**7. Discussion**

The natural complexity of coastal lagoons is the result of a wide range of situations, mainly driven by the degree of confinement, the ionic composition and the salinity variations depending on evaporation vs. inputs from rivers and seas. This determines the absence of a clear or unidirectional salinity gradient and leads to a three-dimensional heterogeneity, influencing the species composition and favouring the dominance of a few ecological guilds, such as, for fish fauna, the marine migrants [106]. However, several studies demonstrated the significant role of lagoon geomorphological features to explain fish assemblages in terms of both productivity, taxonomic and functional diversity ([107–109] and references therein). According to the Ramsar convention, wetlands (including coastal lagoons) are "areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six metres" [110]. Tidal wetlands have a significant role in selectively trapping fine sediments, influencing the water residence time and sequestering nutrients and pollutants [111]. In the present paper, the main focus is on the importance of fish fauna in two structures that characterize coastal lagoons of middle and high latitudes: salt marshes and mudflats. Marshes can be found in all aquatic environments where the erosive forces, produced by waves and currents, are weak and a sheltered and sedimentary environment prevails. Along coasts, the amplitude of the tidal regime determines the salt marsh structure, extension and the habitats within a salt marsh, that can be characterized by creeks and pools, which provide diverse habitats for aquatic vegetation and animals [112]. Nearby salt marshes, mud flats are the structures periodically inundated by the tide and their characteristics vary depending on the tide excursion. According to Wolanski and Elliott [111], "Mudflats located below neap high tide are wetted daily by the tides and the top layer of the mud is commonly soft and unconsolidated. Mudflats between the neap and spring tide high water marks are wetted on alternate weeks, whereas those in supratidal areas are tidally inundated only at the highest astronomical tides."

Multifactorial approaches are recommended to describe the ecological status of coastal lagoons [106], due to the high heterogeneity described above and also considering that the larger the lagoon, the greater the habitat diversity [108]. Traditional metrics or the indices developed in the framework of National and International laws mainly focus on the water quality or on rapid environmental changes. Often, the information on the rate and impact of erosive processes that affect lagoon morphology on a long-term scale is missed. Therefore, the use of indicator species can add additional support to decision-makers, targeting specific managemen<sup>t</sup> actions to habitat restoration [15], focusing on morphological alterations, salinity changes, and the presence of invasive alien species. Ecological indicator species are sensitive to particular environment attributes [113], so their presence and abundance can provide information on it [15]. Despite that marine migrants, mainly juveniles, received much more attention in coastal lagoon research, due to their fishery importance, the few fish species of lagoon residents are more abundant [91,114–116] and can be more suitable indicators

due to their low dispersal abilities and strong relationships with lagoon morphological structures during their entire life cycle. Some evidence on the relationship between morphological degradation and the status of nekton community highlighted that the e ffects of pressures acting on the lagoon morphology are stronger for resident species than for marine migrants [117]. As an example, the populations of *Atherina boyeri* (Risso, 1810), a small estuarine resident of commercial interest, resulted to be significantly a ffected by pressures deriving from the morphological (hydrological) alteration in Marano-Grado lagoon system (Northern Adriatic Sea) [118].

### *7.1. Cyprinodontiformes (Aphaniidae and Valenciidae)*

Among the lagoon residents included in Annex II of the Habitat Directive here presented, *A. fasciatus* is recognized to be a typical component of the fish community of saltmarsh systems [114,119], which generally represent important transition areas in bu ffering flood phenomena [31] or in supporting the morphological complexity that counteracts the increasing inputs of seawaters. The in-depth study of *A. fasciatus* bio-ecology could provide the necessary tools to monitor and manage the erosion processes that a ffect its habitat.

The present overview highlights the occurrence of *A. fasciatus* in brackish to hyperhaline systems characterized by shelter habitats: such as small-sized intertidal creeks (200–250 m long, 2–4 m width), with a maximum depth of 0.7 m [49]. *A. fasciatus* seems to be able to tolerate eutrophic waters, but it needs small tidal creeks or sheltered canals to maintain successful reproductive and survival rates, indicating a significant relationship with the structure of saltmarshes. Moreover, sites with around 15% of organic matter were found to better support the local population than the zones with a percentage around 7% [28]. Locally, healthy populations were found to reach an annual mean higher than 100 ind/100 m<sup>2</sup> [28], while in the areas where a decreasing trend was observed, the catching accounts for few specimens [34,44]. The presence of aquatic vegetation coverage is another fundamental factor.

Beyond salt marsh degradation, and due to an increasing erosive process, habitat fragmentation also tends to accentuate population isolation and weaken their genetic heritage and, in the long term, adaptive capacity. The e ffects of habitat fragmentation and the consequent increase of isolation would be evident in the future, when the intraspecific genetic diversity will be drastically reduced.

Furthermore, *A. fasciatus* was demonstrated to be strongly related to artificial habitats, such as salt works and small man-made creeks [28,49], that are important components of the lagoon landscape, as the historical product of the interaction between traditional human activities and the lagoon coastal habitats. This species could, therefore, be considered indicator species for these peculiar components of the lagoon landscape, contributing to support its conservation and management.

The use of *A. fasciatus* as a sentinel of anthropogenic impacts in Mediterranean coastal lagoons is well-documented [51,120,121]. In fact, Kessabi et al. [51] believe that it may be a good indicator for detecting metal and organic pollutants by using mRNA biomarkers. Heavy metal pollution e ffects were, in fact, documented in both reproductive function [50] and spinal deformities [51]. Mossesso et al. [120] proposed *A. fasciatus* like a promising "sentinel organism" to detect the genotoxic impact of complex mixtures in coastal lagoon ecosystems.

As endemic species, *A. iberus*, that is, the most endangered species among lagoon residents, and *V. hispanica* and *V. letourneuxi*, which are priority species, can provide information on local habitats of high ecological value. In particular, *A. iberus* can be used as an indicator of the functional complexity of the lagoon ecosystem, with a habitat a ffinity similar to that of *A. fasciatus*, while the *Valencia* spp. are sensitive to hydrological variations and salinity increase. *A. iberus* was found to be more abundant far from the sea and to prefer vegetated areas [54]. *V. letourneuxi* was found to tolerate a salinity up to 4 in the wild [103]. Therefore, the increase of seawater intrusion due to infrastructural intervention or aquifer managemen<sup>t</sup> could have further compromised its presence.
