Abundance and Diversity of Crypto- and Necto-Benthic Coastal Fish Are Higher in Marine Forests than in Structurally Less Complex Macroalgal Assemblages

Predation risk of the sea urchin Paracentrotus lividus juveniles in an overfished area reveal system stability mechanisms and restocking challenges

Where sea urchin harvest has been so intense that populations have drastically regressed, concerns have arisen about the effectiveness of harvesting management. According to the theory of phase transition in shallow rocky reefs between vegetated and barren habitats, sea urchin recruitment, a key population structuring process, seems hampered by some stabilizing feedback despite an end to local human harvest of sea urchins. To shed a light on predation effects on sea urchin recruits, a 27-day field experiment was conducted using mega-predator exclusion cages (40x40x40 cm, 1 cm in mesh size) in barren and turf substrates. To facilitate this, 672 recruits (1.1 ± 0.02 cm in size) reared under control conditions were positioned in groups of 42 in each experimental unit (n = 4). Exclusion of mega-predators had a significant effect regardless the substrate, since a higher number of recruits was found under cages both in turf and barren. However, the results showed that in uncaged treatments the size of recruits that survived was larger in turf than in barren, as in the former substrate predation had reduced the abundance of the smallest recruits, highlighting that mega-predator presence affects differently the size of the recruits that had survived depending on the substrate. Overall, these results provide valuable information to address restocking actions of sea urchin populations in overharvested areas, where algal turfs are widespread, and assist studies on habitat stability mechanisms.

Unveiling functional linkages between habitats and organisms: Macroalgal habitats as influential factors of fish functional traits

Understanding the relationship between the characteristics of habitats and their associated community is essential to comprehend the functioning of ecological systems and prevent their degradation. This is particularly relevant for in decline, habitat-forming species, such as macroalgae, which support diverse communities of fish in temperate rocky reefs. To understand the link between the functional habitats of macroalgae and the functional dimension of their associated fish communities, we used a standardized underwater visual census to quantify the macroalgal functional diversity, as well as the functional diversity, redundancy, and richness of fish communities in 400 sites scattered in three southern temperate marine realms. Our findings reveal that functional macroalgal habitats can be classified into three groups that shape the functional diversity, redundancy, and richness of fish when considering trait commonness. These results enhance our comprehension of the functional connections between the habitat and coexisting fish within marine ecosystems, providing valuable insights for the preservation of these habitats.

Diversity of Molluscan Assemblage in Relation to Biotic and Abiotic Variables in Brown Algal Forests

Canopy-forming macroalgae, mainly those belonging to the order Fucales, form the so-called brown algal forests, which are among the most productive assemblages in shallow coastal zones. Their vertical, branching canopies increase nearshore primary production, provide nursery areas for juvenile fish, and sustain understory assemblages of smaller algae and both sessile and vagile fauna. The majority of benthic invertebrates inhabiting these forests have larval stages that spend some time floating freely or swimming in the plankton. Therefore, canopy-forming macroalgae play an important role as species collectors related to larval supply and hydrodynamic processes. During the past several decades, brown algal forests have significantly reduced their extension and coverage in the Mediterranean basin, due to multiple interacting natural and anthropogenic pressures, with negative consequences also for the related fauna. The aim of this research was to examine how differences in macrophyte abundance and structure, as well as environmental variables, affect the associated molluscan communities in the shallow northern Adriatic Sea. Sampling sites with well-developed vegetation cover dominated by different canopy-forming species were selected in the shallow infralittoral belt of the northern Adriatic Sea in the spring–summer period of the years 2019 and 2020. Our results confirm the importance of algal forests for molluscan assemblage, with a total of 68 taxa of molluscs found associated with macrophytes. Gastropods showed the highest richness and abundance, followed by bivalves. Mollusc richness and diversity (in terms of biotic indices) were not related with the degree of development of canopy-forming species (in terms of total cover and total volume), nor with the ecological status of benthic macroalgae at different depths. On the contrary, the variability in molluscan taxa abundances was explained by some environmental variables, such as temperature, pH, light, and nitrates concentration.

How to quantify algal turf sediments and particulates on tropical and temperate reefs: An overview

Algal turfs are the most abundant benthic covering on reefs in many shallow-water marine ecosystems. The particulates and sediments bound within algal turfs can influence a multitude of functions within these ecosystems. Despite the global abundance and importance of algal turfs, comparison of algal turf-bound sediments is problematic due to a lack of standardisation across collection methods. Here we provide an overview of three methods (vacuum sampling, airlift sampling, and TurfPods), and the necessary equipment (including construction suggestions), commonly employed to quantify sediments from algal turfs. We review the purposes of these methods (e.g. quantification of standing stock versus net accumulation) and how methods can vary depending on the research question or monitoring protocol. By providing these details in a readily accessible format we hope to encourage a standardised set of approaches for marine benthic ecologists, geologists and managers, that facilitates further quantification and global comparisons of algal turf sediments.

Adaptive Vertical Positioning as Anti-Predator Behavior: The Case of a Prey Fish Cohabiting with Multiple Predatory Fish within Temperate Marine Algal Forests

Simple Summary

Fish cohabiting within structurally complex habitats (e.g., coral reefs, seagrass meadows, algal forests) include abundant small-bodied prey fish and specialized piscivorous fish. Habitat structural complexity mediating fish predator–prey interactions has been shown to be an important mechanism sustaining this coexistence. However, the effect of the vertical stratification of habitat structure on predator–prey interactions remains poorly known, especially within a forest-like marine habitat, i.e., a habitat containing three vertical strata (understory, canopy and open-water). We set up tank experiments to test how such habitat vertical stratification affects predator–prey lethal and behavioral interactions, using one prey and two predator model species cohabiting in Mediterranean algal forest. We found that prey anti-predator behavior was predator-specific. When exposed to a sit-and-wait predator, the prey increased its vertical distance from the predator, regardless of the habitat structure. Conversely, when exposed to a stalk-and-attack predator, the prey sought refuge within forest structures. Prey hide motionless within the canopy, the most complex strata, while they avoid and escape from predators within the understory, which is a less complex stratum allowing for fast prey movements but still protected from predators by the canopy above. Our results suggest the crucial role of habitat vertical stratification in influencing predator–prey interactions, which should be studied in three dimensions.

Abstract

Prey fish cohabit with specialized predator fish within structurally complex habitats. How the vertical stratification of the habitat affects lethal and behavioral predator–prey interactions and contributes to explaining these patterns has never been investigated within a forest-like marine habitat, i.e., a habitat containing three vertical strata (understory, canopy, open-water above). We studied this in tank experiments, with a model prey (the wrasse Symphodus ocellatus) and two model predators (the stalk-and-attack comber Serranus cabrilla and the sit-and-wait scorpionfish Scorpaena porcus), which are among the most abundant prey and predators cohabiting in Mediterranean Cystoseira forests. Wrasse anti-predator behavior was predator-specific. When exposed to the scorpionfish, the wrasse increased its vertical distance from the predator, regardless of the habitat structure. Conversely, when exposed to the comber, the wrasse sought refuge within forest structures: (1) the canopy provides more hiding opportunities due to its high complexity, and (2) the understory provides more escape/avoidance opportunities due to (a) its low complexity that allows for fast prey movements, and (b) the presence of the canopy above that limits the comber’s access to the understory. Our results suggest that habitat vertical stratification mediates predator–prey interactions and potentially promotes the co-existence of prey and multiple predators within marine forests.

The role of habitat in the facilitation of Ostreopsis spp. blooms

In recent decades, recurrent Ostreopsis spp. blooms have been recorded throughout the globe, causing public health issues and mass mortalities of invertebrates. Ostreopsis species are benthic and develop in shallow waters in close relation with a substrate, but possible substrate preferences are still ambiguous. Bloom develops on both living and dead substrates and several interacting biotic and abiotic factors acting at different spatial scales can potentially foster or regulate Ostreopsis spp. development. The objective of this review is to collect and summarize information on Ostreopsis spp. blooms related to the habitat at different spatial scales, in order to assess preferences and trends. References including Ostreopsis spp. samplings in the field were analysed in this review, as potentially including information about the micro- (substrate), meso‑ (community) and macrohabitat (ecosystem) related to Ostreopsis spp. blooms. The sampled substrate and the ecosystem where Ostreopsis spp. were collected were generally reported and described in the studies, while the description of the mesohabitat was rarely reported. Ostreopsis spp. were generally described as attached to biotic substrates and in particular, macroalgae, even in studies conducted in coral reefs, where macroalgae are generally not dominant (but they can be in case of coral reef degradation). In both temperate and tropical areas, Ostreopsis spp. were mostly sampled on algal species usually forming medium or low complexity communities (erect or turf-forming algae), often characteristic from post-regime shift scenarios, and rarely on canopy-forming species (such as fucoids and kelps). This literature review highlights the need of collecting more information about the mesohabitat where important Ostreopsis spp. blooms develop, as much as of the underlying mechanisms driving eventual differences on Ostreopsis spp. abundances. This knowledge would allow a better risk assessment of Ostreopsis spp. blooms, identifying areas at high risk on the base of the benthic habitats.

Coastal armouring affects intertidal biodiversity across the Alboran Sea (Western Mediterranean Sea)

Intertidal ecosystems are key habitats that are being replaced by artificial hard substrates due to the increment of human activities in coastal areas. These new substrates host generally less biodiversity mainly due to differences in complexity and composition. This is a global phenomenon and has led to the development of strategies in the framework of eco-engineering. However, mitigating measures, such as new eco-designs, must cope with the high spatial variability of the region where they are applied. Therefore, in order to assess if differences in biodiversity detected at local scales in previous studies could be scaled up to predict patterns at a wider scale, we studied taxa richness and taxonomic structure of intertidal communities across the Alboran Sea (western Mediterranean Sea). We compared four different types of artificial substrates (cubes, rip-raps, seawalls and tetrapods) to assess which produces less impact. Overall, artificial substrates host low benthic biodiversity, specially on seawalls, whereas boulder-like artificial structures such as rip-raps were more similar to natural ones. Nevertheless, the effect of a particular type of artificial structure at a regional scale seems unpredictable, highlighting the challenge that eco-engineering measures face in order to establish global protocols for biodiversity enhancement and the importance of local scale in management programmes.

All shallow coastal habitats matter as nurseries for Mediterranean juvenile fish

Coastal zones are ecosystems of high economic value but exposed to numerous disturbances, while they represent nurseries for many fish species, raising the issue of the preservation of their functions and services. In this context, the juvenile fish assemblages of all types of habitats present in shallow coastal zones were studied on the south-east coast of France using underwater visual censuses in warm (June–July 2014) and cold (April 2015) periods. A total of fourteen habitat types were characterized, which could be grouped into three broad categories, rocky substrates (natural and artificial), sedimentary bottoms with all levels of granulometry, and seagrass beds including Cymodocea nodosa and Posidonia oceanica meadows; the ecotones or interfaces between the three broad habitat categories were individualized as particular habitat types. The abiotic and biotic descriptors of the 14 habitat types individualized did not vary with time, except for a higher cover percentage and canopy height of macrophytes in the warm period, which increased the three-dimensional structure of some habitats. The taxonomic composition and density of juvenile fish assemblages were analyzed using both multivariate and univariate descriptors, after grouping the 57 fish species recorded into 41 well-individualized taxa. Juvenile fishes were recorded in all habitat types, with higher mean species richness and abundance during the warm than the cold period. The richest habitats in terms of both fish species richness and abundance were the natural rocky substrates and the interfaces between Posidonia beds and the other habitats. Although juvenile fish assemblage composition differed among habitat types and between periods, the most abundant fish species were Atherina sp., Sarpa salpa, Gobiidae, Symphodus spp., Pagellus spp. and several Diplodus species, which colonized 7 up to 14 different habitat types (depending on taxa) during their juvenile life. Most species settled in one or a few specific habitats but rapidly colonized adjacent habitats when growing. This study provided evidence of the role of all types of shallow coastal habitats as fish nurseries and their varying pattern of occupation in space and time by the different juvenile stages. It highlighted the importance of the mosaic of habitats and interfaces for the complete development of all juvenile life stages of fishes, and for the preservation of a high diversity of coastal fish assemblages and fisheries resources in the Mediterranean Sea.

Unravelling the taxonomy of an interstitial fish radiation: Three new species of Gouania (Teleostei: Gobiesocidae) from the Mediterranean Sea and redescriptions of G. willdenowi and G. pigra

The clingfish (Gobiesocidae) genus Gouania Nardo, 1833 is endemic to the Mediterranean Sea and inhabits, unlike any other vertebrate species in Europe, the harsh intertidal environment of gravel beaches. Following up on a previous phylogenetic study, we revise the diversity and taxonomy of this genus by analysing a comprehensive set of morphological (meristics, morphometrics, microcomputed tomography imaging), geographical and genetic (DNA-barcoding) data. We provide descriptions of three new species, G. adriatica sp. nov., G. orientalis sp. nov. and G. hofrichteri sp. nov., as well as redescriptions of G. willdenowi (Risso, 1810) and G. pigra (Nardo, 1827) and assign neotypes for the latter two species. In addition to elucidating the complex taxonomic situation of Gouania, we discuss the potential of this enigmatic clingfish genus for further ecological, evolutionary and biodiversity studies that might unravel even more diversity in this unique Mediterranean fish radiation.

The date mussel Lithophaga lithophaga: Biology, ecology and the multiple impacts of its illegal fishery

The date mussel Lithophaga lithophaga is an edible endolithic bivalve, protected by the EU Habitats Directive and other international agreements, living inside carbonate rocks. Its illegal harvesting is carried by breaking the rocks where the bivalve grows. The impact has cascade consequences as it causes permanent changes in the substrate characteristics, the removal of benthic species, a shift from highly complex to structurally simplified habitats. As a result, the rich biodiversity of rocky reefs turns into a biological desert, named "barren". Along with the over exploitation of fish, this practice leads to the increase of sea urchin density and grazing pressure on habitats, hampering the resilience of the associated biodiversity and functions. This paper summarizes the information on date mussel biology, ecology, ecotoxicology, fishery and the legal framework regulating its protection. Evidence indicates that illegal harvesting is still operated and widespread along the Mediterranean and has huge costs in terms of loss of natural capital and ecosystem services, and in terms of active ecological restoration. Two case study areas (the Sorrento and Salento peninsulas) were selected to assess the economic costs of this practice. Tangible economic costs in terms of ecosystems services' loss are huge (from ca. 35,000 to more than 400,000 euros/year in 6.6 km of Sorrento and ca. 1.8 million euros/year along the 69 km of Salento). These costs are, on average, ca. 30 times lower than those of ecosystem restoration. Data mining from websites indicates that date mussels are presently commercialized in hundreds of restaurants in Greece, Balkan countries, Spain and Italy, favoured also by the lack of appropriate consumer information. This practice should be controlled and contrasted at local scale, enforced by national legislations, and implemented by transnational initiatives. Social campaigns are needed to increase public awareness of the serious consequences of date-mussel fishery and consumption.

Impairment of microbial and meiofaunal ecosystem functions linked to algal forest loss

Habitat loss is jeopardizing marine biodiversity. In the Mediterranean Sea, the algal forests of Cystoseira spp. form one of the most complex, productive and vulnerable shallow-water habitats. These forests are rapidly regressing with negative impact on the associated biodiversity, and potential consequences in terms of ecosystem functioning. Here, by comparing healthy Cystoseira forests and barren grounds (i.e., habitats where the macroalgal forests disappeared), we assessed the effects of habitat loss on meiofaunal and nematode biodiversity, and on some ecosystem functions (here measured in terms of prokaryotic and meiofaunal biomass). Overall, our results suggest that the loss of Cystoseira forests and the consequent barren formation is associated with the loss of meiofaunal higher taxa and a decrease of nematode biodiversity, leading to the collapse of the microbial and meiofaunal variables of ecosystem functions. We conclude that, given the very limited resilience of these ecosystems, active restoration of these vulnerable habitats is needed, in order to recover their biodiversity, ecosystem functions and associated services.

Macroalgal forest vs sea urchin barren: Patterns of macro-zoobenthic diversity in a large-scale Mediterranean study

The study aimed at contributing to the knowledge of alternative stable states by evaluating the differences of mobile and sessile macro-zoobenthic assemblages between sea urchin barrens and macroalgal forests in coastal Mediterranean systems considering a large spatial scale. Six sites (100 s km apart) were selected: Croatia, Montenegro, Sicily (Italy), Sardinia (Italy), Tuscany (Italy), and Balearic Islands (Spain). A total of 531 taxa, 404 mobile and 127 sessile macro-invertebrates were recorded. Overall, 496 and 201 taxa were found in macroalgal forests and in barrens, respectively. The results of this large-scale descriptive study have met the expectation of lower macrofauna complexity and diversity in barrens rather than in macroalgal forests, and have allowed estimating the differences in levels of diversity and the consistency of variability across Mediterranean sites. Some peculiar patterns in barrens, related to both abundance of specific taxa and to high values of beta diversity, have been evidenced.

Fish nursery value of algae habitats in temperate coastal reefs

The nursery function of coastal habitats is one of the most frequently mentioned and recognized ecosystem services in the valuation of coastal ecosystems. Despite its importance our understanding of the precise habitat parameters and mechanisms that make a habitat important as a nursery area is still limited for many species. The study aimed to establish the importance of different algae morphotypes in providing shelter and food for juvenile coastal fish during the main settlement peaks, in early spring and late summer, in littoral rocky reef systems in the Northwestern Mediterranean. The results of our study showed strong seasonal differences in algae cover, composition and height between the two sampling periods. Overall, during spring the algae were well developed, while in late summer, both density and height, of most algae decreased considerably. Equally, prey biomass, in form of suitable sized invertebrate fauna associated to the algae, decreased. Accordingly, the shelter and food for the fish settling in this habitat during late summer were less abundant, indicating a mismatch between the observed presence of juvenile fish and optimal habitat conditions. Differences in prey densities were detected between algae morphotypes, with structurally more complex algae, such as Cystoseira spp. and Halopteris spp. consistently containing more prey, independent of season, compared to simpler structured morphotypes such as Dictoytales. The study furthermore related juvenile fish density to habitats dominated by different algae morphotypes. Out of the three-study species (Diplodus vulgaris, Symphodus ocellatus, Coris julis) only S. ocellatus showed a significant association with an algae habitat. S. ocellatus related positively to habitats dominated by Dictoytales which provided the highest cover during late summer but had the lowest prey densities. A strong association of this species with Cystoseira, as reported by other studies, could not be confirmed. Cystoseira was abundant within the study area but in a state of dieback, showing loss and reduced height of foliage, typical for the time of year within the study area. It is therefore likely that algae-fish associations are context-dependent and that several algae species may fulfil similar functions. We also discovered that prey biomass did not appear to have an important effect on juvenile abundances. Nevertheless, the availability of prey may influence juvenile fish condition, growth performance and ultimately long-term survival. We therefore suggest that future studies on habitat quality should also include, besides abundance, indicators related to the condition and growth of juveniles.

Exploring the relationships between marine protected areas and invasive fish in the world's most invaded sea

Marine Protected Areas (MPAs) are an essential tool for marine biodiversity conservation. Yet, their effectiveness in protecting marine ecosystems from global stressors is debated. Biological invasions are a major driver of global change, causing biodiversity loss and altering ecosystem functioning. Here, we explored the relationships between MPAs and alien/native range-expanding fishes in the Mediterranean Sea, the world's most invaded sea. We surveyed fish and benthic communities in nine MPAs and adjacent unprotected areas across six countries. In the South and Eastern Mediterranean MPAs, the biomass of alien and native range-expanding fishes often exceeded 50% of the total fish biomass. Conversely, in the North and Western Mediterranean, alien fishes were absent. A negative relationship was found between native and alien species richness. Average and minimum sea surface temperature (SST) over six consecutive years were positively correlated with the total biomass of alien species; no alien fishes were recorded below 20.5°C average SST and 13.8°C minimum SST. We also found a negative relationship between alien fishes' biomass and the distance from the Suez Canal, which is the main pathway for the introduction of alien fish in the Mediterranean Sea. The biomass of alien and native range-expanding fishes was found to be higher in the South and Eastern Mediterranean MPAs than in adjacent unprotected areas. The association of barrens (rocky reefs deprived of vegetation) and invasive herbivores was observed at all eastern sites, regardless of protection status. Currently, the level of fishing pressure exerted on alien and native range-expanding fishes seems to be the most influential factor determining the lower biomass of invasive fishes at unprotected sites compared to MPAs. Our findings suggest that complementary management actions, such as species-targeted removals, should be taken in MPAs to effectively control invasive fish populations.

Hydroacoustics as a tool to examine the effects of Marine Protected Areas and habitat type on marine fish communities

Hydroacoustic technologies are widely used in fisheries research but few studies have used them to examine the effects of Marine Protected Areas (MPAs). We evaluate the efficacy of hydroacoustics to examine the effects of closure to fishing and habitat type on fish populations in the Cabo Pulmo National Park (CPNP), Mexico, and compare these methods to Underwater Visual Censuses (UVC). Fish density, biomass and size were all significantly higher inside the CPNP (299%, 144% and 52% respectively) than outside in non-MPA control areas. These values were much higher when only accounting for the reefs within the CPNP (4715%, 6970% and 97% respectively) highlighting the importance of both habitat complexity and protection from fishing for fish populations. Acoustic estimates of fish biomass over reef-specific sites did not differ significantly from those estimated using UVC data, although acoustic densities were less due to higher numbers of small fish recorded by UVC. There is thus considerable merit in nesting UVC surveys, also providing species information, within hydroacoustic surveys. This study is a valuable starting point in demonstrating the utility of hydroacoustics to assess the effects of coastal MPAs on fish populations, something that has been underutilised in MPA design, formation and management.

Juvenile fish assemblages in temperate rocky reefs are shaped by the presence of macro-algae canopy and its three-dimensional structure

Arborescent macro-algae forests covering temperate rocky reefs are a known habitat for juvenile fishes. However, in the Mediterranean, these forests are undergoing severe transformations due to pressures from global change. In our study, juvenile fish assemblages differed between pristine arborescent forests (Cystoseira brachycarpa var. balearica) versus an alternate state: bushland (Dictyotales - Sphacelariales). Forests hosted richer and three-fold more abundant juvenile assemblages. This was consistent through space, whatever the local environmental conditions, along 40 km of NW Mediterranean subtidal rocky shores (Corsica, France). Among Cystoseira forests, juvenile assemblages varied through space (i.e. between localities, zones or sites) in terms of total abundance, composition, richness and taxa-specific patterns. More than half of this variability was explained by forest descriptors, namely small variations in canopy structure and/or depth. Our results provide essential cues for understanding and managing coastal habitats and fish populations. Further studies are needed to explain the residual part of the spatial variability of juvenile fish assemblages and to help focus conservation efforts.

Threats to large brown algal forests in temperate seas: the overlooked role of native herbivorous fish

Canopy-forming algae are declining globally due to multiple disturbances. This decline has recently been on the increase due to the spread of some tropical herbivorous fishes. This new phenomenon has drawn attention to the effects of fish herbivory in temperate areas, which have been assumed to be negligible compared to that of invertebrates, such as sea urchins. In this study, the impact of a Mediterranean native herbivorous fish (Sarpa salpa, salema) was assessed on the canopy-forming seaweed Cystoseira amentacea var. stricta. Cystoseira amentacea forms belts in the infralittoral fringe of wave-exposed shores, which has so far been considered a refuge from fish herbivory. To test the effects of salema feeding on natural C. amentacea belts, an innovative herbivore deterrent device was conceived. Salema had a significant effect on C. amentacea by decreasing algal size, biomass and fertility, by up to 97%. The results suggest that the contribution of salema feeding to the loss of Cystoseira forests in the Mediterranean may have been overlooked. In addition, the analysis of temporal and spatial patterns of salema landings in the Mediterranean Sea suggests that salema abundance may have increased recently. Thus, along with invertebrate herbivory and anthropogenic stressors, fish herbivory may also represent a potential threat to algal forests in temperate areas.