Digenea community structure of the Salema, Sarpa salpa (Linnaeus, 1758) (Teleostei, Sparidae), from the Central coast of Algeria.

The main goal of the current work was to describe and analyse the Digenean community structure in Sarpa salpa. A total of 114 specimens of S. salpa were collected from the Central Coast of Algeria at six localities. The fish were immediately transported to the laboratory to be examined for digeneans. From 107 of the 114 S. salpa examined, a total of 8,722 specimens of seven species belonging to three families were recovered. Among them, Robphildollfusium fractum and Mesometra orbicularis were the most prevalent and abundant species. Moreover, Lepocreadium album was found for the first time in S. salpa from the coast of Algeria, and Centroderma spinosissima and Wardula capitellata represent new parasites to the Algerian digenean fauna that infect teleost fish. The component community of these Digeneans is characterised by R. fractum as its dominant species representing 62% of the total number of collected Digeneans, as well by a Shannon diversity index (H') and Dominance Simpson index (D) of 1.03 and 2.23, respectively. According to Spearman's correlation test, Mesometra brachycoelia and W. capitellata were negatively correlated to the host biological factors. On the other hand, R. fractum showed a positive correlation between its mean abundance and the total length of the fish. With these findings, we provided the component structure of the Digenean fauna of S. salpa and highlighted their diversity, contributing to the biodiversity of the parasitic Platyhelminthes in Algeria.

Organic pollutants in marine samples from Tunisian coast: Occurrence and associated human health risks.

140 contaminants belonging to various classes (organochlorine and organophosphorus pesticides, pyrethroid insecticides, carbamates, fungicides, acaricides, herbicides, synergists, insect growth regulators, polychlorobiphenyls, polycyclic aromatic hydrocarbons) were simultaneously analysed by GC-MS/MS in marine sediments, aquatic plant leaves and fish tissues samples. A total of 260 samples from five stations along the coast of Tunisia were evaluated. The results highlight that only 28 residues (12 polychlorobiphenyls, 8 organochlorine pesticides, 7 polycyclic aromatic hydrocarbons and triphenyl phosphate) were detected at levels higher than relative LOQ values. The amounts in sediment samples were compared with Sediment Quality Guidelines (SQGs) showing that the values are acceptable and no toxic effect is expected on aquatic organisms. A little variation of contaminant residues in sediment samples among coastal stations was recorded. Namely, with respect to almost all polychlorobiphenyls and organochlorine pesticides, higher values were recorder in summer. With respect to almost all polycyclic aromatic hydrocarbons, higher values were recorder in autumn. Aquatic plant leaves showed a residue accumulation higher than that of other compartments of marine system. The data about fish samples (Sparus aurata and Sarpa salpa, the two most frequently caught fish species at five sites on the central coast of Tunisia) do not pose direct hazard to human health because values were lower than protection limits.

Monitoring of Environmental Hg Occurrence in Tunisian Coastal Areas.

Total mercury (Hg) was determined in 450 environmental samples (seawater, sediment plant and fish) from five Mahdia coastal areas (Tunisia). Tolerable Weekly Intake% (TWI) values, according to the European Food Safety Authority (EFSA), were calculated based on the average metal concentration in fish and the average weekly fish consumption rate. Hg was accumulated mainly in fish and in Posidonia oceanica leaves. Hg in sediment ranged from 1.88 µg/kg dry weight (d.w.) to 7.48 µg/kg d.w., while it was between 0.32 µg/kg and 0.19 µg/kg in seawaters. Our study showed high concentration in Posidonia oceanica in S3 (plant = 16.76 ± 4.48 µg/kg d.w.) as compared to those in S4 sites (plant = 5.33 ± 0.05 µg/kg d.w.). Concentrations for S. aurata and S. salpa in the Rejiche area exceeded the EC 1881/2006 legislation with values of 1.9 mg/kg and 2.5 mg/kg, respectively, and consumers may be exposed to high concentrations of Hg that exceeds the EFSA. The results showed that the fish species should be constantly monitored due to their TWI% of 154.5% for S. aurata and 209.8% S. salpa respectively.

Boosted fish abundance associated with Posidonia oceanica meadows in temperate shallow CO2 vents.

Ocean acidification (OA) may induce major shifts in the structure and function of coastal marine ecosystems. Studies in volcanic CO2 vents, where seawater is naturally acidified, have reported an overall simplification of fish assemblages structure, while some primary producers are likely to increase their biomass under elevated concentration of CO2. Here we used temperate shallow CO2 vents located around the coast of Ischia island (Italy) to assess the effects of OA on necto-benthic fish assemblages associated with the foundation seagrass species Posidonia oceanica in the Mediterranean Sea. We compared P. oceanica meadow structure, its epiphytic community and the associated fish assemblage structure and diversity at vents with low pH sites and reference sites with ambient pH using underwater visual census strip transects, in two seasons (fall 2018 and summer 2019). Data were analysed using both univariate and multivariate statistical techniques. Results showed greater P. oceanica habitat complexity (i.e. shoot density) and lower abundance of epiphytic calcareous species (e.g. coralline algae) at the vents than reference sites. Total abundance of adult and juvenile fish was higher at vents than reference sites, while no differences were found for species richness and composition. Overall, the herbivore Sarpa salpa stands out among the species contributing the most to dissimilarity between vents and reference sites, showing higher abundances under OA conditions. This pattern could be explained by the combined effect of a positive response to the higher structural meadows complexity and the greater seagrasses palatability/nutritional value occurring at the vents, which may help herbivores to withstand the higher energetic cost to live under high pCO2/low pH conditions. Our results indicate that necto-benthic fish assemblages associated with the Mediterranean P. oceanica ecosystem may cope with OA under the CO2 emission scenarios forecasted for the end of this century.

The role of competition and herbivory in biotic resistance against invaders: a synergistic effect.

Invasive species pose a major threat to global diversity, and once they are well established their eradication typically becomes unfeasible. However, certain natural mechanisms can increase the resistance of native communities to invaders and can be used to guide effective management policies. Both competition and herbivory have been identified as potential biotic resistance mechanisms that can limit plant invasiveness, but it is still under debate to what extent they might be effective against well-established invaders. Surprisingly, whereas biotic mechanisms are known to interact strongly, most studies to date have examined single biotic mechanisms separately, which likely influences our understanding of the strength and effectiveness of biotic resistance against invaders. Here we use long-term field data, benthic assemblage sampling, and exclusion experiments to assess the effect of native assemblage complexity and herbivory on the invasion dynamics of a successful invasive species, the alga Caulerpa cylindracea. A higher complexity of the native algal assemblage limited C. cylindracea invasion, probably through competition by canopy-forming and erect algae. Additionally, high herbivory pressure by the fish Sarpa salpa reduced C. cylindracea abundance by more than four times. However, long-term data of the invasion reflects that biotic resistance strength can vary across the invasion process and it is only where high assemblage complexity is concomitant with high herbivory pressure, that the most significant limitation is observed (synergistic effect). Overall, the findings reported in this study highlight that neglecting the interactions between biotic mechanisms during invasive processes and restricting the studied time scales may lead to underestimations of the true capacity of native assemblages to develop resistance to invaders.

Reduction of herbivorous fish pressure can facilitate focal algal species forestation on artificial structures.

Coastal areas have been transformed worldwide by urbanization, so that artificial structures are now widespread. Current coastal development locally depletes many native marine species, while offering limited possibilities for their expansion. Eco-engineering interventions intend to identify ways to facilitate the presence of focal species and their associated functions on artificial habitats. An important but overlooked factor controlling restoration operations is overgrazing by herbivores. The aim of this study was to quantify the effects of different potential feeders on Cystoseira amentacea, a native canopy-forming alga of the Mediterranean infralittoral fringe, and test whether manipulation of grazing pressure can facilitate the human-guided installation of this focal species on coastal structures. Results of laboratory tests and field experiments revealed that Sarpa salpa, the only strictly native herbivorous fish in the Western Mediterranean Sea, can be a very effective grazer of C. amentacea in artificial habitats, up to as far as the infralittoral fringe, which is generally considered less accessible to fishes. S. salpa can limit the success of forestation operations in artificial novel habitats, causing up to 90% of Cystoseira loss after a few days. Other grazers, such as limpets and crabs, had only a moderate impact. Future engineering operations,intended to perform forestation of canopy-forming algae on artificial structures, should consider relevant biotic factors, such as fish overgrazing, identifying cost-effective techniques to limit their impact, as is the usual practice in restoration programmes on land.