found significant effects of the depth on the length of the older size classes fraction (larger sizes at deeper stations) and an effect of time of the day on the abundance (less abundant during the evenings), and an opposite pattern was observed for early juvenile’s abundances. The MPA protection had no effect in D. annularis population structure. Interestingly, the relative abundance of D. annularis early juveniles was comparable at all depths, from 2 to 20 m, which suggests a potentially higher nursery value of P. oceanica meadows than earlier thought.

Variability in habitat positioning within seascapes (over kilometres) influences fauna assemblage composition, but the characteristics of a habitat patch (10–100s of metres) influence how species use that patch and how variable assemblages are within habitats. Understanding the relative influence of these two contrasting scales is crucial to improving the management of marine habitats. We used baited remote underwater video systems (BRUVS) to quantify seagrass fish assemblages, and took seagrass cores to quantify seagrass metrics, at ten sites across three seasons in Moreton Bay, Australia to determine if fish are influenced more by seascape context or metrics of seagrass habitat complexity. We found that fish species richness and assemblage composition are most influenced by large-scale variability in seascape (e.g. proximity to ocean and mangroves). However, variability in habitat complexity (e.g. seagrass blade length and density) and proximity to mangrove forests had the greatest effect on assemblage beta diversity. Connectivity with other habitats plays a vital role in structuring the fish community, as it is crucial for daily feeding excursions (mangrove forests) and spawning/reproduction (proximity to ocean). Continuous, non-patchy seagrass beds are however vital to how individuals use a seagrass meadow, with beta diversity being higher in seagrass meadows that were less patchy. Identifying how habitat attributes and context influence fish assemblages is vital for optimizing conservation initiatives. Therefore, we suggest that monitoring populations with biodiversity metrics such as beta diversity can be effective in determining areas that are critical for conservation.

International audience; Alterations in seagrass epiphytic communities are expected under future ocean acidification conditions, yet this hypothesis has been little tested in situ. A Free Ocean Carbon Dioxide Enrichment system was used to lower pH by a ~0.3 unit offset within a partially enclosed portion (1.7 m3) of a Posidonia oceanica meadow (11 m depth) between June 21 and November 3, 2014. Leaf epiphytic community composition (% cover) and bulk epiphytic mineralogy were compared every 4 weeks within three treatments, located in the same meadow: a pH-manipulated (experimental enclosure) and a control enclosure, as well as a nearby ambient area. Percent coverage of invertebrate calcifiers and crustose coralline algae (CCA) did not appear to be affected by the lowered pH. Furthermore, fleshy algae did not proliferate at lowered pH. Only Foraminifera, which covered less than 3% of leaf surfaces, declined in manner consistent with ocean acidification predictions. Bulk epiphytic magnesium carbonate composition was similar between treatments and percentage of magnesium appeared to increase from summer to autumn. CCA did not exhibit any visible skeleton dissolution or mineral alteration at lowered pH and carbonate saturation state. Negative impacts from ocean acidification on P. oceanica epiphytic communities were smaller than expected. Epiphytic calcifiers were possibly protected from the pH treatment due to host plant photosynthesis inside the enclosure where water flow is slowed. The more positive outcome than expected suggests that calcareous members of epiphytic communities may find refuge in some conditions and be resilient to environmentally relevant changes in carbonate chemistry.

Posidonia oceanica meadows host a huge number of shoots and their dynamics is strictly related to the spatial distribution patterns of those shoots. To investigate the structure of P. oceanica meadows at very small spatial scale (i.e. in the 100–102 cm range), point patterns of shoot micro-distribution were analyzed. Spatial distribution of shoots was recorded by cutting all the leaves and by digitizing shoot location from pictures of square frames (1 m2) that were randomly positioned in seemingly uniformly dense stands. Ten frames were sampled, all from Southern Italian meadows, and the position of 7828 shoots was recorded. Nearest neighbour distance (NNd) statistics revealed recurring patterns at the different spatial scales: regular patterns were recognized among shoots at smaller spatial scale (100–101 cm), while aggregated shoot distribution emerged in the 101–102 cm range and an important stochastic component was observed at larger spatial scales. Reasons underpinning different spatial point patterns in P. oceanica meadows were discussed by relating the observed patterns to ecological processes (i.e. competition among shoots, role of “species-specific” drivers or “site-specific” features), also including relationships between shoot NNd and shoot density counts. The raw data, provided as supplementary material, are currently the first and the only source of information available about shoot spatial micro-distribution. In this regard, although our data set cannot represent the whole spectrum of variability in P. oceanica meadows, it can be regarded as a first step towards a better knowledge of small scale shoot point patterns in P. oceanica meadows.

To investigate the spatial variation in bivalve food sources along a pollution gradient and assess bivalve contribution to biogeochemical cycles in tropical coastal ecosystems, the δ13C and δ15N values of bivalves and their potential food sources were studied in northwestern Philippines. In a semi-enclosed embayment affected by river discharge and mariculture activities, bivalves depended primarily on 13C-depleted suspended particulate organic matter such as phytoplankton and/or fish feeds. However, toward the relatively oligotrophic seagrass beds, the bivalve food source gradually shifted to more 13C-enriched resuspended and/or settled particles. Furthermore, in the outer seagrass beds exposed to the open ocean, bivalves mainly relied on similar food sources, such as detritus of microalgae, regardless of the distance from the embayment. These trends appear to reflect the ready availability of the food sources. Especially in the outer seagrass beds, a semi-closed material cycle within the vicinity of the sea bottom likely emerged between bivalves and algae, but not between the phytoplankton in the overlying water column. This resulted in a relatively weak benthic-pelagic coupling for bivalves. These cycles would need to be taken into account when estimating the biogeochemical cycles in eutrophicated coastal areas.

Seagrass meadows are among the most efficient and long-term carbon sinks on earth, but disturbances could threaten this capacity, so understanding the impacts of disturbance on carbon stored within seagrass meadows—‘blue carbon’—is of prime importance. To date, there have been no published studies on the impacts of seagrass loss on ‘blue carbon’ stocks. We experimentally created several kinds of small-scale disturbances, representative of common grazer and boating impacts, within seagrass (Zostera nigracaulis) meadows in Port Phillip Bay (Australia) and measured the impacts on sediment organic carbon stocks (‘C org’, and other geochemical variables—%N, δ13C, δ15N). Disturbance had no detectable effect on C org levels within seagrass sediments, even for high-intensity disturbance treatments, which remained bare (i.e. no seagrass recovery) for 2 years after the disturbance. These findings challenge the widely held assumption that disturbance and concomitant loss of seagrass habitat cause release of carbon, at least for small-scale disturbances. We suggest that larger (e.g. meadow scale) disturbances may be required to trigger losses of ‘blue carbon’ from seagrass meadows.

The influence of dietary elemental contents on consumer stoichiometry was investigated in selected and combined soft tissues (as a proxy of the whole individual) of the omnivorous sea urchin, Lytechinus variegatus. We raised urchins for 4 months in controlled seawater tanks using three different diets with different nutritional contents (from lower to higher: seagrass, red macroalgae, and a formulated diet). Individuals fed the different diets varied an average of 19.7, 19.4, and 38 % in C:N, C:P, and N:P ratios, respectively, with stronger temporal variability for C:P and N:P ratios across tissues and whole individuals. This resulted in homeostasis parameters (1/H) of −0.45, 0.09, and 0.38, respectively, for C:N, C:P, and N:P, indicative of homeostatic to weakly homeostatic organisms, at least for C:P and N:P ratios. Individuals fed the nutrient-rich formulated diet had higher growth rates (14 ± 0.83 g WW month−1) than those fed macroalgae or seagrass (9.3 ± 0.57 and 3.4 ± 0.33 g WW month−1, respectively). However, rapid body increments in more nutritional diets caused both a decrease in the %N and an increase in the %P of soft tissues, which resulted in significant but opposite effects of diet stoichiometry and growth in sea urchin C:N (R = −0.74 and R = 0.93, for diet and growth effects, respectively) and N:P ratios (R = 0.60 and R = −0.63, also, respectively, for diet and growth effects). Among potential compensatory mechanisms helping to preserve certain levels of homeostasis, ingestion rates (g WW diet per g WW of urchin) were higher for seagrass and macroalgae diets than for the nutrient-rich formulated diet. In contrast, absorption and growth efficiencies displayed significant negative associations with nutrient contents in diets and did not exhibit nutritional compensation. Overall, our results suggest that resource stoichiometry strongly determines the growth rate of individuals (R = 0.88, P

Seagrass beds are highly important for tropical ecosystems by supporting abundant and diverse fish assemblages that form the basis for artisanal fisheries. Although a number of local- and regional-scale variables are known to influence the abundance, diversity and assemblage structure of seagrass-associated fish assemblages, few studies have evaluated the relative and joint (interacting) influences of variables, especially those acting at different scales. Here, we examined the relative importance of local- and regional-scale factors structuring seagrass-associated fish assemblages, using a field survey in six seagrass (Thalassodendron ciliatum) areas around Unguja Island (Zanzibar, Tanzania). Fish density and assemblage structure were mostly affected by two regional-scale variables; distance to coral reefs, which positively affected fish density, and level of human development, which negatively affected fish density. On the local scale, seagrass biomass had a positive (but weaker) influence on fish density. However, the positive effect of seagrass biomass decreased with increasing level of human development. In summary, our results highlight the importance of assessing how multiple local and regional variables, alone and together, influence fish communities, in order to improve management of seagrass ecosystems and their services.

Plants evolve the production of wound-activated compounds (WACs) to reduce grazing pressure. In addition, several plant-produced WACs are recognized by various invertebrates, playing the role of infochemicals. Due to co-evolutionary processes, some invertebrates recognize plant infochemicals and use them to identify possible prey, detect the presence of predators or identify algae containing various classes of toxic metabolites. Different metabolites present in the same algae can play the role of toxins, infochemicals or both simultaneously. We investigated the infochemical activity of compounds extracted from three diatoms epiphytes of the seagrass Posidonia oceanica, by conducting choice experiments on invertebrates living in the same community or in close proximity. Furthermore, the specific toxicity of the extracts obtained from the same algae was tested on sea urchin embryos using a standard bioassay procedure, to detect the presence of toxins. The comparison of the two effects demonstrated that invertebrates are subjected to diatom wound-activated toxicants when these algae are not associated with their own habitat, but they are able to recognize volatile infochemicals derived from diatoms associated with their habitats. The specific toxicity of WACs was shown to be inversely correlated to the perceptive ability of invertebrates towards volatile compounds liberated by the same algae. Hence, when the recognition of specific algae by a given invertebrate species evolves, their detrimental effects on the receiving organism may be lost.

The effects of air exposure on the desiccation rate and photosynthetic activity of Thalassia hemprichii and Enhalus acoroides were studied using chlorophyll fluorescence. The desiccation rate of T. hemprichii was higher than that of E. acoroides, while the critical threshold of relative water content (RWCcritical) showed opposite trend. Neither the desiccation rate nor RWCcritical of T. hemprichii was significantly affected by temperature, while the desiccation rate of E. acoroides was affected significantly. For both species, the effective photochemical efficiency of PSII decreased gradually above the RWCcritical, while declined rapidly below the RWCcritical. After initial air exposure, the relative maximum electron transport rate of both seagrasses increased gradually to a maximum and then decreased to initial value near the RWCcritical. Therefore, short-term air exposure above the RWCcritical is beneficial for both seagrasses, and the RWCcritical should be considered when examining the response of seagrass photosynthesis to air exposure and selecting transplantation sites.

Macroalgal fields are a feature of the shallow tropical benthos, yet their importance for coral reef fish population dynamics remains poorly understood. The abundance of fish recruits was recorded using underwater visual census at six macroalgal and 11 coral reef sites in the Montebello and Barrow Islands. Surveys identified 6,935 individual recruit fish from 105 species, 54 genera and 20 families. Of these, 1,401 recruits from 48 species, 31 genera and 14 families were observed in macroalgal sites. Sixteen of the 105 recruit species (15.2 %) were observed exclusively at macroalgal sites. Forty-two (87.5 %) of these species have been observed as adults on adjacent coral reefs. Species composition of fish recruits differed significantly between the two habitats. Corallivore, small omnivore and zooplanktivore recruits had significantly higher numbers in the coral sites, while the results clearly demonstrate that juveniles, within the genera Lethrinus and Choerodon, as well as large algal croppers, are predominantly found at macroalgal (74–100 %) rather than coral-dominated sites. High-canopy macroalgae cover was positively correlated with abundance of these taxa, particularly Lethrinids (r 2 = 0.40). This study is the first to highlight the important attributes of tropical macroalgal fields and suggests that they have a similar role to seagrass meadows as essential juvenile habitat, thus warranting greater attention in conservation planning and ecological studies.

Body size during larval development is one of the most important attributes of aquatic animals. The optimal size for changing form or habitat may vary according to ecological traits of species, while phylogenetic constraints also play a significant role. The major goal of this study is to reveal the patterns in the settlement size of planktotrophic larvae in an archaic gastropod superorder Neritimorpha. We take advantage of the fact that size at various ontogenetic stages of neritimorphs can be rigorously estimated by measuring features of the adult opercula. This unique feature of neritimorphs has allowed us to generate the largest data set so far on larval settlement sizes within a group of marine invertebrates that recruit into very different post-metamorphic habitats. Eighty-eight species that represent most extant genera from rocky shores, seagrass beds, mangroves, estuaries, streams, submarine caves, deep-sea vents or seeps showed negligible intraspecific variation and considerable interspecific differences in settlement size, particularly between genera or families. Settlement size is determined primarily by phylogenetic constraints, while parallel evolution toward smaller sizes was shown to occur exclusively in four independent clades (two living and two extinct) of amphidromous snails with a marine larval period followed by a limnic adult phase. The smaller settlement size may possibly reduce the risk of being wafted away from the estuaries of their natal streams through less time achieving metamorphic competence, while ability to make occasional long-distance trips is retained by the presence of a sufficiently long delay period. This delay period also seems to obscure the possible correlation between settlement size and geographic distribution range of neritimorph species, both fully marine and amphidromous.